- Disc brake system
- Disc brake operation
- Disc brake rotors
- Disc brake pads
- Disc brake calipers
- Proportioning valves
- Proportioning valve operation
Disc brake system
Modern passenger vehicles are usually equipped with disc brakes on at least two wheels.
The primary components of the disc brakes are:
The rotor is the main rotating part of this brake system. It is hard wearing and resists the high temperatures that occur during braking. Its manufacturer will specify the minimum thickness for the rotor. Rotors can be of a solid construction or slotted. The slotted rotor is referred to as a "ventilated disc".
| Rotor | |
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Disc brakes have rotors to dissipate heat so the brakes work efficiently. In high performance vehicles, the rotors are made from composite materials, ceramics, or carbon fiber; otherwise, they're made of iron. 
Some of these rotors have directional vanes, which means the disc can only be fitted to one side of the vehicle.
| Caliper | |
The caliper straddles the rotor, and houses the disc brake pads and activating pistons. The caliper is usually bolted to the steering knuckle or, in the case of a non-steer axle, to a suspension component. | |
| Brake pads | |
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The disc pads are located inside the caliper. The pads clamp onto the rotor to slow or stop the vehicle. The disc pad consists of a friction material bonded to a steel backing plate.
Disc brake operation
Disc brakes can be used on all 4 wheels of a vehicle, or combined with disc brakes on the front wheels and drum brakes on the rear.
When the brake pedal is depressed, a push rod transfers the force through a brake booster to a hydraulic master cylinder. The master cylinder converts the force into hydraulic pressure, which is then transmitted via connecting pipes and hoses to one or more pistons at each brake caliper.
The pistons operate on friction pads to provide a clamping force on a rotating flat disc that is attached to the wheel hub. This clamping tries to stop the rotation of the disc, and the wheel.
On non-driving wheels, the centre of the brake disc or hub contains the wheel bearings. The hub can be part of the brake disc or a separate assembly between the wheel and hub with nuts or bolts.
On driving wheels, the disc is mounted onto the driving axle and may be held in place by the wheel.
On front wheel drive vehicles, it can be mounted on the front hub and wheel bearing assembly.
The brake caliper assembly is bolted to the vehicle axle housing or suspension.
In most cases the brake is positioned as close as possible to the wheel, but there are exceptions. Some high-performance cars use inboard disc brakes on its rear wheels. The makers claim improved vehicle handling for this design because it reduces unsprung weight.
Applying brakes can absorb a lot of vehicle energy so friction between braking surfaces generates great heat. Brake parts withstand very high temperatures.
Most of the friction area of a disc is exposed to air so cooling is far more rapid than for a drum brake. Unlike with drum brakes, brake fade is rare.
Because of their shape, discs tend to throw off water. So after being driven through water, they operate almost immediately.
Disc brakes need much higher pressures to operate than drum brakes, so almost all disc brake systems need a power brake booster to help reduce the pedal forces that are needed from the driver.
Because of the high forces needed to apply a disc brake, using it as a handbrake is less common. Some vehicles build a drum brake into the centre of the rear disc to provide for park brake operation.
Mechanism
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Disc brake open | Inner shoe pushes against disc |
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Pressurized brake fluid travels along the brake line to the caliper. The pressurized fluid pushes the piston and inner brake pad against the disc . Pressure against the disc pushes the caliper away from the piston, pulling the outer brake pad against the disc. As the brake pads clamp together, friction slows the rotation of the disc and wheel. |
Discs
The design of the disc varies somewhat. Some are simply solid steel, but others are hollowed out with fins joining together the disc's two contact surfaces (usually included as part of a casting process). This "ventilated" disc design helps to dissipate the generated heat. Many motorcycle and sports car brakes instead have many small holes drilled through them for the same purpose. Additionally, the holes aid the pads in wiping water from the braking surface. Other designs include "slots" - shallow channels machined into the disc to aid in removing used brake material from the brake pads. Slotted discs are generally not used on road cars because they quickly wear down brake pads. However this removal of material is beneficial to race cars since it keeps the pads soft and avoids vitrification of their surfaces. Some discs are both drilled and slotted.
Disc damage modes
Discs are usually damaged in one of three ways:
- warping,
- scarring,and
- cracking.
In addition, the useful life of the discs may be greatly reduced by excessive machining.
Warping
Warping is caused by excessive heat build up, which softens the metal and can allow it to be disfigured. This can result in wheel shimmy during braking. The likelihood of warping can be reduced if the car is being driven down a long grade by several techniques. Use of a lower gear to obtain engine braking will reduce the brake loading. Also, operating the brakes intermittently - braking to a slower than cruising speed for a brief time then coasting will allow the brakes to cool between applications. The suitability of this is of course, dependent upon traffic conditions. Riding the brakes lightly will generate a great amount of heat with little braking effect and should be avoided. The wheel shimmy during braking is caused by thickness variation of the disc. Tests have shown that high temperature does not permanently warp discs.
