Many drivers have suffered a rollover and even more have lifted an axle or two, unaware that it happened. These drivers are frequently driving their trucks on the limit without realising it because most drivers never get rollover prevention training. Many things contribute to a rollover – the primary ones are the height of the centre of gravity and the speed of the vehicle – but there are other factors that come into play. These are suspension and tyres.
Suspension soaks up bumps on the road. If it’s two hard, it’s uncomfortable to drive with and can damage goods in transit. If it’s too soft, the truck or trailer will lean towards the outside of the bend. If the truck leans too far, the centre of gravity can move outside the limits of the suspension and the vehicle will tip.
The suspension is optimised to dampen movement. Airbags help with suspension rebound to help prevent scenarios where a truck might lean one way because it hits a bump and then tilts back the other way as it corrects. Trucks have been known to overturn on straight roads simply because of dips and bumps that cause the truck to rock side to side. This could be something as innocuous as a railway level crossing that is diagonal to the direction of the road. If there centre of gravity is high enough and the frequency of the dips and bumps matches the natural sway of the truck, it will tip.
Tyres play their part, too. If the tyres are too soft, they will compress more, exacerbating the lean. If they are too hard, they don’t provide enough grip or cushioning (more air pressure means less tyre in contact with the road.)
If one tyre in a dual set is low, the remaining tyre on that side is compressed more, causing a slight lean. This is why it’s important that a truck’s tyre pressures must be checked every day as part of the pre-start inspection. While the actual pressure won’t be taken every day, the driver can use a metal bar or a mallet to hit to tyres and listen for a change in the tone in comparison with the rest.