DISCLAIMER:I am in no way affiliated with any branch of the motor industry.

I am
just a pro-car, pro-motorbike petrolhead :-) The information on these pages is
the result of a lot of information-gathering and research. This website was orig
inally established in 1994 to answer a lot of FAQs from posters on the old trans
port-related usenet groups. By reading these pages, you agree to indemnify, defe
nd and hold harmless me (Christopher J Longhurst), any sponsors and/or site prov
iders against any and all claims, damages, costs or other expenses that arise di
rectly or indirectly from you fiddling with your car or motorbike as a result of
what you read here. In short : the advice here is worth as much as you are payi
ng for it. One more thing : the Google ads are only at the top of the page here
- I need to pay for my site space and bandwidth somehow. The rest of the page is
ad-free for your reading pleasure.
Are you confused by your car's tyres? (or tires if you're American). Don't know
your rolling radius from your radial? Then take a good long look through this pa
ge where I hope to be able to shift some of the mystery from it all for you. At
the very least, you'll be able to sound like you know what you're talking about
the next time you go to get some new tyres.
Decoding all that information on the sidewall
It's confusing isn't it? All numbers, letters, symbols, mysterious codes. Actual
ly, most of that information is surplus to what you need to know. So here's the
important stuff:
Key A Description Manufacturers or brand name, and commercial name or identity.
Tyre size, construction and speed rating designations. Tubeless designates a tyr
e which requires no inner tube. See tyre sizes and speed ratings below. Denotes
type of tyre construction. M&S denotes a tyre designed for mud and snow. Reinfor
ced marking only where applicable.
B and J C D
1
Key
Description
A F
ECE (not EEC) or brand name, and Manufacturers type approval mark and number. co
mmercial name or identity. North American Dept of Transport compliance symbols a
nd identification numbers. Tyre size, construction and speed rating designations
. Tubeless designates a tyre which requires no inner tube. See tyre sizes and sp
eed ratings below. Denotes type of tyre construction.
G B and J
C
H D
Country of manufacture. M&S denotes a tyre designed for mud and snow. Reinforced
marking only where applicable.
Also on the sidewall, you might The temperature rating - an indicator highest ra
ting; "C" is the lowest. The traction rating - an indicator of how well the tire
is capable of stopping on wet pavement. "A" is the highest rating; "C" is the l
owest. The tread-wear rating - a comparative rating for the useful life of the t
ire's tread. A tire with a tread-wear rating of 200, for example, could be expec
ted to last twice as long as one with a rating of 100. Tread-wear grades typical
ly range between 60 and 600 in 20-point increments. It is important to consider
that this is a relative indicator, and the actual life of a tire's tread will be
affected by quality of road surfaces, type of driving, correct tire inflation,
proper wheel alignment and other variable factors. In other words, don't think t
hat a tread-wear rating of 100 means a 30,000 mile tyre. Encoded in the US DOT i
nformation (G on the diagram above) is a two-letter code that identifies where t
he tyre was manufactured in detail. In other words, what factory and in some cas
es, what city it was manufactured in. It's the first two letters after the 'DOT'
- in this case "FA" denoting Yokohama. This two-letter identifier is worth know
ing in case you see a tyre recall on the evening news where they tell you a cert
ain factory is recalling tyres. Armed with the two-letter identifier list, you c
an figure out if you are affected. It's a nauseatingly long list, and I've not p
ut it on this page. But if you click here it will popup a separate window with j
ust those codes in it.
Load and pressure marking requirement (not applicable in the find the following
E info UK). These go from a load rubber. embossed in the index of 50 (190kg) up
to an index of 169 of how well the tire withstands heat buildup. "A" is the (580
0kg).
DOT Codes and the 6-year shelf life
As part of the DOT code (G above), there is a tyre manufacture date stamped on t
he sidewall. Take a look at yours - there will be a three- or four-digit code. T
his code denotes when the tyre was manufactured, and as a rule-of-thumb, you sho
uld never use tyres more than 6 years old. The rubber in tyres degrades over tim
e, irrespective of whether the tyre is being used or not. When you get a tyre ch
ange, if you can, see if the tyre place will allow you to inspect the new tyres
first. It's not uncommon for these shops to have stuff in stock which is more th
an 6 years old. The tyre might look brand new, but it will delaminate or have so
me other failure within weeks of being put on a vehicle. Reading the code. The c
ode is pretty simple. The three-digit code was for tyres manufactured before 200
0. So for example 1 7 8 means it was manufactured in the 17th week of 8th year o
f
2
the decade. There was no way of determining which decade, so in fact, 1 7 8 coul
d mean the 17th week of 1988....Good tip : if the tyre has a 3-digit code, don't
buy it!! After 2000, the code was switched to a 4-digit code. Same rules apply,
so for example 3 0 0 3 means the tyre was manufactured in the 30th week of 2003
.
DOT Age Code Calculator
The calculation built in to this page is up-to-date based on today's date. If th
e DOT age code on your tyres is older than this code, change your tyres.
DOT AGE CODE: 11 00
Interesting note : in June 2005, Ford and GM admitted that tyres older than 6 ye
ars posed a hazard and from their 2006 model year onwards, started printing warn
ings to this effect in their drivers handbooks for all their vehicles.
A Word on "guaranteed" tyres
When I moved to America, I noticed a lot of tyre shops offering tyres with x,000
mile guarantees. It's not unusual to see 60,000 mile guarantees on tyres. It am
azed me that anyone would be foolish enough to put a guarantee on a consumable p
roduct given that the life of the tyre is entirely dependent on the suspension g
eometry of the car it is being used on, the style of driving, the types of road,
and the weather. Yet many manufacturers and dealers offer an unconditional* gua
rantee. There's the catch though. The '*' after the word "unconditional" takes y
ou elsewhere on their information flyer, to the conditions attached to the uncon
ditional guarantee. If you want to claim on that guarantee, typically you'll hav
e to prove the tyres were inflated to the correct pressure all the time, prove t
hey were rotated every 3000 miles, prove the suspension geometry of your car has
always been 100%, prove you never drove over 80mph, prove you never left them p
arked in the baking hot sun or freezing cold ice, and prove you never drove on t
he freeways. Wording in the guarantee will be similar to: "used in normal servic
e on the vehicle on which they were originally fitted and in accordance with the
maintenance recommendations and safety warnings contained in the attached owner
's manual" and "The tyres have been rotated and inspected by a participating (ty
re brand) tyre retailer every 7,500 miles, and the attached Mounting and Rotatio
n Service Record has been fully completed and signed" There will typically also
be a long list of what isn't covered. For example:
Road hazard injury (e.g., a cut, snag, bruise, impact damage, or puncture), inco
rrect mounting of the tire, tire/wheel imbalance, or improper repair, misapplica
tion, improper maintenance, racing, underinflation, overinflation or other abuse
, uneven or rapid wear which is caused by mechanical irregularity in the vehicle
such as wheel misalignment, accident, fire, chemical corrosion, tire alteration
, or vandalism, ozone or exposure to weather. Given that you really can't prove
any of this, the guarantee is, therefore, worthless because it is left wide open
to interpretation by the dealer and/or manufacturer. For a good example, check
out the Michelin warranty or guarantee, available on their website (PDF file). D
on't be taken in by this - it's a sales ploy and nothing more. Nobody - not even
the manufacturers - can guarantee that their tyre won't de-laminate or catch a
puncture the moment you leave the tyre shop. Buy your tyres based on reviews, re
commendations, previous experience and the recommendation of friends. Do not buy
one simply because of the guarantee.
Big-chain dealers vs. manufacturer warranties.
3
A reader pointed out to me that the dealer he worked for honoured tyre warrantie
s in a no-fuss manner requiring simply the original receipt for when they were p
urchased and one small form to be filled out. They then typically used a pro-rat
ed refund applied to the new tyre. For example if someone paid $100 for a tyre g
uaranteed for 60,000 miles and it was dead after 40,000, pro-rata the customer h
ad 34% of the warranty mileage left in the tyre. They would either refund $34 (3
4% of $100) or apply it against the cost of a replacement. I suspect this no-fus
s attitude is down to buying power. Large chain stores like CostCo or Sears will
have far more clout with the manufacturers than you or I with our 4 tyres. Afte
r all they buy bulk in he hundreds if not thousands. For the consumer, it makes
them look good because you get a fair trade. They can argue the toss with the ma
nufacturers later, leveraging their position as a bulk buyer in the market to ge
t the guarantees honoured.
Tyre sizes and what they mean.
Okay, so you look at your car and discover that it is shod with a nice, but worn
set of 18565HR13's. Any tyre mechanic will tell you that he can replace them, a
nd he will. You'll cough up and drive away safe in the knowledge that he's just
put some more rubber on each corner of the car that has the same shamanic symbol
s on it as those he took off. So what does it all mean?
185 This is the width in mm of the tyre from sidewall to sidewall when it's unst
ressed and you're looking at it head on (or topdown). This is known as the secti
on width. 65 This is the ratio of the height of the tyre sidewall, (section heig
ht), expressed as a percentage of the width. It is known as the aspect ratio. In
this case, 65% of 185mm is 120.25mm the section height. H R 13 This is the diam
eter in inches of the rim of the wheel that the tyre has been designed to fit on
. Don't ask me why tyre sizes mix imperial and metric measurements. They just do
. Okay?
This is the speed rating of the tyre.
This tells you that the tyre is a radial construction. Check out tyre constructi
on if you want to know what that means.
More recently, there has been a move (especially in Europe) to adjust tyre desig
nations to conform to DIN (Deutsche Industrie Normal). This means a slight chang
e in the way the information is presented to the following:
185 Section width 65 Aspect ratio R Radial 13 Rim diameter 91 load rating V spee
d rating.
Classic / vintage / imperial crossply tyre sizes.
