Chassis 23 PDF
Chassis 23 PDF
Chassis 23 PDF
Chassis 101
Spool operation
Terminology
TIRES
Coefficient of friction
Slip angle
Percent slip (slip ratio)
Cornering stiffness
Camber stiffness
Self aligning torque
Normal load sensitivity
Load transfer sensitivity
Pneumatic trail
Tires Choose wisely.
Based on your design
and performance goals
Learn all you can about
your tire properties
Follow tire makers
recommendations
Tire mounting
Some constructions are
directional
Chassis setup
Rim width
From Lucasfilms, Indiana Jones and the Last Crusade, 1989 Wheel alignment
Cold inflation pressure
New tire break-in
Scuffing
Heat cycling
Post test storage
Tires
Historically,
dominating race and
street vehicles have
been designed
around their tires.
From Paul Henri Cahier, 1977
From Paul Van Valkenburgh, Race Car Engineering and Mechanics, 2000
There Are Many Solutions
It depends.
Everything is a compromise.
Suspension 101
Ride Frequency/ Balance (Flat Ride)
Motion Ratios
Ride Friction
Suspension Geometry Selection
Suspension Layouts- Double A Arm Variations and
Compromises
Dampers- A Really Quick Look
The thing we had missed was that the excitation at
front and rear did not occur simultaneously. The
actual case was more like this--
Front Rear
Suspension Travel
Time
Lag
Tim e
1.5
1
Suspension Travel
0.5
Pitch (deg)
0
-0.5
-1
-1.5
-2
Tim e
(From Chassis Design: Principles and Analysis, Milliken & Milliken, SAE 2002)
Ride Summary
Flat Ride
Improves handling, acceleration, braking performance
Plenty of suspension travel
Allows lower spring rates & ride frequencies
Allows progressive jounce bumper engagement
Good motion ratio
Reduces loads into vehicle structure
Increases shock velocity, facilitates shock tuning
1.00:1 is ideal, 0.60:1 minimum design target
Stiff structure (The 5th Spring)
Improves efficiency of chassis and tire tuning
Provides more consistent performance on the track
Applies to individual attachment compliances, 5:1 minimum design
target, 10:1 is ideal
Successful SAE designs in the 2000-3000 ft-lbs/deg range (static
torsion), 3X for static bending (lbs/in),
Low Friction
Permits dampers to provide consistent performance
Not masked by coulomb friction (stiction)
40:1 minimum (corner weight to frictional contribution for good SLA
suspension
Suspension Layout
Many contemporary Formula SAE Cars use:
Double A arm (pushrod spring/damper actuation)
Double A arm (pullrod spring/damper actuation)
Double A arm (rocker arm spring/damper actuation)
Double A arm (outboard coilover actuation)
Parallel A arms
Non parallel A arms
Equal length A arms
Unequal length A arms
Solid axle
There is no right or wrong answer
Be prepared to sell your decision to the design judges.
Suspension Geometry Setup
Front Suspension 3 views
Rear Suspension 3 views
Front Suspension Front View
Start with tire/wheel/hub/brake rotor/brake
caliper package.
pick ball joint location.
pick front view instant center length and
height.
pick control arm length.
pick steering tie rod length and orientation.
pick spring/damper location.
FSFV: wheel/hub/brake
package
Ball joint location establishes:
King Pin Inclination (KPI): the angle between
line through ball joints and line along wheel
bearing rotation axis minus 90 degrees.
Scrub radius: the distance in the ground plane
from the steering axis and the wheel centerline
intersections with the ground.
Spindle length: the distance from the steer
axis to the wheel center.
Spindle Length
Spindle
Length
King Pin
Inclination Angle
Scrub Radius
(positive shown)
Scrub Radius
From The Automotive Chassis: Engineering Principles, (negative shown)
J. Reimpell & H. Stoll, SAE 1996
FSFV: wheel/hub/brake
package
Scrub radius determines:
the sign and magnitude of the forces in the
steering that result from braking.
a small negative scrub radius is desired.
Scrub radius influences brake force steer
FSFV: wheel/hub/brake
package
KPI effects returnability and camber in
turn.
KPI is a result of the choice of ball joint
location and the choice of scrub radius.
FSFV: wheel/hub/brake
package
KPI effects returnability and camber in
turn.
KPI is a result of the choice of ball joint
location and the choice of scrub radius.
FSFV: wheel/hub/brake
package
Spindle length determines the magnitude
of the forces in the steering that result
from:
hitting a bump
drive forces on front wheel drive vehicles
Spindle length is a result of the choice of
ball joint location and the choice of scrub
radius.
