Better Fundamental Knowledge Required for
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Dr. Steinitz Analyses the Brake Improvement Basis of Braking Prob A NUMBER of exceedingly interest tt points coneerning the study of braking problems is put forward by Dr. Dipl. Ing. Otto Steinitz, a wellknown authority on such matters. By analysing what may be described as the first principles of brake testing, he attempts to clear up certain confusing issues, and to form a more definite basis for investigation and comparison.
He begins by defining reliability as applied to brakes. One aspect, he says, implies the avoidance of side skidding, turning over and other undesirable movements, together with the avoidance of making excessive demands on the material. Another further implies that reliability is required not only under favourable or ordinary conditions, but also in every instance, and with certainty.
Standards for Comparison.
The shortness of the "brake track," he continues, is an expression for the speed of braking, and in order to form comparisons it is necessary to know what the standard would be for definite travelling speeds. These are governed by natural laws and the possibilities of science.
Brake track we understand to mean the actual marks on the road surface made by the tyre treads; the natural laws, presumably, are physical and can be reduced to terms of straightforward mathematics, whilst the big possibilities of science, we surmise, refer to brake mechanisms and matters of friction between facing and drum and tyre and road.
Dr. Steinitz comments on the ignorance existing regarding conditions of brake application. The formula for the stopping distance, namely the square )f the speed divided by 2g times the z:oefficient of friction (between tyre and road), is based upon the lastnamed being constant. It is, in fact, Sot constant, even with similar conditions of road and brake system. Careful tests have revealed big dis:repancies.
In exploring the reasons for these, se remarks, "We are only at the seginning." It is certain, however, tat the friction coefficient is dependent ns the speed and is at its highest at iverage speed (presumably about 30 n.p.h.).
The drop with rise of speed is attriE30 butable to the burning of the rubber on the surfaces coming into contact with the road, which is said to give rise to fluid friction. In this case the weight on the wheel must also have some influence, as the heating of the rubber must increase with added weight.
This explanation, however, does not account for the coefficient having also a lower value at speeds under average.
The effect upon retardation of locking the wheels is highly controversial. Some hold that the highest braking speed is attained by doing so ; others that it reduces the friction coefficients. There is little doubt that it accelerates tyre wear and promotes skidding. The issue is yet more complicated because— although at first sight it seems rather contradictory—there are degrees of locking. To our mind, then, it seems that locking is an inappropriate word.
If the theory about the burning of the rubber be correct, the doctor reasons, then the favourable action of the wheels would he partly sliding and partly rolling, but, for the shares contributed to the retardation by the sliding movement and the rolling movement respectively, there are absolutely no measurements. There is ample scope here for further study.
Braking Processes.
Analysing the operation of braking, Dr. Steinitz divides the process into three sections :—(l) From the idea, of braking to the commencement of movement of the pedal or lever; (2) from that point to the grip of the shoes on the drums ; (3) from that point to the vehicle coming to rest.
This analysis assists in establishing definite basic meanings of stopping distance .and stopping time. The latter may include all three sections or only 2 and 3. The former may be the distance covered during all three periods, during periods 2 and 3, or during period 3 alone.
Again a complication arises. The stopping distance may be that travelled from the beginning of the slowing down of the vehicle. The braking track usually starts after the slowing down process has commenced, and the starting points of neither necessarily coincide with those of 2 or 3. The study of the variations is difficult because of the shortness of the periods involved.
Process 1 is usually neglected by the "scientific motor authorities "as being a matter of psychology. (In conducting The Commercial Motor tests we endeavour td make a rough correction for the personal equation of the driver.) It should not be neglected, however, for the technical equipment of the vehicle has a definite bearing upon the driver's reactions. The mechanism may assist or hinder the realization of the
idea of braking and may make easier or more difficult the application of the brake_ There seems to be a possibility of important progress in this matter_ A suggestion quoted in illustration of this point was made by a Roumanian doctor, in a German patent specification, whose idea was to arrange the brake control in such a way that it could be worked in the direction away from the impending danger.
Slowing down should begin, at the latest, at the starting of process 2. This will always be the case if the accelerator pedal be released simultaneously with braking movement. Loss of speed during process 2 depends upon normal running assistance, and on the retarding effect of the motor and transmission. Whilst contributing towards the same result, these items are distinct from brake action.
A Matter of Design.
The period from starting to move the pedal to the gripping of the shoes is determined, less than in operation 1, by psychological resistances and more by technical equipment.
We suggest the time-lag of a servo motor as an example, or just the clearance between shoes and drum in conjunction with the weight of the parts and the friction of the bearings.
In so far as the differences of brake systems are concerned, the dependence of process 2 on mechanical equipment is the most important factor. The differences, however, have little hearing on processes 1 and 3, and the length of the period from the grip of the shoes to the vehicle coming to rest is, as a rule, the dominant factor as regards both tim.e and space.
Indeed, irrespective of the mechanism by which the wheels are held from rotating, the coefficient of friction between tyre tread and road surface becomes the determining factor. Thus we are back again at the locked-wheel problem.
Tests have shown that, probably, 30 m.p.h. is the speed at which the coefficient of friction for sliding wheels is at a maximum. For speeds in excess of this rolling friction is greater.
A tech niral method of measuring the degree of locking would require to be
developed. As matters are to-day, drivers have to rely upon instinct.
The scientific study of braking, Dr. Steinitz concludes, is only at its commencement. In spite of the greatest care, experiments have produced results with differences so great as to be absolutely inexplicable. Only progress in research, in the ways indicated, will yield useful results for the improved construction of vehicles (and, we might add, roads), and will make security in driving less dependent on individual capacity than is the case now.