||This article relies largely or entirely upon a single source. (July 2008)|
The term drum brake usually means a brake in which shoes press on the inner surface of the drum. When shoes press on the outside of the drum, it is usually called a clasp brake. Where the drum is pinched between two shoes, similar to a conventional disc brake, it is sometimes called a pinch drum brake, though such brakes are relatively rare. A related type called a band brake uses a flexible belt or "band" wrapping around the outside of a drum.
The modern automobile drum brake was invented in 1902 by Louis Renault. He used woven asbestos lining for the drum brakes lining, as no alternative dissipated heat like the asbestos lining, though Maybach has used a less sophisticated drum brake. In the first drum brakes, levers and rods or cables operated the shoes mechanically. From the mid-1930s, oil pressure in a small wheel cylinder and pistons (as in the picture) operated the brakes, though some vehicles continued with purely mechanical systems for decades. Some designs have two wheel cylinders.
The shoes in drum brakes wear thinner, and brakes required regular adjustment until the introduction of self-adjusting drum brakes in the 1950s. In the 1960s and 1970s, disc brakes gradually replaced drum brakes on the front wheels of cars. Now practically all cars use disc brakes on the front wheels, and many use disc brakes on all wheels. However, drum brakes are still often used for handbrakes, as it has proven very difficult to design a disc brake suitable for holding a parked car. Moreover, it is very easy to fit a drum handbrake inside a disc brake so that one unit serves as both service brake and handbrake.
Early brake shoes contained asbestos. When working on brake systems of older cars, care must be taken not to inhale any dust present in the brake assembly. The United States Federal Government began to regulate asbestos production, and brake manufacturers had to switch to non-asbestos linings. Owners initially complained of poor braking with the replacements; however, technology eventually advanced to compensate. A majority of daily-driven older vehicles have been fitted with asbestos-free linings. Many other countries also limit the use of asbestos in brakes.
Drum brake components include the backing plate, brake drum, shoe, wheel cylinder, and various springs and pins.
Backing plate 
The backing plate provides a base for the other components. It attaches to the axle and forms a solid surface for the wheel cylinder, brake shoes, and assorted hardware. Since all braking operations exert pressure on the backing plate, it must be strong and wear-resistant. Levers for emergency or parking brakes, and automatic brake-shoe adjuster were also added in recent years.
Brake drum 
The brake drum is generally made of a special type of cast iron that is heat-conductive and wear-resistant. It is positioned close to the brake shoe without actually touching it, and rotates with the wheel and axle. When a driver applies the brakes, the lining pushes against the inner surface of the drum, generating friction heat that can reach as high as 600 °F (316 °C).
Wheel cylinder 
One wheel cylinder operates the brake on each wheel. Two pistons operate the shoes, one at each end of the wheel cylinder. Hydraulic pressure from the master cylinder acts on the piston cup, pushing the pistons toward the shoes, forcing them against the drum. When the driver releases the brakes, the force of the brake shoe return springs returns the piston to its original position. The parts of the wheel cylinder are as follows:
Brake shoe 
Brake shoes are typically made of two pieces of sheet steel welded together. The friction material is either riveted to the lining table or attached with adhesive. The crescent-shaped piece is called the Web and contains holes and slots in different shapes for return springs, hold-down hardware, parking brake linkage and self-adjusting components. All the application force of the wheel cylinder is applied through the web to the lining table and brake lining. The edge of the lining table generally has three “V"-shaped notches or tabs on each side called nibs. The nibs rest against the support pads of the backing plate to which the shoes are installed. Each brake assembly has two shoes, a primary and secondary. The primary shoe is located toward the front of the vehicle and has the lining positioned differently than the secondary shoe. Quite often the two shoes are interchangeable, so close inspection for any variation is important.
Linings must be resistant against heat and wear and have a high friction coefficient unaffected by fluctuations in temperature and humidity. Materials that make up the brake shoe include, friction modifiers (which can include graphite and cashew nut shells), powdered metal such as lead, zinc, brass, aluminium and other metals that resist heat fade, binders, curing agents and fillers such as rubber chips to reduce brake noise.
Automatic self-adjuster 
The self-adjuster is used to adjust the distance between the brake shoe and the drum automatically as brake shoes wear.
In operation 
Normal braking 
When the brakes are applied, brake fluid is forced under pressure from the master cylinder into the wheel cylinder, which in turn pushes the brake shoes into contact with the machined surface on the inside of the drum. This rubbing action reduces the rotation of the brake drum, which is coupled to the wheel. Hence the speed of the vehicle is reduced. When the pressure is released, return springs pull the shoes back to their rest position.
Automatic self-adjustment 
As the brake linings wear, the shoes must travel a greater distance to reach the drum. When the distance reaches a certain point, a self-adjusting mechanism automatically reacts by adjusting the rest position of the shoes so that they are closer to the drum. Here, the adjusting lever rocks enough to advance the adjuster gear by one tooth. The adjuster has threads on it, like a bolt, so that it unscrews a little bit when it turns, lengthening to fill in the gap. When the brake shoes wear a little more, the adjuster can advance again, so it always keeps the shoes close to the drum.
