KPI – Bump/Roll Steer

Bump steer is a measure of the suspension toe angle change with suspension vertical travel.  Bump steer, given in Deg/m, is a measure of the toe angle change in degrees versus suspension travel in metres.  Roll steer is calculated in the same way as bump steer except the wheel travel is generated by applying a roll angle to the vehicle body.

Positive bump steer gives a toe in tendency with suspension bump and negative bump steer gives a toe out tendency with suspension bump.  As suspension systems are typically tuned to have a level of understeer, front axles typically have negative bump steer (toe out, understeer tendency on the front axle) and rear axles typically have positive bump steer (toe in, understeer tendency on the rear axle).

A common perception among tuners is that all bump steer is bad and zero bump steer is the idea scenario.  In reality it is a key tuning lever to deliver a predictable handling balance on a vehicle.  All  production cars have a combination of suspension understeer from compliance steer and/or bump steer.  A suspension system with no understeer will feel very responsive, maybe even too responsive for non-professional drivers.  Having a level of understeer in a suspension system delivers more intuitive handling and also improves high speed stability (reduces vehicle nervousness at high speed).

Typically production cars have front bump steer in the -2 to -10 Deg/m range and rear bump steer between 0 and 4 Deg/m.  The linearity of the bump steer curve is also important to ensure linear vehicle response during cornering.  A typical front suspension bump steer curve is shown below from RACE.  The on-centre bump steer is -2.5 Deg/m.  The curve also has excellent linearity for wheel travel between 50mm rebound and 50mm bump.