Introduction:
For many years drum brakes were used on all four wheels on virtually every vehicle on the road. Today disc brakes have largely replaced drum brakes on the front wheels of most vehicles, and some models now have disc brakes on all four wheels. Despite this shift, drum brakes remain in widespread use for several practical reasons. They can easily handle the 20% to 40% of total braking load that is typically placed on the rear wheels. In addition, drum brakes can be built with a simple parking brake mechanism which makes them a cost effective and space saving choice for rear applications. This article examines both drum and disc brake systems in detail covering their operation, components, design variations and parking brake functions.Drum and disc brakes each have distinct strengths. Drum brakes are cost effective for rear applications, handle moderate loads, and easily integrate parking brakes. Their self energising action multiplies braking force. Disc brakes excel in heat dissipation, water resistance, and consistent performance, making them the preferred choice for front wheels and increasingly for all four corners. Modern vehicles may use either or both, depending on performance, weight, and cost considerations.
Drum Brake Operation:
The operation of a drum brake is straightforward. The key factor in its effectiveness is the brake shoe pressure directed against the drum. When the brakes are applied and the vehicle moves forward or backward, the force of the shoe pressing against the drum multiplies itself a phenomenon called self energizing. This occurs because the anchor pin acts as a stop, preventing the shoe from following the rotation of the drum. The resulting wedging action, combined with the applied force, creates a self multiplied braking force significantly increasing stopping power without requiring extra hydraulic pressure.Drum Brake Components:
Backing Plate and Foundation:
The backing plate is the foundation for the brake shoes and associated hardware. It is bolted to the axle flange or spindle. The wheel cylinder, which is mounted on the backing plate uses hydraulic pressure to force the shoes against the drum. Two linked brake shoes are attached to the plate, they support the lining and distribute pressure across the lining surface during braking. Shoe return springs and holddown parts maintain correct shoe position and clearance. Some drum brakes have self adjusting mechanisms while others rely on manual adjustment.Brake Shoes and Linings:
Brake shoes are the backbone of the system. The shoe rim is welded to the web to provide a stable surface for the lining. Web thickness, cutout shapes and reinforcements can vary between shoes of the same size to provide the needed stiffness or flexibility for a specific application. Many shoes have nibs or indented spots along the rim that rest against support ledges on the backing plate preventing the shoe from hanging up.In a servo system, each drum contains a set of shoes: the primary (leading) shoe is toward the front of the vehicle, and the secondary (trailing) shoe is toward the rear. Friction between the primary shoe and the drum forces it to shift slightly limited by the anchor pin, which then pushes against the secondary shoe increasing braking action. In forward braking, the secondary lining develops greater friction and is therefore thicker and has more surface area. These roles reverse when braking in reverse.
The lining itself contains heat resistant fibers and is molded with a high temperature synthetic bonding agent. Linings are attached to the shoe either by riveting or bonding. In the unapplied position, return springs hold the shoes against the anchor pin, holddown springs or clips hold them to the backing plate. Opposite the anchor pin, a star wheel adjuster links the shoe webs and provides threaded adjustment to expand or contract the shoes, which is held in place by a spring.
Wheel Cylinders:
Wheel cylinders convert hydraulic pressure from the master cylinder into mechanical force. The cylinder bore is filled with fluid. When the brake pedal is depressed, additional fluid forces the cups and pistons outward, pushing the shoes against the drum. Piston stops prevent fluid leakage or air entry when the pistons reach the end of their bores.Shoe Return Springs, Holddowns, and Anchors:
Return springs can be hooked into a link or guide, or strung between the shoes. They are normally installed in a specific order. Although similar in appearance, springs are usually not interchangeable; they are sometimes colour coded to help distinguish them. Various holddown designs exist to unlock or lock a straight pin holddown, depress the locking cup and coil spring (or clip) and rotate the pin 90°. On GM lever adjusters, the inner (bottom) cup has a sleeve that aligns the adjuster lever.Shoe anchors come in several types: fixed non‑adjustable, self‑centring sliding, and adjustable fixed types with eccentric or slotted adjustment. On some front brakes, fixed anchors are threaded into or bolted through the steering knuckle and also support the wheel cylinder. For adjustable anchors, when recentring is needed, loosen the locknut enough to let the anchor slip but not tilt.