What ho. Fabulous morning for a ride in the Bentley. Problem is your 1955 Bentle
y is running on 7.6x15 tyres. What, you ask, is 7.6x15? Well it's for older vehi
cles with imperial measurements and crossply tyres. Both measurements are in inc
hes - in this case a 7.6inch tyre designed to fit a 15inch wheel. There is one p
iece of information missing though - aspect ratio. Aspect ratios only began to b
e reduced at the end of the 1960s to improve cornering. Previously no aspect rat
io was given on radial or crossply tyres. For crossply tyres, the initial number
is both the tread width and the sidewall height. So in my example, 7.6x15 denot
es a tyre 7.6 inches across with a sidewall height which is also 7.6 inches. Aft
er conversion to the newer notation, this is the equivalent to a 195/100 15. If
you're plugging numbers into the tyre size calculator lower down this page, I've
included an aspect ratio value of 100 for imperial calculations. Note: I put 19
5/100 15 instead of 195/100R15 because technically the "R" means radial. If you'
re trying to get replacement crossply tyres, the "R" won't be in the specificati
on. However if you're trying to replace your old crossply tyres with metric radi
al bias tyres, then the size does
4
have the "R" in it. Here is a javascript calculator to turn your imperial tyre s
ize into a radial metric tyre size:
7.6 15
Your
imperial
tyre /100 R
size:
x
Equivalent standard tyre size is :
Classic / vintage radial tyre sizes.
Remember above that I said aspect ratios only started to come into play in the 1
960s? Unlike the 100% aspect ratio for crossply tyres, for radial tyres, it's sl
ightly different - here an aspect ratio of 80% is be assumed. So for example, if
you come across on older tyre with 185R16 stamped on it, this describes a tyre
with a tread width of 185mm and a sidewall height which is assumed to be 80% of
that; 148mm. The question of the aspect ratio for radial sizes has been the subj
ect of a lot of email to me. I've had varying figures from 80% up to 85% and eve
ryone claims they're right. Well one reader took it to heart and did some in-dep
th research. It seem there is actually no fixed standard for aspect ratio when i
t is not expressly stated in the tyre size. Different manufacturers use slightly
different figures. The english MOT (road-worthiness test) manual states: Unless
marked otherwise, "standard" car tyres have a nominal aspect ratio of 82%. Some
tyres have an aspect ratio of 80%. These have "/80" included in their size mark
ing e.g. 165/80 R13. Note: Tyres with aspect ratios of 80% and 82% are almost id
entical in size and can be safely mixed in any configuration on a vehicle. See h
ttp://www.motuk.co.uk/manual_410.htm for the online version. If you're plugging
vintage radial numbers into the tyre size calculator, I've included aspect ratio
s of 80 and 82 for these calculations.
Metric Tyre sizes and the BMW blurb.
Fab! You've bought a BMW 525TD. Tyres look a bit shoddy so you go to replace the
m. What the....? TD230/55ZR390? What the hell does that mean? Well my friend, yo
u've bought a car with metric tyres. Not that there's any real difference, but c
ertain manufacturers experiment with different things. For a while, (mid 1990s)
the 525TD came with arguably experimental 390x180 alloy wheels. These buggers re
quired huge and non-conformal tyres. I'll break down that classification into ch
unks you can understand with your new-found knowledge: TD - ignore that. 230 = c
ross section 230mm. 55 = 55% sidewall height. Z=very high speed rating. R390=390
mm diameter wheels. These are the equivalent of about a 15.5" wheel. There's a n
ice standard size for you. And you, my friend, have bought in to the long-raging
debate about those tyres. They are an odd size, 180x390. Very few manufacturers
make them now and if you've been shopping around for them, you'll have had the
odd heart-stopper at the high price. The advice from the BMWcar magazine forum i
s to change the wheels to standard sized 16" so there's more choice of tyres. 21
5-55R16 for example. The technical reason for the 390s apparently is that they s
hould run flat in the event of a puncture but that started a whole debate on the
ir forum and serious doubts were expressed. You've been warned... If you're Euro
pean, you'll know that there's one country bound to throw a spanner in the works
of just about anything. To assist BMW in the confusion of buyers everywhere, th
e French, or more specifically Michelin have decided to go one step further out
of line with their Pax tyre system. See the section later on to do with run-flat
tyres to find out how they've decided to mark their wheels and tyres.
Land Rovers and other off-road tyre sizes.
5
On older Land Rovers, you'll often find tyres with a size like 750x16. This is a
nother weird notation which defies logic. In this case, the 750 refers to a deci
malised notation of an inch measurement. 750 = 7.50 inches, referring to the "no
rmal inflated width" of the tyre - i.e. the external maximum width of the inflat
ed, unladen tyre. (This is helpfully also not necessarily the width of the tread
itself). The 16 still means 16 inch rims. Weird eh? The next question if you ca
me to this page looking for info on Land Rover tyres will be "What size tyre is
that the equivalent of in modern notation?". Simple. It has no aspect ratio and
the original tyres would likely be cross-ply, so from what you've learned a coup
le of paragraphs above, assume 100% aspect ratio. Convert 7.5inches to be 190mm.
That gives you a 190/100 R16 tyre. (You could use the calculator in the section
on Classic / vintage / imperial crossply tyre sizes above to get the same resul
t.) Generally speaking, the Land Rover folks reckon a 265/65R16 is a good replac
ement, although the tread is slightly wider and might give some fouling problems
on full lock. It's also 5% smaller in rolling radius so your speed will over-re
ad by about 4mph at 70mph. If you're really into this stuff, you ought to read T
om Sheppard's Off Roader Driving (ISBN 0953232425). It's a Land Rover publicatio
n first published in 1993 as "The Land Rover Experience". It's been steadily rev
ised and you can now get the current edition from Amazon. I've even helpfully pr
ovided you with this link so you can go straight to it....
Lies, Damn Lies and Speed ratings.
All tyres are rated with a speed letter. This indicates the maximum speed that t
he tyre can sustain for a ten minute endurance without coming to pieces and dest
roying itself, your car, the car next to you and anyone else within a suitable r
adius at the time.
Speed Symbol Max Car Speed Capability Km/h L M N P Q R 120 130 140 150 160 170 M
PH 75 81 87 95 100 105 S T U H V W Z Speed Symbol Max Car Speed Capability Km/h
180 190 200 210 240 270 240+ MPH 113 118 125 130 150 168 150+
'H' rated tyres are becoming the most commonplace and widely used tyres, replaci
ng 'S' and 'T' ratings. Percentage-wise, the current split is something like thi
s: S/T=67%, H=23%, V=8%. Certain performance cars come with 'V' or 'Z' rated tyr
es as standard. This is good because it matches the performance capability of th
e car, but bad because you need to re-mortgage your house to buy a new set of ty
res.
UTQG Ratings
6
The UTQG - Uniform Tyre Quality Grade - test is required of all dry-weather tyre
s ("snow" tyres are exempt) before they may be sold in the United States. This i
s a rather simple-minded test that produces three index numbers : Tread life, Tr
action and Temperature.
• The tread life index measures the relative tread life of the tyre compared to
a
"government reference". An index of 100 is equivalent to an estimated tread life
of 30,000 miles of highway driving. • The traction test is a measure of wet bra
king performance of a new tyre. There is no minimum stopping distance, therefore
a grade "C" tyre can be very poor in the wet. • The temperature test is run at
high speeds and high ambient temperatures until the tyre fails. To achieve a min
imum grade of "C" the tyre must safely run at 85mph for 30 minutes, higher grade
s are indicative of surviving higher speeds (a rating of "B" is, for some reason
, roughly equivalent to a European "S" rating, a rating of "A" is equivalent to
an "H" rating.) There are some exceptions: Yokohama A008's are temperature rated
"C" yet are sold as "H" speed rated tyres. These UTQC tests should be used only
as a rough guide for stopping. If you drive in the snow, seriously consider a p
air of (if not four "Snow Tyres" Like life, this tyre test is entirely subjectiv
e.
Load indices.
The load index on a tyre is a numerical code associated with the maximum load th
e tyre can carry. These are generally valid for speed under 210km/h (130mph). On
ce you get above these speeds, the load-carrying capacity of tyres decreases and
you're in highly technical territory the likes of which I'm not going into on t
his page. The table below gives you most of the Load Index (LI) values you're li
kely to come across. For the sake of simplicity, if you know your car weighs 2 t
ons - 2000kg - then assume an even weight on each wheel. 4 wheels at 2000kg = 50
0kg per wheel. This is a load rating of 84. The engineer in you should add 10% o
r more for safety's sake. For this example, I'd probably add 20% for a weight ca
pacity of 600kg - a load rating of 90. Generally speaking, the average car tyre
is going to have a much higher load rating than you'd ever need. It's better to
have something that will fail at speeds and stress levels you physically can't a
chieve, than have something that will fail if you nudge over 60mph with a six pa
ck in the trunk.
LI kg LI kg LI kg LI kg LI kg LI kg 50 190 51 195 52 200 53 206 54 212 55 218 56
224 57 230 58 236 59 243 60 250 61 257 62 265 63 272 64 280 65 290 66 300 67 30
7 70 335 71 345 72 355 73 365 74 375 75 387 76 400 77 412 78 425 79 437 80 450 8
1 462 82 475 83 487 84 500 85 515 86 530 87 545 90 600 91 615 92 630 93 650 94 6
70 95 690 96 710 97 730 98 750 99 775 100 800 101 825 102 850 103 875 104 900 10
5 925 106 950 107 975 110 1060 111 1090 112 1120 113 1150 114 1180 115 1215 116
1250 117 1285 118 1320 119 1360 120 1400 121 1450 122 1500 123 1550 124 1600 125
1650 126 1700 127 1750 130 1900 131 1950 132 2000 133 2060 134 2120 135 2180 13
6 2240 137 2300 138 2360 139 2430 140 2500 141 2575 142 2650 143 2725 144 2800 1
45 2900 146 3000 147 3075 150 3350 151 3450 152 3550 153 3650 154 3750 155 3875
156 4000 157 4125 158 4250 159 4375 160 4500 161 4625 162 4750 163 4875 164 5000
165 5150 166 5300 167 5450
7
68 315 69 325
88 560 89 580
108 1000 109 1030
128 1800 129 1850
148 3150 149 3250
168 5600 169 5800
Tyre types for passenger cars.
