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Abstract
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Cervical spine distortion injuries (CSD) in passenger car rear impact accidents, in particular at low speeds with a ∆v of 15 km/h or less, are becoming increasingly problematic. Since only few vehicle models are equipped with a CSD protection system to date, the aim of Autoliv and Winterthur Insurance Corp. was to develop a system for aftermarket vehicle installation, especially because the vehicles being sold today or being only few years old will be on the roads for many more years. The system will be available for a first car model from fall 2001.
Basically, there are by now two different systems for reduction of CSD injuries on the market: active headrest (e.g. SAHR, Saab) and yielding backrest (e.g. WHIPS, Volvo). Both of these systems reduce relative movement between the head and thorax in rear impact; the first one by increasing head acceleration, the latter one by reducing thorax acceleration. The yielding backrest has the advantage to reduce head rebound speed and to offer protection with wrongly adjusted headrest, as well. The working principle is as follows: A deformation element is located in the recliner. During rear impact, a parallel backwards movement of the seat back begins at a point of critical load, which motion is then transformed into rotation. The backwards movement is limited so that the seat back will offer sufficient protection in a high- speed impact. Our CSD protection system, AWS (Anti Whiplash System), is based on this principle. Because aftermarket installation of an energy-absorbing recliner in an existing seat design appeared unsuitable to us, we decided to integrate the deformation element in the seat rail (Fig. 1). In this solution, the entire seat moves backwards with the desired motion. This installation site is also special for another reason: Different levering rations result from the position of the seat in the longitudinal direction. The CSD protection system is more readily released when the seat is positioned at the front than at the back - a simple mechanical adaptation of the release threshold to the size of the occupant. This means optimum protection for small, lightweight persons as well as large, heavy persons. This was proved by MADYMO simulation at ∆v 15 km/h using HIII 5th percentile female and 95th male. For the female dummy, the thorax acceleration is reduced by 30% to 65% depending on the seating position, for the large male by around 10% in the most rearward position. As there is no biofidelic neck for these dummy percentiles available, a benefit in terms of reducing injury risk can be assumed but not quantified.
AWS ensures nearly constant NIC in the ∆v range from 9 km/h to 16 km/h, i.e. in the speed range in which CSD injuries occur most frequently. The NIC value in the standard test configuration at ∆v 15km/h is 17.6. Comparison testing of 13 different seat designs with the BioRID shows, that NIC values of current seats with CSD protection systems are between 9 and 19, and from 15 to 33 without such systems (Zellmer et al., to be published). This means that AWS is well in the range of current CSD protection systems on the market, although the backrest of the seat under investigation is not optimised for such impact conditions. |