The braking performance of tractor-trailer combinations

Abstract

Equations are derived for the maximum decelerations which can be obtained with balanced and unbalanced trailers with and without trailer brakes. The equations are used to produce graphs showing the maximum decelerations of trailers having typical dimensions, of different weights relative to the towing tractor on surfaces of different adhesion coefficients and with different types and degrees of braking on the trailers. Results measured during braking tests on tractors and trailers are given. Unbalanced trailers are shown to have a better braking performance than balanced trailers and it is therefore, recommended that trailers should be designed to carry the maximum permissible proportion of their weight on the tractor. Both over-run and power brakes on trailers are shown to provide very useful improvements in braking performance. Power brakes are preferred because of the difficulty of fitting over-run brakes on unbalanced trailers. The tractor's external hydraulics are considered to be the most convenient source of power. On-off brakes providing a fixed braking force equal to approximately 5–10% of the trailer maximum gross laden weight are considered to be a useful and simple means of improving existing trailers, but the development of trailer braking systems providing a braking force proportional to the tractor brake pedal force up to a maximum of approximately 20–30% of the trailer maximum gross laden weight is considered to be very desirable.