Whiplash and Car Design


Whiplash and Car Design

Whiplash and Car Design: Does modern design increase the incidence of whiplash?
Matthew Avery

The following article was presented at the recent Bristol conference on FM sponsored by Lyons Davidson solicitors. A large number of FM sufferers blame their whiplash accident with the onset of the condition.

The Motor Insurance Repair Research Centre, Thatcham, England Material damage, the sort that commonly occurs at road intersections and traffic lights accounts for the majority of financial claims costs incurred by British motor insurers. Seventy percent of the 8 billion currently paid out covers the dents, dings and scrapes that inevitably occur in everyday driving, most claims involving no personal injury. However, claims costs are rising steadily, hence vehicle damage is an area of great concern to British Insurers.In 1969 British Insurers formed The Motor Insurance Repair Research Centre, known as Thatcham, the name of the small town in Berkshire, where the centre is situated. The centre's major task is to control the costs to British Motor Vehicle Insurers by researching into safe, cost effective and high quality methods of vehicle body repair - reinstating any damage to a pre-accident condition. Along with its state-of-the-art research workshop and training centre, Thatcham's 110 staff aim to set the standards and raise the skills of the entire repair industry.

The principal weapon in the arsenal of British insurers is the vehicle group rating system. Comprising 20 groups, each vehicle is categorised into a particular group depending upon performance, security, and most important of all, its damagability and repairability.

All large-volume vehicles must undergo a low-speed crash test to the front and rear. Run to international, RCAR standards (the Research Council for Automobile Repairs) these tests comprise two impacts, both at 15 km/h (9 mph). The front impact is into a rigid wall, 40% offset to the RH corner. The rear impact is by a mobile barrier of 1000 kg, 40% offset to the LH corner.These tests recreate average urban impact damage costs and so enable Thatcham to help insurers to assess and rate a model's damage risk.

Since insurance and cost of ownership play a very important role in a vehicle's sales prospects, this test encourages the manufacturer to fit counter measurers to improve a vehicle's low speed crash performance, thus lowering its insurance costs. In addition to the 70% of claims costs relating to material damage, vehicle theft, (theft front and theft of) accounts for some 6%. This figure was 20% in the late 80's and early 90's until British Insurers launched their vehicle security scheme which involves Thatcham testing new vehicle security systems - such as alarms, immobilisers and door locks. These results also go into the group rating formula. Such is the success of the security scheme for passenger cars that theft of newer vehicles has almost been eradicated, thieves now targeting older vehicle designs with poorer protection.The final 24% of claims costs relate to personal injury (PI) and this figure continues to rise. Of this 24%, over 0% comes from soft-tissue neck injuries - the so called whiplash injury - costing UK insurers an estimated 800 million annually. In the USA, due to their more litigious society, the PI figure for American insurers runs at 50% of all claims. As the UK adopts their compensation culture the UK PI claims are no doubt set to rise further and it will not be long before the PI claims in the UK alone top 1 billion annually.

Thatcham is therefore undertaking a large scale research project into whiplash causation - to attempt to reduce injury levels through engineering solutions. Vehicle design, in particular vehicle stifiness, head restraint geometry - and seat yield strength can all play their role in injury causation and severity.Thatcham's whiplash prevention work is mutli-faceted.

Firstly, all vehicles undergo a static head restraint measurement. This procedure rates the geometry (height and backset when measured against a 50% percentile male dummy) of a head restraint by placing it geometrically into one of 4 zones, rated as - poor, marginal, acceptable, or good. This rating depends upon the size and shape of the head restraint, its adjustability and, of great importance, its ability to lock. There are a number of published works that have illustrated the importance of head restraint geometry in reducing injury levels. In 1999 Chapline et al of Rochester University, New York State and the Institute of Highway Safety published. their report titled "Neck Pain and Head Restraint Position Relative to the Driver's Head in Rear End Collisions". In it they stated "Head Restraint designs that have rated as good reduce the likelihood of neck injury relative to those rated as poor". The static rating of head restraints are now undertaken not only by Thatcham but globally.

