Bump Steer
Encyclopedia
Bump steer is the term for the tendency of a wheel to steer as it moves upwards into jounce. It is typically measured in degrees per metre or degrees per foot.
On modern cars the front of the tire moves outwards, as the suspension is raised, a process known as the front wheels "toeing out". This gives roll understeer
. The rear suspension is usually set up to minimise bump steer, where possible.
A typical value is two degrees per metre, or perhaps more, for the front wheels.
Excessive bump steer increases tire wear and makes the vehicle more difficult to handle on rough roads.
Solid axles generally have zero bump steer, but still have roll steer, in most cases. That is, if the wheels move upwards by the same amount, they tend not to steer.
Roll steer is an important part of the budget used to define a vehicle's understeer
, known as a Bundorf analysis
.
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.
Bump steer can be made more toe out in bounce 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.
On modern cars the front of the tire moves outwards, as the suspension is raised, a process known as the front wheels "toeing out". This gives roll understeer
Understeer
Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Simply put, oversteer is what occurs when a car turns by more than the amount commanded by the driver...
. The rear suspension is usually set up to minimise bump steer, where possible.
A typical value is two degrees per metre, or perhaps more, for the front wheels.
Excessive bump steer increases tire wear and makes the vehicle more difficult to handle on rough roads.
Solid axles generally have zero bump steer, but still have roll steer, in most cases. That is, if the wheels move upwards by the same amount, they tend not to steer.
Roll steer is an important part of the budget used to define a vehicle's understeer
Understeer
Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Simply put, oversteer is what occurs when a car turns by more than the amount commanded by the driver...
, known as a Bundorf analysis
Bundorf analysis
A Bundorf analysis is a way of describing the characteristics of a vehicle that govern its understeer balance. The understeer is measured in units of degrees of additional yaw per g of lateral acceleration.-An imaginary example:...
.
Difference between Bump Steer and Roll Steer
In a bump steer, both wheels rise together. In roll steer, one wheel rises as the other falls. Typically this produces more "toe in" on one wheel, and more "toe out" on the other, thus producing 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 barSway bar
A sway bar or anti-roll bar or stabilizer bar is a part of an automobile suspension that helps reduce the roll of a vehicle that is induced by cornering or road irregularities. It connects opposite wheels together through short lever arms linked by a torsion spring...
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.
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 bounce 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.