GB2374575A - A bumper assembly for a motor vehicle - Google Patents

Abstract

A bumper assembly (11, Fig.4) for a motor vehicle 10 is disclosed having a foam member 13 located in a recess 18 in a front wall 16 of a thin walled box section cross-member 14. The cross-member 14 is secured to two spaced apart front rails 15 forming part of the body structure of the motor vehicle 10. By locating the foam member 13 in the recess 18 the crush performance of the foam member can be maximised without unduly increasing the length of the motor vehicle 10. A front portion (120, Fig.5) of the foam member (113) may have slots (121) formed therein to define separate laminations (122) which enable an impact load to be absorbed in a more adaptive manner.

Description

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A bumper assemblv for a motor vehicle This invention relates to motor vehicles and in particular to a bumper assembly for attachment to a motor vehicle.

It is well known for example from GB-2,322, 602 to provide a bumper assembly for attachment to a motor vehicle having a foam member to absorb energy in the event of a collision of the motor vehicle with another object. Such foam shock absorbers are primarily provided to reduce the injuries that would otherwise occur if a rigid bumper were to impact a pedestrian.

A prior art bumper assembly is shown in Figs 1 and 2 and comprises a facia member 2 overlying a foam member 3 and a box section cross-member 4. The crossmember 4 is attached at two spaced apart positions to two front rails 5 forming part of a body structure of a motor vehicle 1 by means of threaded fasteners (not shown).

As is well known in the art foam material becomes increasingly rigid as it is compressed and will eventually become virtually incompressible after a predetermined amount of compression. The ratio of uncompressed to compressed thickness varies with the type of foam used but for a particular foam is relatively predictable. It is desirable from a pedestrian impact viewpoint to use a relatively thick foam member as this provides less damage to a pedestrian hit by the vehicle.

At the point when the foam becomes incompressible it has effectively no further beneficial effect so far as pedestrian impact is concerned.

In Fig. 2"Z"is the thickness of the cross-member 4,"X"represents the uncompressed thickness of the foam and"Y"represents the thickness of the foam when it becomes substantially incompressible. The minimum distance of the facia

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member 2 from the cross-member 4 is determined by the thickness"X"of the foam member 3.

There is an intrinsic conflict in that the thickness of the foam needs to provide sufficient crush depth ("X"-"Y") but the distance of the facia member 2 from the cross- member 4 needs to be a small as possible in order to minimise the length of the motor vehicle 1.

It is an object of this invention to provide an improved bumper assembly for a motor vehicle.

According to a first aspect of the invention there is provided a bumper assembly for attachment to a motor vehicle the bumper assembly comprising a thin walled box section cross-member having a rear wall adapted for attachment to two or more front rails of the motor vehicle and arranged to extend transversely with respect to the motor vehicle, at least one foam member to provide a progressively increasing resistance in the event of an impact and a facia member overlying the cross-member and the or each foam member, wherein the cross member has a recess formed in a front wall thereof in which the or each foam member is located.

The thin walled box section member may have an internal cavity defined by the front wall, the rear wall and two end walls.

The cavity may be a U shaped cavity having a narrow central cavity joining two larger cavities.

Preferably, there may be a small gap between the front wall and the rear wall in the region of the central cavity.

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Preferably, end portions of the front wall are inclined forwardly so as to define a flat bottomed V shaped recess.

Each outer end of the end portions of the front wall may be connected to a respective one of the side walls.

The depth of the recess may be the same as the compressed thickness of the foam material.

Alternatively, the depth of the recess may be slightly less than the compressed thickness of the foam.

The cross member may be a one-piece aluminium alloy extrusion or altematively, the cross-member may be made from steel.

Preferably, the foam is a closed cell foam.

According to a second embodiment of the first aspect of the invention the foam member may comprise of a first solid foam portion located within the recess and a second lamella portion extending outwardly from the solid portion.

The lamella portion of the foam member may have a plurality of vertical slots therein defining separate laminations.

The lamella portion may be formed from the same material as the solid portion.

According to a second aspect of the invention there is provided a motor vehicle having a body structure and at least one bumper assembly according to said first aspect of the invention.

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The invention will now be described by way of example with reference to the accompanying drawing in which :- Fig. 1 is an exploded representation of part of the front end of a motor vehicle showing part of a prior art bumper assembly ; Fig. 2 is a cross-section through the prior art bumper assembly shown in Fig1 ; Fig. 3 is an exploded representation of part of the front end of a motor vehicle according to a second aspect of the invention showing part of a bumper assembly according to a first aspect of the invention; Fig. 4 is a cross-section through a bumper assembly according to the first aspect of the invention; and Fig. 5 is a pictorial representation of a second embodiment of a bumper assembly according to the invention.

