The lateral stability of tractor and trailer combinations

A model has been developed to predict the lateral stability characteristics of tractor and unbalanced trailer combinations. For present combinations, stability deteriorates with speed culminating in instability at forward speeds in the region of 18 m/s. The effect of tractor and trailer size and other parameter variations on this speed dependent instability are examined.The effect of braking with and without axle locking are analysed. The stability of the combination is sensitive to the braking distribuion between the axles, which affect the hitch forces developed. Locking the tractor rear or trailer axle results in instabilities, commonly termed jack-knifing and trailer swing respectively. Jack-knifing is the more hazardous instability, whereas trailer swing although potentially dangerous has a divergence approximately an order of magnitude less.The potential of the model for predicting lateral dynamic behaviour of design concepts for future high speed farm transport which would operate at higher speeds than the current maximum of 9 m/s for tractor and trailer combinations is discussed. The scope for generalizing the model to examine other aspects of lateral behaviour, such as steering response is restricted by the limited amount of data available on the side force generated by tyres in agricultural conditions.