Petrol Motors and Motor Cars: A Handbook for Engineers, Designers, and ... by Thomas Hyler White

Author:Thomas Hyler White
Language: eng
Format: epub
Publisher: Longmans
Published: 1905-03-25T05:00:00+00:00

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BRAKES.

Pbobablt no part of an automobile reciuiiea more careful design than the brakes, in view of the probability that efBcient stopping powei may on occasion prevent loss of life. A h^h &ctor of safety is desirable, and in calculating strengths the weakest plaoea should receive ohief consideration.

The duty of the brakes is to dissipate the energy stored in the movii^ vehicle, and in the shortest possible time. Hence, before we can calculate the strei^th required for any part of the brake gear, the energy contained in the moving mass must be ascertained.

Denoting the energy in foot-pounds by E, the weight of the vehicle in pounds by W, and the velocity in feet per second by «, we have—

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g representing tiie accelerataon due to gravity a 33-2 feet per second. It will be more convenient to modify Uie expression to ^ree with miles per hour, as the speed of motor vehicles is generally expressed in these terms. !nierefore, aa 1 mile is = 5280 feet, and 1 hour is = 3600

seconds, 1 mile per hour ia = -^sfi = 1-466 feet per

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IIJ PETROL MOTORS AND MOTOR CARS. second. Hence for 1 mile per hour ve have, anbatitDtuig

and pntling S = mileB per lioar, the expression 64 becomea—

(55) E = WS" X 0-0334

As an example, ve will assnme a vehiole weighing, inolnding its load, 16 owt, nmning at a speed of 15 miles per hour. Ignoring road reaistance, and substituting known values in 56, we obtain—

E =■ 1792 X 225 y. 00334 = 13466 foot-pounds

A fiustor of importance to be considered when calculating the stoppii^ power of the brakes is the coefficient of friction between the tyre of the wheel and the road surface. For want of precise information on this head we may assnme this coefficient to be 0'4 for iron tyres and 0*7 for rubber tyres. These values will vary according to the condition and material of the road surface. For wood and aaphalte roads, when the surface is greasy, the coefficients should be taken as not more than one-half the above values. On the mi^'oiity of automobiles the brakes only act upon two of the wheels, usually the driving wheels. Therefore we shall require to know the proportion of the total weight of the vehicle carried by the wheels to which the brakes are applied. Fen: the example selected above we wiU assume this to be one-half of the total weight = 896 lbs. Then for the minimum distance in which the car can be stopped we have the expression—

T "WS* X 0-0334 E ^^^> ^ klT^ ftini

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in which L = the mimmatii distance in which the car can be stopped, k = the coefficient of friction between the tyte and the road, and w = the weight in pounds carried by the wheels to which the brakes are applied; the other factors being aa before. Sabatituting known values in 56, we

L = 0.4 y QQg = 37-5 feet for iron tyres

L = Tr= —

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