108 



Popular Science Monthly 



load in the car. The load on the front tires 

 and the load on the rear tires is not equal. 

 To secure the greatest service from the 

 tires the air pressure of front 

 and rear wheels cannot be 

 the same. 



The first step, then, is to 

 measure the load borne by 

 front and rear wheels. That 

 is done by running first the 

 front wheels of a car on plat- 

 form scales and noting the 

 weight, and then the rear 

 wheels. The front tires 

 should be inflated to about 

 15-18 pounds per inch of tire 

 section, and the rear wheels 

 to about 15-20 pounds. 



The inflation tables pub- 

 lished by all tire manufac- 

 turers apply only to unloaded 

 cars. Cars are driven loaded , 

 not empty. The wear on the 

 tires of a light car, when a 

 single extra passenger is car- 

 ried, is anything but negligi- 

 ble. In a small roadster, 

 equipped with three-inch 

 tires, that third man who 

 always insists on riding with 

 you, even though there is 

 obviously no room for him, 

 increases the load by 

 about forty pounds per 

 tire, which means that 

 the tires are under- 

 inflated by that 

 amount so long as he 

 is a passenger. 



To make a pneu- 

 matic which will carry 

 its proportionate share 

 of a load, which com- 

 prising vehicle and pas- 

 sengers, weighs con- 

 siderably over a ton, 

 for as much as ten 

 thousand miles 

 (the earth, 

 mind you, is 

 only twenty- 

 four thousand 

 miles in cir- 

 cumference) is 

 about as diffi- 

 cult a technical 

 problem as ever a 

 manufacturer solved. 



The tread must act 



To remedy a small cut in this tire an 

 inside patch was applied, which acted 

 as a wedge, resulting in a blow-out 



Note how badly the tread shown be- 

 low is cut and torn from the use of 

 chains, evidently fastened tightly to 

 the spokes and not soon removed 



as a kind of armor, and yet the tire must 

 be very resilient — two quite incompatible 

 ideals. To obtain elasticity and also re- 

 sistance to sharp gravel, the 

 tread is made very thick and 

 the sides as thin as is con- 

 sistent with strength and 

 flexibility, to keep heat at 

 a minimum and so that 

 they may bend and recover 

 their shape as the tire rolls 

 along. 



The Action of a Tire 



Energy is lost in bending 

 the rubber in the side walls, 

 but not much, inasmuch as 

 the pressure is constant and 

 the air around the wheel is 

 continuous. Although air is 

 being constantly displaced, 

 the rapidity with which ex- 

 pansion succeeds compres- 

 sion, as the tire revolves, 

 overcomes the effect of this 

 work almost wholly. In the 

 solid tire, work is done by the 

 rubber, theinherentcompres- 

 sibility of the material being 

 constantly tested by repeated 

 compression and expansion 

 and attendant heating. 



A tire which is blown 



up tight is not so 



yielding a cushion as 



one which is softer. 



The higher the air 



pressure within, the 



less opportunity have 



the sides for bending. 



Hence, the temptation 



is strong to let out a 



little air. The car 



rides more easily. But 



all the time the side 



walls bend back and 



forth, back and forth, 



thousands and 



thousands of 



times. You 



know what 



happens when 



you bend a 



piece of wire 



back and forth 



with your fingers. 



It becomes hot 



long before it breaks. 



The side walls of a 



