Bump Steer

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Bump Steer is the term for the tendency of a wheel to steer as it moves upwards into jounce. It is measured in degrees per metre, or degrees per foot, typically.

On modern cars the front wheels usually toe out, that is the front of the tire moves outwards, as the suspension is raised. This gives roll understeer. The rear suspension is usually set up to minimise bump steer, where possible.

Typical values are 2 degrees per metre, or perhaps more, for the front wheels.

Excessive bump steer increases tire wear and makes the vehicle darty on rough roads.

Solid axles have zero bump steer.

Roll steer is an important part of the budget used to define a vehicle's understeer, known as a Bundorf analysis.

[edit] Difference between Bump Steer and Roll Steer

In a bump, both wheels rise together. In roll one wheel rises as the other falls. Typically this produces more toe in on one wheel, and more toe out on the other, thus giving a steering effect. In a simple analysis you can just assume that the roll steer is the same as bump steer, but in practice things like the Sway bar geometry have an effect that modifies it.

Roll steer is usually measured in degrees of toe per degree of roll, but can also be measured in degrees of toe per metre of wheel travel.

[edit] Method of Adjustment

The linearity of the bump steer curve is important. If it is not straight then the length of the tie rod needs to be adjusted.

Bump steer can be made more toe out in jounce by lifting the rack or dropping the outer tie rod, if the rack is in front of the axle. The reverse applies if the rack is behind the axle. Usually only small adjustments (say 3mm) are required.