There are several different types of tyre that you, the humble consumer, can buy
for your car. What you choose depends on how you use your car, where you live,
how you like the ride of your car and a variety of other factors. The different
classifications are as follows, and some representative examples are shown in th
e image on the right. Performance tyres or summer tyres Performance tyres are de
signed for faster cars or for people who prefer to drive harder than the average
consumer. They typically put performance and grip ahead of longevity by using a
softer rubber compound. Tread block design is normally biased towards outright
grip rather than the ability to pump water out of the way on a wet road. The ext
reme example of performance tyres are "slicks" used in motor racing, so-called b
ecause they have no tread at all. All-round or all-season tyres These tyres are
what you'll typically find on every production car that comes out of a factory.
They're designed to be a compromise between grip, performance, longevity, noise
and wetweather safety. For increased tyre life, they are made with a harder rubb
er compound, which sacrifices outright grip and cornering performance. For 90% o
f the world's drivers, this isn't an issue. The tread block design is normally a
compromise between quiet running and water dispersion - the tyre should not be
too noisy in normal use but should work fairly well in downpours and on wet road
s. All-season tyres are neither excellent dry-weather, nor excellent wet-weather
tyres. Wet-weather, snow & mud or winter tyres Winter tyres come at the other e
nd of the spectrum to performance tyres, obviously. They're designed to work wel
l in wintery conditions with snow and ice on the roads. Rather than use an even
harder rubber compound than all-season tyres, winter tyres actually use a softer
compound than performance tyres. The rubber needs to heat up quicker in cold co
nditions and needs to have as much mechanical grip as possible. Winter tyres typ
ically have larger, and thus noiser tread block patterns. They'll normally also
have a lot more siping to try to disperse water and snow. In extreme climates, t
rue snow tyres have tiny metal studs fabricated into the tread for biting into t
he snow and ice. The downside of this is that they are incredibly noisy on dry r
oads and wear out both the tyre and the road surface extremely quickly if driven
in the dry. Aquachannel tyres are a subset of winter or wet-weather tyres and I
have a little section on them further down the page. All-terrain tyres All-terr
ain tyres are typically used on SUVs and light trucks. They are larger tyres wit
h stiffer sidewalls and bigger tread block patterns. The larger tread block mean
s the tyres are very noisy on normal roads but grip loose sand and dirt very wel
l when you take the car or truck off-road. As well as the noise, the larger trea
d block pattern means less tyre surface in contact with the road. The rubber com
pound used in these tyres is normally middle-of-the-road - neither soft nor hard
. Mud tyres At the extreme end of the all-terrain tyre classification are mud ty
res. These have massive, super-chunky tread blocks and really shouldn't ever be
driven anywhere other than loose mud and dirt. The tread sometimes doesn't even
come in blocks any more but looks more like paddles built in to the tyre carcass
.
8
Tyre constructions.
Simply put, if you bought a car in the last 20 years or so, you should be riding
on radial tyres. If you're not, then it's a small miracle you're still alive to
be reading this. Radial tyres wear much better and have a far greater rigidity
for when cars are cornering and the tyres are deforming.
Cross-ply components
Radial components
The tread consists of specially compounded/vulcanised rubber which can have uniq
ue characteristics ranging from wear resistance, cut resistance, heat resistance
, low rolling resistance, or any combination of these. The purpose of the
9
tread is to transmit the forces between the rest of the tyre and the ground. The
sidewall is a protective rubber coating on the outer sides of the tyre. It is d
esigned to resist cutting, scuffing, weather checking, and cracking. The chafer
of a radial tire acts as a reinforcement. It increases the overall stiffness of
the bead area, which in turn restricts deflection and deformation and increases
the durability of the bead area. It also assists the bead in transforming the to
rque forces from the rim to the radial ply.
The chafer protects the bead and body from chafing (wear from rubbing) where the
tyre is in contact with the rim.
The liner is an integral part of all tubeless pneumatic tires. It covers the ins
ide of the tire from bead to bead and prevents the air from escaping through the
tire. The bead of a cross-ply tyre consists of bundles of bronze coated high te
nsile strength steel wire strands which are insulated with rubber. A cross-ply t
yre designed for off-road use typically has two or three bundles. A radial on-ro
ad tyre normally only has one. The bead is considered the foundation of the tire
. It anchors the bead on the rim. The cord body is also known as the tyre carcas
s. It consists of layers of nylon plies. The cord body confines the pressure, wh
ich supports the tyre load and absorbs shocks encountered during driving. Each c
ord in each ply is completely surrounded by resilient rubber. These cords run di
agonally to the direction of motion and transmit the forces from the tread down
to the bead. The body ply of a radial tire is made up of a single layer of steel
cord wire. The wire runs from bead to bead laterally to the direction of motion
(hence the term "radial plies"). The body ply is a primary component restrictin
g the pressure which ultimately carries the load. The body ply also transmits th
e forces (torque, torsion, etc.) from the belts to the bead and eventually to th
e rim. The belts are layers of steel cord wires located between the tread and th
e body ply. Off-road tyres can have up to five belts. Road tyres typically have
one or two. The steel wire of the belts run diagonally to the direction of motio
n. The belts increase the rigidity of the tread which increases the cut resistan
ce of the tire. They also transmit the torque forces to the radial ply and restr
ict tire growth which prevents cutting, cut growth and cracking.
The breakers are also know as belts. They provide protection for the cord body f
rom cutting. They also increase tread stability which resists cutting. Breakers
can be made of nylon, aralon, or steel wire.
Comparison of Radial vs. Cross-ply performance
This little table gives you some idea of the advantages and disadvantages of the
two types of tyre construction. You can see the primary reasons why radial tyre
s are almost used on almost all the world's passenger vehicles now, including th
eir resistance to tearing and cutting in the tread, as well as the better overal
l performance and fuel economy.
Cross-ply Vehicle Steadiness Cut Resistance - Tread Cut Resistance - Sidewall Re
pairability Self Cleaning Traction Heat Resistance Wear Resistance Flotation Rad
ial
10
Fuel Economy
A subset of tyre construction : tyre tread.
You thought tread was the shape of the rubber blocks around the outside of your
tyre didn't you? Well it is, but it's also so much more. The proper choice of tr
ead design for a specific application can mean the difference between a comforta
ble, quiet ride, and a piss poor excuse for a tyre that leaves you feeling exhau
sted whenever you get out of your car. A proper tread design improves traction,
improves handling and increases Durability. It also has a direct effect on ride
comfort, noise level and fuel efficiency. Believe it or not, each part of the tr
ead of your tyre has a different name, and a different function and effect on th
e overall tyre. Your tyres might not have all these features, but here's a rundo
wn of what they look like, what they're called and why the tyre manufacturers sp
end millions each year fiddling with all this stuff.
Sipes are the small, slit-like grooves in the tread blocks that allow the blocks
to flex. This added flexibility increases traction by creating an additional bi
ting edge. Sipes are especially helpful on ice, light snow and loose dirt. Groov
es create voids for better water channeling on wet road surfaces (like the Aquac
hannel tyres below). Grooves are the most efficient way of channeling water from
in front of the tyres to behind it. By designing grooves circumferentially, wat
er has less distance to be channeled. Blocks are the segments that make up the m
ajority of a tyre's tread. Their primary function is to provide traction. Ribs a
re the straight-lined row of blocks that create a circumferential contact "band.
" Dimples are the indentations in the tread, normally towards the outer edge of
the tyre. They improve cooling. Shoulders provide continuous contact with the ro
ad while maneuvering. The shoulders wrap slightly over the inner and outer sidew
all of a tyre. The Void Ratio is the amount of open space in the tread. A low vo
id ratio means a tyre has more rubber is in contact with the road. A high void r
atio increases the ability to drain water. Sports, dry-weather and high performa
nce tyres have a low void ratio for grip and traction. Wet-weather and snow tyre
s have high void ratios.
Tread patterns
There are hundreds if not thousands of tyre tread patterns available. The actual
pattern itself is a mix of functionality and aesthetics. Companies like Yokoham
a specialise in high performance tyres with good-looking tread patterns. Believe
it or not, the look of the pattern is very important. People want to be safe wi
th their new tyres, but there's a vanity element to them too. For example, in th
e following comparison, which would you prefer to have on your car?
11
The thought process you're going through whilst looking at those two tyres is an
example of the sort of thing the tyre manufacturers are interested in. Sometime
s they have focus groups and public show-and-tells for new designs to gauge publ
ic reaction. For example, given the choice, I'd prefer the tread pattern on the
right. The challenge for the manufacturers is to make functionally safe tyres wi
thout making them look like a random assortment of rubber that's just been glued
to a wheel in a random fashion. In amongst all this, there are three basic type
s of tread pattern that the manufacturers can choose to go with: Symmetrical: co
nsistent across the tyre's face. Both halves of the treadface are the same desig
n. Asymmetrical: the tread pattern changes across the face of the tyre. These de
signs normally incorporates larger tread blocks on the outer portion for increas
ed stability during cornering. The smaller inner blocks and greater use of groov
es help to disperse water and heat. Asymmetrical tyres tend to also be unidirect
ional tyres. Unidirectional: designed to rotate in only one direction, these tyr
es enhance straight-line acceleration by reducing rolling resistance. They also
provide shorter stopping distance. Unidirectional tyres must be dedicated to a s
pecific side of the vehicle, so the information on the sidewall will always incl
ude a rotational direction arrow. Make sure the tyres rotate in this direction o
r you'll get into all sorts of trouble.
