The disc brake is a device for slowing or stopping the rotation of a wheel. A brake disc (or rotor in US English), usually made of cast iron or ceramic, is connected to the wheel or the axle. To stop the wheel, friction material in the form of brake pads (mounted in a device called a brake caliper) is forced mechanically, hydraulically or pneumatically against both sides of the disc. Friction causes the disc and attached wheel to slow or stop.
Experiments with disc-style brakes began in England in the 1890s; the first ever automobile disc brakes were patented by Frederick William Lanchester in his Birmingham factory in 1902, though it took another half century for his innovation to be widely adopted. The first modern disc brakes appeared on the Jaguar C-Type racing car in 1951. In 1950, disc brakes appeared on the first production car, the Crosley Hotshot, followed by the Chrysler Imperial in 1951.
These brakes offered much greater stopping performance than comparable drum brakes, including much greater resistance to "brake fade" (caused by the overheating of brake components), and recovered quickly from immersion (drum brakes were ineffective for some time after a water crossing, an important factor in off-road vehicles). Disc brakes are also more reliable than drum brakes due to the simplicity of their mechanics, the low number of parts compared to the drum brake, and ease of adjustment. Unlike a drum brake, the disc brake has no self-servo effect and the braking force is always proportional to the pedal force being applied by the driver.
Disc brakes were most popular on sports cars when they were first introduced, since these vehicles are more demanding about brake performance. Many early implementations located the brake disc on the inboard side of the driveshaft, near the differential, but most discs today are located inside the wheels. (An inboard location reduces the unsprung weight and eliminates a source of heat transfer to the tires, important in Formula One racing.) Discs have now become standard in most passenger vehicles, though some retain the use of drum brakes on the rear wheels to keep costs and weight down as well as to simplify the provisions for a parking brake or emergency brake. As the front brakes perform most of the braking effort, this can be a reasonable compromise.
The design of the disc varies somewhat. Some are simply solid cast iron, 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 or cast through them. This "cross drilling" was originally done in the 1960's on racing cars. Brake pads of that era would outgas at the elevated temperatures found in racing. This boundary layer of gas between the pad and the rotor hurt braking performance, and so, cross drilling was created to provide the gas someplace to escape. Today's more advanced brake pads (both racing and street) do not suffer from outgassing problems, and so, the holes' purpose is largely cosmetic today. Poorly-made cross drilled rotors (such as those made by simply drilling through a plain faced rotor) may crack at the holes under severe use, such as on the track. 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 brake rotors are manufactured out of a material called grey iron. SAE (Society of Automotive Engineers) maintains a specification for the manufacture of grey iron for various applications. For normal car and light truck applications, the SAE specification is J431 G3000 (superseded to G10). This specification dictates the correct range of hardness, chemical composition, tensile strength, and other properties that are necessary for the intended use.
Historically disc brake rotors were manufactured throughout the world with a strong concentration in Europe, and America. During the period from 1989 to 2005, manufacturing of brake rotors has migrated predominately to China. Today, almost 90% of brake discs and brake drums are now manufactured in China and exported globally.
Leading manufacturers in China include Laizhou Sanli, MAT (Midwest Air Technology), Winhere, Longji, and Haimeng.
Disc damage modes
Discs are usually damaged in one of three ways: warping, scarring, and cracking. Machining the discs to correct these problems also leads to reduced life.
Warping is primarily caused by excessive heat, which softens the metal and allows it to be reshaped. The main causes of overheating are: undersized/overmachined brake discs, excessive braking (racing, descending hills/mountains), "riding" the brakes, or a "stuck" brake pad (pad touches disc at all times).
Another cause of warping is when the disc is overheated and the vehicle is stopped. When keeping the brakes applied, the area where the pads contact the disc will cause uneven cooling and lead to warping.
Several methods can be used to avoid overheating brake discs. Use of a lower gear when descending steep grades to obtain engine braking will reduce the brake loading. Also, operating the brakes intermittently - braking to slower speed for a brief time then coasting will allow the brake material to cool between applications. Riding the brakes lightly will generate a great amount of heat with little braking effect and should be avoided. High temperature conditions as found in automobile racing can be dealt with by proper pad selection, but at the tradeoff of everyday driveability. Pads that can take high heat usually do best when hot and will have reduced braking force when cold. Also, high heat pads typically have more aggressive compounds and will wear discs down more quickly.
