POLICE ACTIVITY IN TESTING BRAKES.
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The Importance of Carrying Out Tests of Brake Efficiency in a Proper Scientific Manner and not by the Methods Which Brought Police Trap Timing Into Disrepute, By L. A. LEGROS, M.I.Mech.E.
THE INCREASING weights and numbers of commercial vehicles, whether of the lorry, char-hbanes, or motorbus types, and the great differences that are found between the working conditions, supervision, and maintenance of fleets that range from thousands of vehicle to units, have caused the question of brakes and their effectiveness to assume an importance as great at the present time as was that of railway brakes in 1878.
It is well known that the brakes of many motor vehicles have small power and little wearing capacity, and, further, that in some cases they do not admit of ready and rapid adjustment. It is generally recognized that a motor vehicle of inadequate brake power, whether this arises from error in design or from insufficient care in inspection and maintenance, is a source of danger not only to its crew, but also to other users of the road. In the ease of large fleets, regular and systematic examinations and tests of the brakes are made, and accidents arising from inefficient brakes are not numerous; hut in the case of vehicles not subject to such technical supervision troubles are more common, and their occurrence, announced in the Press under such headings as "runaway lorries " or " vehicles not under control," has become so frequent of late as to have attracted the attention of the police in many areas.
Speed in the early days of the motorcar was singled out as the chief factor in producing danger, and many years passed before the general public recognized 20 miles an hour on the open road as a safe speed. Motorists of the early days will remember the extraordinary timing methods that were adopted and accepted as evidence of speed. Length and time can he measured with great accuracy by the use of sensitive or delicate apparatus in skilled ha,nds, and with fair accuracy by observers who have had little training, but so soon as length and time are either or both
involved more than once, as in the measurement of rate (or speed), and still more in the case of change of rate, greater skill and knowledge are required to obtain even a rough approximation and, generally, the difficulty increases and the accuracy diminishes with increased complexity of the combination. Thus, it is much more difficult to measure " brake power,' or negative acceleration, with an error of less than 5 per cent, than to measure speed with an error of less than 1 per cent. ; or time, or length (except for
small intervals) with an error of less than a tenth of 1 per cent.
Obtaining reliable ineasureni. ents of the effectiveness of brakes depends upon the coefficient of friction of the brakes on the drums, and also.u.pon the adhesion between wheel and road surface—two factors that vary between wide limits. On the one hand, tests made with four-wheel brakes have given a value exceeding
unity for the ratio of brake effort to weight ; and, on the other hand, on asphalt moistened by slight rain or on greasy mud of the proper consistency, and even on asphalt under very heavy rain, the coefficient of friction may be so small that a very gentle application of the brakes will lock the wheels. The careful driver reduces his speed on such surfaces to the limit which his opera] experience tells him is compatible with ability to stop, and he takes account of the gradient, a high speed being as safe uphill as a low speed downhill.
Apart from adhesion, there is the resistance to traction offered by certain surfaces, which varies from GO lb: per ton to 120 lb. per ton on paved or macadam roads, and is greater still on loose stones. This resistance considerably assists in stopping a ear, and further help may be bbtained by leaving the engine engaged in low gear and using the clutch to check any tendency to gather speed—a method recommended and used by most experienced motorists on bad hills and on some kinds of treacherous surfaces, because it is less liable to cause skidding than the sudden application of the brakes. Agricultural tractors, quite devoid of brakes, may even be stopped by the judicious use of the clutch. Another factor that affects the effectiveness of the brakes is that of whether the hand brake is applied by pulling or pushing the lever. In the former case the driver must lean forward to pull the brake lever, and the effect of momentum tends to keep the body forward and to diminish the force applied to the lever, whereas with the " push-on " type the action of the brake tends to be exaggerated by the momentum of the driver's body. The cases resemble those of stable and unstable equilibrium, but, in the case of the 'push-on" brake, the driver's arm assumes a position
in which it has greater freedom when the brake is on than when it is off.
The War Office subsidy specification for commercial vehicles separates construction from performance, and the question of effectiveness of brakes may well be examined under both of these heads. Under the head of construction, it can he ascertained from the results of trials whether the design provides for efficient )..rakes, for what range of wear they Will remain efficient, •whether proper provision has been inade for taking up wear within the travel of the hand lever or pedal, and whethef the brakes will keep reasonably cool after long application. It must be remembered that the coefficient of friction is low at high seed and risesat slow speeds (1), and that the coefficient is high on cold metal and falls as the temperature rises. On railways the rise in the coefficient due to reduction of speed during the period of stopping generally exceeds the fall in the coefficient due 1 rise of temperature ; but the accelerations on railways are small compared with those that are common in heavy motor vehicles. Moreover, the gradients on railways are very much easier than those of average roads.
