The increased braking performance required on air-brake equipped commercial vehicles by the Federal Motor Vehicle Safety Standard 121 results in vehicles with higher front brake torque capacity and greater deceleration capability. Using a simple analytical model, certain mechanisms by which handling during braking is influenced by tire characteristics, load transfer during braking, steering system characteristics, brake imbalance, and other factors are demonstrated. In addition, analysis of the steering system shows how steer angle deviations arise from braking and lateral forces acting against compliance of the steering linkage, and the influence of caster geometry on these deviations.
To investigate certain quantifiable characteristics of handling performance, the HSRI Directional Response Computer Program for predicting the longitudinal and directional response behavior of trucks was modified to include the effects of a compliant steering system subject to the force and moment inputs of the front tires. Measurement of bias ply truck tire force and moment characteristics for use in the computer simulation revealed that tire aligning torque characteristics reverse in direction at high braking levels and may dominate the effect of geometric caster built into the steering system.
Studies utilizing the modified program indicate that (a) no vehicle will stop perfectly straight without driver steering corrections because of steer angle deviations and (b) the steering reactions fed back to the steering wheel during braking may reverse direction with antilock brake cycling, largely because of the reversal of tire aligning torques. A relationship between these steering reactions and front brake torque level is shown.