in traffic on the third speed the driver may set the second speed button and by depressing the clutch pedal fully will automatically shift into second speed. In a similar manner all other changes may be made. The driver need not lift his hand from the wheel in order to accomplish any change in the gears. Those who have witnessed the operation of this device state that the system is quiet, the only noise being a slight click as the gears engage when changed. This system is also used on the Haynes 1914 automobiles and several other types.

NOTE. For complete discussion of all standard gear shifting systems see "The Modern Gasoline Automobile."-By Pagé.

LESSON TWENTY-EIGHT

UTILITY OF BRAKES AND THEIR USES

Q. What is the purpose of the braking system?

A. The brakes of an automobile or any other vehicle are used to arrest its motion when desired.

Q. How do motor car brakes differ from those used on wagons?

A. The shoe brake, which consists of a friction block bearing against the wheel tire, is generally used on wagons and in railway practice. Automobile brakes act on special drums attached to some part of the power transmission system and not on the wheel tire.

Q. Why is the shoe and tire brake undesirable on automobiles?

A. If the automobile, which is a self-propelled vehicle, and which can be stopped only by application of the brakes, utilized shoes bearing against the wheel tires the constant application of the braking member would result in rapid depreciation of the tires which deteriorate rapidly enough from normal usage.

Q. What forms of brakes are used on motor car chasses? A. Brakes employed in motor cars are of the band or shoe form and may be applied to the exterior or interior of the brake drums. The brake shown at Fig. 259 is an internal form in which two brake shoes are spread by a cam until they engage the inner surface of a steel brake drum usually attached to the propeller shaft or to the rear wheel hub. The brake outlined at Fig. 260 is a constricting form applied to the outer periphery of the brake drum and which arrests motion of the drum by binding a band tightly around it. Sometimes the cast shoes of the internal brake are replaced by a

band of steel faced with some frictional material such as Raybestos and in some cars the external brake is composed of a pair of cast shoes hinged to clamp the brake drum between them when applied. Q. Where are brakes applied?

A. The braking members of modern automobiles are generally attached to the rear axle and are applied directly against drums carried by the wheel hubs. A typical assembly is shown at Fig 261 with the drum removed in order to outline clearly the construc

Brake Drum,
Brake Lining
Brake Shoe

Hinge

Cam
Brake Shoe

Fig. 259. Internal Expanding or Shoe Brake.

tion of the brake members. The internal brake which is cam expanded is composed of a steel band faced with Raybestos or other woven asbestos fabric, the action of the cam being to spread out the brake against the inner periphery of the brake drums. The external brake band is made of lighter material than the internal member and is constricted around the brake drum by a simple bell crank

lever. Both inner and outer brakes are supported by spring members in order to keep them out of engagement with the brake drum unless applied and to prevent rattle.

Q. Can brakes be fitted to other parts of a car besides the wheels?

A. In some forms of pleasure cars the regular or service brake is applied on a drum at the drive end of the transmission to which the driving shaft is attached as shown at Fig. 262, while in some

Sector for coarse Adjustment

Brake Band

Brake Lining
Brake Drum

Brake Rod

Locking Nut) for fine
Adj. Nut) Adj.

External Brake
Band Lever

Fig. 260.-External Constricting Band Brake.

motor trucks, in addition to the rear wheel location a service brake may be attached to drums that are in direct mechanical connection with the driving sprockets on the countershaft. A brake of this nature is shown at Fig. 263 and is used as a service brake while the emergency braking effect is obtained by a pair of shoes expanded by a cam.

Q. What are the advantages of countershaft and driving shaft brakes?

A. An important advantage gained by applying a brake to a

transmission member revolving at higher speed than the rear wheels is that a smaller brake can be used to retard the vehicle on account of the multiplication of power obtained by the difference in ratio between the driving and driven sprockets or the driving pinion and This is considered by some designers to be a construc

driven gear.

Fig. 261.-Double Brake Assembly Intended for Rear Wheel Hub Drum.

tion subject to criticism inasmuch as it is contended that the driving elements are subjected to considerable stress by using them for both driving and braking purposes that they would not be called upon to take if used only for driving purposes.