With particular reference to Figs. 3 and 4 there is shown the front portion of a motor vehicle 10 having two spaced apart front rails 15 that form part of the body structure of the motor vehicle 10.

A first embodiment of a bumper assembly 11 is attached to the motor vehicle 10 and comprises a facia member 12, a structural foam member 13 and a thin walled box section cross-member 14.

The facia member 12 is made from a resilient plastics material and overlies the foam member 13 and the cross-member 14 so as to conceal them during normal use.

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The cross-member 14 is a thin walled aluminium extrusion defining a longitudinally extending cavity 17 and is attached to the motor vehicle such that it extends transversely with respect to the motor vehicle 10.

The internal cavity17 is defined by a front wall, a rear wall and two end walls.

The cavity 17 is a substantially U shaped cavity having a narrow central cavity joining two larger cavities. There is a small gap between the front wall and the rear wall in the region of the central cavity.

The use of a thin walled structure is important because motor vehicles have to be able to withstand impacts from various angles. If a solid or channel shaped crossmember is used then the collapse performance of the cross-member is adversely affected if an impact occurs at an angle to the front of the motor vehicle. With such an impact a thin walled box section structure will tend to buckle and collapse thereby absorbing the impact but a flat plate or channel section will not.

A number of complimentary apertures 19, 20 are provided in the rear wall of the cross-member 14 and the two front rails 15 to accommodate threaded fastening means (not shown) used to secure the cross-member 14 to the front rails 15.

The front wall 16 of the cross-member 14 is formed so as to define a recess 18.

The recess 18 is in the form of a flat bottomed V shaped recess defined by forwardly inclined end portions of the front wall 16 which are connected to a respective one of the side walls and a flat central portion of the front wall which also defines one side wall of the central portion of the cavity.

The foam member 13 is secured in the recess 18 by adhesive bonding.

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"U"on Fig. 4 indicates the uncompressed thickness of the foam member 13 and the thickness of the foam member 13 when it becomes substantially incompressible is indicated by"C"which corresponds approximately to the depth of the recess 18. The reference letter "0" indicates the thickness of the cross-member 14.

The facia member 12 must be large enough to clear the foam member 13 but in this case the distance of the facia member from the front face of the cross-member 14 is not determined by the uncompressed thickness"U"of the foam member 13 but by the distance"P"which the foam member 13 extends in front of the front face 16 of the cross-member 14.

Ideally, the depth of the recess 18 and the substantially incompressible thickness "C"of the foam member 13 when the foam reaches its fully compressed state should be the same. This because, with such an arrangement, the crush performance of the foam material 13 is maximised and the distance from the facia member 12 to the front face 16 of the cross-member is minimise. If the depth of the recess 18 is greater than the thickness "CD then the foam material will never be fully compressed because the facia member will contact the front face 16 of the cross-member 14 before this can occur. Alternatively, if the depth of the recess 18 is less than the compressed thickness"C"of the foam material 13 then the distance the facia member 12 can travel is reduced and so the thickness of the foam material itself has to be increased in order to achieve the same crush performance and, in this case, the distance of the facia member 12 from the cross-member is greater than it would be using an optimum depth recess.

Therefore, It is preferred that the depth of the recess 18 be the same as the compressed thickness"C"of the foam material 13. However, because it is difficult to accurately predict the exact compressed thickness of the foam in a production

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environment the depth of the recess may be slightly less than the compressed thickness of the foam.

The shape of the front wall 16 and the internal cavity 17 are important. With the arrangement shown when the foam has been fully compressed by an impact the central portion of the front wall is pushed back towards the rear wall in the region of the central cavity. The presence of a gap between the front and rear walls in this region permits this movement and allows the front wall to be deflected so as to further absorb energy. If there was no cavity in this region then the front wall would abut against the rear wall and the applied load would be immediately transferred to the rear wall reducing or eliminating the energy absorbing effect that can be achieved by a deformation of the front wall In addition the inclined side portions of the front wall which are connected to the side walls tend to pull the front edges of side walls inwardly as the front wall is pushed back towards the rear wall and so the deformation of the side walls can be used to provide a further impact absorbing effect.

The energy absorbing effects created by the deformation which can occur to the front face and side walls are important because they provide a gradual transfer from the relatively easy to deform characteristics of the foam to the very hard to deform characteristics of the front rails 5. In addition, the deformation of these parts may absorb sufficient energy that no damage occurs to the side rails 5 thereby significantly reducing the cost of repair.

With particular reference to Fig. 5 there is shown a second embodiment of the invention which is in most respects the same as that previously described.

A structural foam member 113 is located in a recess 118 formed in a crossmember 114.

