The True Definition and Measurement of Oversteer and Understeer 2015-01-1592

The concept of vehicle understeer and oversteer has been well studied and equations, test methods, and test results have been published for many decades. This concept has a specific definition in the steady-state driving range as opposed to quantification in highly transient limit handling events. There have been specific test procedures developed and employed by automotive engineers for decades on how to quantify understeer. These include the constant radius method, the constant steering wheel angle/variable speed method, the constant speed/ variable radius method, and the constant speed/variable steer method. These methods are very good for calculating the understeer gradient but care must be taken in interpreting the result at the limits of tire traction since lateral tire forces can be reduced on a drive axle when significant throttle is applied.

This paper focusses on the mainstream definition of understeer and oversteer and how that objective metric should be measured on a motor vehicle and where care should be taken in interpreting results. A proposed “Transient Oversteer metric” was also evaluated and was found to not have mainstream acceptance and was very dependent on driver inputs and not vehicle handling characteristics. In fact, the results could not be repeated.