Rollover - Dynamics

Dynamics

Vehicles can roll over in several ways. These include excessive cornering speed, tripping, collision with another vehicle or object, or traversing a critical slope.

Rollovers caused by excessive cornering speed occur when cornering forces destabilize the vehicle. As a vehicle rounds a corner, three forces act on it: tire forces, inertial effects, and gravity. The cornering forces from the tire push the vehicle towards the center of the curve. This force acts at ground level. The force of inertia acts horizontally through the vehicle's center of mass in the direction opposite to the one it is turning. These two forces make the vehicle roll towards the outside of the curve. The force of the vehicle's weight acts downward through the center of mass in the opposite direction. When the tire and inertial forces are enough to overcome the force of gravity, the vehicle starts to turn over. Most passenger vehicles will slide or spin before this happens, but this is a common type of rollover for taller vehicles, including light trucks (SUVs, vans and pickup trucks), buses, and heavy trucks.

A vehicle may perform a complete rollover, rolling onto its side, roof, far side and finally ending up on its wheels again, or even going on to roll further.

Tripping rollovers occur when a vehicle is sliding sideways, and the tires strike a curb, dig into soft ground, suddenly regain traction, or a similar event occurs that results in a sudden lateral force. The physics are similar to cornering rollovers.

A collision with another vehicle or object can cause a rollover. These occur when the collision causes the vehicle to become unstable, such as when a narrow object causes one side of the vehicle to accelerate upwards, but not the other, causing the vehicle to rotate along its long axis. A side impact can accelerate a vehicle sideways. The tires resist the change, and the coupled forces rotate the vehicle. In 1983, crash tests showed that light trucks were prone to rolling over after colliding with certain early designs of guide rail.

A rollover can also occur as a vehicle crosses a ditch or slope. Slopes steeper than 33 percent (one vertical unit rise or fall per three horizontal units) are called 'critical slopes' and often contribute to rollovers.

A vehicle may roll over for other reasons, such as when hitting a large obstacle with one of its wheels or when manoeuvring over uneven terrain.