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The cross member is formed by a front plate 123 and a rear plate 124 which are joined together along their upper and lower edges by welding and are both made from thin gauge steel.. The front plate 123 has the recess 118 formed in it for accommodating the foam member 113. As previously discussed the depth of the recess 118 is substantially the same as the compressed thickness of the foam member 113.

The structural foam member 113 has two portions, a solid portion 119 located within the recess 118 and a lamella portion 120 extending forwardly from the solid portion 119 so as to project from the recess 118.

The lamella portion 120 is made from the same foam as the solid portion 119 and differs from it in that it has a number of vertically extending slots 121 formed therein which form a plurality of separate laminations 122 all joined together at their base by the solid portion 119.

This structure provides a more adaptive energy absorbing structure than the solid foam based arrangement described with reference to the first embodiment. This is because the provision of the slots 121 enables the foam member 113 to react differently depending upon the type of impact load applied to it.

If the load is concentrated in one area then only the laminations 122 in that area will be deflected. The stiffness of each of the laminations is relatively low compared to the stiffness of the foam member 113 along its entire length and so the effect of a narrow or localised impact is that the resistance to deformation is relatively low providing a soft impact. This is important if the vehicle collides with a pedestrian because the damage sustained by the pedestrian will be less. The role of the bumper in a pedestrian impact is primarily to tip the pedestrian up onto the bonnet of the vehicle so as to prevent the pedestrian being trapped under the motor vehicle. In

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such circumstances the need to absorb large forces is not present but the minimising of injury is important.

However if the vehicle impacts with another vehicle then contact is likely to be spread out along a wide contact area and in this case the foam insert 113 provides more resistance to crushing as a large number of laminations 122 have to be deformed.

In this way the foam member 113 adapts itself to the type of impact and provides good pedestrian protection by being relatively soft in the event of such an impact but has sufficient energy absorbing properties to reduce the damage to the vehicle in the event of an inter-vehicular collision.

It will be appreciated that for either of the embodiments described the foam could be moulded to the cross-member, fixed in some other way or if the facia member abuts the foam during normal use then there is no need to secure the foam to the cross-member it could be placed in position upon assembly of the bumper assembly and held in place by the facia.

Although the invention has been described with reference to the use of a single foam member it will be appreciated that there could be two or more separate foam members positioned at critical positions along the length of the cross-member.

Claims (17)

1. A bumper assembly for attachment to a motor vehicle, the bumper assembly comprising a thin walled box section cross-member having a rear wall adapted for attachment to two or more front rails of the motor vehicle and arranged to extend transversely with respect to the motor vehicle, at least one foam member to provide a progressively increasing resistance in the event of an impact and a facia member overlying the cross-member and the or each foam member, wherein the cross member has a recess formed in a front wall thereof in which the or each foam member is located.

2. A bumper assembly as claimed in claim 1 in which the thin walled box section member has an internal cavity defined by the front wall, the rear wall and two end walls.

3. A bumper assembly as claimed in claim 2 in which the cavity is a U shaped cavity having a narrow central cavity joining two larger cavities.

4. A bumper assembly as claimed in claim 3 in which there is a small gap between the front wall and the rear wall in the region of the central cavity.

5. A bumper assembly as claimed in claim 2 or in claim 3 in which end portions of the front wall are inclined forwardly so as to define a flat bottomed V shaped recess.

6. A bumper assembly as claimed in claim 5 in which each outer end of the end portions of the front wall are connected to a respective one of the side walls.

7. A bumper assembly as claimed in any preceding Claim in which the depth of the recess is the same as the compressed thickness of the foam material.

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8. A bumper assembly as claimed in any of claims 1 to 7 in which the depth of the recess is slightly less than the compressed thickness of the foam.

9. A bumper assembly as claimed in any of claims 1 to 8 in which the cross member is a one-piece aluminium alloy extrusion.

10. A bumper assembly as claimed in any of claims 1to 8 in which the cross-member is made from steel.

11. A bumper assembly as claimed in any preceding claim in which the foam is a closed cell foam.

12. A bumper assembly as claimed in any preceding claim in which the foam member comprises a first solid foam portion located within the recess and a second lamella portion extending outwardly from the solid portion.

13. A bumper assembly as claimed in claim12 in which the lamella portion of the foam member has a plurality of vertical slots therein defining separate laminations.

14. A bumper assembly as claimed in claim 12 or in claim 13 in which the lamella portion is formed from the same material as the solid portion.

15. A bumper assembly substantially as described herein with reference to the accompanying drawing.

16. A motor vehicle having a body structure and at least one bumper assembly as claimed in any of claims 1 to 14 attached thereto.

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17. A motor vehicle substantially as described herein with reference to the accompanying drawing.