VEHICLE ROLL

By Rod Cross, Physics Department, Sydney University, May 2007

When a vehicle turns a corner too fast, it either slides out of control or tips over, depending on whether the vehicle has a low or a high centre of gravity. The diagram below illustrates the physics. If you push a box of cornflakes along a table, then the box will slide if you push near the bottom but it will tip over if you push near the top, at least if the table is not too slippery. The same thing happens when a vehicle turns a corner. The centrifugal force acts through the centre of mass. If the vehicle is travelling too fast or if the radius through the turn is too small, then the vehicle will either slide or tip over depending on (a) friction between tyres and road (b) distance between wheels, (c) and height of centre of gravity.

I once wrote a report on a case where a van rolled over with 7 drunk passengers. Two were mooning out the back and two were were attempting to pull them back into the van, at the instant when the driver turned a corner. The vehicle was top heavy and side heavy. In addition, the driver turned the corner too fast. The question was, was it the driver’s fault or the passengers’ fault?

Vehicle here is turning to the RIGHT, and will either slide or tilt to the LEFT if it turns too fast. The simplest way to explain this is in terms of the CENTRIFUGAL force, despite the fact that there is no such thing! See American Journal of Physics, 67, 447 (1998) and May 2002 or download a pdf version here.