This paper is concerned with in-plane (horizontal and vertical) tire vibrations due to “impulsive” input forces which result when tires impact small or cleat-like bumps (e.g., tar strips). Such high frequency vibrations (20-100 Hz) are commonly associated with vehicle harshness.Quantitative information relating output spindle forces to different cleat inputs is obtained from rolling drum experiments on radial tires. An examination of the data reveals a nonlinear behavior not included in conventional tire equations. This phenomenon is discussed in conjunction with recent advances in tire modeling. A physical explanation is offered which attempts to describe and isolate the major source of the nonlinearity.