Frequently the tire is too small for the rim, and this is always bad for it. Heavy loads will cause cutting, because the tire will be flattened out, although inflated to the proper tension. It is good practice to turn a tire, when one side wears more than the other. This wearing on one edge excessively, shows some defect in the wheel alinement, which needs correcting. Possibly the wheels may not be parallel. This is a frequent trouble with front wheels, on account of the bending of the arm which runs from the knuckle. INFLATION PRESSURES.--Manufacturers of tires furnish data with respect to the proper pressures for their products, and these vary somewhat, and it is wise to observe the pressures which they indicate for the different sizes. EXPANSION OF HEATED AIR.--There is another cause of tire expansion, not generally considered, which is due to the expansion of heated air. It is not infrequently the case that a tire will, in running, heat up fifty or sixty degrees, which means an expansion of one-eighth the volume of air within the tube. If, therefore, there is any weakness in the walls of the tire, a blowout follows. As this heating is liable to take place to a greater extent in the summer than in winter, it is obvious that it is better to under inflate during that period, than to have an over pressure, particularly with old, or considerably worn, or injured tires. CHAPTER IV THE STEERING GEAR AND BRAKES THE STEERING COLUMN.--This is a very important mechanical element of the car. Its direct useful functions are to carry or hold the mechanism for steering the machine, and for the motor control, controlling the air supply for the fuel, as well as for regulating the sparking mechanism. MOTOR CONTROL.--Some machines are provided with a foot lever mechanism (accelerator) as well as the throttle lever on the steering wheel. This is advantageous, because in moving through crowded streets, where frequent and quick changes are necessary, the foot is the most convenient for controlling purposes. THROTTLE MOVEMENT.--A downward pressure of the foot opens the throttle, and a spring returns it to its normal position. The foot throttle is also convenient when shifting the transmission gear, as both hands are otherwise engaged, one to operate the gear-shifting levers, and the other for steering. The hand throttle on the steering wheel, however, is most convenient for long runs, when little change is required, and it can then be set so as to avoid the use of the foot lever. The levers are so arranged that they do not entirely close the throttle, but, when fully thrown to a closed position, will still provide a sufficient opening to keep the engine running light. [Illustration: Fig. 20. Steering Wheel.] STEERING WHEEL TYPE.--The drawing, Fig. 20, shows a type of steering wheel, which has a segment A. The long lever B is for throttling purposes, as above described, and the short lever C for operating the sparking device.