Scarring
Scarring can occur if brake pads are not changed promptly, all the friction material will wear away and the caliper will be pressed against the metal backing, reducing braking power and making scratches on the disc. If not excessive, this can be repaired by machining off a layer of the disc's surface. This can only be done a limited number of times as the disc has a minimum safe thickness. For this reason it is prudent to periodically inspect the brake pads for wear (this is done simply on a vehicle lift when the tires are rotated without disassembly of the components). When practical they should be replaced before the pad is completely worn.
Cracking
Cracking is limited mostly to drilled discs, which get small cracks around the drilled holes. These cannot be repaired.
Unnecessary resurfacing machining
Resurfacing machining has three purposes; to remove warps (restore planarity to the discs), to remove scoring, and to remove a glazed surface when new pads are installed. Brake shops will often resurface through a machining operation regardless of the need to do so due to warping or scarring. This can reduce the useful life of the disc in cases where only a light glaze removal (using emery cloth) would suffice. Reducing the life of the discs is of little concern to many brake shops as they can make money on replacing discs worn (or machined) below the manufacturer's minimum specified thickness.
Calipers
The brake caliper is the assembly which houses the brake pads and pistons. The pistons are usually made of aluminum or chrome plated iron. There are two types of calipers: floating or fixed. A fixed caliper does not move relative to the disc. It uses one or more pairs of pistons to clamp from each side of the disc, and is more complex and expensive than a floating caliper. A floating caliper (also called a "sliding caliper") moves with respect to the disc; a piston on one side of the disc pushes the inner brake pad till it makes contact with the braking surface, then pulls the caliper body with the outer brake pad so pressure is applied to both sides of the disc.
Floating caliper (single piston) designs are subject to failure due to sticking. This can occur due to dirt or corrosion if the vehicle is not operated. This can cause the pad attached to the caliper to rub on the disc when the brake is released. This can reduce fuel milage and cause excessive wear on the affected pad.
Pistons & cylinders
The most common caliper design uses a single hydraulically actuated piston within a cylinder, although high performance brakes use as many as 8. Modern cars use different hydraulic circuits to actuate the brakes on each set of wheels as a car safety|safety measure. The hydraulic design also helps multiply braking force.
Failure can occur due to failure of the piston to retract - this is usually a consequence of not operating the vehicle during a time that it is stored outdoors in adverse conditions. For high milage vehicles the piston seals may leak, which must be promptly corrected.
Brake pads
The brake pads are designed for high friction with the disc, while wearing evenly. The brake pads must be replaced regularly, and most are equipped with a method of alerting the driver when this needs to take place. Some have a thin piece of soft metal that causes the brakes to squeal when the pads are too thin, while others have a soft metal tab embedded in the pad material that closes an electric circuit and lights a warning light when the brake pad gets thin. More expensive cars may use an electronic sensor.
Early brake pads (and shoes) contained asbestos. When working on older car's brakes, care must be taken not to inhale any dust present in the caliper (or drum).
Parking brakes
Most vehicles include a mechanical parking brake system (also called an "emergency brake") which operates on the rear wheels. These systems are very effective with drum brakes, since these tend to lock. The adoption of rear-wheel disc brakes caused concern that a disc-based parking brake would not effectively hold a vehicle on an incline. Though some early vehicles (like the Toyota 2000GT) did use the disc for the parking brake, others used a tiny drum brake embedded inside the rear disc.
Today, most cars use the disc for parking, though some still rely on separate drums. The advent of electric parking brakes will change the rear caliper configuration substantially.
Materials advances
The Porsche Carrera GT's carbon-ceramic brake disc
Recently, carbon-ceramic and carbon-carbon composite brakes have been used in racing, sport car, and even high speed
railroad train applications. This should not be confused with ceramic brake pads for use with standard steel discs, which are simply high quality brake pads. Carbon-carbon brake discs are composed of carbon fiber within a carbon matrix, exploiting the excellent thermal conductivity of graphite. Among other things, they have been used in airplane brakes. Moisture can reduce the braking power of carbon-carbon brakes. Another major problem with carbon-carbon lies in its reactivity under high temperature. Additionally, carbon-carbon pads do not perform at their full capacity till they reach 300 °C (572°F). Above 500 °C (932°F) the carbon will react with the air and burn, and even at normal braking temperatures there will be some burning of the outer layers. This is minimized by coating the disc, sometimes with carbon-ceramic.
