Effects of Suspension Tuning on Off-Road Vehicle Operating Speeds Over Wavy Terrain and Occupant Endurance

Occupants of off-road vehicles are very susceptible to the effects of vertical accelerations. Root mean square values normally accepted are 0.25 g's for longer duration and 0.4 g's for a shorter duration (up to two hours per day) (ref. 1). Typical off-road vehicles stiffen the suspension in order to prevent bottoming of the vehicle. This causes the ride to be extremely rough, meaning the vertical accelerations are at the limit of the human endurance. The speed of the vehicle is then limited by the endurance of the individual. By lengthening the suspension travel and tuning the spring stiffness and damping coefficients, a smooth and controllable ride is achieved thus increasing the natural limit speed.

“Whoop-de-doos” are referred to by off-road drivers as a set of evenly spaced waves in the path of travel. These are often caused by repeated traffic over a soft surface. These bumps are sinusoidal in nature and are usually spaced 6 meters apart and 0.2 meters high. Research has determined that acceptable accelerations over sinusoidal bumps are about 1.4 times the RMS values stated above. Handling these bumps is one of the most severe of off-road challenges.

This paper will discuss the effects of suspension tuning of an off-road vehicle to reduce vertical accelerations. It will be shown that by designing the vehicle to control the pitch response, one also reduces the vertical accelerations felt by the operator. The results of this study will be given as a tuning of the front and rear natural frequency for a given suspension travel and a percentage of critical damping in compression and rebound in the front and rear.