Aquachannel tyres.
12
In the last few years, there has been a gradually increasing trend for manufactu
rers to design and build so-called aquachannel tyres. Brand names you might reco
gnise are Goodyear Aquatread and Continental Aquacontact. These differ noticeabl
y from the normal type of tyre you would expect to see on a car in that the have
a central groove running around the tread pattern. This, combined with the new
tread patterns themselves lead the manufacturers to startling water-removal figu
res. According to Goodyear, their versions of these tyres can expel up to two ga
llons of water a second from under the tyre when travelling at motorway speeds.
My personal experience of these tyres is that they work. Very well in fact they
grip like superglue in the wet. The downside is that they are generally made of
a very soft compound rubber which leads to greatly reduced tyre life. You've got
to weigh it up - if you spend most of the year driving around in the wet, then
they're possibly worth the extra expense. If you drive around over 50% of the ti
me in the dry, then you should think carefully about these tyres because it's a
lot of money to spend for tyres which will need replacing every 10,000 miles in
the dry.
TwinTire(tm) tyres.
This is an idea from the USA based on the twin tyres used in Western Australia o
n their police vehicles. It's long been the practice for closed-wheel racing car
s, such as Nascar vehicles, to use two inner tubes inside each tyre, allowing fo
r different pressures inside the same tyre. They also allow for proper run-flat
puncture capability. Well, it seems that TwinTires have put the same principle i
nto effect for those of us with road-going cars. Their system uses specially des
igned wheel rims to go with their own unique type of tyres. Each wheel rim is ac
tually molded as two half-width rims joined together. The TwinTires tyres then f
it these double rims. Effectively, you're getting two independent tyres per whee
l, each with their own inner tube or tubeless pressure. The most obvious advanta
ge of this system is that it is an almost failsafe puncture proof tyre. As most
punctures are caused by single objects entering the tyre at a single point, with
this system, only one tyre will deflate, leaving the other untouched so that yo
ur vehicle is still controllable. TwinTires themselves actually claim a reductio
n in braking distance too. Typically from 150ft down to 120ft when braking from
a fixed 70mph. The other advantage is that the system is effectively an evolutio
n of the Aquatread type single tyres that can be bought over the counter. In the
dry, you have more or less the same contact area as a normal tyre. In the wet,
most of the water is channeled into the gap between the two tyres leaving (suppo
sedly) a much more efficient wet contact patch. Time will tell whether this syst
em is just a passing fad or if it will take off as a viable alternative to the s
tandard wheel/tyre combos that we all use. Typical tyre sizes are 125/85-R16 and
125/90-R16 (Yokohama and Avon). For an independent opinion on TwinTyre systems
from someone who's been using them since the year dot, have a read of his e-mail
to me which has a lot of information in it.
13
Run-Flat Tyres.
Yikes! Tyres for the accident-prone. As it's name implies, it's a tyre designed
to run when flat. ie. when you've driven over a cunningly placed plank full of n
ails, you can blow out the tyre and still drive for miles without needing to rep
air or re-inflate it. I should just put one thing straight here - this doesn't m
ean you can drive on forever with a deflated tyre. It means you won't careen out
of control across the motorway and nail some innocent wildlife when you blowout
a tyre. It's more of a safety thing - it's designed to allow you to continue dr
iving to a point where you can safely get the tyre changed (or fixed). The way i
t works is to have a reinforced sidewall on the tyre. When a normal tyre deflate
s, the sidewalls squash outwards and are sliced off by the wheel rims, wrecking
the whole show. With run-flat tyres, the reinforced sidewall maintains some heig
ht in the tyre allowing you to drive on. A pressure sensor is strapped
to the inside of the wheel rim and is activated by centrifugal forces once the s
peed of the vehicle is above 5mph. It then samples the pressure once a minute fo
r 4 minutes, and then the temperature once every 5 minutes. The information from
all 4 wheels is relayed by radio to a dash-mounted readout for the driver's inf
ormation. Of course, in normal use, this also means that the driver knows what a
ll 4 tyre pressures are for everyday use. It means they're far less likely to ge
t up one day and find one tyre with such low pressure that it's not possible to
drive to a garage to re-inflate it. With run-flat tyres, that also becomes a bit
of a moot point. Both Goodyear (Run-flat Radials) and Michelin (Zero Pressure S
ystem) have introduced run-flat tyres to their ranges this year. The Michelin ty
re technology cutaway explains it all much better than I can. Check it out here.
Not content with their Zero Pressure System, Michelin developed the PAX system t
oo in late 2000 which is a variation on a theme. Rather than super-supportive si
dewalls, the PAX system relies on a wheel-rim and tyre combination to provide a
derivative run-flat capability. As well as the usual air-filled tyre, there is n
ow a reinforced polymer support ring inside. This solid ring clips the air-fille
d tyre by it's bead to the wheel rim which is the first bonus - it prevents the
air-filled tyre from coming off the rim. The second bonus, of course, is that if
you get a puncture, the air-filled tyre deflates, and the support ring takes th
e strain. Michelin say this system is good for over 100 miles at 80km/h (50mph)!
Remember up the top of this page where I was talking about tyre sizes and menti
oned that Michelin had come up with a new 'standard' ? Imagine you're used to se
eing tyre sizes written like this : 205/65 R15. If you've read my page this far,
you ought to know what that means. But for the PAX system, that same tyres size
now becomes : 205-650 R440 A. Decoding this, the 205 is the same as it always w
as - tyre width in mm. The 650 now means 650mm in overall diameter, rather than
a sidewall height of 65% of 205mm. The 440 is the metric equivalent of a 15inch
wheel rim - and metric is no bad thing - and finally the 'A' means "This is a PA
X system wheel or tyre". What about the criminals? My immediate thought when I h
eard about run-flat tyres was "so now criminals can outfit their cars with these
, and not be prone to the police stinger devices used to slow down getaway cars.
" I e-mailed all the major tyre companies for their response on this matter, and
so far have only had one reply - from Michelin. Here's what they have to say on
the matter: "Michelin's aim is to propose products allowing people to drive in
enhanced conditions of security. From this point of view, run-flat tyres and PAX
System represent great progress in the history of the automotive industry. Inde
ed, these two developments allow drivers to go on driving even after a puncture,
if, for instance, they do not feel safe to stop on the hard shoulder
14
of a highway to repair their tyre, or they are in a hazardous area. Michelin is
of course aware that such inventions, like any other innovations can be used in
a distorted way : cheques for example are meant to facilitate transactions, howe
ver the signature on a cheque can be falsified and money can go into the wrong h
ands ; run flat tyres are designed to provide better security to a driver, but c
ould be used for other purposes by somebody having other intentions. Michelin is
very sorry that it is unable to control any abuses made of its tyres by individ
uals intent on breaking the law."
Michelin Tweels.
In 2005, Michelin unveiled their "Tweel" concept - a word made up of the combina
tion of Tyre and Wheel. After decades of riding around on air-filled tyres, Mich
elin would like to convince us that there is a better way. They're working on a
totally air-less tyre. Airless = puncture proof. The Tweel is the creation of Mi
chelin's American technology centre - no doubt working with the sound of the For
d Explorer / Bridgestone Firestone lawsuit still ringing in their ears. The Twee
l is a combined single-piece tyre and wheel combination, hence the name, though
it actually begins as an assembly of four pieces bonded together: the hub, a pol
yurethane spoke section, a "shear band" surrounding the spokes, and the tread ba
nd - the rubber layer that wraps around the circumference and touches the road.
The Tweel's hub functions just like your everyday wheel right now - a rigid atta
chment point to the axle. The polyurethane spokes are flexible to help absorb ro
ad impacts. These act sort of like the sidewall in a current tyre. But turn a tw
eel side-on and you can see right through it. The shear band surrounding the spo
kes effectively takes the place of the air pressure, distributing the load. Fina
lly, the tread is similar in appearance to a conventional tyre. The image on the
right is my own rendering based on the teeny tiny images I found from the Miche
lin press release. It gives you some idea what the new Tweel could look like. On
e of the basic shortcomings of a tyre filled with air is that the inflation pres
sure is distributed equally around the tire, both up and down (vertically) as we
ll as side-to side (laterally). That property keeps the tire round, but it also
means that raising the pressure to improve cornering - increasing lateral stiffn
ess - also adds up-down stiffness, making the ride harsher. With the Tweel's inj
ection-molded spokes, those characteristics are no longer linked. Only the spoke
s toward the bottom of the tyre at any point in its rotation are determining the
grip / ride quality. Those spokes rotating around the top of the tyre are free
to flex to full extension without affecting the grip or ride quality. The Tweel
offers a number of benefits beyond the obvious attraction of being impervious to
nails in the road. The tread will last two to three times as long as today's ra
dial tires, Michelin says, and when it does wear thin it can be retreaded. For m
anufacturers, the Tweel offers an
15
opportunity to reduce the number of parts, eliminating most of the 23 components
of a typical new tire as well as the costly air-pressure monitors now required
on all new vehicles in the United States. (See TPMS below). Another benefit? No
spare wheels. That leaves more room for boot/trunk space, and reduces the carrie
d weight in the vehicle. Reporters who took the change to drive an Audi A4 sedan
equipped with Tweels early in 2005 complained of harsh vibration and an overly
noisy ride. Michelin are well aware of these shortfalls - mostly due to vibratio
n in the spoke system. (They admit they're in extremelyalpha-test mode.) Another
problem is that the wheels transmit a lot more force and vibration into the cab
in than regular tyres. A plus point though is cornering ability. Because of the
rigidity of the spokes and the lack of a flexing sidewall, cornering grip, respo
nse and feel is excellent. There are other negatives: the flexibility, at this e
arly stage, contributes to greater friction, though it is within 5% of that gene
rated by a conventional radial tyre. And so far, the Tweel is no lighter than th
e tyre and wheel it replaces. Almost everything else about the Tweel is undeterm
ined at this early stage of development, including serious matters like cost and
frivolous questions like the possibilities of chrome-plating. Either way, it's
a promising look into the future. Tweels are being tested out on the iBot - Dean
Kamen's (the Segway inventor) new prototype wheelchair, and by the military. Th
e military are interested because the Tweel is incredibly resistant to damage, e
ven caused by explosions. Michelin hope to bring this technology to everyday roa
d car use, construction equipment, and potentially even aircraft tyres.