FSFV: wheel/hub/brake
package
Front view instant center is the
instantaneous center of rotation of the
spindle (knuckle) relative to the body.
Front view instant center length and height
establishes:
Instantaneous camber change
Roll center height (the instantaneous center of
rotation of the body relative to ground)
From Car Suspension and Handling 3rd Ed, D. Bastow & G. Howard, SAE 1993
FSFV: wheel/hub/brake
package
The upper control arm length compared to
the lower control arm length establishes:
Roll center movement relative to the body
(vertical and lateral) in both ride and roll.
Camber change at higher wheel deflections.
(From Suspension Geometry and Design, John Heimbecher, DaimlerChrysler Corporation)
FSFV: Roll Center
Movement
Ride and roll motions are coupled when a
vehicle has a suspension where the roll center
moves laterally when the vehicle rolls.
The roll center does not move laterally if, in ride,
the roll center height moves 1 to 1 with ride (with
no tire deflection).
FSFV: wheel/hub/brake
package
The steering tie rod length and orientation
(angle) determines the shape (straight,
concave in, concave out) and slope of the
ride steer curve.
FSFV: wheel/hub/brake
package
The spring location on a SLA suspension
determines:
the magnitude of the force transmitted to the body
when a bump is hit (the force to the body is higher than
the force to the wheel)
the relationship between spring rate and wheel rate
(spring rate will be higher than wheel rate)
how much spring force induces c/a pivot loads
An offset spring on a strut can reduce ride
friction by counteracting strut bending (Hyperco
gimbal-style spring seat).
Spring axis aligned
with kingpin axis
(not strut CL)
From The Automotive Chassis: Engineering Principles, J. Reimpell & H. Stoll, SAE 1996
Front Suspension Side View
Picking ball joint location and wheel center
location relative to steering axis
establishes:
Caster
Caster trail (Mechanical Trail)
From The Automotive Chassis: Engineering Principles, J. Reimpell & H. Stoll, SAE 1996
Front Suspension Side View
Picking the side view instant center
location establishes:
Anti-dive (braking)
Anti-lift (front drive vehicle acceleration)
Front Suspension Side View
Anti-dive (braking):
Instant center above ground and aft of
tire/ground or below ground and forward of
tire/ground.
Increases effective spring rate when braking.
Brake hop if distance from wheel center to
instant center is too short.
Front Suspension Plan View
Picking steer arm length and tie rod
attitude establishes:
Ackermann
recession steer
magnitude of forces transmitted to steering
Front Suspension: Other
Steering Considerations
KPI and caster determine:
Returnability
The steering would not return on a vehicle with
zero KPI and zero spindle length
camber in turn
Camber
Caster
Steer Angle
From The Automotive Chassis: Engineering Principles, J. Reimpell & H. Stoll, SAE 1996
Front Suspension: Other
Steering Considerations
Caster and Caster Trail establish how
forces build in the steering
Caster gives effort as a function of steering
wheel angle (Lotus Engineering).
Caster Trail gives effort as a function of lateral
acceleration (Lotus Engineering).
Spindle offset allows picking caster trail
independent of caster.
Rear Suspension Rear View
Start with tire/wheel/hub/brake rotor/brake
caliper package.
pick ball joint (outer bushing) location
pick rear view instant center length and height.
pick control arm length.
pick steering tie rod length and orientation.
pick spring/damper location.
RSRV: wheel/hub/brake
package
Ball joint location establishes:
Scrub radius: Scrub radius determines the sign and
magnitude of of the forces in the steering that result
from braking.
Spindle length: Spindle length determines the
magnitude of the steer forces that result from hitting a
bump and from drive forces. Spindle length is a result
of the choice of ball joint (outer bushing) location and
the choice of scrub radius.
RSRV: wheel/hub/brake
package
Rear view instant center length and height
establishes:
Instantaneous camber change
Roll center height
RSRV: wheel/hub/brake
package
The upper control arm length compared to
the lower control arm length establishes:
Roll center movement relative to the body
(vertical and lateral) in both ride and roll.
Camber change at higher wheel deflections.
RSRV: wheel/hub/brake
package
Some independent rear suspensions have
a link that acts like a front suspension
steering tie rod. On these suspensions,
steering tie rod length and orientation
(angle) determines the shape (straight,
concave in, concave out) and slope of the
ride steer curve.
