Traction determination by changing wheel normal force

A control system for controlling an active suspension system of a vehicle to determine relative traction levels, the control system comprising at least one controller, wherein the control system is configured to:

• control the active suspension system to change normal force through a first subset of one or more wheels;
• determine a traction-dependent variable at each of the first subset of wheels to which known torque is applied;
• control the active suspension system to change normal force through a second subset of one or more wheels;
• determine a traction-dependent variable at each of the second subset of wheels to which known torque is applied;
• and
• obtain an initial approximation of relative traction levels based on wheel speed measurements of the first and second subsets of wheels prior to changing normal force through the first and second subsets of wheels, wherein the traction-dependent variables determined during the normal force changes refine the initial approximation,
• wherein the traction-dependent variables indicate relative traction levels.