Carbon-ceramic brake discs are composed of carbon fiber within a silicon carbide matrix (C/SiC). Carbon ceramic brakes are lightweight and have a very high specific heat and thermal conductivity, making them ideal as brake discs able to withstand over 1600 °C (2912°F). They are also very expensive and require special pads, delegating them for use mostly on high end applications such as the Porsche Carrera GT. The lifespan of carbon-ceramic brakes is limited by cracking that occurs because of the different coefficient of thermal expansion between the carbon and the silicon carbide. These cracks slowly allow air to come in contact with the carbon, resulting in burning.
The early Lotus Elise models came with Aluminium metal matrix composite (MMC) brake discs. These brakes were also lightweight and a cost effective alternative to the carbon/ceramic variations available but they cannot operate at the same temperatures. However, the manufacturer for these discs closed down, and Lotus was forced to switch to a iron disc once again. Brakepads are still available for the MMC discs.
Disc brake rotors
The brake disc or rotor is the main rotating component of the disc brake unit.
It’s usually made of cast iron because it’s hard-wearing and can resist high temperatures.
On motorcycles, it is often made of stainless steel.
Most brake discs are stamped with the manufacturer’s minimum thickness specification. When the pad wears, if the thickness of the disc were below this minimum, the piston may go beyond the sealing edge.
Ventilated discs can be used to improve cooling. These slots are designed to use centrifugal force to cause airflow when the disc is rotating.
Some discs are drilled or slotted on their friction surface to improve cooling and assist with removing water.
Disc brake pads
Friction Materials
The lining material is the most important aspect of any braking system. They are variations in the composition and quality levels.
Previously, there were basically two types of friction material:
- organic and
- semi-metallic.
Under present government regulations, manufacturers are using a variety of different friction materials such as:
- organic,
- non-asbestos organic,
- low metallic,
- semi-metallic, and
- ceramic.
Often these materials differ in name only. However, there are very few restrictions on what you can put in friction material so you may find any number of different raw materials in friction material.
Use of Various Lining Compositions
Generally:
- semi-metallic - generic industry term with virtually no specific meaning - pads are used when an vehicle is likely to generate more heat and the higher metal content will be needed the to stop in a reasonable distance while providing acceptable pad life; while
- ceramic/ non-asbestos organic /low metallic material are generally used on lighter weight vehicles where heat buildup is not as great. Consequently, these materials usually sacrifice stopping power and pad life to achieve greater noise control.
Therefore, all linings should contain the following characterists to be an optinum performer:
- Resistance to heat fade - the ability to absober and expel large quantities of heat while still maintaining its effectiveness at these higher temperatures.
- Fast recovery qualities - the lining materials ability to return to its specified pre-fade specification (friction) under cooling conditions.
- Resistance to water fade - the recovery time for lining material due to frictional losses after being immersed in water or cooling medium.
Overview
Disc brake pads consist of friction material bonded onto a steel backing plate. The backing plate has lugs that locate the pad in the correct position in relation to the disc.
Calipers are usually designed so that the condition of the pads can be checked easily once the wheel has been removed, and to allow the pads to be replaced with a minimum of disassembly.
Some pads have a groove cut into the friction surface. The depth of this groove is set so that when it can no longer be seen, the pad should be replaced.
Some pads have a wire in the friction material at the minimum wear thickness. When the pad wears to this minimum thickness, the wire touches the disc as the brakes are applied. A warning light then tells the driver the disc pads are due for replacement.
The composition of the friction material affects brake operation. Materials which provide good braking with low pedal pressures tend to lose efficiency when they get hot. This means the stopping distance will be increased.
Materials which maintain a stable friction co-efficient over a wide temperature range generally require higher pedal pressures to provide efficient braking.
Brake Pads
On the majority of automobiles built today have two disc brake pads on each caliper. Today’s preferred method of construction is a lining bonded to a metal backing plate. A disc brake pad is mounted in the caliper, one on each side of the rotor. Older disc brake linings used to be made primarily of asbestos because of its heat absorbing properties and quiet operation; however, due to health risks, asbestos has been banned by governments, so new materials are now being used they also must absorb and disperse enormous amount of heat generation. Brake pads are a consumable item that wears out with use and must be replaced periodically. There are many types and qualities of pads available. The differences have to do with:
- brake life (how long the new pads will last) and
- noise (how quiet they are when you step on the brake).
It is important that worn disc brake pads be replaced before they cause extensive rotor damage. Some manufacturers have built-in wear sensors that notify you when the pads are worn.
In vehicles without wear sensors, the pads should be inspected visually at manufacturer’s specified intervals for both normal driving conditions and severe operating conditions.
There are three types of wear sensors. They are:
- Audible
This sensor is a soft metal tab attached to the edge of the pad backing plate that emits a high-pitched squeal when it contacts the rotating rotor face.
