OFFSET CRASH TESTS

OBSERVATIONS ABOUT VEHICLE DESIGN AND STRUCTURAL PERFORMANCE

Sixteenth International Technical Conference of the Enhanced Safety of Vehicles (ESV)
Windsor, Canada, June 1998
Paper Number 98-S1-W-21  (PDF version)  Published by NHTSA, Washington, USA.

Contents

The offset crash test

Aggressivity

Footwell intrusion
Seat movement
Upward movement of steering column
Sideways movement of steering column
Seat belt upper anchorages

ABSTRACT

This paper sets out some general observations about the structural performance of cars, passenger vans and four-wheel-drive vehicles in offset crash tests. The design features which appear to contribute to good structural performance are discussed. Likely reasons for poor performance are noted.

INTRODUCTION

The Offset Crash Test

The vehicle structure affects the outcome of an offset frontal crash in two main ways:

• Absorption and dissipation of crash energy
• Integrity of the passenger compartment

Figure 1. Overhead view of an offset test into a deformable barrier at 64km/h (ANCAP).
 
 
 
 

ABSORPTION OF CRASH ENERGY

The increasing use of engine/suspension cradles has allowed designers to better control this deformation and to by-pass very rigid components such as engine blocks which are not effective energy absorbers.

To avoid load concentrations it is important that the crash forces are spread across the face of the deformable barrier. In several cases it has been observed that box-section structures at the front of the vehicle have punched through the barrier and relatively little energy is absorbed through deformation of the barrier. These box section structures appear to be designed to achieve better performance during a full-frontal crash into a rigid barrier but they can be much less effective during offset crashes into deformable objects, including other motor vehicles. Conversely, some box sections which crush efficiently in a full-frontal crash do not perform as well under the asymmetric loads of an offset crash - they tend to buckle rather than concertina (also a problem with bull-bars).

Some four-wheel-drive recreational vehicles have relatively stiff front structures. This can result in a very high deceleration of the passenger compartment and high loads on the occupants. A stiff front structure can also place excessive demands on the deformable barrier, causing the barrier to bottom out early in the crash sequence. In a collision between two vehicles the occupants of the heavier vehicle would generally be better off, due to the physics of the collision. In the case of four-wheel-drive vehicles colliding with passenger cars, however, this advantage can be diminished by a stiff front structure. Analysis of crashes in Australia have shown that, on average, the driver of a four-wheel-drive vehicle has a greater likelihood of being killed or seriously injured in a crash than the driver of a large car (Newstead et al , 1997).

Aggressivity

• There is the expected trend of increased aggressivity with increased vehicle kerb mass but there is a large amount of scatter, with some high-mass vehicles showing low (good) aggressivity.
• Four-wheel-drives generally show high aggressivity but there is a large amount of scatter and some commendable exceptions. Some of the lighter four-wheel-drives are no more aggressive than cars of the same mass.
• Passenger vans and commercial vans tend to have high aggressivity for their kerb mass.

One criticism of consumer offset crash tests is that they are claimed to push manufacturers towards building stiffer vehicles, in order to protect their own occupants, and that these stiffer vehicles are more aggressive to other vehicles. The MUARC study, and results of recent offset crash tests suggest that this criticism is unfounded. Some vehicles performed exceptionally well at protecting their occupants in the offset test while apparently having low aggressivity towards the occupants of other vehicles. Evidently this was achieved by efficiently absorbing crash energy in the front structure while retaining the integrity of the passenger compartment.

Occupants of other vehicles should be at less risk in collisions with these low-aggressivity vehicles. Vehicle design is therefore a crucial factor in achieving good crashworthiness and low aggressivity.

If the all-vehicle average aggressivity observed in the MUARC study had been reduced to the average for small cars then it is estimated there would have been a 30% reduction in fatal/serious injuries to the occupants of "other" vehicles. Since aggressivity is not a quality easily affected by consumer pressure this may be an area which requires legislation.
 

INTEGRITY OF THE PASSENGER COMPARTMENT

There is a temptation, when observing a frontal crash test, to concentrate on what is happening at the front of the vehicle, since this is where most of the deformation is occurring. This might give the impression that the front of the vehicle is being forced back into the passenger compartment. While this is an important source of passenger compartment intrusion it is only part of the story.

At the height of a frontal crash test the front of the vehicle has come to a halt but the remainder of the vehicle is still undergoing a high deceleration - typically around 40g (up to 60g with some four-wheel-drive vehicles). Substantial compression forces are generated between the front and rear of the vehicle at this time.

Consider a transverse vertical plane in line with the dash. The resulting cross-section might include the a-pillars, side doors, door sills and floor. About 50% of the vehicle's weight will usually be rearward of this plane. The compression forces arising in these components due to a 40g deceleration are therefore equivalent to about 20 times the weight of the vehicle. This places a severe demand on the structure. Furthermore, there is usually very little structural redundancy in the design and if any one of these components has a major failure then catastrophic collapse can occur (and has been observed).

For commercial vehicles and four-wheel-drives the proportion of mass in the rear is usually greater than with passenger cars, particularly when laden. This, combined with a stiffer front structure, can place a very severe demand on the structure of the passenger compartment .

Figure 2. Unladen light commercial vehicle during an offset crash test at 64km/h (ANCAP).
 
