Road collisions: a question of grip
Earlier this month we had the chance to attend a road crash investigation event which took place on a long stretch of disused airport runway. The crashes we saw were as life-like as possible, using dummies of correct size and weight, as well as remote-controlled vehicles that crashed into each other at speeds of up to 45mph.
We were not there just for some sinister spectacle. In fact, the vast majority of those attending the crashes were police investigators and ambulance professionals, all looking to understand better some of the more common collisions their vehicles can become involved with. On that basis, we identified some interesting take-away messages:
Check your tyre pressures frequently: we had a chance to drive a slalom course at 40mph three times, each time with gradually reduced pressure in the rear offside tyre. First time was fine, second time was much harder, and third time (with only 15PSI in the rear wheel) proved impossible. We found ourselves spinning out of control after negotiating just two of the slalom cones.
Familiarise yourself with ABS: we watched three vehicles accelerate to 50mph. One was a fully laden ambulance (with ABS switched on), the second was an ambulance car (with ABS on) and the third an ambulance car (with ABS switched off). At a pre-agreed point on the runway, and all at the same time, all three drivers applied maximum braking. The non-ABS car stopped in the shortest distance because its wheels locked up. The ambulance and the other car both stopped in the same distance, around 20 metres further along the runway. However, completing the same exercise on a wet surface quickly changed the result, and the ABS vehicles stopped in a shorter distance – and retained the ability to steer.
Respect the road surface: overall stopping distance can be compromised when the road surface is poor, or when there is a lot of loose material on it, because grip on the road is reduced. Snow, ice, water, mud and leaves will also affect a vehicle’s grip on the road surface.
Be aware of ‘split friction’: this is when the friction differs on each side of the vehicle. In heavy braking, this can have the effect of making a vehicle rotate towards the sider that offers the better grip. In extreme cases, it’s this sort of scenario that can lead a trailer or caravan to swing or an articulated truck to jack-knife.
If you’re interested in more of the science surrounding road collisions, then click here and take a look at this excellent video.