High sided vehicles may suffer from ‘roll steer’

The centre of gravity of a high vehicle is usually quite high and its large springs that cushion the heavy weights it is required to carry inorder to render the use of such vehicles profitable to the operator allow considerable movement of the load carrying space. In normal circumstances this is within the design capabilities of the vehicle however in unusual circumstances or if the vehicle is poorly maintained the effect of roll steer can be dangerous.

The diagram indicates a solid beam axle with elliptic springs as this is the simplest form to describe. They show the rear of the vehicle in a straight line as it moves away from us.

As the vehicle enters a bend centripetal force acts on the centre of gravity as it meets the resistance of the tyres to distortion and the load carrying compartment leans as the spring suspension tries to absorb the moving load. One side becomes more heavily loaded than the other and the suspension mountings move allowing steering to be applied to the course of the vehicle artificially. As a result the vehicle starts to describe a circle without the aid of the driver.

If the spring deflection caused by the body roll on a right hand bend is similar front and rear the following situation will occur because both axles remain almost parallel, this is known as neutral roll steer.

Roll understeer is experienced if the front nearside spring is deflected more than the rear nearside spring assuming a right hand bend. In this condition the vehicle will tend to veer off the road to the left.

Likewise if the rear nearside spring deflects more than the front nearside spring when negotiating a right hand bend then the vehicle will veer off to the offside of the road.

Simply reverse the situation for each example if the bend is left handed.

The effect of the manner in which a load has been placed in a vehicle can have a similar effect, it may be that the load started life off in the best position but has either moved in transit or as items have been removed the load has become unstable. Apply the same principles to the movement of the load carrying compartment and the distortion of the spring suspension this can cause and a similar result can be achieved. Of course it would be the owner/operator and/or his agent the driver who would be responsible for the state of the load and should the police officer attending the accident scene be as well informed as yourself he will have identified the cause of the accident and prosecuted the offenders accordingly making your life easy. If only that were true you would not need to know about any of the topics covered on this site.