‘Bump steer’ is a term used by road testers, but often misunderstood by some of the people who use it – let alone readers to interpret it. Bump steer describes a steering effect caused by suspension movement as the vehicle passes over undulations.
Bump steer is often confused with axle hop or axle tramp.
The classic bump steer effect is noticed in leaf-sprung 4WDs, because a leaf spring is longer when flat than it is when positively or negatively curved. The lengthening and shortening takes place all the time there is suspension movement and that fore and aft movement causes the axles to change their ideal right angle position across the chassis.
If you look at a leaf-sprung 4WD from the side as it moves over a hump, you can see the slight rearward movement of the axles. The more the suspension deflects, the greater the angling effect of the axles and the more self-steering effect on the vehicle.
When bump steer is more noticeable in leaf-sprung vehicle, you can have bump steering effect with coil-sprung axles, because their pivot on control arms which move through arcs, which can impart some steering effect.
Bump steer is often noticed at the front of 4WDs that have a ‘live’ front axle, rather than independent, wishbone suspension. Because the two front wheel are linked by a tubular axle beam and a steering tie rod, when one wheel encounters a bump, the reaction is fed to the other wheel.
Bump steer, axle hop and axle tramp can be experienced virtually any 4WD if you push hard enough on rough roads, so it’s important to know your vehicle’s degree of axle control. Driving technique – minimizing savage acceleration and braking – can reduce axle misbehavior.
Another value technique is to find the ‘sweet spot’ when running on corrugated roads – that speed which provides the best ride and good directional control.
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