Effects of Load and Gradient on Braking
Page 56
If you've noticed an error in this article please click here to report it so we can fix it.
XP ER I MENTAL investigations which had been carried out showed that brake performance on vehicles which had been operated for some time was much lower than that of new vehicles, said Dr. Starks and Mr. Lister.
The design of brakes for commercial vehicles, except those for light vans, presented more difficulty than in the case of private cars. This was because of the greater braking forces required and the difference between the laden and unladen weights., With new vehicles, or with correctly adjusted brakes, it was found that, in the lightest categories, brake performance was similar to that of the private car, the chassis of which formed the basis of the commercial vehicle. When the vehicles were fully laden the performance was. usually lower than that of the private car.
The Heavier the Longer In the case of vehicles with an unladen weight of under three tons, braking performance approached that of private cars, but the effect of loading reduced this performance to a greater extent than with the lighter types. Similarly, with the heavy types, the braking distance of all the vehicles tested was greater when they were laden.
The effect of disengaging the clutch during braking was to give a more rapid rise in the deceleration-time curve, the measured braking distance being 10 per cent. shorter.
In the case of a vehicle with compressed-air braking, maximum deceleration was about the same as that of the lighter vehicles, but because the time taken to reach maximum deceleration was greater, the average deceleration was lower and the braking distance longer.
An investigation of the effect of load and gradient on the braking distance from different initial speeds was made with a heavy 6 x 6 vehicle. The braking system, which was overhauled by the manufacturer before the tests began, was of the vacuum-assisted hydraulic type.
The unladen weight was 111 tons and the gross vehicle weight about 20 tons. Emergency brake applications were made from different speeds on a dry, level aerodrome runway and on a number of roads having a uniform gradient.
When braking from 20 m.p.h. with the vehicle unladen, the stopping distance was about 45 ft., corresponding to an average deceleration of about 0.29g (9.3 ft. per sec. per sec.). When fully laden the distance was about 80 ft., corresponding to an average deceleration rate of about 0.17g (5.5 ft. per sec. per sec.).
None of the wheels locked during the tests and the ratio of the braking distance in the unladen and laden states was approximately proportional to the ratio of the unladen and laden weights.
The effect of a down gradient was to increase the braking distance by an amount depending on the steepness of the grade. Thus, the average deceleration on a slope of 1 in 10 was about 0.1g less than the average rate on the level.
The Problem of Fade
In' braking on gradients, fade had sometimes to be considered. Tests in which the brakes were not allowed time to cool between successive stops showed that the braking distance increased progressively.
Dr. Starks and Mr. Lister referred to the different results obtained by members of the technical Press. The Laboratory had co-operated with several testers in making measurements on new commercial vehicles, in particular, those conducted by Mr. L. J. Cotton, when he was technical editor of The Commercial Motor. The method used by Mr. Cotton during the past three years was similar to that adopted by the Laboratory.