Page:Popular Science Monthly Volume 90.djvu/124

From Wikisource
Jump to navigation Jump to search
This page needs to be proofread.


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

��� �