Car Parts and Accessories

Shoes Press against Car Drums to slow the Car wheels

A car drum brake consists of a cast-iron car drum containing a pair of semicircular car brake shoes. The car drum is attached to a car wheel and revolves with it, so that when the car drum is slowed and halted, the car wheel slows and stops too. Friction to slow the car drum is applied from inside by the shoes, which do not rotate but are mounted on a stationary metal back-plate. Each shoe consists of a curved length of steel or light-alloy casting faced with a hard-wearing lining.

In most car drum car brakes, the shoes are forced against the rotating car drum by a pivoting arrangement; one end of each shoe hinges on a pivot point and the other end can be moved by a cam, or by hydraulic fluid which is forced into the car wheel cylinder from the carbrake’s master cylinder.

In one car hydraulic system, the car wheel cylinder is rigidly fixed to the back-plate and contains two pistons which operate the shoes individually. An alternative plan is to use a single piston in a car cylinder which can slide on the car back-plate. When the car brakes are applied, fluid pressure acts equally on the piston and on the closed end of the car cylinder, pushing them apart. They in turn force the shoes apart, so that the car linings rub against the car drum.

Return springs, stretched between the shoes, ensure that when the car brake pedal is released the shoes are retracted until the linings are clear of the car drum.

When two shoes are hinged on the same pivot point, the car braking system has one leading shoe and one trailing shoe. An alternative arrangement is to hinge the shoes separately, at opposite points on the back-plate. Both then act as leading shoes when the car is running forwards.

A leading shoe tends to be forced into closer contact by the frictional drag of the rotating car drum—a ’self-applying’ action which increases the car braking force on the car wheel. A trailing shoe tends to be pushed away from the car drum, and so does considerably less work than a leading shoe.

Different types of Car Drum Braking

A two leading shoes arrangement gives an augmented response to car pedal effort because of its self-applying action. This car system is usually used on the front car wheels, where extra weight is transferred during car braking and where there is less likelihood of the car wheels locking and skidding.

The two-leading-shoe car system is not suitable for rear car brakes which, applied by hand, may have to hold the car against running backwards on a slope; in reverse, leading shoes become trailing shoes.

A leading-trailing shoe brake is a cheaper and better alternative for the rear car wheels, since it is equally effective whether the car is going forwards or backwards.

In another arrangement, called a duo- servo brake, the leading shoe is hinged to the trailing shoe. When the leading shoe is forced against the car drum by hydraulic pressure, the frictional drag is transferred to the trailing shoe, which is pushed into contact with the car drum.

Car Brake linings

Car brake linings are riveted to the shoes (or bonded to them with an adhesive) after which the car working surface is ground to precise limits. The car linings are likely to be moulded, or may be of the woven type. Both kinds are likely to contain similar materials, like asbestos, although some later vehicles may be fitted with asbestos- free linings.

It is important when replacing linings to use only the type recommended by the car manufacturers. The safest plan is to use car replacement shoes which can be obtained (with linings already fitted) in part exchange for shoes with worn car linings. In handling brake shoes, take care to avoid contaminating the linings with oil.

Self-adjusting Car Drum Car Brakes

Cars fitted with front disc car brakes, which need no adjustment, often have self-adjusting rear car drum car brakes, so that in the car brakes off’ position the shoes always travel through the same distance to contact the car drum. In one system a screw-and‑ratchet car wheel is the adjuster; a pawl, or pivoted lever attached to the handbrake operating lever, engages with the ratchet. As the handbrake is applied, the shoes move outwards and the pawl rides up the back of a ratchet tooth.

If the shoes are sufficiently worn, the pawl will engage the next tooth. When the handbrake is released, the pawl returns, turning the adjuster.

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