Brake Drum

 
A drum brake is a brake in which the friction is caused by a set of shoes or pads that press against the inner surface of a rotating drum. The drum is connected to a rotating wheel.

Servo Design

Drum brakes, depending on the way the shoes are hinged, can have a "self-servo" characteristic. This increases stopping power without any additional effort by the driver because the rotation of the drum drags the shoes around with it, increasing the force holding them together. In rear brakes only one shoe will have this characteristic. Front drum brakes may use two actuating cylinders which allow both shoes to utilize the servo characteristic and which also increase the front axle braking force, required to compensate for forward weight shift and also to avoid premature rear-wheel locking. Servo action can be used to make a very powerful brake (as on the rear axles of large commercial vehicles), but it does reduce the ability of the driver to modulate the brakes sensitively. (The disc brake has no self-servo effect because the pads act perpendicularly to the rotating disc.)

Advantages

Drum brakes are still used in modern cars. There can be engineering and cost advantages. Drum brakes allow simple incorporation of a parking brake. They are often applied to the rear wheels since most of the stopping happens in the front of the vehicle and therefore the heat generated in the rear is significantly less. Drum brakes are also occasionally fitted as the parking (and emergency) brake even when the rear wheels use disk brakes as the main brakes. In this situation, a small drum is usually fitted within or as part of the brake disk.

In hybrid vehicle applications, wear on braking systems is greatly reduced by energy recovering motor-generators. An example of a hybrid car using drum rear brakes is the Toyota Prius.

Disadvantages

Drum brakes with internal shoes have a particular disadvantage; when the drums are heated by hard braking, the diameter of the drum increases due to thermal expansion of the material, and the brakes must be further depressed to obtain effective braking action. Due to the design of drum brakes, the shoe (friction material) is in contact most of the way around the drum, reducing cooling effectiveness compared to disc brakes which have a much lower contact ratio. The higher contact ratio leads to an overheating or glazing of the brake lining adhesive (which in the 1960's and earlier were composed of asbestos and a binding adhesive). This effect is known as brake fade and can lead to driver panic and brake failure in extreme circumstances. Under normal driving conditions it is seldom noticed, especially when drums of appropriate size are fitted. The Pontiac GTO is one vehicle often cited as having undersized drums. Before disc brakes became common metal brake liners were used to combat this problem, but the high cost and poor performance under light braking cause most vehicle manufactures to continue using asbestos linings.

Before 1984, it was common to re-arc brake shoes to match the arc within brake drums. This practice, however, was controversial as it removed friction material from the brakes and caused a reduction in the life of the shoes as well as creating hazardous asbestos dust. Current design theory is to use shoes for the proper diameter drum, and to simply replace the brake drum when necessary, rather than perform the re-arcing procedure.

Adjustment

Early drum brakes (before about 1955) required periodic adjustment to compensate for drum and shoe wear. If not done sufficiently often the symptom would be long brake pedal travel ("low pedal"). Low pedal can be a severe hazard when combined with brake fade as the brakes can become ineffective when the pedal bottoms out.

Self adjusting brakes may use a mechanism that engages only when the vehicle is being stopped from reverse motion. This is a traditional method suitable for use where all wheels use drum brakes (most vehicles now use disc brakes on the front wheels). By operating only in reverse it is less likely that the brakes will be adjusted while hot (when the drums are expanded), which could cause dragging brakes that would accelerate wear and reduce mileage.

Self adjusting brakes may also operate by a ratchet mechanism engaged as the hand brake is applied, a means suitable for use where only rear drum brakes are used. If the travel of the parking brake actuator lever exceeds a certain amount, the ratchet turns an adjuster screw that moves the brake shoes toward the drum.

Percussive Uses

The brake drum can be very effective in modern and film music to provide a non-pitched metal effect. Some drums have more resonance than others and the best method of producing the clearest sound is to hang the drum with nylon cord or to place it on foam. Either way, the brake drum is struck with hammers of various weight