Ackermann at 50mm Rack Travel

Ackermann in a suspension system determines how much the the inside and outside wheels steer during cornering.  As a vehicle corners, the inside and outside wheels travel along different paths, with the inside wheel scribing an arc with a smaller radius than the outside wheel.  To follow this tighter turn radius without scrubbing the tyre, the inside wheel steer angle needs to be greater than the outside wheel.  The geometry which defines the exact steer angles required for the kinematic steer condition with no tyre scrub is called Ackermann steer geometry.  Ackermann is reported as a percentage, 0% being parallel steer (i.e. inside and outside wheels steer to the same toe angles) and 100% being perfect Ackermann steering.

In every day driving the Ackermann in a car’s suspension is most noticeable during low speed manoeuvring (e.g. during parking).  When turning on full lock in a car with low levels of Ackermann, excessive tyre scrub can be felt as a judder in the suspension and steering.  The motion of the car doesn’t feel smooth through the turn as the tyres are fighting the turning motion of the car.

In racing or track applications, low speed manoeuvring is not a concern and Ackerman is often set to zero or negative (i.e. inside wheel steers less than the outside wheel).  This geometry optimises cornering potential by reducing the slip angle on the inside (unloaded tyre).  Tyres with lower vertical load generate their maximum cornering potential at lower slip angles, so by running zero or negative Ackermann the overall cornering potential of the inside tyre can be increased.

In production cars, most would have greater than 50-60% Ackermann at high steering lock angles (e.g. >30-35 Deg).  Sports cars can sometimes prioritise the handling gains of lower levels of Ackermann and can be somewhere between 30-50%.

In RACE, Ackermann is reported at 50mm rack travel.  The steering lock angles at 50mm rack travel are also reported.  It is worth noting that the level of Ackerman in a suspension system changes with steering lock angle.  This can be seen in the curves produced as part of the RACE pdf report.