Roll Steer

Geometry being what it is, all suspension systems have a steering effect that is induced when one wheel is higher in relation to the chassis than the other – otherwise known as body roll. For those of you who prefer choices, roll-steer comes in two different flavors, roll over-steer and roll under-steer. Over steer will cause the vehicle to turn further in to the corner, making it feel as though you have turned the steering wheel too far, while under steer does the exact opposite, requiring more input from the wheel to keep on course as the vehicle rolls to the outside of the corner. As a general rule of thumb, under steer is more desirable for a stable feeling vehicle, but of course it is not always quite that simple. Roll steer is moderately important to be aware of, as a poorly set up system can become nearly undriveable if there is too much roll steer of either type.

In the diagram, you can see how the wheel moves forward and back in the chassis as the suspension runs through its travel. If one wheel was up and one wheel was down, the rearend would have its own steering input, similar to a radio flyer wagon.

While this is a very crude suspension design and goes against everything I support, this is an extreme example or how roll-steer works. If the vehicle were to lean far enough while cornering, the front-to-rear movement of the axle, caused by this suspension design following an arc, would induce a severe roll-OVERsteer situation – which can feel quite sketchy and possibly even cause the rearend to slide out much easier than expected. To avoid this, most manufacturers try to create a safer, more confident feeling, roll-UNDERsteer situation instead… It should be noted that too much roll-understeer can cause a very dangerous driving situation as well.