Page 566 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Vehicle safety C HAPTER 17.1

Fig. 17.1-10 The Head Impact Test System (HITS) rig used by MIRA for assessing the occupant-friendliness of interior components
and trim.

any other hard component. On the other hand, the lid vehicle inherently safe before it occurs. If the ride is good
must not be too unyielding, or it will itself cause injury. and the road holding is such that the wheels never lose
To assess the effectiveness of lids in decelerating the contact with the ground, the driver will more easily be
heads gently enough to avoid brain damage, the dummy able to maintain control in difficult circumstances, in-
heads are fully instrumented with triaxial accelerome- cluding in an emergency. Too little roll can give the driver
ters. These indicate the rates of deceleration experienced a false sense of security when cornering, while too much
by the dummy heads when they are propelled at could lead to instability.
predetermined speeds, by an air-actuated gun, at the lids. As regards handling, there should be no sudden
The same dummies, similarly propelled, are also changes in either steering or braking characteristics. For
employed to determine the forces to which heads of the example, some slight oversteer may be desirable, but it
occupants of the car are subjected when they strike should not occur suddenly, nor should there be a change
windscreens or any other parts of the vehicle structure, from under to oversteer, or vice versa, while the car is
Fig. 17.1-10. cornering. Tyres too play a major part. In extreme cir-
Propulsion by air-actuated gun is also used to de- cumstances they will inevitably break away into a slide,
termine the deceleration and bending forces to which the but this should occur progressively: sudden breakaway
upper (femural) and lower part of the leg are subjected in while cornering can cause an inexperienced driver to lose
an accident. In this case, a dummy of only the part of the control. Furthermore, the tread pattern should be such as
leg to be tested is propelled by the gun against the to eject water rapidly from between the contact patch
bumper. It comprises an instrumented steel core repre- and road. This not only increases the speed of onset of
senting the relevant bone structure, covered with a spe- aquaplaning, but also helps to maintain a high coefficient
cial foam plastic to represent the flesh. The steel cores of friction between tyre and road. Incidentally, a useful
are instrumented for assessing the shear, bending and rule of thumb for estimating aquaplaning speeds for tyres
acceleration forces, while secured to their ends are without treads is:
masses for simulating the reactions of the parts of the p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
body to which they would, in real life, be connected. Aquaplaning speed ¼ 9 tyre pressure
It is based on experimental data and the fact that
recommended tyre pressures are a function of, among
17.1.5 Active safety other things, the load on the tyre and the area of its
contact patch with the road.
Whereas passive safety entails introducing measures for Aquaplaning occurs when the film of water on the
protecting those involved from injury when an accident road is driven by the forward rolling motion of the wheels
occurs, active safety entails, in effect, rendering the into the wedge-shaped-gap between the tyre and the

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