Quantitative Measure of Transient Oversteer of Road Vehicles

When discussing oversteer of a vehicle, reference is made to results of the SAE J266 circle test or gradually increasing steer test. However, these tests demonstrate the vehicle’s characteristics at a quasistatic condition and do not consider the dynamic effects of the moment of inertia of the vehicle or of the wheelbase and tire characteristics during yaw accelerations occurring in transient maneuvers. Frequently, there are discussions of the transitional effects on oversteering of the vehicle and reference may be made to the radius of gyration squared versus the product of the front and rear distances from the axles to the CG. This particular relationship, however, assumes that the tire lateral capabilities on the front and the rear are the same. This paper will discuss the comparison of the “Ackermann yaw rate” versus the measured yaw rate in transient steer maneuvers such as the step steer. The Ackermann yaw rate will be the yaw rate developed if the vehicle were to track exactly along the direction that the wheels are pointing. If this theoretical yaw rate is compared to the measured yaw rate, a vehicle’s transitional handling characteristics can be quantified. An example where there has been considerable discussion is with the 15-passenger van. Loss of control of these vans, attributed to oversteer when attempting an accident avoidance maneuver, has been discussed extensively by government and private groups. That oversteer occurs even though these vans exhibit understeering characteristics when tested with the J266 protocol up to a transition to oversteer at the vehicle’s lateral adhesion limit. The technique described here allows the transitional oversteer characteristic of any vehicle to be quantified. This will help to explain and quantify the characteristic causing loss of control of these vans and other similar vehicles.