The other institutes include the Insurance Institute for Highway Safety in the USA, ICBC in Canada and NRMA in Australia, (now an integral part of their NCAP programme). In the UK over 400 vehicles have so far been rated, the results of which are to go into the insurance group rating scheme in 2002. This will have the effect of inducing vehicle manufacturers to introduce head restraint designs that offer more protection for their occupants and thus lowering claims frequency. Whiplash is however, a dynamic event, and so Thatcham's main testing focus is with the dynamic evaluation of head restraints and seat systems. All vehicles that undergo the damagability impact are assessed for their whiplash protection ability within a full scale crash test. Several recent studies have noted a lack of biofidelity with the traditional Hybrid 3, dummy even when fitted with a so called 'whiplash' neck. Subsequent, research from a Swedish consortium, including the Insurer Tolksam', has seen the development of an entirely new dummy; the BioRID. This dummy has been developed to more accurately reproduce the complex kinematics of the human occupant when involved in a typical rear crash. Thatcham is one of only a handful of research centres around the world with such a state-of-the-art test devise and so is therefore well placed to lead in the field of dynamic whiplash research. However, since the mechanism of whiplash injury is still not fully understood it is not yet possible to 'rate' a vehicle's dynamic performance accurately. There appears to be little correlation between dummy response and real world injury severites.

However, since we are aware that whiplash is an acceleration injury, no acceleration should mean no injury. Therefore, at this stage the lowering, by good seat and head restraint design, of all measurable dummy response criteria, serves as the only realistic means to reduce injury probability. Thatcham is a founder member of the IIWPG, the International Insurance Whiplash Prevention Group, a collection of insurer-supported centres which are coordinating their research efforts into whiplash injury prevention. The group aims to have a dynamic whiplash test standard published by the end of 2002 and to begin rating seats and head restraints shortly after that date for inclusion within vehicle group rating. In addition to this, the group is in contact with the European Commission with the aim of integrating the group's test standard within the European type approval process. We are also in contact with the Euro NCAP authorities who are anxious to expand the breadth of testing currently undertaken by their organisation to encompass soft tissue neck injuries. It is envisaged that these efforts will begin to level out or even reduce the cost of PI claims, by simply engineering out many of the poor designs evident in the current vehicles.

Thatcham has seen a steady decrease in the level of material damage over the recent years. Manufacturers, keen to gain commercial advantage have worked hard to improve their vehicle's low speed crash performance - resulting, in many cases, in a lowering of the insurance group. An example of this general reduction in material damage severity can be seen in this comparative example.

Vehicle A, manufactured in 1996, was subjected to the low speed insurance test. Damage from the rear crash was quite severe and included rear lamps, bumper cover plus it's reinforcement, the rear panel and luggage compartment floor. However the damage was not untypical for a vehicle from this period.

Vehicle B, manufactured in 2000, has been specifically engineered to perform well in this test displaying typically low damage levels to the rear. The sacrificial aluminium armature being designed to deform and absorb the impact energy of the crash, thus preventing damage to the components seen on Vehicle A. Vehicle B's repair time was over a third less than that of Vehicle A. The front damage followed a similar pattern. Sensors were fitted to these vehicle during the crash to enable measurements to be taken, of vehicle forces. The crash pulses produced enabled a direct comparison of energy management characteristics to be analyzed and to illustrate the difference in their respective performance.

The crash performance of these two vehicles display a marked contrast. Vehicle B manages the energy of the impact more effectively than Vehicle A through the use of a very stiff vehicle structure together with softer sacrificial components thus preventing more severe damage and leading to a reduction in repair costs. But this increase in efficient energy management leads to a corresponding increase in acceleration levels for the vehicle's occupants. Therefore, the whiplash protection performance of Vehicle B must be considerably enhanced to offer the same level of protection as that of Vehicle A.

In a further comparison, two different vehicles, with similar mass were subjected to identical impacts.

During this test however the speed was 18 km/h with 100% overlap, giving a change of speed (delta V) of 8 km/h (5 mph). Both vehicles utilised a Hybrid III dummy fitted with a special 'whiplash' neck, known as the TRJD. The dummy responses measured included; head acceleration (g), NIC, (the relative acceleration and speed between the OC and Tl) and neck moment, forces and angles. Both vehicles have head restraint geometry that is rated 'poor'. The time/history from Vehicle C manufactured in 1983 displays a pulse duration of 100 ms, and a peak acceleration of 7g at 50 ms, (similar to that of Vehicle A).