Tinder the head of performance, it is possible to arrange a series of testa for brakes which will show the efficiency of each brake separately on very steep" pitches and on long hilh. Such trials can be carried out on a specially selected road, with adequate assistance at hand in case of any failure, but these severe trials, intended to settle the acceptance of a type, are not such as can be applied by anyone at any time and anywhere. In the case of brakes of small surface or fitted with narrow linings, an unreasonably severe application may destroy the linings and render the brakes useless.
It•is not sufficient to require a driver who has been stopped for brake inspection to start his car, and then at some unknown speed to apply the foot brake or the hand brake until the ear comes to rest, and for a conclusion to be drawn—from the distance that has apparently been run after the order to apply the brake was given—as to whether the brake was effective or not. A lorry running at 14 miles an hour will run more than 20 ft. (or more than its own length) in one second, and tbe operations to be performed in removing the hand from the change lever to the brake lever and pulling or pushing the latter, as the case may be, into the " on" position involve several movements, all taking time to perform, to which time must be added the normal reaction period of the driver. Results so obtained might serve for a caution, but they cannot form a fair basis for a conviction. .
The regulations—The Motor Cars (Use and Construction) Order, 1904—stipulate, under art. 11, clause 4, par, 1, that:—
" The motorcar shall have two independent
brakes, in good working order andof such
efficiency that the application of either to the motorcar shall cause two. of its wheels on the same axle to be so held that the wheels shall be effectually prevented from revolving, or shall have the same effect in stopping the motorcar as if such wheels were so held."
This clause, however, is extremely vague, because it takes no account of several variables—namely, adhe sion, gradient or load. It states that the wheels must be " efficiently prevented from revolving" ; but, on the one hand, it does not f0::ognize the assistance that may be obtained from the clutch and switching all the engine, and, on the other hand, it says nothing as to the load on the vehicle.
For the determination of speed in cases of exceeding the speed limit, it was found necessary to require the evidence of two representatives of the police. In the case of brake testing there is only an implied necessity for two observers, because speed is aVactor in the distance run after application ef the brake. There can be no doubt as to the position of the ear when it has come to fest, but, while it is in motion, it is difficult to ascertain its position when any manceuvre is performed. A single observer riding on the car cannot simultaneously watch the application of the brake and locate the position occupied by the car when the brake was applied. An observer on the road cannot see the movements of the hands and feet of the driver, and, in the case of a brake test of the kind in question, it is necessary for him to occupy a previously arranged position.
The effectiveness of brakes is not a. question of opinion. It can be measured, and the measurement recorded so that—provided the instrument is properly adjusted—it is beyond dispute. To do this, however, requires the use of an accelerometer, of which a recording type has been devised by Dr. F. W. tanehester (2) and a non-recording type by Mr. H. E. Wimperis. Neither of these appliances, however, could be entrusted to an untrained observer.
It is of the greatest importance to the public and to the commercial motor user that the brakes of motor vehicles should be properly designed and maintained in a state of efficiency. It is also of interest to the insurance companies and underwriters that accidents due to defective brakes should diminish.
it is highly ,undesirable that attempts to measure the effectiveness of brakes should be made by haphazard methods and used for the conviction of drivers. The application of such "tests " by the police must inevitably lead to an outcry against persecution, and the real offenders, as usual, will escape. The danger due to ineffective brakes is far too serious to be made the subject of jests against police methods, as was the ease with speed. Speed involved the idea of distance divided by time. Acceleration involves the • idea of speed divided by time ; that is, the rate at which speed is varied. The fact that the actual speed of a car varies from instant to instant is not generally recognized. The lay mind confuses the terms " first speed," " second speed," etc. (which refer to various gear ratios engaged for corresponding positions of the change-gear lever), with the actual speed.
The question of brake efficiency is of too great public importance for it to be turned to ridicule after the fashion of amateur efforts at timing. Inquiry by the Ministry of Transport into an accident after the event (as made by the Board of Trade in the case of boiler explosions) will have little effect on the owners or drivers of vehicles, and the treatment by the insur. ance companies of accident risks as pure matters of chance will merely affect the premiums. It would appear that a matter of this magnitude can only be dealt with by the authority directly responsinie, on the recommendation of a committee, which committee should consist of representatives of all the interests involved.