Is 100 per cent. Braking Efficiency Attainable?
If you've noticed an error in this article please click here to report it so we can fix it.
A Suggested System in Which the Braking Pressure is Governed by the Extent of the Wheel Loading HERE is widespread mis
understanding of what repre
sents an efficient braking system Ffficiency figures as such do not interestthe average driver; all he is concerned about is the response he gets, in retardation, to the pressure he applies to the brake pedal. '
Such response will, of course, vary with the load and its distribution, the degree of inflation of the tyres and the condition of their treads, the nature of the road surface, and whether it be wet or dry.
Reference to road-test figures as published in "The Commercial Motor" will show that even under identical conditions there is a wide variation in stopping distances between vehicles of different makes, but coming in the same unladenweight class. It is not immediately apparent why such wide differences should exist.
If a vehicle could be brought to rest from 30 m.p.h. in a distance of 30.2 ft, the braking system would be said to have an efficiency of 100 per cent. In practice, such a high rate of efficiency could be obtained only if the coefficient of adhesion between the tyre and the road surface were at unity. In more simple language, 100 per cent, efficiency is possible 54 only when the braking pressure exerted at the wheel and gravitational force exerted (the weight carried by the wheel) are equal.
Whilst the braking pressure may be constant, the weight carried by any individual wheel at any given moment is a variable, being dependent upon weight transference as controlled by road gradient and camber, -acceleration and retardation. So far as its effect on braking is concerned, acceleration can be ignored, as brake applications are not made under such conditions.
Weight transference, however, can be of considerable magnitude and is • in direct proportion to the total weight of the vehicle, the height of the centre of gravity, and the rate of
deceleration Weight transference (7) can be calculated thus: y= E
where E = the percentage of WB braking efficiency, W, weight in cwt., He, height of centre of gravity in feet, and WB wheelbase also in feet. • Applying this formula to a vehicle weighing 10 tons, with a wheelbase of 16 ft, centre of gravity 5 ft. from the ground, and it braking efficiency of 60 per cent., the weight-trusference figure comes out at 37.5 cwt Assuming that at rest the weight distribution between the axles was equal, the proportions would he changed to 6 tons I75 cwt on the front axl: and 3 tons 2.5 cwt on 1.h. rear axle A Variable Factor With a normal braking system the calculations for weight transference are usually a matter of compromise, and as no effort is made to match up brake pressure with individual wheel loading, braking efficiency, as practical experience proves, remains a variable factor.
In an endeavour to provide a system in which the braking pressure on all wheels is proportionate. to the individual wheel loading at any given
moment, Mr. H. R. Morgan, 44, Market Street, Eastleigh, Hants, offers as a suggestion a form of brake in which the energy for operating the shoes is dependent upon the rate of revolution of the wheel being braked, in conjunction with the extent of the loading on the wheel A Suggested Arrangement Reference to the accompanying drawing shows an arrangement of two diaphragm-pumps located within a front-wheel -hub, and a piston pump arranged between the chassis frame and the spring. The twin pumps are driven by an eccentric, and the piston pump by the relative movement between the frame and the spring.
In action, fluid from the diaphragm, pumps is fed under pressure, via a non-return valve, to a high-pressure cylinder, within which are two opposed pistons with a calibrated compression spring between them. The primary piston, on the right, faces pressure from the hub pumps, and the secondary piston, on the left, is acted upon by potential pressure from the piston pump.
The strength of the compression spring is governed by the potential
weight of the vehicle, arid for explanatory -purposes it can be assumed to compress' 1 in. under a load of 320 lb: with a hydraulic and mechanical leverage of 7.25 to 1; this is adequate for a vehicle of 6-7 tons unladen weight. With the machine at rest and unladen, and with the secondary, piston adjustingscrew in the " off " position,this piston will be Subjected to only gravity. pressure.
Maximum Load Control
If the wheel be revolved; pressure will build up in front of the primary piston; _forcing it against the compression spring. When this spring becomes compressed to the extent of 1 in.; the primary piston will uncover a port in the high-pressure cylinder leading to a gravity supply reservoir via a reserve-pressure cylinder:' From From this it will be seen that the maximum load which can be imposed on the primary Piston is 320 lb.. pe:' sq. in., and as the piston is 0.75 in. diameter, this is equivalent to approximately 725 lb. per sq. in fluid pressure. Any weight applied
to the wheel, either by normal loading or by weight transference, Will have the effect of operating the piston pump, and thus a counteracting pressure will In exerted on the primary piston, via the, secondary, piston and the compression spring. The capacity of the road-spring
pump is such that it displace the secondary piston I in, at full road-spring deflection.
Thus, when the wheel is loaded to its maximum, the pressure in the high-pressure cylinder will be raised to 1,450 lb. per sq. in. before the primary, piston can uncover the relief port.
From this brief description it will be appreciated that the greater the wheel loading the higher will be the potential pressure in the.. highpressure cylinder available for brake application.
If .Mr. Morgan"s reasoning be correct, the result will be automatic compensation between the tractive effort erected and the braking pressure presented. An obvious criticism which at once springs to the mind is that of steering instability as ,the result of uneven braking as between the four, six or eight wheels of the vehicle. •
This point was raised with Mr. Morgan, Who countered with the accepted fact that it is the tendency towards wheel locking that affects steering stability.; As the greater the wheel loading the more the tractive effort -presented, it follows that such a wheel will withstand increased braking effort without showing a tendency to lock, and therefore it is reasonable to suppose that such a system would-have no adverse effect on stability.
Is It Practicable ?
Whilst the idea is based on a theory that appears to be fundamentally sound, there might be diflicollies in putting it into practice. Mr. Morgan has, however, prepared a. series of. drawings included . which are prOposed practical arrangement details.
Readers of " The Commercial Motor" may care to give their views as to whether they, consider the scheme practicable or not