Coloured dots and stripes - whats that all about?
16
When you're looking for new tyres, you'll often see some coloured dots on the ty
re sidewall, and bands of colour in the tread. These are all here for a reason,
but it's more for the tyre fitter than for your benefit. The dots on the sidewal
l typically denote unformity and weight. It's impossible to manufacture a tyre w
hich is perfectly balanced and perfectly manufactured in the belts. As a result,
all tyres have a point on the tread which is lighter than the rest of the tyre
- a thin spot if you like. It's fractional - you'd never notice it unless you us
ed tyre manufacturing equipment to find it, but its there. When the tyre is manu
factured, this point is found and a coloured dot is put on the sidewall of the t
yre corresponding to the light spot. Typically this is a yellow dot (although so
me manufacturers use different colours just to confuse us) and is known as the w
eight mark. Typically the yellow dot should end up aligned to the valve stem on
your wheel and tyre combo. This is because you can help minimize the amount of w
eight needed to balance the tyre and wheel combo by mounting the tire so that it
s light point is matched up with the wheel's heavy balance point. Every wheel ha
s a valve stem which cannot be moved so that is considered to be the heavy balan
ce point for the wheel. As well as not being able to manufacture perfectly weigh
ted tyres, it's also nearly impossible to make a tyre which is perfectly circula
r. By perfectly circular, I mean down to some nauseating number of decimal place
s. Again, you'd be hard pushed to actually be able to tell that a tyre wasn't ro
und without specialist equipment. Every tyre has a high and a low spot, the diff
erence of which is called radial runout. Using sophisticated computer analysis,
tyre manufacturers spin each tyre and look for the 'wobble' in the tyre at certa
in RPMs. It's all about harmonic frequency (you know - the frequency at which so
mething vibrates, like the Tacoma Narrows bridge collapse). Where the first harm
onic curve from the tyre wobble hits its high point, that's where the tyre's hig
h spot is. Manufacturers typically mark this point with a red dot on the tyre si
dewall, although again, some tyres have no marks, and others use different colou
rs. This is called the uniformity mark. Correspondingly, most wheel rims are als
o not 100% circular, and will have a notch or a dimple stamped into the wheel ri
m somewhere indicating their low point. It makes sense then, that the high point
of the tyre should be matched with the low point of the wheel rim to balance ou
t the radial runout.
What if both dots are present?
17
Generally speaking, if you get a tyre with both a red and a yellow dot on it, it
should be mounted according to the red dot - ie. the uniformity mark should lin
e up with the dimple on the wheel rim, and the yellow mark should be ignored.
What about the coloured stripes in the tread?
Often when you buy tyres, there will be a coloured band or stripe running around
the tyre inside the tread. These can be any colour and can be placed laterally
almost anyhwere across the tread. Some are on the tread blocks whilst others are
on the tyre carcass. For ages I thought this was a uniformity check - a painted
mark used to check the "roundness" of the tyre. But I had a tyre dealer contact
me with a far more feasible answer. The same tyre is often made with slightly t
weaked specifications for different vehicles. To easily identify these same labe
lled tyres when they are warehoused or in storage, different markings and stripe
s are used. Sometimes stripes are added for huge bulk orders to various manufact
ures. Eg All the red outside stripes are for Toyota next week. This gives anyone
in the warehouse a very quick visual check of the different types of tyres with
out needing to pull them all down and read the sidewall on each one. As well as
the colour, the actual position of the lines is something to take note of too. T
hey're a measure of something called runout. Depending on how the belts are laid
on the tyre during manufacturing, they can cause the tire to "run out" - to not
track perfectly straight, but pull to the left or right. The closer to the cent
re of the tyre that these lines are, the less runout the tyre has and the straig
hter it will track when mounted on your car. So for example, if you were looking
at your car from the front and you saw the coloured striped running around the
right side of both your front tyres, the car would likely have a tendency to pul
l to that side. The best thing is to have the coloured stripes on opposite sides
of the tyres for opposite sides of the car, so that the runout on each side wil
l counteract the other and help maintain a good straight running. This is someth
ing that not many tyre fitting places know about or take any notice of. The obvi
ous solution to having the stripes both on one side is to flip one of the tyres
around, but that will only work if they're not unidirectional tyres. If they are
unidirectional (and thus must be mounted to rotate a specific way) then you sho
uld try to find another tyre from the same batch with the stripe on the opposite
side.
18
Wheel Information.
Okay. If you want to change the wheels on your car, you need to take some things
into consideration.
• Number
of bolts or studs It goes without saying that you can't fit a 4-bolt wheel onto
a 5-bolt wheel hub. Sounds obvious, but people have been known to fork out for a
n expensive set of wheels only to find they've got the wrong number of mounting
holes. Circle Diameter • Pitch Right. So you know how many holes there are. Now
you need to know the PCD, or Pitch Circle Diameter. This is the diameter of the
invisible circle formed by scribing a circle that passes through the centre poin
t of each mounting hole. If you've got the right number of holes, but they're th
e wrong spacing, again the wheel just won't fit.
4 stud (bolt) PCD
5 stud (bolt) PCD
• Inset
or outset This is very important. Ignore this and you can end up with all manner
of nasty problems. This is the distance in mm between the centre line of the wh
eel rim, and the line through the fixing face. You can have inset, outset or nei
ther. This determines how the suspension and self-centring steering behave. The
most obvious problem that will occur if you get it wrong is that the steering wi
ll either become so heavy that you can't turn the car, or so light that you need
to spend all your time keeping the bugger in a straight line. More mundane prob
lems through ignoring this measurement can range from wheels that foul parts of
the bodywork or suspension, to high-speed judder in the steering because the sus
pension setup can't handle that particular type of wheel. This figure will be st
amped on the wheel somewhere as an ET figure.
No offset
Inset wheel
Outset wheel
19
• A
real
example
They say a picture is equivalent to a thousand words, so study this one carefull
y. It's one of the wheels off my own car. Enlarged so you can read it is the whe
el information described above. You'll notice it reads "6J x 14 H2 ET45". The "6
J x 14" part of that is the size of the wheel rim - in this case it has a depth
of 6 inches and a diameter of 14 inches (see the section directly below here on
wheel sizes for a more in-depth explanation). The "J" symbolises the shape of th
e tyre bead profile. (More on that later) The "H2" means that this wheel rim is
designed to take "H" speed-rated tyres. The "ET45" figure below that though symb
olises that these wheels have a positive offset of 45mm. In other words, they ha
ve an inset of 45mm. In my case, the info is all stamped on the outside face of
the wheel which made it nice and easy to photograph and explain for you. On most
aftermarket wheels, they don't want to pollute the lines and style of the outsi
de of the wheel with stamped-on information - it's more likely to be found insid
e the rim, or on one of the inner mounting surfaces.
Matching your tyres to your wheels.
Okay. This is a biggie so take a break, get a hot cup of Java, relax and then wh
en you think you're ready to handle the complexities of tyre matching, carry on.
This diagram should help you to figure out what's going on.
20
Wheel sizes
Wheel sizes are expressed as WWWxDDD sizes. For example 7x14. A 7x14 wheel is ha
s a rim width of 7 inches, and a rim diameter of 14 inches. The width is usually
below the width of the tyre for a good match. So a 185mm tyre would usually be
matched to a wheel which is 6 inches wide. (185mm is more like 7 inches, but tha
t's across the entire tyre width, not the bead area where the tyre fits the rim.
)
Rolling Radius
The important thing that you need to keep in consideration is rolling radius. Th
is is so devastatingly important that I'll mention it in bold again:rolling radi
us!. This is the distance in mm from the centre of the wheel to the edge of the
tread when it's unladen. If this changes because you've mismatched your new whee
ls and tyres, then your speedo will lose accuracy and the fuel consumption might
go up. The latter reason is because the manufacturer built the engine/gearbox c
ombo for a specific rolling radius. Mess with this and the whole thing could sta
rt to fall down around you. It's worth pointing out that the actual radius the m
anufacturers use for speedo calculation is the 'dynamic' or the 'laden' radius o
f the wheel at the recommended inflation pressure and 'normal' loading. Obviousl
y though, this value is entirely dependent on the unladen rolling radius.
J, JJ, K, JK, B, P and D : Tyre bead profiles / rim contour designations.
