A Load Off the Right Foot
Page 78
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BRAKES on British commercial chassis of under 1 ton carrying capacity are adequate for normal operation in this country. Some people may, therefore, feel that the offering of a servo unit to boost the brakes of these vehicles is wishful thinking on the part of the Clayton Dewandre Co., Ltd., Lincoln.
After testing the servo unit, which is known as the Mot-A-Vac, fitted to a Bedford CA 10-12-cwt. van I suggest that at an installed cost of £17 10s., the servo can provide a worthwhile return by reducing driver fatigue and giving improved braking performance under difficult road conditions, The servo is, of course, not solely for commercial vehicles, its main market being in the private-car field. However, it is ideallysuited to light commercials incorporating car-type running units.
Fitting the servo should cause little difficulty, as it is a compact unit and is provided with a comprehensive kit of piping and brackets to allow universal installation.
On the Bedford CA van the Clayton experimental department had attached it to the cruciform chassis member under the floorboards, where it was adjacent to an existing brake pipe junction into which the outlet from the servo hydraulic cylinder was inserted. Steei piping connected the servo control valve to the master cylinder on the original braking system and similar piping, with a rubber section interposed to allow for relative movement between the engine and chassis, carried the vacuum supply.
Rapid Reaction
The servo itself is of the suspendedvacuum-type to give quick reaction. Vacuum is applied to both sides of the diaphragm when the servo is inactive and it is brought into action by the admission of air to the control side of the unit.
The air inlet valve is actuated by pressure build-up in the master cylinder and, in order not to reduce brake pedal pressure abnormally, does not come into service until pressure in the system has reached 8 p.s.i. Below this pressure hydraulic fluid merely by-passes the valve to give normal braking. In the event of vacuum power failure, similar by-passing takes place to allow full unassisted braking.
No additional vacuum tank is fitted, as the servo is capable of retaining vacuum sufficient for about three normal brake applications after the engine has been stopped. The servo is sealed and can, therefore, be mounted in an exposed position on a vehicle without fear of damage by water.
The test van was about four years old and was operated by the Clayton service department. It had completed some Es44 75,000 miles in the hands of a variety of drivers and bore the marks of a hard life. The brakes were considered to need relining. Tests were conducted on the perimeter track of a disused airfield and for the loaded stops 11 cwt. of iron formed the payload.
Attempting a trial stop from 30 m.p.h.
with the servo disengaged (a stop-cock in the vacuum line was provided for this purpose), it seemed doubtful whether the test could be conducted at all, because the brakes displayed extreme unbalance. Two more stops using full pedal pressure were made with the same result and then, inexplicably, the brakes came back into balance and behaved impeccably for the remainder of the tests.
Clayton claim that the servo will reduce the required pedal pressure for a given deceleration rate to half that of an unassisted system. Thus, for my tests from all speeds I used maximum pressure without the servo—about 240 p.s.i. —and about half this pressure with the servo. Two stops were made from each speed and the stopping distances were measured by means of the electrically detonated chalk gun used normally for brake tests by The Commercial Motor.
The surface on which the tests were carried out was abnormally good and resulted in exceptional stopping distances. For instance, the unassisted stopping distance from 30 m.p.h. was more than 6 ft. less than that achieved with the latest Bedford CA running at about the same weight.
From 20 m.p.h. the average stopping distance without the servo was 15 ft. 4 in., whilst with assistance, a distance of 15 ft. 6 in. was achieved at 120 p.s.i. pedal pressure. There was less than 2 ft. difference between the results from 30 m.p.h.-35 ft. 7 in. unassisted and 37 ft. 6 in. with the servo.
Average pedal pressure with the servo at 115 p.s.i., was found to be slightly lower in this case. Without the servo a trial application from 30 m.p.h., using a pressure of 120 p.s.i., returned a stopping distance of 45 ft: The Quicker the Better As speed increases so the value of the servo becomes more apparent, as all the small vans when loaded take determined pedal pressure to slow them rapidly from high speeds. It must be remembered that there are vans in this category which will, with ease, top 70 m.p.h. The Bedford would reach only 40 m.p.h. in the distance available, but from this speed the normal braking system required all the pressure I could apply to pull the vehicle up in 65 ft., whilst, with the servo, a pressure of 140 p.s.i. sufficed to halt it in 60 ft.
This was proof that the servo can do what Clayton Dewandre claim for it in respect of reducing pedal pressures. What is equally important is whether the servo can also give a progressive action to ensure that braking will be smooth and safe undet all conditions, laden or unladen, When laden the assisted braking system of the van displayed no vices, giving a fully proportional return for pedal movement and quite sufficient " feel."
With the load removed from the van the servo caused no embarrassment. In this condition the pedal pressure required is not unduly low and one becomes accustomed to it quickly. Even with the well-worn braking system of the van there was no discernible snatch or any other unpleasant characteristic.
In hilly areas or for multi-stop urban operation the addition of a servo to light commercial vehicles can, I am now convinced, he of great benefit.—A.E.