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WHY FRONT-WHEEL BRAKES ARE NECESSARY.

20th May 1924, Page 10
20th May 1924
Page 10
Page 10, 20th May 1924 — WHY FRONT-WHEEL BRAKES ARE NECESSARY.
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Which of the following most accurately describes the problem?

A Reply to Dr. T. Blackwood Murray's Recent Article. The Speed of Commercial Motor Vehicles, the Danger of Side Slip and the Question of Weight Distribution.

B) (opt, 14/. 0. Bent.ey.

IAM naturally most interested in the views on the above subject put forward by your eminent eontributor, Dr. T. Blackwood Murray, D.Sc., M.I.O.E., and, although I myself cannot claim to be aS deeply versed as he in the technique of the industrial motor vehicle, I feel that hi ii statement of the case leaves so much unsaid, and is, in fact, so incomplete, that some rejoinder is necessary.

It is not to be denied that, in the case of the commercial vehicle, there are two matters of great importance calling. for consideration, which enter only to a minor degree into the conditions of the touring car, namely (1) the necessity for keeping the cost of the vehicle as low as possible, and (2) the effect of the distribution of weight upon the axles.

With regard to the first, it can be said at once that feent-wheel brakes are "worth while" always provided that they enhance safety and effectiveness. As to this I do not think there can be a shadow of doubt. I have no statistics at hand to prove this point, but my impression, founded upon observation, is that est of the accidents in which industrial vehicles figure (I am, of 'course, not suggesting -that in this bonnection they are any worse than so-called pleasure cats) have been due to' ma inability to stop quickly . , in case of emergency. •

Commercial Vehicles Generally Over-driven.

A vehicle driven for dividend-earning purposes is, in all probability, habitually driven, may I say, more forcefully than the touring car, owing, first to the fact that the driver is merely an employee having no personal interest in the property under his control, and, secondly, to the fact that the heavy lorry is almost certain to emerge practically unscathed from any highway trouble into which it contrives to get. Its invulnerability in case of -collision is an encouragement to careless driving, and I may add that this view seems to find support in the formidable arrangements for protecting the radiator which are seen on so many vehicles of thie type.

The great weight of the lorry makes it a source of serious potential damage, and of all vehicles, therefore, it should be equipped with the means of the greatest possible control. From this standpoint, front-wheel brakes are decisively indicated.

Under conditions F of deceleration, a considerable fraction of the effective weight borne by the rear axle is transferred to the front axle, se that, whatever may be the static distribution of weight, a distinct advantage is to be gained by braking the front wheels. Dr. Murray, in his argument, assumes that the industrial vehicle is always loaded. I cannot believe that the average distribution of: weight for the whole of such a vehicle's working life is 75 per cent. to SO per eent, on the rear axle. In my opinion, the conditions of, say, a three-ton lorry running light point unmistakably nie, only to the desirability, but even to the n6cessity, of front brakes.

So far as pulling-up power is concerned, on a dry road a four-wheelbraked car, from 30 m.p.h., should be able to pull up in a little over 40 ft. Dr. Murray quotes the caw of a commercial vehicle pulling up with rear brakes alone in a distance of 50 ft. to .54 ft. I have not the slightest doubt that this record was obtained on a dry road surface. Now, the great advantage of the front-wheel brake is that it is effective 'on a slippery surface, •and is, indeed (as 1326

has been amply proved), a powerful and valuable corrective of any tendency for the rear wheels to skid. Tarmac, asphalt, and concrete constructions are steadily making roads more slippery—in many cases alarmingly so—in addition to which it is to be conjectured that, thanks to their being in industrial areas,* most 'of the reads on, which lorries run their greatest mileage . are bad from this point of .viewetram-lines, greasy stone setts, and so on. Hence, again, the front-wheelebrake is directly pointed to, since it is the• best corrective of skidding that has yet been inteoclueed„

The Ever-present Risk of Side Slip on Greasy Surfaces.

HasDr. Murray ever studied the behaviour of London motorbuses and other commercial vehicles on slippery surfaces? If so, he cannot deny the danger that they represent. How is it possible, then, for him to deny the utility of front-wheel brakes ? Such an attitude suggests that the " wish is father to the thought," All engineerieg progress demands certain sacrifices and compromises, and to eegard presentday transport conditions as being as good as they could possibly be With regard to the braking power of commercial vehicles strikes me as being, to say the

-least of it, unduly optimistic. . • Dr. Murray has one sentence -which, to any candid and unprejudiced observer, is simply indefensible. May I quote it in full?

" Thirty m.p.h. is an abnormal speed for anything but a very exceptional commercial vehicle, and, therefore, if the braking is good enough at 30 m.p.h., as it obviously is, it is relatively much more satisfactory at lower speeds." I have rarely come across a statement so indefensible and provocative. There are large numbers of eommercial vehicles which both can and do attain their 30 miles -an hour (I hope Dr. Murray is not placing his faith in governors:). And the braking, whilst it may he "good enough" tinder ideal conditions, is certainly not good enough under average conditions. According to my experience, . the only Obvious thing is that the braking of commercial vehicles ought to be improved.

Better Weight Distribution Demanded.

I, for one, do not hesitate to express the hope that the technical Press will do nothing to prevent vehicle users being stampeded" into demanding front-wheel brakes. If, at the saine' time, they demand a better distribution of weight than that which Dr. Murray quotes, they will he not only doing something to enhance the safety of mechanical transport in general, but they will also be bearing a hand in the excellent work of relieving some of the inordinate stress to which the roads of this country are subjected.

He dismisses front-Wheel brakes as an " unnecessary amplification." It is not so long ago that multicylinder engines, pump cooling„ four-speed gearboXes, pressure lubrication and many other things 'accepted as cemmonidace to-day -were stigmatized in the same opprobrious..terins. It Dr Murray thinks that the present-day industrial .vehicle represents finality in design I regret I Must disagree with him, and I am quite satisfied to leave the confirmation of my views to the decision of the future.

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