Effect of weight distribution on the slip efficiency of a four-wheel-drive skidder

Abstract

On four-wheel-drive skidders (FWDS) the front and rear axles are rigidly coupled and the tyres are the same size. This means that angular speeds of the front and rear tyres are the same. Therefore, variations in the rolling radii of the front and rear tyres, due to changes in inflation pressure and tyre deflection, and different sinkage, etc., could cause significant torsional wind-up. A theoretical analysis and experiments showed that the coefficient of weight distribution influenced the efficiency of slip of a FWDS. In cases where the coefficient of weight distribution decreased, wheel slip was predicted and the ratio of the theoretical speed also increased leading to a loss of slip efficiency and the risk of torsional wind-up between both axles. Greater efficiency of slip was obtained with equal tyre inflation pressures in front and rear tyres. The efficiency of slip increased when tyre inflation pressures and drawbar pull were reduced. This can be explained by the fact that the larger the tyre-soil contact area, the higher the traction and proper ratio of theoretical speed of the wheels and the lower torsional wind-up. The lowest efficiency of slip gets when inflation pressures in front tyres is on high level and in rear tyres on low level due to highest torsional wind-up. This is because that soil shear strength beneath the front wheels is lower than that beneath the rear wheels, due to lower soil bearing capacity of non-compacted forest soil, which requires higher slip in the front wheels than rear wheels which moves on compacted ruts. To reduce the effect of torsional wind-up and to enhance the efficiency of slip in given forest condition inflation pressures should be kept at lower but permissible levels and equal in all tyres. It was proposed that the ratio of gross traction coefficients for the front and rear wheels best represents the effects of weight distribution on gross traction and facilitates modelling.