- Visual
This type consists of a sensor recessed in the back of the brake pad and a warning light on the dashboard. When the rotor wears through and contacts the face of the sensor, the light is activated.
- Tactile
This is actually two sensors; one on the rotor face and one on the lower portion of the brake pad. When the two sensors contact, a pedal pulsation is created to warn the driver.
If a visual check of the pads must be made, there are a few guidelines to keep in mind. For example, disc brake pads that are bonded to the backing plate, the amount of friction material should equal the thickness of the backing plate. If the pads fail meet these specifications, replace them, It is recommended by brake component manufacturer’s that brake pads should be replace on both axles at the same time. In other words, replaced as matched sets.
Harder linings tend to last longer and stop better under heavy use but they may produce an irritating squeal when they are applied. If the lining wears down to the metal brake shoe, then you will have a "Metal-to-Metal" condition where the shoe rubs directly against the rotor causing severe damage and loss of braking efficiency. This noise will usually be heard when your foot is on the brake and disappear when you release the brake pedal. If you hear this noise, have your brakes checked as soon as possible.
Disc brake calipers
The disc brake caliper assembly is bolted to the vehicle axle housing or suspension.
There are 2 main types:
Fixed calipers can have 2, 3, or 4 pistons. 2-piston calipers have one piston on each side of the disc. Each piston has its own disc pad.
When the brakes are applied, hydraulic pressure forces both pistons inwards, causing the pads to come in contact with the rotating disc.
The sliding or floating caliper has 2 pads but only 1 piston. The caliper is mounted on pins or bushes that let it move from side to side.
When the brakes are applied, hydraulic pressure forces the piston inwards. This pushes the pad against the disc. The caliper is free to move on slides, so there is a clamping effect between the inner and outer pads. Equal force is then applied to both pads which clamp against the disc.
In disc brake calipers, the piston moves against a stationary square section sealing ring.
When the brakes are applied, the piston slightly deforms the seal.
When the brakes are released, the seal returns to its original shape. The action of this sealing ring retracts the piston to provide a small running clearance between the disc and pads. It also makes the brake self-adjusting.
Proportioning valves
The proportioning valve divides up the braking effort applied to front and rear wheels under heavy braking, according to how load is distributed across a vehicle.
The effectiveness of braking force is determined by tire-to-road friction. And this increases as load increases.
Applying the brakes causes the front of this vehicle to dip. This causes greater tire-to-road friction on the front tires, and less on the rear. This kind of change of load is called load transfer.
So, if equal braking force is applied to the front and rear wheels, the smaller rear load can make the rear wheels lock, and perhaps skid.
The braking force applied to the wheels needs to be adjusted to allow for changes in load.
Proportioning valve operation
The proportioning valve adjusts braking force to allow for load transfer. It can be pressure-sensitive, or load-sensitive.
The pressure-sensitive valve can be in the master cylinder, or in a separate unit in the rear brake circuit.
The load-sensitive type can be in the body or the axle, where it can respond to load changes, and change the braking effort as needed.
Master cylinder applications usually combine the proportioning valve with a pressure differential switch.
In normal braking, the poppet piston is held in a relaxed position by a large pressure spring. The poppet valve is held against its retainer by a light return spring, and fluid passes freely through the valve to the rear brakes.
In heavy braking, master cylinder pressure can reach a valve’s crack-point. The pressure applied to the 2 different areas of the poppet piston creates unequal forces. That moves the poppet piston against the large pressure spring. This action holds the conical section of the valve against the seat, which limits the pressure increase to the rear brakes.
As greater pedal force increases pressure in the master cylinder, fluid pressure rises on the smaller end of the piston. This combines with the force of the pressure spring to overcome the lower pressure now on the larger end. This forces the piston back, clear of the poppet valve.
The increased pressure now acts on the larger end of the poppet piston and again forces the piston forward to contact the valve.
When the pedal is released, the pressure of the rear brake fluid unseats the poppet valve, letting fluid return to the master cylinder. The pressure spring now returns the poppet piston to its relaxed position.
Should the front brake system fail, the warning lamp spool moves forward, taking the poppet valve with it. Pressure in the rear brakes rises and the piston moves forward but it can’t seal on the valve.
Should the rear brake system fail, the warning lamp spool will move backwards to activate the warning light. The proportioning valve doesn’t operate in this situation.
On a diagonally-divided system, the pressure-sensitive proportioning valve is usually located away from the master cylinder. There is one for each circuit. They each operate in a similar way to the pressure-sensitive proportioning valve located in the master cylinder, but without the pressure-differential warning light circuit.
The load-sensing proportioning valve is usually located in the rear brake circuit, on the chassis. A diagonally-split system may have 2 load-sensing proportioning valves, one for each brake. The unit is mounted on the chassis, around the rear suspension.