Brake Drums:
Modern brake drums are made of heavy cast iron, some are aluminium with an iron or steel liner and have a machined internal surface against which the linings rub. This friction generates substantial heat. The inability of drums to dissipate heat as effectively as disc brakes is a major reason why discs have replaced drums at the front of all late model cars and light trucks, and at the rear of some sports and luxury cars. In some front wheel drive cars, the rear drums are integral with the hub and cannot be removed without disassembling the wheel bearing; on other FWD and most RWD cars, the drums are held by wheel lugs and can be removed without disturbing the bearings.Drum Brake Designs:
There are two common drum brake designs: duo servo (self‑energising) and nonservo (leading‑trailing). Most large American cars use the duo servo design, while the nonservo type has become popular on smaller, lighter cars because it helps reduce rear brake lockup without sacrificing braking ability.Duo Servo Drum Brakes:
The name duo servo comes from the fact that the self energising force transfers from one shoe to the other in either direction of wheel rotation. Both shoes are actuated by a double piston wheel cylinder. The upper end of each shoe is held against a single anchor by a heavy return spring while an adjusting screw assembly and spring connect the lower ends. When the brakes are applied, hydraulic pressure forces both pistons outward. As the shoes contact the rotating drum, they tend to move with it until one shoe contacts the anchor and the other is stopped by the star wheel adjuster link. During forward rotation, frictional forces on the primary shoe create a force through the adjuster link that applies the secondary shoe, this force is many times greater than the wheel cylinder input. That force is again multiplied by friction on the secondary lining, and the resultant force is taken on the anchor pin. In normal forward braking, the secondary lining produces more friction, hence it is thicker and has more surface area. These roles reverse when backing up.Automatically Adjusted Servo Brakes:
Since the early 1960s, automatic adjusters have been used on all American and most import vehicles. Several variations exist.Basic Cable System:
The cable eye is attached to the anchor. As the shoe moves, the cable pulls over a guide on the shoe web and operates a pawl that engages a star‑wheel tooth. The pawl acts as a ratchet lock. When lining wear permits sufficient shoe movement, the pawl engages the next tooth; on release, the adjuster spring returns the pawl, advancing the star wheel one notch. On most vehicles, this system is on the secondary shoe and operates when braking while backing up on a few, it is on the primary shoe and operates during forward braking. Left hand and right hand threaded star wheels are used on opposite sides if the wrong thread is installed, the system either fails to adjust or unadjusts with every application. Another version uses a cable and pawl, with left hand threads on the left brake and right hand on the right, the adjuster works in either direction.Cable with Overtravel Spring:
This has an upstroke pawl advance. The left brake has left hand threads and the right has right hand threads. The pawl is mounted on a web pin with an additional mousetrap spring. The cable hooks to the pawl via an overtravel spring, which dampens movements and prevents unnecessary adjustment during hard braking that causes drum deflection.Lever with Override:
This downstroke pawl system has right‑hand threads on the left brake and left hand on the right. The lever pivots on a cup sleeve and has a separate return spring. A pivot lever and override spring dampen movement to avoid over adjustment from drum deflection.Lever and Pawl:
Also downstroke, with right hand on left and left hand on right. The lever engages the pawl with a separate return spring between them.Nonservo Drum Brakes:
Often used on small cars, the nonservo brake has both shoes held against a fixed anchor at the bottom by a retaining spring, there is no servo action. On forward braking, the leading shoe develops friction forces from wheel cylinder pressure, working against the anchor pin. The trailing shoe is also actuated but can only support a force equal to the piston force, it carries no friction load at the anchor. Thus, the leading shoe does most of the braking. In reverse, the roles swap.Automatically Adjusted Nonservo Brakes:
Some small cars use similar cable adjusters, but others have unique mechanisms like expanding struts or ratchets.Automatic Cam Adjusters:
Used with front disc brakes, this rear drum has one leading and one trailing shoe. Shoes rest against the wheel cylinder pistons at the top and are held to the anchor plate by a pull back spring. Cam adjusters attach to each shoe via a pin through a slot in the web. As shoes move outward, the pin rotates the cam adjusting the shoes. The pin diameter is smaller than the slot width, so shoes always return to proper clearance.
