May 1, 1917.] 



THE INDIA RUBBER WORLD 



447 



Power Losses In Pneumatic Tires. 



THE following abstract is from a paper by E. M. Lockwood, 

 assistant professor of mechanical engineering, Sheffield 

 Scientific School, Yale University. The tests were made 

 on an apparatus for power measurement at the rear tires of an 

 automobile, and for the purpose of measuring the internal fric- 

 tion losses of tires and transmission system when driven by 

 an external source, a calibrated motor was attached to the 

 drums. The experiments have the limitation of being made on 

 one size of tire only, and no claim is made that the entire ques- 

 tion of tire resistance has been solved. 



The method of obtaining the rolling resistance was to anchor 

 the car with either pair of wheels on the center of the test 

 drums, and then rotate the latter by means of the calibrated 

 motor. Two readings were needed, one with the car on the 

 drums and one with the car removed, when the difference in 

 power measures the rolling resistance of a pair of wheels. The 

 available range of speed of the motor is from 20 to 40 miles 

 per hour at the circumference of the drums, and its maximum 

 power is IS horse power. The diameter of the drums is such 

 that 300 revolutions are equal to one mile. 



ROLLING RESISTANCE AT VARIOUS SPEEDS. 

 One surprising result of all measurements of rolling resistance 

 at speeds from 20 to 40 miles per hour has been the very slight 

 increase of resistance at the higher speed. 



.A.S an example of the amount of variation of resistance with 

 speed, the followin.g tests are reported with readings exactly as 

 recorded : 



ROLLING RESISTANCE OF FRONT WHEELS. 



Roiling Resistance. 

 Infla- Miles per Hour. Aver- 



tion. Load. .- ^ ^ age. 



Front Tires. Pounds. Pounds. 30 to 40 Pounds. 



32 X 4 Goodyear Cord.... 60 1,120 11.7 11.7 13.8 .... 12.4 



32 X 4 Fabric 55 1,145 24.1 25.0 23.5 24.4 24.25 



35 X 5 Goodyear Cord 60 2,100 33.0 33.6 34.2 33.8 33.65 



These cars differed considerably in rolling resistance, but in 

 every case the speed of the car had little or no effect on the 

 result. Rear wheel tests were made confirming the results ob- 

 tained with the front wheels, hence the conclusion from these 

 tests that the .internal rolling resistance of an automobile is 

 practically constant at all speeds up to 40 miles per hour. 



DISTRIIiUTION OF POWER LOSSES. 

 The internal j ower losses of the six-cylinder Chalmers chassis 

 have been determined in this way. The tires are 32 by 4, load on 

 front wheels, 1,145 pounds, load on rear 1,545 pounds, tire infla- 

 tion 75. 



POWER LOSSES IN 6-CYLINDER CHASSIS (2.700 POUNDS). 



Pounds. Per Cent. 



Front tires only 18.2 31.2 



Rear tires only 17.7 30.3 



Front bearings 4.7 8. 



Rear bearings and transmission 17.9 30.5 



Totals 58.5 100. 



Dividing the above four items nito groups we find that the 

 tires cause 61 per cent of the total resistance, while the axle 

 bearings and transmission up to neutral consume only 39 per 

 cent. The conclusion is that the tires are responsible for nearly 

 two-thirds of the power lost in the car itself. 



The above values are averages for speeds from 20 to 40 miles 

 per hour, and, as before stated, the resistance chan,ged little 

 with the speed. 



TIRE RESISTANCE AND PRODUCTION OF HEAT. 



The discovery that 61 per cent of the internal power loss of 

 this car is expended in the tires leads naturally to what becomes 

 of this power. 



It is probable that friction of the material within the tire 

 structure is continually taking place, due to flexure at the con- 



tact with the road. This absorption of work and production 

 of heat is similar to the friction brake, except that the friction 

 is produced by bending the fabric, not by sliding it. 

 - -Assuming the truth of these assumptions, it ought to be possi- 

 ble to calculate from the work absorbed the amount of heat 

 generated; also the temperature of the tire surface when it 

 reaches thermal equilibrium. The solution of this problem is 

 worked out like a hot water radiator ; or, rather, as the wheel 

 is revolving, like a hot blast heater. 



These tires have 5.83 square feet of surface, and the force of 

 resistance at the circumference is 8.85 pounds. At a speed of 

 30 miles per hour there will be generated 1,810 British thermal 

 units i.er hour, and to dissipate this heat to the air will require 

 a rise of temperature of 39 degrees F. above room temperature, 

 based on a heat transfer coefficient of 8 Britisli thermal units 

 per degree per hour. 



This calculation was made in advance of any experiments, but 

 was later verified by running one of the 32 by 4 tires for half 

 an hour at 30 miles per hour, after which the car was stopped 

 and the surface temperature taken with a mercurial thermometer. 

 The room temperature was 70 degrees P., and the tire tempera- 

 ture was 107.5 degrees, or a rise of 37.5 degrees as compared 

 with the calculated rise of 39 degrees F. 



This experiment confirms the theory that tire resistance is 

 caused by flexure of the material, and leads logically to the con- 

 clusion that resistance will be increased by under inflation and 

 by overloading. 



INFLATION PRESSURE AND LOAD. 



Of the factors affecting rolling resistance, inflation pressure 

 is perhaps the most important. Very considerable changes of 

 resistance may be caused by moderate changes of pressure. 



The load carried by the Ijre is also an important factor, which 

 must be considered along with the inflation. It seems probable 

 that for a given load there is a region of inflation below which 

 the resistance increases very fast. 



This relationship can best be shown by a series of character- 

 istic curves, where rolling resistance is plotted against inflation 



DiAGR.\.M Showing Relation Between Rolling Resistance 

 AND Inflation Pressure. 



SOi-rr 



40 50 

 nnXATIOB PBESStJIE 



DESCRIPTKIN OF 



Wheels. Tirr^. 



Rear Fabi ic . 



Rear Fabric . 



Front F'nbric 



60 



eo 70 



1AS.VZSI so-n. 



CUK\'ES, 



Load on Each 

 Wheel in Pounds. 



772 



550 

 570 



Front Silvertown Cord 570 



for a number of loads. A diagram showing an incomplete series 

 of such characteristic curves for 32 by 4 tires, is shown herewith. 