The dummy responses measured were;

Head Acceleration 9 g
NIC 9 m2s2
Neck Moment 3.3 Nm

The pulse from Vehicle D manufactured in l999 displays similar performance to that of Vehicle B - with a very fast onset of acceleration and a rapid dissipation of energy, in that the pulse had entirely decayed after only 50 ms, but with a peak acceleration almost twice that of Vehicle C (1983). Damage levels of the later vehicle were again only superficial. Analysis of the dummy measurements shows significant rise in response, indicating a significant increase in the risk and severity of injury.

Head Acceleration 17 g
NIC 12 m2s2
Neck Moment 4 Nm

When we compare vehicle accelerations from a wide range of vehicles manufactured between 1980 and 2001, we can see a large increase in vehicle structural stiffness. This suggests that there will be corresponding increase in the frequency and severity of real world 'Whiplash' injuries occurring in the near future. Real world Insurance data from Folksam (Sweden) and GDV (Germany) from the early '90's suggests that the majority of whiplash injuries occur during rear crashes in the 15-25 km/h deltaV range (change of speed). Indeed some German Insurers work on the premise that neck injuries cannot occur at speeds less than this, and if proven will limit payouts accordingly. However with the advent of these stiffer modern vehicle structures, Thatcham expect this "average" to fall below 10 km/h as newer and stiffer designs become more commonplace on the UK roads. The phenomenon of a rising frequency of claims will be exacerbated frirther through the use of stiffer, less compliant seat backs with higher yield thresholds. Beyond 30 km/h deltaV the frequency and severity of injuries subside dramatically due to the design of current seat backs which absorb the higher impact forces seen in these crashes. Although we have seen an improvement in head restraint geometry over recent years most of the current new vehicle pare has geometry that rates only 'marginal'. Current EC regulations stipulate only a minimum height for the top of the restraint and pays no attention to backset - now indicated by the facet joint and hydrodynamic pressure change injury hypothesis - as a significant factor in the causation of a 'whiplash injury'.

Although some newer head restraints gain a 'good' rating statically, this encouraging performance is not always seen under dynamic test conditions, especially when tested with the more biofidelic - BioRID dummy. This is due to the more human-like posture that the dummy's flexible spine adopts when placed in the vehicle seat compared with the Head Restraint Measuring Device was designed to replicate the posture of the Hybrid III. This trend is also evident in the latest 'active' head restraint designs. These seats have a cantilever mechanism in the upper seat back, which is designed to interact with the upper torso of the occupant during a rear impact by moving the head restraint forward, and up thus improving the 'backset' and lowering the probability of injury during a rear crash.

However, many of these designs were validated with the Hybrid III dummy and the solid spine box engages the actuation mechanism early enough to significantly reduce injury response by lowering the relative acceleration levels seen at the OC and Tl. The BioRID's articulated spine, although more human like, does not interact with such mechanisms as effectively as the Hybrid III and as such, fails to be of significant benefit. It is suspected that this will be the likely outcome in the real world. Thatcham's current research work is highlighting these problems and by working together with the manufacturers will be helping to improve their seating designs. Is it possible to engineer-out injury, but still have good low speed performance? The answer appears to be yes. This vehicle performed well in its damagability test and gained a four-star Euro NCAP safety rating, yet it attained a bench-mark status in the whiplash test.

Why? The answer is that the vehicle has good seat and head restraint geometry and has an anti-whiplash system that progressively absorbs the accelerations seen in a typical rear impact. Real-world data is still scarce but what is available suggests that such a system has positive benefits in the rear world. There appears to be a correlation between low speed crash performance and dummy response in rear- impact situations, especially where head restraint geometry remains poor. However, until vital research is undertaken to link real-world injury with dummy responses, advances in seat design are as yet unclear. Thatcham's fixture research is designed to bridge these gaps in knowledge, leading to safer seats and lower 'whiplash' claims, but still retaining good damagability and repairability performance.


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