Ratchet Automatic Adjuster:
This leading trailing design has a spacer strut and toothed ratchets attached to the secondary shoe. The parking brake lever pivots on the strut. As linings wear, the strut’s effective length increases: wear causes greater shoe movement, and the strut moves with the primary shoe to close the gap. Further movement rotates the large ratchet against a spring‑loaded small ratchet and the serrations hold the new setting until more wear occurs. Upon release, return springs bring the shoes to the strut shoulders, restoring clearance proportional to the gap.Drum Parking Brakes:
The parking brake keeps the vehicle from rolling while parked. It is entirely mechanical not hydraulic using a lever assembly and cable system to actuate the rear drum service brakes. Parking brakes can be hand operated or foot operated. Downsized cars and light trucks typically use hand operated self adjusting levers; full size vehicles usually have a foot‑operated pedal that latches and is released by a handle or button. Some vehicles have a vacuum power unit linked to the release lever, it releases the brake when the engine is running and the transmission is in a forward gear with a manual override in case of vacuum failure. Hoses connect the unit to a release valve on the steering column.The system starts with the pedal or hand lever, which is a variable ratio mechanism that converts input effort and travel into output force with less travel. The front cable pulls the equaliser lever which multiplies force and pulls the rear cables through an equaliser that balances pull between both sides. When the pedal is applied, the cables and equaliser exert balanced pull on the parking brake levers of both rear brakes. The levers and struts move the shoes outward against the drums, and they stay until the pedal is released. Each rear cable enters through a conduit, the cable end engages the lower end of the parking brake lever, which is hinged to the secondary shoe web and linked to the primary shoe by a strut. The lever and strut expand both shoes away from the anchor and wheel cylinder, bringing them into drum contact. Return springs reposition the shoes when cable tension is released. To replace the brake shoes, one may need to back off the equaliser adjusting nuts to relieve cable tension. An electronic switch triggers the brake indicator light when the pedal is applied and the ignition is on, the light goes out when the pedal is released or ignition off.
Disc Brake Systems:
Disc brakes work like bicycle brakes: friction pads are squeezed against a rotating disc (rotor). The rotor is typically cast iron and has both sides machined smooth. Pads are attached to metal backings and are actuated by pistons inside a caliper a housing that wraps around the rotor and is mounted to the steering knuckle. Disc brakes have four major advantages over drums, better resistance to heat fade means more exposed surface, better cooling, resistance to water fade means rotation throws off moisture and pads wipe the surface, more straight line stopping (less pulling), and automatic adjustment as pads wear.Disc Brake Components:
Disc brakes come in fixed caliper or floating caliper designs. All consist of a hub/rotor assembly, a caliper, and brake pads.Rotors:
The rotor has two main parts: the hub where the wheel mounts and bearings reside and the braking surface machined on both sides. Rotor diameter determines size; larger cars with greater braking needs have larger rotors. A splash shield bolted to the steering knuckle protects the rotor from water and dirt, the wheel shields the outboard side, and both direct airflow for cooling.Fixed vs. Floating Rotors:
A fixed rotor has the hub and rotor cast as one unit. A floating rotor has separate hub and rotor; the wheel studs are in the hub and pass through the rotor. This design is cheaper and easier to replace.Composite Rotors:
To reduce weight, composite rotors use cast iron for friction surfaces and the hub but steel stampings for supporting parts, bonded together under heat and pressure. They can be fixed or floating and have the same wear standards as other rotors.Ceramic Rotors:
