The current paper investigates the nonlinear stationary oscillations of a quarter vehicle model with two degrees of freedom subjected to a vertical road excitation. The damping of the wheel suspension has a bilinear characteristic, so that the damping strength is larger during compression than during restitution of the damper. For the optimization of the damping behavior the peak-to-peak swings have to be as small as possible. The unevenness of the road was approximated by filtered white noise which was modelled numerically using pseudorandom sequences. The first order form of the governing equations was transformed to hyperspherical representation. The stability was determined according to the largest Liapunov exponents obtained from the numerical simulation. For a chosen parameter range stability charts were constructed both in the stochastic and harmonic case (for comparison).