Emergency brake 
The parking brake (emergency brake) system controls the brakes through a series of steel cables that are connected to either a hand lever or a foot pedal. The idea is that the system is fully mechanical and completely bypasses the hydraulic system so that the vehicle can be brought to a stop even if there is a total brake failure. Here the cable pulls on a lever mounted in the brake and is directly connected to the brake shoes. This has the effect of bypassing the wheel cylinder and controlling the brakes directly.
Self-applying characteristic 
Drum brakes have a natural "self-applying" characteristic, better known as "self-energizing."  The rotation of the drum can drag either one or both of the shoes into the friction surface, causing the brakes to bite harder, which increases the force holding them together. This increases the stopping power without any additional effort being expended by the driver, but it does make it harder for the driver to modulate the brake's sensitivity. It also makes the brake more sensitive to brake fade, as a decrease in brake friction also reduces the amount of brake assist.
Disc brakes exhibit no self-applying effect because the hydraulic pressure acting on the pads is perpendicular to the direction of rotation of the disc. Disc brake systems usually have servo assistance ("Brake Booster") to lessen the driver's pedal effort, but some disc braked cars (notably race cars) and smaller brakes for motorcycles, etc., do not need to use servos.
Note: In most designs, the "self-applying" effect only occurs on one shoe. While this shoe is further forced into the drum surface by a moment due to friction, the opposite effect is happening on the other shoe. The friction force is trying to rotate it away from the drum. The forces are different on each brake shoe resulting in one shoe wearing faster. It is possible to design a two-shoe drum brake where both shoes are self-applying (having separate actuators and pivoted at opposite ends), but these are very uncommon in practice.
Drum brake designs 
Rear drum brakes are typically of a leading/trailing design (for non-servo systems), or primary/secondary (for duo servo systems) the shoes being moved by a single double-acting hydraulic cylinder and hinged at the same point. In this design, one of the brake shoes always experiences the self-applying effect, irrespective of whether the vehicle is moving forwards or backwards. This is particularly useful on the rear brakes, where the parking brake (handbrake or footbrake) must exert enough force to stop the vehicle from travelling backwards and hold it on a slope. Provided the contact area of the brake shoes is large enough, which isn't always the case, the self-applying effect can securely hold a vehicle when the weight is transferred to the rear brakes due to the incline of a slope or the reverse direction of motion. A further advantage of using a single hydraulic cylinder on the rear is that the opposite pivot may be made in the form of a double-lobed cam that is rotated by the action of the parking brake system.
Front drum brakes may be of either design in practice, but the twin leading design is more effective. This design uses two actuating cylinders arranged so that both shoes use the self-applying characteristic when the vehicle is moving forwards. The brake shoes pivot at opposite points to each other. This gives the maximum possible braking when moving forwards, but is not so effective when the vehicle is traveling in reverse.
The optimum arrangement of twin leading front brakes with leading/trailing brakes on the rear allows more braking force at the front of the vehicle when it is moving forwards, with less at the rear. This helps prevent the rear wheels from locking up, but still provides adequate braking at the rear.
The brake drum itself is frequently made of cast iron, though some vehicles have used aluminum drums, particularly for front-wheel applications. Aluminum conducts heat better than cast iron, which improves heat dissipation and reduces fade. Aluminum drums are also lighter than iron drums, which reduces unsprung weight. Because aluminum wears more easily than iron, aluminum drums frequently have an iron or steel liner on the inner surface of the drum, bonded or riveted to the aluminum outer shell.
Roller brakes are specially designed drum brakes for bicycles, mounted to the side of the wheel's hub.
Drum brakes are used in most heavy duty trucks, some medium and light duty trucks, and few cars, dirt bikes, and ATVs. Drum brakes are often applied to the rear wheels since most of the stopping force is generated by the front brakes of the vehicle and therefore the heat generated in the rear is significantly less. Drum brakes allow simple incorporation of a parking brake.
Drum brakes are also occasionally fitted as the parking (and emergency) brake even when the rear wheels use disc brakes as the main brakes. The vast majority of rear disc braking systems use a parking brake in which the piston in the caliper is actuated by a cam or screw. This compresses the pads against the rotor. However, this type of system becomes much more complicated when the rear disc brakes use fixed, multi-piston calipers. In this situation, a small drum is usually fitted within or as part of the brake disc. This type of brake is also known as a banksia brake.
In hybrid vehicle applications, wear on braking systems is greatly reduced by energy recovering motor-generators (see regenerative braking), so some hybrid vehicles such as the GMC Yukon Hybrid and Toyota Prius (except the third generation) use drum brakes.
Disc brakes rely on pliability of caliper seals and slight runout to release pads, leading to drag, fuel mileage loss, and disc scoring. Drum brake return springs give more positive action and, adjusted correctly, often have less drag when released.
Certain heavier duty drum brake systems compensate for load when determining wheel cylinder pressure; a feature rare when discs are employed (Hydropneumatic suspension systems as employed on Citroën vehicles adjust brake pressure depending on load regardless of if drum or discs are used). One such vehicle is the Jeep Comanche. The Comanche can automatically send more pressure to the rear drums depending on the size of the load.
