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New Formula For Brake Testing Suggested

3rd October 1941, Page 25
3rd October 1941
Page 25
Page 25, 3rd October 1941 — New Formula For Brake Testing Suggested
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Which of the following most accurately describes the problem?

MAY I be permitted more of your space to reply to AVI Mr. F. J. Hughes and " P.G."? (Issue July 18.)

Mr. Hughes has appreciated the points I wished to make, but does he realize that many hundreds of prosecutions for inefficient brakes have been brought because the hand brake did not attain the required 30 per cent. and convictions have been recorded merely because some authority has stated that this is the standard to be maintained?

" P.G." has gone a little hay-wire in his calculations —he states that .30 per cent. 'efficiency gives a stopping distance of 124 ft. from 30 m.p.h. and 40 per cent. gives 93 ft. Allow me to inform hits that if he drives a vehicle which cannot pall up in under these distances he will very soon be prosecuted for inefficient brakes, although 30 per cent. and 40 per cent. are allowed.

The braking formula is derived from the fact that a deceleration of 32.2 ft. per sec. per sec. is accepted as 100 per cent., and converting this to terms of velocity and distance we get: Efficiency (%) — (approx.) 3d where V equals speed of vehicle in m.p.h. and d equals stopping distance in feet, and it is from this formula that the braking charts issued to police and p.s.v. examiners are calculated. This gives the correct stopping distances from 30 m.p.h. for 30 per cent. and 40 • per cent. as 100 ft. and 75 ft., respectively. I agree that Coefficients of friction in excess of unity

have, under certain conditions, been obtained, but these conditions obviously go outside the bounds of pure friction, and we are dealing with normal conditions of roads and tyres.

Your readers might be interested to know that the following are actual values found under conditions of

' new tyres of approved tread design and dry road surfaces

New concrete, 0.9; tarred macadam, 0.8; tarred grit, 0.7; asphalt, 0.7; and plain grit, 0.6.

To assume, as the official formula does, a coefficient of friction of 1, is not practicable, and in any case completely impossible unless all road wheels be braked.

• I therefore suggest that if a standard is to be laid down, the people concerned see that a practicable standard be made, and not a percentage of something impossibl,e demanded.

As a guide, may I suggest that a formula such as the following would meet all requirements—the authorities, of course, deciding what percentage is to be attained:—

Efficiency (%) = where V = speed of

vehicle, in m.p.h., W. = number of road wheels, d distance in feet, and B = number of wheels braked. This assumes a coefficient of friction between tyres and road of .83—quite a fair value. G. FREEND.

Cambridge. •

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People: F. J. Hughes
Locations: Cambridge

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