Brakes may be divided roughly into two classes—the external contracting and the internal ex panding brakes. Fig. 22 shows the external type, and fig. 23 shows the internal type. The for mer type consists of a band at tached to the stationary part of the axle, and acting on the outer surface of a drum attached to the wheel. The internal expanding brake uses a similar drum, the inner surface of which is gripped by two or more shoes which are forced outward by the driver through a suitable mechanism connected to the brake pedal or lever; in some installations a flexible band is used, which is forced outward against the drum by separating its ends.
Two general methods of operating the brakes are commonly used—mechanical and hydraulic. In the mechanical systems mo tion of the brake pedal is transferred to the brakes at the wheels by means of rods or cables, with suitable shafts and levers to compensate for the motion of the wheels relative to the frame and sometimes to equalize pressure between the different brakes. In the hydraulic system, used almost universally on American cars, motion of the brake pedal applies the brakes through an oil pipe to each wheel. A master cylinder and piston is actuated by the pedal, and the oil displaced from this cylinder is forced through a system of conduits to each wheel, where it actuates the pistons in such a way as to apply the brakes. A mixture of glycerine and alcohol is commonly used for the liquid; this mixture changes in viscosity but slightly with change of temperature, and freezes far below any usual operating temperature of the car. Suitable means are provided to refill automatically or manually the liquid lost from the system. A brake is often fitted to the drive shaft which acts on the rear wheels through the driving mechanism. Since the retarding forces are in general more severe than the driving forces, use of this brake involves a certain amount of extra wear and tear on the driving gear, and for this reason it is most often used only as a parking brake. The shoes or bands which press against the brake drum are ordinarily faced with some material having a high value of friction. This is ordinarily made of woven asbestos, which is treated with various materials to act, as a binder and to secure desirable friction characteristics.
proper relative positions. The type of frame which has become practically standard, consists of two longitudinal beams, usually pressed-steel channel sections, with cross members spanning be tween them at intervals. Brackets are provided for the mounting of the various parts of the car. The frame with the mechanical parts of the car assembled upon it is called the chassis. Due to inequalities of road surface there is considerable tendency to dis tort the frame, which places strains upon the mechanism and body-work; so it is important that both longitudinal and cross members be of stout construction to avoid bending and twisting.
The side members are almost always of pressed steel and are from 4.5 to 9.5 in in depth, of material from to -A in. in thickness. The cross members may be of almost any section, but are most commonly either channel or I-beam. Departures from this type of construction, which are relatively few, include designs which com bine the functions of body and frame and others which substitute tubular longitudinal members. The front axle, which was com monly used on most cars until recent years, supports the front end of the car as its name implies. It is a steel beam, usually of I or tubular section, which joins the front wheels of the vehicle; and is ordinarily attached to the frame of the vehicle through springs. These springs prevent the irregularities and shocks from the road being transmitted to the car and passengers. Instead of turning with respect to the vehicle, as in the case of the horse drawn carriage, the axle of the motor car is always fixed in atti tude and direction, and the vehicle is steered by moving the wheels about pivots vertically disposed at the ends of the axle. On many motor cars the front axle is omitted, and each wheel is independ ently attached to the car frame.
The earls steered with a hand wheel which is connected to the steering pivots by means of gears and linkages. The steering gear, as this system is called, may be any one of quite numerous forms of construction. The rotation of the hand wheel is communicated to a horizontal shaft at the base of the hand wheel, or steering column, through a worm gear, screw or cam mechanism. A lever at the end of this horizontal shaft transmits the turning motion, through an intervening link, to a pivot on which a front road wheel is mounted. The other front wheel is also mounted on a similar pivot and the two pivots are linked together by means of a cross rod so that the two front wheels move simultaneously, keeping their proper relative position.