No, my keyboard letters weren't stuck down when I typed this. The letter that ty
pically sits between the rim width and diameter figures stamped on the wheel, an
d indicates the physical shape of the wheel where the tyre bead meets it. In the
cross-section on the left you can see the area highlighted in red. Like so many
topics, the answer as to which letter represents which profile is a long and co
mplicated one. Common wisdom has it that the letter represents the shape. ie. "J
" means the bead profile is the shape of the letter "J". Not so, although "J" is
the most common profile identifier. 4x4 vehicles often have "JJ" wheels. Jaguar
vehicles (especially older ones) have "K" profile wheels. Some of the very old
VW Beetles had "P" and "B" profile wheels. Anyway the reason it is an "awkward t
opic to find definitive data on" is very apparent if you've ever looked at Stand
ards Manual of the European Tyre and Rim Technical Organisation. It is extremely
hard to follow! There are pages and pages (64 in total) on wheel contours and b
ead
21
profiles alone, including dimensions for every type of wheel you can think of (a
nd many you can't) with at least a dozen tabled dimensions for each. Casually lo
oking through the manual is enough to send you to sleep. Looking at it with some
concentration is enough to make your brain run out of your ears. To try to boil
it all down for you, it seems that they divide up the rim into different sectio
ns and have various codes to describe the geometry of each area. For example, th
e "J" code makes up the "Rim Contour" and specifies rim contour dimensions in a
single category of rims called "Code 10 to 26 on 5deg. Drop-Centre Rims". To giv
e you some idea of just how complex / anal this process is, I've recreated one s
uch diagram with Photoshop below to try to put you off the scent.
From the tables present in this manual, the difference in dimensions between "J"
and "B" rims is mainly due to the shape of the rim flange. This is the part in
the above diagram defined by the R radius and B and Pmin parameters. Hence my so
mewhat simpler description : tyre bead profiles. Note that in my example, the di
fference between "J" and "B" rims is small but not negligible. This area of rim-
to-tire interface is very critical. Very small changes in a tyre's bead profile
make large differences in mounting pressures and rim slip. "A" and "D" contour d
esignations come under the category of "Cycles, Motorcycles, and Scooters" but a
lso show up in the "Industrial Vehicles and Lift Trucks" category. Naturally, th
e contours have completely different geometry for the same designation in two di
fferent categories. The "S", "T", "V" and "W" contour designation codes fall int
o the "Commercial Vehicles, Flat Base Rims" category. The "E", "F", "G" and "H"
codes fall into the "Commercial Vehicles, SemiDrop Centre Rims" category. Are yo
u beginning to see just how complex this all is? I think the best thing for you,
dear reader, is a general rule-of-thumb, and it is this : if your wheels are st
amped 5J15 and you buy 5K15 tyres, rest assured they absolutely won't fit. If yo
u're obsessive-compulsive and absolutely must know everything there is to know a
bout bead profiles and rim flanges, you can check out the ETRTO (European Tyre a
nd Rim Technical Organisation website from where you can purchase their manuals
and documents. Go nuts. Meanwhile, the rest of us will move on to the next topic
.
Why would I want to change my rims and tyres anyway?
A good question. Styling and performance are the only two reasons. Most cars com
e with horrible narrow little tyres and 13 inch rims. More recently the manufact
urers have come to their senses and started putting decent combinations on facto
ry cars so that's not so much of a problem any more. The first reason is perform
ance. Speed in corners more specifically. If you
22
have larger rims, you get smaller sidewalls on the tyres. And if you have smalle
r sidewalls, the tyre deforms less under the immense sideways forces involved in
cornering.
So how does it all figure out?
Point to note: 1 inch = 25.4mm. You need to know that because tyre/wheel manufac
turers insist on mixing mm and inches in their ratings. Also note that a certain
amount of artistic licence is required when calculating these values. The tyre'
s rolling radius will change the instant you put load on it, and calculating val
ues to fractions of a millimetre just isn't worth it - tyre tread wear will more
than see off that sort of accuracy. Lets take an average example: a car with fa
ctory fitted 6x14 wheels and 185/65 R14's on them.
• Radius of wheel = 7 inches (half the diameter) = 177.8mm • Section height = 65
% of 185mm = 120.25mm • So the rolling radius for this car to maintain is 177.8+
120.25=298.05mm
With me so far? Good. Now lets assume I want 15 inch rims which are slightly wid
er to give me that nice fat look. I'm after a set of 7x15's First we need to det
ermine the ideal width of tyre for my new wider wheels. 7 inches = 177.8mm. The
closest standard tyre width to that is actually 205mm so that's what we'll use.
(remember the tyre width is larger than the width of the bead fitting.)
• • • • • •
Radius of wheel = 7.5 inches (half of 15) = 190.5mm We know that the overall rol
ling radius must be as close to 298.05mm as possible So the section height must
be 298.05mm-190.5mm = 107.55mm Figure out what percentage of 205mm is 107.55mm.
In this case it's 52.5% So combine the figures - the new tyre must be 205/50 R15
....giving a new rolling radius of 293mm - more than close enough.
A tyre size calculator.
Well if all that maths seems a little beyond you, and judging by the volume of e
-mails I get on this subject, it might well be, I've made a little Javascript ap
plication below to help you out. Select the tyre size you currently have, and th
en the size you're interested in. Calculate each tyre size and then click on the
click to calculate the difference button. It will show you all the rolling radi
i, circumferences, percentage differences and even speedometer error. Enjoy. Not
e:
For some reason, this little java app doesn't work with Netscape 4.x. IE is fine
and Netscape 6.x is fine. I'm working on it.
Current wheel/tyre New wheel/tyre
185
/
65
R
14
205
/
50
R
15
Current RR: Current circumference:
mm mm
New RR: New circumference:
mm mm
23
Difference in circumference:
mm or
% mph
So when your speedo reads 70mph, you're actually travelling at
Aspect Ratio and Rim / Pan Width.
Aspect ratio is, as you know if you read the bit above, the ratio of the tyre's
section height to its section width. The aspect ratio is sometimes referred to a
s the tyre 'series'. So a 50-series tyre means one with an aspect ratio of 50%.
The maths is pretty simple and the resulting figure is stamped on all tyres as p
art of the sizing information:
Section height Aspect ratio = Section width
The actual dimensions of a tyre are dependent on the rim on which it is mounted.
The dimension that changes the most is the tyre's section width; a change of ab
out 0.2" for every 0.5" change in rim width. The ratio between the section width
and the rim width is pretty important. If the rim width is too narrow, you pinc
h the tyre in and cause it to balloon more in cross-section. If the rim width is
too wide, you run the risk of the tyre ripping away at high speed. For 50-serie
s tyres and above, the rim width is 70% of the tyre's section width, rounded off
to the nearest 0.5. For example, a P255/50R16 tyre, has a design section width
of 10.04" (255mm = 10.04inces). 70% of 10.04" is 7.028", which rounded to the ne
arest half inch, is 7". Ideally then, a 255/50R16 tyres should be mounted on a 7
x16 rim. For 45-series tyres and below, the rim width is 85% of the tyre's secti
on width, rounded off to the nearest 0.5. For example, a P255/45R17 tyre, still
has a design section width of 10.04" (255mm =
24
10.04inces). But 85% of 10.04" is 8.534", which rounded to the nearest half inch
, is 8.5". Ideally then, a 255/45R17 tyre should be mounted on an 8½x17 rim.
An ideal rim-width calculator
Blimey I'm good to you. Can't figure that maths out either? Click away my friend
and Chris's Rimwidthulatortm will tell you what you need to know.
185 65 14
Your tyre size:
/
R
x
up to
x
Too wide or too narrow - does it make a difference?
Given all the information above, you ought to know one last thing. A rim that is
too narrow in relation to the tyre width will allow the tyre to distort excessi
vely sideways under fast cornering. On the other hand, unduly wide rims on an or
dinary car tend to give rather a harsh ride because the sidewalls have not got e
nough curvature to make them flex over bumps and potholes. That's why there is a
range of rim sizes for each tyre size in my Rimwidthulator above. Put a 185/65R
14 tyre on a rim narrower than 5inches or wider than 6.5inches and suffer the co
nsequences.
The Plus One concept
The plus one concept describes the proper sizing up of a wheel and tyre combo wi
thout all that spiel I've gone through above. Basically, each time you add 1 inc
h to the wheel diameter, add 20mm to the tyre width and subtract 10% from the as
pect ratio. This compensates nicely for the increases in rim width that generall
y accompany increases in diameter too. By using a larger diameter wheel with a l
ower profile tyre it's possible to properly maintain the overall rolling radius,
keeping odometer and speedometer changes negligible. By using a tyre with a sho
rter sidewall, you gain quickness in steering response and better lateral stabil
ity. The visual appeal is obvious, most wheels look better than the sidewall of
the tyre, so the more wheel and less sidewall there is, the better it looks.
Tyre size table up to 17" wheels
25
Here, for those of you who can't or won't calculate your tyre size, is a table o
f equivalent tyres. These all give rolling radii within a few mm of each other a
nd would mostly be acceptable, depending on the wheel rim size you're after.
80 SERIES 75 SERIES 70 SERIES 65 SERIES 60 SERIES 55 SERIES 50 SERIES
135/80 R 13 -
-
145/70 R 13 155/70 R 13 -
165/65 R 13 175/65 R 13
175/60 R 13 -
-
-
145/80 R 13 -
-
155/70 R 13 165/70 R 13 175/70 R 13
175/65 R 13 165/65 R 14 -
185/60 R 13 175/60 R 14 -
185/55 R 14 -
-
155/80 R 13 -
165/75 R 13 -
175/70 R 13 185/70 R 13 165/70 R 14
165/65 R 14 175/65 R 14 -
175/60 R 14 185/60 R 14 195/60 R 14
195/55 R 14 185/55 R 15 -
195/50 R 15 -
165/80 R 13 -
-
185/70 R 13 165/70 R 13 175/70 R14 -
175/65 R 14 185/65 R 14 -
195/60 R 14 205/60 R 14 -
205/55 R 14 185/55 R 15 195/55 R 15 205/55 R15
205/50 R 15 195/50 R 16 -
175/80 R 13 -
175/75 R 14 -
175/70 R 14 185/70 R 14 -
185/65 R 14 195/65 R 14 185/65 R 15
205/60 R 14 215/60 R 14 195/60 R 15
195/55 R 15 205/55 R 15 -
215/50 R 16 195/50 R 16 205/50 R 16
185/80 R 13 -
185/75 R 14 -
185/70 R 14 195/70 R 14 -
195/65 R 14 185/65 R 15 195/65 R 15 -
215/60 R 14 225/60 R 14 195/60 R 15 205/60 R 15 215/60 R 15
205/55 R 16 -
205/50 R 16 225/50 R 16 205/50 R 17 -
So that's it then?
