Ackermann Steering Geometry

In order for a car to turn smoothly, the inside wheel must travel on a smaller radii arc than the outer wheel. This is achieved by the inside wheel turning more than the outside wheel. This concept is known as Ackermann steering geometry.

Perfect Ackermann geometry can be devised using the wheel base of a car and knowing the the location of the lower wishbone hub pick-up point. A line can then be traced from the lower wishbone pick-up point to the centre of the rear driveline.

The diagram below shows these lines drawn on a vehicle layout. If the tie rods of the car are placed upon this line, the car will have perfect Ackermann geometry, allowing the car to turn on a very smooth arc without scrubbing the tyres.

The main purpose of a perfect Ackermann system is to reduce tyre scrub when cornering by making the inside wheel turn on a tighter radius than the outer wheel. Without this the inside tyre would have too much slip angle to turn and would be forced to give way, resulting in the tyre scrubbing. A scrubbing tyre has reduced grip and and limits the speed at which a car can turn.

The Effects Of Tyre Characteristics on Ackermann Geometry

Ackermann was designed for low speed manoeuvres, and as such, Ackermann geometry in high speed situations, such as motorsport, creates problems with tyre temperatures and grip levels. These issues occur due to tyre slip angles.

When a car is moving at high velocity racing speeds, the Ackermann angle in the steering system is heavily affected by the tyre slip angles, resulting in a reduced Ackermann angle between the two contact patches of the tyre.

When observing the slip angle vs. lateral force curve for a racing tyre, the differential between the lateral load and the slip angle is a measure of the responsiveness of the tyre to steering inputs. The larger the differential, the less responsive the tyre is to steering inputs. As the graph in the “slip angles” section shows: as normal force increases, so does the gradient of the slip angle curve giving a higher differential. This directly relates to a high speed cornering situation where the outside tyre exhibits a large proportion of normal force and the inside tyre experiences a much lower amount of normal force.

Whilst cornering, optimum slip angle must be achieved. So, during a high speed corner it is possible for the outer wheel to take 75% of the normal load, and the inside wheel to take 25% of the normal load. The optimum slip angle of the tyre must also be achieved at both wheels to generate maximum grip from the tyres. Often at lower normal forces, the optimum slip angle for the inner tyre is a smaller number. This suggests that the inner wheel requires less steering angle than the outside wheel to achieve optimum slip angle. The resulting geometry caused by the slip angle theory is often parallel or anti-Ackermann geometry.

On the other hand, some tyres show data the opposite to this suggesting that at a lower normal load, the slip angle needs to increase to obtain maximum lateral force which would suggest that positive Ackermann should be tuned into the steering geometry. Therefore, a thorough understanding of the tyre dynamics is highly important in order to correctly design the Ackermann geometry for the car.

Turning the inside wheel past its optimum slip angle will also cause the tyre to start scrubbing which will increase the inside tyre temperature and induce understeer into the car.

Aerodynamics and Ackermann Geometry

Ackermann geometry can be influenced by the aerodynamics on a car and it can influence the airflow around a car. These reasons more recently have been determining and selecting factors for Ackermann geometry within Motorsport.

With increased downforce from wings and under body aero, the lateral load transfer around high velocity corners has been rapidly reduced. This has changed the tyre dynamics during cornering and has altered the optimum operating slip angles for tyres. Due to this one of the reasons for justifying anti-Ackermann within motorsport has been ruled out as the new slip angles work in favour of a range for parallel Ackermann to positive Ackermann geometry for the optimum grip levels.

The way in which Ackermann can affect airflow around a car works at both high speed cornering and low speed cornering. During high speed cornering, if the inside wheel has more lock (positive Ackermann) then it can deflect the air away from side pods, air inlets and wings that could be fundamental to the performance of the car. Therefore sacrifice of some front end grip might be done to improve airflow. With relation to slow speed cornering, if the inside or outside wheel has a high amount of lock then it could possibly act as an air dam to the side pod or air inlet situated behind it. This problem is solved by the use of parallel Ackermann.