First offered by Porsche on the 911 GT2 in the late 1990s and later on the 911 Turbo, ceramic brakes are now options on Ferraris, Lamborghinis, Porsches and Bentleys. They are costly but weigh about half of a conventional rotor, reducing unsprung weight to improve ride, handling and fuel economy. They last four times longer than steel discs, and pads last about three times longer. The pads contain ceramic powder mixed with metal particles and have heat shields. Ceramic brakes resist fade offer excellent stopping power, and keep wheels cleaner (no black dust). The disc is a two‑piece unit: a ceramic ring (silicon carbide with carbon fibres, resembling stone) bolted to a steel hub.Solid vs. Ventilated Rotors:
A solid rotor is a simple metal disc with friction surfaces on both sides - light, simple and cheap, used on smaller cars or rear brakes. A ventilated rotor has cooling fins between the surfaces to increase cooling area and airflow. Although heavier they dissipate heat much better. Some ventilated rotors have curved fins that increase centrifugal airflow, these are unidirectional and cannot be swapped side to side.Drilled vs. Slotted Rotors:
High performance cars may have cross‑drilled rotors to allow heat, gases, and dirt to escape and give pad edges a grip. However, drilling reduces surface area and thermal capacity, shortening service life. The newer trend is tangential slots or channels, which offer the same benefits without the drawbacks.
Rotor Hubs and Wheel Bearings:
Tapered roller bearings (inner cone and outer cup) are common on front RWD and rear FWD wheels. The cone contains tapered rollers held by a cage; the cup is pressed into the hub. A thrust washer, nut, locknut, and cotter pin secure the bearings, with a dust cap and an inboard seal.Caliper Assembly:
The caliper converts hydraulic pressure to mechanical force. Its housing (cast iron or aluminium) has cylinder bores with a groove for a square cut seal and another groove for the dust boot. A fluid inlet and a bleeder valve are present. Calipers may have one, two, or four pistons each large diameter and short stroke to provide high pressure with minimal fluid displacement. Hydraulics are the same as for drum brakes: master cylinder pressure forces fluid into the caliper. Pistons are steel (nickel‑chrome plated), aluminium, or phenolic resin. The dust boot fits in grooves at the top of the bore and on the piston. The hydraulic seal prevents fluid leakage and acts as a return spring: when pressure releases, the seal retracts the piston slightly. As pads wear, the piston moves further out to maintain adjustment without leaking.Fixed Caliper Disc Brakes:
The caliper is bolted rigidly. Pistons on both sides move inward to clamp the pads against the rotor.
Floating Caliper Disc Brakes:
This one piece casting has a single piston on the inboard side. The caliper slides on locating pins with Teflon sleeves. When pressure builds, the piston pushes the inboard pad against the rotor then the caliper slides to bring the outboard pad into contact with equal force.Sliding Caliper Disc Brakes:
Similar to floating, but the caliper slides on machined surfaces of the anchor plate, not on pins. Some use a support key between caliper and anchor plate; a worn key can cause tapered pad wear, so it must be inspected and lubricated during pad replacement.
Brake Pad Assembly:
Pads are metal plates with linings riveted or bonded, linings are semimetallic or non asbestos.
Disc Pad Wear Sensors:
Three common sensors: audible, visual, and tactile. Some vehicles have electronic wear indicators: when the pad wears to a predetermined point, a warning light illuminates. In some systems, a small pellet in the friction material completes a circuit when it contacts the rotor.
Rear Wheel Disc Brakes:
Rear disc calipers are the same as front ones, except they must incorporate a parking brake. There are two main approaches.Rear Disc/Drum Parking Brake:
Found on some fixed or sliding caliper setups, this uses the inside of the rear rotor hub as a parking brake drum. A pair of small brake shoes on a backing plate operates independently of the service brakes, actuated by cables and linkage from the pedal. This drum in hat system is a smaller drum brake without self adjusters, it must be manually adjusted via star wheels accessible through the backing plate or drum outer surface. Late model GM trucks use an expandable friction‑covered metal band inside the drum.
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