casings should be looked over to see if there are any serious cuts or bruises penetrating to the fabric and allowing water to find its way into the textile material and cause it to rot. The tires should also be properly inflated as it is claimed that fully 75% of all tire troubles outside of those due to natural depreciation are caused by insufficient inflation and overloading of tires.

Q. What are the best tire sizes for various vehicle weights? A. Each size of tire is designed to carry a specified weight and as many times cars are equipped with tires which are barely up to the requirements, the following table recommended by the U. S. Tire Company will act as a guide to enable the motorist to see if the proper size of tire has been provided by the car manufacturer. The cars are to be weighed without passengers or luggage and each end of the car is to be weighed independently of the other. This is done by selecting a point on the frame midway between the two axles, and marking it in some way so that it can be easily distinguished. The front end of the car is then run on the usual form of heavy platform coal or hay scale, until the mark indicating the central position is just over the edge of the scale. This weight gives the approximate amount to be carried by the two front wheels. The car is then reversed and run over the scales until the rear parts rest on the platform. This weight as indicated on the scale beam shows the amount of load carried by the two rear wheels. The entire car is then placed on the platform and the total weight taken to see how it checks up with the sum of the two previous readings, which thus provides an approximately accurate check on the correctness of the first trial.

Q. Is there any relation between tire size and inflation pressure?

A. There is a very definite relation between the amount of air pressure for the various sizes of tires. As a rule, the larger tires which are heavier in construction and are depended on to carry the greater weight must be blown up to a greater degree than the smaller

and lighter tires. The following table shows the proper air pressures for various sizes of tires. A simple way to remember these pressures is to bear in mind that there should be twenty pounds air pressure for each inch of tire diameter.

Q. What happens if tires are not sufficiently inflated?

A. If tires are not filled with the proper amount of air they will be distorted unduly under load and the fabric and rubber plies of which the casing is composed will be subjected to serious bending which may cause the various layers to separate from each other, this weakening the casing.

Q. What is the result if tires are too highly inflated?

A. If tires are pumped up too much the construction is subjected to stress that it was not designed to withstand and the inner tube will "blow out" through any weak point in the casing. Hard tires also subject the mechanism to considerable vibration because they are not sufficiently resilient.

Q. Which tire should be larger, the front or rear?

A. As a rule, the greater strain is taken by the rear tires because these members carry the larger portion of the car weight, and in addition are depended upon to furnish traction and propel the vehicle. The front tires on most cars can be made smaller than the rear ones if desired, because they are called upon to support less weight and are not subjected to the wear resulting from skidding, driving stresses and braking reaction.

Q. Why are tires usually made the same size on automobiles? A. It is common practice to make the front and rear wheels the same size on most American cars because there is very little difference possible between front and rear shoes with the

average weight distribution. If two different sizes of tires are used, one must carry two spare casings and two different sizes of inner tubes to make satisfactory repairs. Then again, there is no way of equalizing the wear on the tires. If the casings are the same size, the front tires may be used to replace the weaker rear shoes and these members in turn transferred to the front wheels after a car has been in operation for a time. Rear tires invariably wear faster than front ones, so that after they have worn to a point where they are considered weak for the rear wheels they may have ample strength for the lighter duties at the front end of the car and the front tires, which are in better condition, may be placed on the rear wheels.

Q. How is air introduced in inner tubes?

A. Air is usually pumped into the inner tubes by hand or power operated air pumps of a portable pattern that is carried as part of the equipment of the car. A simple form of check valve is used at the tube that permits one to pump air into the tire but which seats automatically by pressure from within the tube to prevent air from escaping.

Q. Are there any substitutes for air?

A. Various compounds have been devised that are intended to be introduced into inner tubes or outer casings to take the place of air but none of these are practical because they are not sufficiently resilient and many of them are compounds that are actually injurious to the rubber of which the tire is composed. Some of these are affected by heat while others will flatten out at the point of contact between the tire and the ground if the car is left standing for any length of time.

Q. What are puncture proof pneumatic tires and how are they made?

A. Various attempts have been made to devise puncture proof

pneumatic tires by using leather or leather and metal treads applied to the tires of the conventional pattern or to imbed a series of metal discs overlapping each other like scales on a fish in the tread of a tire before this is cured. Most of these tires have a grave defect of generating considerable heat while in use and owing to the stiffness of the composite metal and leather or metal and rubber tread much resiliency is sacrificed.