Yes - that's it. A little time with a calculator, a pen and some paper will enab
le to you confidently stride into your local tyre/wheel supplier and state exact
ly what you want.
26
Oversizing tyres
If you want the fat look but don't want to go bonkers with new wheels, you can o
versize the tyres on the rims usually by about 20mm (to be safe). So if your sta
ndard tyres are 185/60 R14s, you can oversize them to about 205mm. But make sure
you recalculate the percentage value to keep the sidewall height the same. And
finally, you might like to check out this little program written by Brian Cassid
y (skyline6969 btinternet.com),which helps with tyre size calculation.
Fat or thin? The question of contact patches and grip.
If there's one question guaranteed to promote argument and counter argument, it'
s this : do wide tyres give me better grip? Fat tyres look good. In fact they lo
ok stonkingly good. In the dry they are mercilessly full of grip. In the wet, yo
u might want to make sure your insurance is paid up, especially if you're in a r
ear-wheel-drive car. Contrary to what you might think (and to what I used to thi
nk), bigger contact patch does not necessarily mean increased grip. Better yet,
fatter tyres do not mean bigger contact patch. Confused? Check it out: Pressure=
weight/area. That's about as simple a physics equation as you can get. For the g
eneral case of most car tyres travelling on a road, it works pretty well. Let me
explain. Let's say you've got some regular tyres, as supplied with your car. Th
ey're inflated to 30psi and your car weighs 1500Kg. Roughly speaking, each tyre
is taking about a quarter of your car's weight - in this case 375Kg. In metric,
30psi is about 2.11Kg/cm². By that formula, the area of your contact patch is go
ing to be roughly 375 / 2.11 = 177.7cm² (weight divided by pressure) Let's say y
our standard tyres are 185/65R14 - a good middle-ground, factory-fit tyre. That
means the tread width is 18.5cm side to side. So your contact patch with all the
se variables is going to be about 177.7cm² / 18.5, which is 9.8cm. Your contact
patch is a rectangle 18.5cm across the width of the tyre by 9.8cm front-to-back
where it sits 'flat' on the road. Still with me? Great. You've taken your car to
the tyre dealer and with the help of my tyre calculator, figured out that you c
an get some swanky 225/50R15 tyres. You polish up the 15inch rims, get the tyres
fitted and drive off. Let's look at the equation again. The weight of your car
bearing down on the wheels hasn't changed. The PSI in the tyres is going to be a
bout the same. If those two variables haven't changed, then your contact patch i
s still going to be the same : 177.7cm² However you now have wider tyres - the t
read width is now 22.5cm instead of 18.5cm. The same contact patch but with wide
r tyres means a narrower contact area front-to-back. In this example, it becomes
177.7cm² / 22.5, which is 7.8cm.
27
Imagine driving on to a glass road and looking up underneath your tyres. This is
the example contact patch (in red) for the situation I explained above. The nar
rower tyre has a longer, thinner contact patch. The fatter tyre has a shorter, w
ider contact patch, but the area is the same on both.
And there is your 'eureka' moment. Overall, the area of your contact patch has r
emained more or less the same. But by putting wider tyres on, the shape of the c
ontact patch has changed. Actually, the contact patch is really a squashed oval
rather than a rectangle, but for the sake of simplicity on this site, I've illus
trated it as a rectangle - it makes the concept a little easier to understand. S
o has the penny dropped? I'll assume it has. So now you understand that it makes
no difference to the contact patch, this leads us on nicely to the sticky topic
of grip. The area of the contact patch does not affect the actual grip of the t
yre. The things that do affect grip are the coefficient of friction and the load
on the tyre - tyre load sensitivity. Get out your geek-wear because this is goi
ng to get even more nauseatingly complicated now.
28
The graph up above here shows an example plot of normalised lateral force versus
slip angle. Slip angle is best described as the difference between the angle of
the tyres you've set by steering, and the direction in which the tyres actually
want to travel. Looking at it, you can see that for any given slip angle, a hig
her coefficient of friction is obtained with less vertical load on the tyre.
As the load on the tyre is increased, the peak obtainable lateral force is incre
ased but at a decreasing rate. ie. more load doesn't mean infinitely more latera
l force - at some point it's going to tail off. Rubber friction is broken into t
wo primary components - adhesion and deformation or mechanical keying. Rubber ha
s a natural adhesive property and high elasticity which allows it readily deform
and fill the microscopic irregularities on the surface of any road. This has th
e effect of bonding to various surfaces, which aids in dry weather grip but is d
iminished in wet road conditions. Look at this next drawing - this depicts the d
eformation process as the load varies.
As the load is increased the amount of tire deformation also increases. Increasi
ng the load also increases the contact between the tire and road improving adhes
ion. As the load increases, the rubber penetrates farther into the irregularitie
s, which increases grip but at a diminishing rate. This next little graph shows
the change in deformation friction (Fdef) and the deformation coefficient of fri
ction (Cdef) with change in load.
29
As far as cars are concerned, any reduction in load usually results in an increa
se in the coefficient of friction. So for a given load increasing the contact pa
tch area reduces the load per unit area, and effectively increases the coefficie
nt of friction. If this change in coefficient of friction were not true then loa
d transfer would not be an issue. During acceleration grip is reduced partly fro
m the change is suspension geometry and party from the transfer of load from one
set of tires to another. Since the coefficient of friction is changing (non-lin
early lower for higher loads), the net grip during acceleration is reduced. In o
ther words maximum grip occurs when all four tires are loaded equally. That last
paragraph also explains why dynamic setup on your car is pretty important. In r
eality the contact patch is effectively spinning around your tyre at some horren
dous speed. When you brake or corner, load-transfer happens and all the tyres st
art to behave differently to each other. This is why weight transfer makes such
a difference the handling dynamics of the car. Braking for instance; weight move
s forward, so load on the front tyres increases. The reverse happens to the rear
at the same time, creating a car which can oversteer at the drop of a hat. The
Mercedes A-class had this problem when it came out. The load-transfer was all wr
ong, and a rapid left-right-left on the steering wheel would upset the load so m
uch that the vehicle lost grip in the rear, went sideways, re-acquired grip and
rolled over. (That's since been changed.) The Audi TT had a problem too because
the load on it's rear wheels wasn't enough to prevent understeer which is why al
l the new models have that daft little spoiler on the back. If your brain isn't
running out of your ears already, then here's a link to where you can find many
raging debates that go on in the Subaru forums about this very subject. If you d
ecide to read this, you should bear in mind that Simon de Banke, webmaster of Sc
oobyNet, is a highly respected expert in vehicle dynamics and handling, and is a
lso an extremely talented rally driver. It's also worth noting that he holds the
World Record for driving sideways........... If you decide to fatten up the tyr
es on your car, another consideration should be clearance with bits of your car.
There's no point in getting super-fat tyres if they're going to rub against the
inside of your wheel arches. Also, on cars with McPherson strut front suspensio
n, there's a very real possibility that the tyre will foul the steering linkage
on the suspension. Check it first!
Caster, camber, alignment and other voodoo.
Alignment
30
This is the general term used to gloss over the next three points:
Caster
This is the forward (negative) or backwards (positive) tilt of the spindle steer
ing axis. It is what causes your steering to 'self-centre'. Correct caster is al
most always positive. Look at a bicycle the front forks have a quite obvious rea
rward tilt to the handlebars, and so are giving positive caster. The whole point
of it is to give the car (or bike) a noticeable centre point of the steering -
a point where it's obvious the car will be going in straight line.
Camber
Camber is the tilt of the top of a wheel inwards or outwards (negative or positi
ve). Proper camber (along with toe and caster) make sure that the tyre tread sur
face is as flat as possible on the road surface. If your camber is out, you'll g
et tyre wear. Too much negative camber (wheels tilt inwards) causes tread and ty
re wear on the inside edge of the tyre. Consequently, too much positive camber c
auses wear on the outside edge. Negative camber is what counteracts the tendency
of the inside wheel during a turn to lean out from the centre of the vehicle. 0
or Negative camber is almost always desired. Positive camber would create handl
ing problems. The technical reason for this is because when the tyres on the ins
ide of the turn have negative camber, they will tend to go toward 0 camber, usin
g the contact patch more efficiently during the turn. If the tyres had positive
camber, during a turn, the inside wheels would tend to even more positive camber
, compromising the efficiency of the contact patch because the tyre would effect
ively only be riding on its outer edge.
31
Toe in & out
'Toe' is the term given to the left-right alignment of the front wheels relative
to each other. Toe-in is where the front edge of the wheels are closer together
than the rear, and toe-out is the opposite. Toe-in counteracts the tendency for
the wheels to toe-out under power, like hard acceleration or at motorway speeds
(where toe-in disappears). Toe-out counteracts the tendency for the front wheel
s to toe-in when turning at motorway speeds. It's all a bit bizarre and contradi
ctory, but it does make a difference. A typical symptom of too much toe-in will
be excessive wear and feathering on the outer edges of the tyre tread section. S
imilarly, too much toe-out will cause the same feathering wear patterns on the i
nner edges of the tread pattern.
Diagnosing problems from tyre wear.