RSRV: wheel/hub/brake
package
The spring location on a SLA suspension
determines:
the magnitude of the force transmitted to the body
when a bump is hit (the force to the body is higher than
the force to the wheel)
the relationship between spring rate and wheel rate
(spring rate will be higher than wheel rate)
how much spring force induces bushing loads
An offset spring on a strut can reduce ride
friction by counteracting strut bending.
Rear Suspension Side View
Picking outer ball joint/bushing location
establishes:
Caster
Negative caster can be used to get lateral
force understeer
Rear Suspension Side View
Picking side view instant center location
establishes:
anti-lift (braking)
anti-squat (rear wheel vehicle acceleration)
Remember, you dont know your anti
percentages until you establish the cgh of your
car
Rear Suspension Side View
Anti-lift (braking):
Instant center above ground and forward of
tire/ground or below ground and aft of
tire/ground.
Brake hop if distance from wheel center to
instant center is too short.
Rear Suspension Side View
Anti-squat (rear wheel vehicle
acceleration)
Cars are like primates. They need to squat to go.Carroll Smith
independent
wheel center must move aft in jounce
instant center above and forward of wheel
center or below and aft of wheel center
increases effective spring rate when
accelerating.
beam
instant center above ground and forward of
tire/ground or below ground and aft of
tire/ground.
Rear Suspension
Scrub radius:
small negative insures toe-in on braking
Spindle length:
small values help maintain small acceleration
steer values
Rear Suspension
Camber change:
at least the same as the front is desired
tire wear is a concern with high values
leveling allows higher values
Rear Suspension
Roll Center Height:
independent
avoid rear heights that are much higher than the
front, slight roll axis inclination forward is
preferred
beam axle
heights are higher than on independent
suspensions no jacking from roll center height
with symmetric lateral restraint
Rear Suspension
Roll center movement:
independent:
do not make the rear 1 to 1 if the front is not
beam
no lateral movement
vertical movement most likely not 1 to 1
Rear Suspension
Ride steer / roll steer:
independent
small toe in in jounce preferred
consider toe in in both jounce and rebound
gives toe in with roll and with load
toe in on braking when the rear rises
beam
increasing roll understeer with load desired
10 percent roll understeer loaded is enough
roll oversteer at light load hurts directional
stability
Rear Suspension
Anti-lift:
independent
instant center to wheel center at least 1.5 times
track (short lengths compromise other geometry)
to avoid brake hop
Dampers- A Really Quick Look
Purpose of Dampers
Damper Types and Valving
Performance Testing
Development of Dampers
Introduction
Primary function: dampen the sprung and unsprung
motions of the vehicle, through the dissipation of
energy.
Force = kx + c x&
Real World:
The multi-speed valving characteristics of the damper (low, mid and
high relative piston velocity) permit flexibility in tuning the damper.
Different valving circuits in compression (jounce) and extension
(rebound) of the damper permits further flexibility.
Also generates forces that are a function of position, acceleration and
temperature.
Chamber G Chamber 3
Gas Q13
PG,VG Oil
P1 , V1 Oil
P3,V3
Chamber 1 Separator
Oil, P1,V1 Piston
Piston
Oil Oil Ga
Q12 Q12
s
PG, VG
Chamber 2
P2,V2 P2,V2
Piston rod
Chamber G
Oil Low
Pressure
Deflection Types of orifices:
Disc Stack Deflection Disc
&
X Stop
Hole in piston (with or without one way valve)
Deflection
Disc Spacer
Notch in disc
Coin land
Oil Low
Pressure Types of flow compensating devices:
Deflection Discs ( typically stacked)
&
X
Blow off valve (helical spring)
Schematic of mid speed compression valve flow. Spring, amount of initial deflection.
Torque variation on jam nut can often vary preload.
Undesired for production damper,
&
X Flow restrictions, typically which ever has smaller effective
area:
Limit of disc or blow off valve travel.
Orifice size through piston.
Deflection Disc
High
As per low speed damping, pressure drop and force are
Flow
Pressure proportional to velocity squared.
Oil
Dead Length = A + B + C + D + E + F
3 2
3
2 4 1
4
1 1
Sinusoid, most Common Input form for Shock Testing Typically test at a given stroke and vary
Displacement = X sin (
t) frequency.
Velocity = V = X cos (
t) Suspension normally respondes at forcing
freq. and natural frequencies.
Where w = 2 * * Freq.
So should we test at bounce and wheel hop
Peak Velocity = X * freq.?
Test Outputs
The first slip drawn from the hat with the correct answer and
your name and your school name wins a Bell Sport SA 2010
racing helmet for your team. The drawing will be held before
the Q&A session.