 
 
 

During the 56km/h full-frontal test of several four-wheel-drive vehicles there was a noticeable sleeving effect, where the sides of the passenger compartment tended to slide around the engine compartment, resulting in substantial firewall and dash intrusion at the height of the crash (figure 3). The deformation usually appears to be elastic (noticeable in the Figure 1 animation) and therefore the post-crash residual movement of the firewall and dash might not indicate a problem.

Figure 3. Overhead view of a full-frontal crash test into a solid barrier at 56km/h (ANCAP).
 
 
 
 
 

Footwell intrusion

The better designs tend to locate the suspension mounting points below floor level. In this configuration longitudinal structural members mounted under footwell area can effectively transmit the crash forces past the footwell area and reduce intrusion - this has been observed on several vehicles. However, in some cases such structures have insufficient strength to cope with forces during a 64km/h offset crash test. One possible weakness is the location of joints or welds at a bend or change in cross-section.

These observations also apply where engine and/or suspension components are mounted on a cradle under the front of the vehicle. These cradles are usually attached to the body just ahead of the toepan. Their height and method of attachment can have a significant influence on footwell intrusion.

Road wheels and tyres are relatively rigid when compared with footwell panels and they can contribute to intrusion into the footwell area or separation of footwell panels. Failure of the seams between the floor and door sill or the firewall and a-pillar can greatly reduce the strength of this region.

Seat movement

With seat-mounted seat belt buckles the loads on seat components are much higher. Seat runners have been observed to bend, seat mounting frames have rocked forward and floor panels have deformed. All these problems have contributed to excessive forward movement of the occupants.

Upward movement of the steering wheel

The upward motion of the steering column often coincides with the forward and downwards motion of the occupant's head. Where no airbag is fitted this can cause an increase in the severity of the head strike. In one case, this effect contributed to a head deceleration of 250g - one of the highest ever recorded.

In the absence of an airbag, steering wheel hub design plays an important role in reducing head injuries. Considerable research has been undertaken into effective, energy-absorbing hub designs but these do not appear to have been put into production. In some cases the hub cover has flown off just before the head impact, exposing the head to metal components.

Sideways movement of the steering wheel

Figure 4. Diagram showing possible relative movement of steering wheel
 
 
 
 

This effect usually results from excessive deformation in the region of the a-pillar on the driver's side. Rupture of the join between the dash and the a-pillar, and collapse of the door and door sill can also contribute to this problem.

Seat belt upper anchorages

With the trend towards b-pillars which curve substantially inwards between the door sill and the roof there is another source of seat belt "slack". The webbing follows the curved path between the retractor unit and the D-ring and is usually held in place by the trim. During the crash the plastic trim can give way due to tensile forces in the webbing, which then straightens and feeds through the D-ring.

CONCLUSIONS

• a front structure which absorbs crash energy through controlled deformation and avoids load concentrations on the impacted object,
• structural components which bridge the front footwell area so that compression forces are transmitted directly between the front and rear of the vehicle, resulting in minimal footwell deformation,
• structural components which channel crash forces into the a-pillars, side doors and door sills rather than into the firewall area,
• the join between the top of the a-pillar and the roof is smooth and strong so that upwards buckling of the roof is resisted,
• side doors and door sills which offer resistance to longitudinal compression forces (measures to improve side impact protection appear to have assisted in this regard),
• a steering column designed and mounted to minimise the amount of rearward and upward movement at the height of the crash,

Recent research in Australia indicates that the better designs can provide good protection for their occupants, while having low aggressivity towards the occupants of other vehicles.

ACKNOWLEDGMENTS

REFERENCES

• Cameron M H, Newstead S V and Le C M 1998. The Development and Estimation of Aggressivity Ratings for Australian Passenger Vehicles Based on Crashes During 1987 to 1995, Monash University Accident Research Centre, research report in preparation, March 1998.
• Loo M and Brandini M 1998, Applying Computer Aided Engineering to Improve Vehicle Safety, Proceedings of the Developments in Safer Motor Vehicles Seminar, sponsored by SAE-Australasia, Staysafe, Federal Office of Road Safety. NSW Parliament House, March 1998.
• Lowne R W 1996 , The Validation of the EEVC Frontal Impact Test Procedure, Proceedings of the 15th International Technical Conference on the Enhanced Safety of Vehicles, Melbourne.
• Newstead S, Cameron M & Le 1997, Vehicle Crashworthiness Ratings and Crashworthiness by Year of Vehicle Manufacture: Victoria and NSW Crashes During 1987-95, Monash University Accident Research Centre - Report #107.
• Paine M 1997 Guidelines for Crashworthiness Rating System, Report prepared for Australian NCAP Technical Committee, December 1997.
• Shearlaw A & Thomas P, 1996, Vehicle to Vehicle Compatibility in Real-world Accidents, Proceedings of the 15th International Technical Conference on the Enhanced Safety of Vehicles, Melbourne.
• Traffic and Transport Surveys, 1994. The proportion of bull-bars in the vehicle fleet. Data report commissioned by NRMA, October 1994.
• Zeidler F, Scheunert D, Breitner R & Krajewski R 1996 , The Reduction of the Risk of Lower Leg Injuries by Means of Countermeasures Optimised in Frontal Offset Crash Tests, Proceedings of the 15th International Technical Conference on the Enhanced Safety of Vehicles, Melbourne.