Firstly, let me state my views on rotating your tyres. This is the practice of s
wapping the front and back tyres to even out the wear. I personally don't think
this is a particularly clever thing to do. Think about it: the tyres begin to we
ar in a pattern, however good or bad, that matches their position on the car. If
you now change them all around, you end up with tyres worn for the rear being p
laced on the front and vice versa. The upside of it, of course, (which many peop
le will tell you) is even overall tyre wear. By this, they mean wear in the trea
d depth. This is a valid point, but if you can't be bothered to buy a new pair o
f tyres when the old pair wear too much, then you shouldn't be on the road, let
alone kidding yourself that putting worn front tyres on the back and partly worn
back tyres on the front will cure your problem. But that's only my point of vie
w. Your tyre wear pattern can tell you a lot about any problems you might be hav
ing with the wheel/tyre/suspension geometry setup. The first two signs to look f
or are over- and underinflation. These are relatively easy to spot:
Under-inflation
Correct
Over-inflation
Here's a generic fault-finding table for most types of tyre wear:
Problem Cause
Shoulder Wear Both Shoulders wearing faster than the centre of the tread
Under-inflation Repeated high-speed cornering Improper matching of rims and tyre
s Tyres haven't been rotated recently
Centre Wear The centre of the tread is wearing faster than the shoulders
Over-inflation Improper matching of rims and tyres Tyres haven't been rotated re
cently
One-sided wear One side of the tyre wearing unusually fast
Improper wheel alignment (especially camber) Tyres haven't been rotated recently
Spot wear A part (or a few parts) of the circumference of the tread are wearing
faster
32
A part (or a few parts) of the circumference of the tread are wearing faster tha
n other parts.
Faulty suspension, rotating parts or brake parts Dynamic imbalance of tyre/rim a
ssembly Excessive runout of tyre and rim assembly Sudden braking and rapid start
ing Under inflation
Diagonal wear A part (or a few parts) of the tread are wearing diagonally faster
than other parts.
Faulty suspension, rotating parts or brake parts Improper wheel alignment Dynami
c imbalance of tyre/rim assembly Tyres haven't been rotated recently Under infla
tion
Feather-edged wear The blocks or ribs of the tread are wearing in a feather-edge
pattern
Improper wheel alignment (faulty toein) Bent axle beam
Checking your tyres.
It's amazing that so many people pay such scant attention to their tyres. If you
're travelling at 70mph on the motorway, four little 20-square-centimetre pads o
f rubber are all that sits between you and a potential accident. If you don't ta
ke care of your tyres, those contact patches will not be doing their job properl
y. If you're happy with riding around on worn tyres, that's fine, but don't expe
ct them to be of any help if you get into a sticky situation. The key of course,
is to check your tyres regularly. If you're a motorcyclist, do it every night b
efore you lock the bike up. For a car, maybe once a week. You're looking for sig
ns of adverse tyres wear (see the section above). You're looking for splits in t
he tyre sidewall, or chunks of missing rubber gouged out from when you failed to
negotiate that kerb last week. More obvious things to look for are nails sticki
ng out of the tread. Although if you do find something like this, don't pull it
out. As long as it's in there, it's sealing the hole. When you pull it out, then
you'll get the puncture. That doesn't mean I'm recommending you drive around wi
th a nail in your tyre, but it does mean you can at least get the car to a tyre
place to get it pulled out and have the resulting hole plugged. The more you loo
k after your tyres, the more they'll look after you.
Lies, damn lies, and tyre pressure gauges.
Whilst on the subject of checking your tyres, you really ought to check the pres
sures once every couple of weeks too. Doing this does rather rely on you having,
or having access to a working, accurate tyre pressure gauge. If you've got one
of those free pencil-type gauges that car dealerships give away free, then I'll
pop your bubble right now and tell you it's worth nothing. Same goes for the one
s you find on a garage forecourt. Sure they'll fill the tyre with air, but they
can be up to 20% out either way. Don't trust them. Only recently - since about 2
003 - have I been able to trust digital gauges. Before that they were just junk
- I had one which told me that the air in my garage was at 18psi with nothing at
tached to the valve. That's improved now and current-generation digital gauges a
re a lot more reliable. One thing to remember with digital gauges is to give the
m enough time to sample the pressure. If you pop it on and off, the reading will
be low. Hold it on the valve cap for a few seconds and watch the display (if yo
u can). Generally speaking you should only trust a decent, branded pressure gaug
e that you can buy for a small outlay - $30 maybe - and keep it in your glove bo
x. The best types are the ones housed in a brass casing with a radial display on
the front and a pressure relief valve. I keep one in the car all the time and i
t's interesting to see how badly out the other cheaper or free ones are. My loca
l garage forecourt has an in-line pressure gauge which over-reads by about
33
1.5psi. This means that if you rely on their gauge, your tyres are all 1.5psi sh
ort of their recommended inflation pressure. That's pretty bad. My local garage
in England used to have one that under-read by nearly 6 psi, meaning everyone's
tyres were rock-hard because they were 6psi over-inflated. I've yet to find one
that matches my little calibrated gauge. One reader pointed something else out t
o me. Realistically even a cheap pressure gauge is OK provided it is consistent.
This is easy to check by taking three to five readings of the same tyre and con
firming they are all the same, then confirming it reads (consistently) more for
higher pressure and less for lower pressure. One last note : if you're a motorcy
clist, don't carry your pressure gauge in your pocket - if you come off, it will
tear great chunks of flesh out of you as you careen down the road....
Tyre pressure and gas-mileage.
For the first two years of our new life in America, I'd take our Subaru for its
service, and it would come back with the tyres pumped up to 40psi. Each time, I'
d check the door pillar sticker which informed me that they should be 32psi fron
t and 28psi rear, and let the air out to get to those values. Eventually, seeing
odd tyre wear and getting fed up of doing this, I asked one of the mechanics "w
hy do you always over-inflate the tyres?" I got a very long and technical respon
se which basically indicated that Subaru are one of the manufacturers who've nev
er really adjusted their recommended tyre pressures in line with new technology.
It seems that the numbers they put in their manuals and door stickers are a lit
tle out of date. I'm a bit of a skeptic so I researched this on the Internet in
some of the Impreza forums and chat rooms and it turns out to be true. So I pump
ed up the tyres to 40psi front and rear, as the garage had been doing, and as my
research indicated. The result, of course, is a much stiffer ride. But the odd
tyre wear has gone, and my gas-mileage has changed from a meagre 15.7mpg (U.S) t
o a slightly more respectable 20.32 mpg (U.S). That's with mostly stop-start in-
town driving. Compare that to the official quoted Subaru figures of 21mpg (city)
and 27mpg (freeway) and you'll see that by changing the tyre pressures to not m
atch the manual and door sticker, I've basically achieved their quoted figures.
So what does this prove? Well for one it proves that tyre pressure is absolutely
linked to your car's economy. I can get an extra 50 miles between fill-ups now.
It also proves that it's worth researching things if you think something is a l
ittle odd. It does also add weight to the above motto about not trusting forecou
rt pressure gauges. Imagine if you're underfilling your tyres because of a dodgy
pressure gauge - not only is it dangerous, but it's costing you at the pump too
.
TPMS - Tyre (Tire) Pressure Monitor Systems.
For those of you who live in America and are in to cars, you'll no doubt remembe
r the Ford Explorer / Firestone Bridgestone lawsuits of the early 21st century.
A particular variety of Firestone tyre, sold as standard on Ford Explorers, had
a nasty knack of de-laminating at speed causing high-speed blowouts, which, beca
use the Explorer was an S.U.V, resulted in high-speed
34
rollover accidents. After the smoke cleared, it turned out that the tyres were p
articularly susceptible to running at low-pressure. Where most tyres could handl
e this, the Firestones could not, heated up, delaminated and blammo - instant la
wsuit.
The NHTSA ruling.
The American National Highways and Transport Safety Association made some sweepi
ng regulatory changes in 2002 because of the Ford Explorer case. Section 13 of t
he Transportation Recall Enhancement, Accountability and Documentation (TREAD) A
ct, required the Secretary of Transportation to mandate a warning system in all
new vehicles to alert operators when their tires are under inflated. After exten
sive study, NHTSA determined that a direct tire pressure monitoring system shoul
d be installed in all new vehicles. In a "return letter" issued after meetings w
ith the auto industry, the Office of Management and Budget (OMB) demurred, claim
ing its cost-benefit calculations provided a basis for delaying a requirement fo
r direct systems. The final rule, issued May 2002, would have allowed auto maker
s to install ineffective TPMS and would have left too many drivers and passenger
s unaware of dangerously underinflated tires. The full text of the various rulin
gs and judgments, along with a lot more NHTSA information on the subject can be
found at this NHSA link.
Indirect TPMS
Indirect TPMS works without actually changing anything in the wheel or tyre. It
relies on a component of the ABS system on some cars - the wheel speed sensors.
Indirect TPMS reads the wheel speeds from all 4 ABS sensors and compares them. I
f one wheel is rotating at a different rate to the other three, it means the tyr
e pressure is different and the onboard computer can warn you that one tyre is l
ow. Indirect systems don't work if you're losing pressure in all four tyres at t
he same rate because there is no differential between the rotations. Typically l
osing pressure in all tyres at once is a result of either incredibly bad luck or
driving over a police spike strip.
Current / First / Second generation Direct TPMS.
The current generation of direct tyre pressure monitoring systems all work on th
e same basic principle, but have two distinctly different designs. The idea is t
hat a small sensor/transmitter unit is placed in each wheel, in the airspace ins
ide the tyre. The unit monitors tyre pressure and air temperature, and sends inf
ormation back to some sort of central console for the driver to see. This is a p
rime example of trickle-down technology from motor racing. Formula 1 teams have
been using this technology for years and now it's coming to consumer vehicles. A
t its most basic, the system has 4 lights in the cabin and a buzzer or some othe
r sound. When one of the tyre pressure monitors registers over-temperature or un
der-inflation, the driver is alerted by a sound and a light indicating which tyr
e has the problem.
35