Warping can also be caused by improperly torquing the lug nuts when putting on a wheel. When tightening the lugs nuts, it is best to follow the directions included in your manual. A general rule of thumb is to skip every other lug nut in a clockwise or counter-clockwise direction. Also, tightening to final torque in two or three stages helps to balance the stress and avoid warping the disc.
Warping will often lead to a thickness variation of the disc. If it has runout, a thin spot will develop by the repetitive contact of the pad against the high spot as the disc turns. When the thin section of the disc passes under the pads, the pads move together and the brake pedal will drop slightly. When the thicker section of the disc passes between the pads, the pads will move apart and the brake pedal will raise slightly. This change causes pedal pulsation. The thickness variation can be felt by the driver when it is approximately 0.007 inch or greater.
Not all pedal pulsation is due to warped discs, however. One reason is brake pad material operating outside of its designed temperature range and it has left a thicker than normal deposit in one area of the disc surface, creating a "sticky" spot that will grab with every revolution of the disc. Grease or other foreign materials (usually deposited on the disc during wheel maintenance) can likewise create a slippery spot on the disc, also creating the sensation of a grab or warped brake disc.
Scarring (US: Scoring) can occur if brake pads are not changed promptly when worn out. Once enough of the friction material has worn away, the pad's steel backing plate (for glued pads) or the pad retainer rivets (for riveted pads) will bear directly upon the rotor's wear surface, 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 rated 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; on some vehicles the pads can be visually inspected with the vehicle at rest through the wheel cooling slots or openings). When practical, they should be replaced before the pad is completely worn.
Generally a moderately scarred / scored rotor, which operated satisfactorily with existing brake pads, will be equally useable with new pads.
Cracking is limited mostly to drilled discs, which get small cracks around outside edges of the drilled holes near the edge of the disc due to the rotor's uneven rate of expansion in severe duty environments. Manufacturers that use drilled rotors as OEM are doing so for two reasons: looks if they determine that the average owner of the vehicle model will not overly stress them; or as a function of reducing the unsprung weight of the brake assembly, with the engineering assumed that enough brake rotor mass remains to absorb racing temperatures and stresses. A brake disc is a heat sink, so removing mass increases the heat stress it will have to contend with. Generally an OEM application that is not drilled will crack and could fail catastrophically if used over and above the original equipment design. Once cracked, these discs cannot be repaired.
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 until 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 mileage and cause excessive wear on the affected pad.
Pistons and cylinders
The most common caliper design uses a single hydraulically actuated piston within a cylinder, although high performance brakes use as many as 8. (Some pre-1969 Chrysler and General Motors vehicles had 4-piston calipers - usually sought after by restorers.) Modern cars use different hydraulic circuits to actuate the brakes on each set of wheels as a 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 mileage vehicles the piston seals may leak, which must be promptly corrected.
The brake pads are designed for high friction with brake pad material embedded to the disc in the process of bedding while wearing evenly. It is a common assumption that the pad material contacts the metal of the disc to stop the car. The pads work with a very thin layer of its own material and generate a semi-liquid friction boundary that creates the actual braking force. Of course depending on the properties of the material, disc wear could be faster, or slower than with other pads. The properties that determine material wear revolve around trade offs between performance and longevity. 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).
Sometimes a loud noise or high pitch squeal occurs when the brakes are applied. Most brake squeal is produced due to vibration (resonance instability) of the brake components especially the pads and rotors. This type of squeal does not negatively affect brake stopping performance. Some simple techniques like adding chamfers to linings, greasing or gluing the contact between caliper and the pads (finger to backplate, piston to backplate), bonding insulators (damping material) to pad backplate, etc might help reduce squeal. Many times cold weather combined with high early morning humidity (dew) could make the brake squeal worse and vanishes when the lining reaches regular operating temperatures. However, some lining wear indicators are also designed to squeal when the lining is due for replacement. Overall brake squeal can be annoying to the vehicle passengers, passerby, pedestrians, etc especially as vehicles are designed to be more comfortable and quieter. Hence vehicle NVH (Noise, Vibration and Harshness) is one of the important priorities for today's vehicle manufacturers.
See also: Brake lining.