Due to the fact that a drum brakes friction contact area is at the circumference of the brake, a drum brake can provide more braking force than an equal diameter disc brake. The increased friction contact area of drum brake shoes on the drum allows drum brake shoes to last longer than disc brake pads used in a brake system of similar dimensions and braking force. Drum brakes retain heat and are more complex than disc brakes but are often the more economical and powerful brake type to use in rear brake applications due to the low heat generation of rear brakes, a drum brake self-applying nature, large friction surface contact area, and long life wear characteristics (%life used/kW of braking power).
As a driveshaft parking/emergency brake 
Drum brakes have also been built onto the transmission's driveshaft as parking brakes (e.g., Chryslers through 1956). This provides the an advantage that it is completely independent of the service brakes—but suffers a severe disadvantage in that, when used with a bumper jack (common in that era) on the rear, and without proper wheel blocks, the differential's action can allow the vehicle to roll off the jack.
Drum brakes, like most other brakes, convert kinetic energy into heat by friction. This heat should dissipate into the surrounding air, but can just as easily transfer to other braking system components. Brake drums must be large to cope with the massive forces involved, and must be able to absorb and dissipate a lot of heat. Heat transfer to air can be aided by incorporating cooling fins onto the drum. However, excessive heating can occur due to heavy or repeated braking, which can cause the drum to distort, leading to vibration under braking.
- When the drums are heated by hard braking, the diameter of the drum increases slightly due to thermal expansion, so the shoes must move farther and the driver must press the brake pedal farther.
- The properties of the friction material can change if heated, resulting in less friction. This can be a much larger problem with drum brakes than disc brakes, since the shoes are inside the drum and not exposed to cooling ambient air. The loss of friction is usually only temporary and the material regains its efficiency when cooled, but if the surface overheats to the point where it becomes glazed the reduction in braking efficiency is more permanent. Surface glazing can be worn away with further use of the brakes, but that takes time.
- Excessive brake drum heating can cause the brake fluid to vaporize, which reduces the hydraulic pressure applied to the brake shoes. Therefore, the brakes provide less deceleration for a given amount of pressure on the pedal. The effect is worsened by poor maintenance. Brake fluid that is old and has absorbed moisture has a lower boiling point, so brake fade occurs sooner.
Brake fade is not always due to overheating. Water between the friction surfaces and the drum can act as a lubricant and reduce braking efficiency. The water tends to stay until heated sufficiently to vaporize, at which point braking efficiency returns. All friction braking systems have a maximum theoretical rate of energy conversion. Once that rate is reached, applying greater pedal pressure doesn't change it—in fact, the effects mentioned can substantially reduce it. Ultimately, this is what brake fade is, regardless of the mechanisms of its causes. Disc brakes are not immune to any of these processes, but they deal with heat and water more effectively than drums.
Drum brakes can be grabby if the drum surface gets light rust or if the brake is cold and damp, giving the pad material greater friction. Grabbing can be so severe that the tires skid and continue to skid even when the pedal is released. Grab is the opposite of fade: when the pad friction goes up, the self-assisting nature of the brakes causes application force to go up. If the pad friction and self-amplification are high enough, the brake stay engaged due to self-application, even when the external application force is released.
While disc brake rotors can be machined to clean the friction surface (i.e., 'turning'), the same generally cannot be done with brake drums. Machining the friction surface of a brake drum increases the diameter, which would require oversized shoes to maintain proper contact with the drum. However, since oversized shoes are generally unavailable for most applications, worn or damaged drums generally must be replaced.
Another disadvantage of drum brakes is their relative complexity. A person must have a general understanding of how drum brakes work and take simple steps to ensure the brakes are reassembled correctly when doing work on drum brakes. And, as a result of this increased complexity (compared to disc brakes), maintenance of drum brakes is generally more time-consuming. Also, the greater number of parts results in a greater number of failure modes compared to disc brakes. Springs can break from fatigue if not replaced along with worn brake shoes. And the drum and shoes can become damaged from scoring if various components (such as broken springs or self-adjusters) break and become loose inside the drum.
Also, drum brakes do not apply immediately when the wheel cylinders are pressurized, because the force of the return springs must be overcome before the shoes start to move towards the drum. This means that the very common hybrid disc/drum systems only brake with the discs on light pedal pressure unless extra hardware is added. In practice, a metering valve prevents hydraulic pressure from reaching the front calipers until pressure rises enough to overcome the return springs in the drum brakes. If the metering valve were left out, the vehicle would stop only with the front discs until return spring pressure on the rear shoes were overcome.
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 reduced the life of the shoes as well as created 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 re-arcing shoes.
Use in music 
Brake drums are used in modern concert and film music to provide a non-pitched metal sound similar to an anvil. Some are more resonant than others. To produce the clearest sound, percussionists hang the drum with nylon cord or place it on foam. Other methods include mounting the brake drum on a snare drum stand. Either way, the percussionist strikes the brake drum with hammers or sticks of various weight. The brake drum is also commonly in steelpan ensembles, where it is called "the iron."
See also 
- The AA Book of the car, 1976