Vehicle Braking, Stability and Control

An analysis of straight line vehicle braking performance as it relates to basic vehicle, braking system and tire characteristics has been conducted with information drawn from a review of vehicle precrash accident experience focusing on the “no control” condition. Pertinent motor vehicle attributes were reviewed as a function of vehicle size and vintage. The influence of these attributes on vehicle performance in hard braking was analyzed. Stability and control performance was characterized in terms of axle lockup and axle lockup sequence as a function of vehicle attributes and perturbing torques and forces. Both analytical techniques and test results were used in the evaluation. All motor vehicles of early 1970's vintage appear to be underbraked at the front with the concomitant tendency to lock up the rear axle first, a situation that may compromise stability under hard braking. The extent of front underbraking for these earlier vintage cars appears not to be very sensitive to car size for larger cars, but it increases sharply for smaller cars. Based on measurements of a limited but diverse sample, post 1980 vintage cars during hard braking offer actual front brake proportioning of 75 to 85 percent, which represents a level 20 to 40 percent higher than that of early 1970's vintage cars. Because of these differences in front brake proportioning, post 1980 model year cars have a much lower propensity to rear axle lockup under hard braking than cars of 1970 vintage. This is particularly true of a comparison of small cars of these vintages. Based on an examination of a sample of post 1980 production cars, there is considerable variability in brake proportioning by comparison with models of the early 1970's. As regards trucks, light duty trucks of recent vintage appear also to be moving toward higher levels of front brake proportioning, whereas the heavier medium and heavy duty trucks remain much underbraked at the front. The paper concludes with an agenda for further attention.