A disc brake is a wheel brake that slows rotation of the wheel by the friction caused by pushing brake pads against a brake disc with a set of calipers. The brake disc (or rotor in American English) is usually made of cast iron, but may in some cases be made of composites such as reinforced carbon–carbon or ceramic matrix composites. This is connected to the wheel and/or the axle. To stop the wheel, friction material in the form of brake pads, mounted on a device called a brake caliper, is forced mechanically, hydraulically, pneumatically, or electromagnetically against both sides of the disc. Friction causes the disc and attached wheel to slow or stop. Brakes convert motion to heat, and if the brakes get too hot, they become less effective, a phenomenon known as brake fade.
History
Disc-style brakes development and use began in England in the 1890s. The first caliper-type automobile disc brake was patented by Frederick William Lanchester in his Birmingham factory in 1902 and used successfully on Lanchester cars. However, the limited choice of metals in this period meant that he had to use copper as the braking medium acting on the disc. The poor state of the roads at this time, no more than dusty, rough tracks, meant that the copper wore quickly, making the disc brake system non-viable (as recorded in The Lanchester Legacy). It took another half century for his innovation to be widely adopted.
The 1950 Crosley Hot Shot is often given credit for the first U.S. production disc brakes but the Chrysler Crown Imperial actually had them first as standard equipment at the beginning of the 1949 model year.The Crosley disc was a Goodyear development, a caliper type with ventilated rotor, originally designed for aircraft applications.Only the Hot Shot featured it.Lack of sufficient research caused enormous reliability problems, especially in regions requiring the use of salt on winter roads, such as sticking and corrosion.Drum brake conversions for Hot Shots were quite popular.
The Chrysler four-wheel disc brake system was more complex and expensive than Crosley's, but far more efficient and reliable.It was built by Auto Specialties Manufacturing Company (Ausco) of St. Joseph, Michigan, under patents of inventor H.L. Lambert, and was first tested on a 1939 Plymouth.Unlike the caliper disc, the Ausco-Lambert used twin expanding discs that rubbed against the inner surface of a cast-iron brake drum, which doubled as the brake housing.The discs spread apart to create friction against the inner drum surface through the action of standard wheel cylinders.
Chrysler discs were "self energizing," in that some of the braking energy itself contributed to the braking effort.This was accomplished by small balls set into oval holes leading to the brake surface.When the disc made initial contact with the friction surface, the balls would be forced up the holes forcing the discs further apart and augmenting the braking energy.This made for lighter braking pressure than with calipers, avoided brake fade, promoted cooler running, and provided one-third more friction surface than standard Chrysler twelve-inch drums.But because of the expense, the brakes were only standard on the Chrysler Crown Imperial through 1954 and the Town and Country Newport in 1950.They were optional, however, on other Chryslers, priced around $400, at a time when an entire Crosley Hot Shot retailed for $935.Today's owners consider the Ausco-Lambert very reliable and powerful, but admit its grabbiness and sensitivity.
Reliable caliper-type disc brakes were developed in the UK by Dunlop and first appeared in 1953 on the Jaguar C-Type racing car. The 1955 Citroën DS featuring powered inboard front disc brakes was the first French application of this technology, while the 1956 Triumph TR3 was the first English production car to feature modern disc brakes.The first production car to have disc brakes at all 4 wheels was the Austin-Healey 100S in 1954.The first British company to market a production saloon (sedan) fitted with disc brakes to all four wheels was Jensen Motors with the introduction of a Deluxe version of the Jensen 541 with Dunlop disc brakes.The first German production car with disc brakes was the 1961 Mercedes-Benz 220SE coupe featuring British-built Girling units on the front.The next American production automobile equipped with caliper-type disc brakes was the 1963 Studebaker Avanti. Front disc brakes became standard equipment in 1965 on the Rambler Marlin as well as on the Ford Thunderbird,and the Lincoln Continental.A four-wheel disc brake system was also introduced in 1965 on the Chevrolet Corvette Stingray.
Compared to drum brakes, disc brakes offer better stopping performance, because the disc is more readily cooled. As a consequence discs are less prone to the "brake fade" caused when brake components overheat; and disc brakes recover more quickly from immersion (wet brakes are less effective). Most drum brake designs have at least one leading shoe, which gives a servo-effect; see leading/trailing drum brake. By contrast, a disc brake has no self-servo effect and its braking force is always proportional to the pressure placed on the brake pad by the braking system via any brake servo, braking pedal or lever; this tends to give the driver better "feel" to avoid impending lockup. Drums are also prone to "bell mouthing", and trap worn lining material within the assembly, both causes of various braking problems.
Many early implementations for automobiles located the brakes on the inboard side of the driveshaft, near the differential, but most brakes today are located inside the road wheels. (An inboard location reduces the unsprung weight and eliminates a source of heat transfer to the tires.)
Disc brakes were most popular on sports cars when they were first introduced, since these vehicles are more demanding about brake performance. Discs have now become the more common form in most passenger vehicles, although many (particularly light weight vehicles) use drum brakes on the rear wheels to keep costs and weight down as well as to simplify the provisions for a parking brake. As the front brakes perform most of the braking effort, this can be a reasonable compromise.
The first motorcycles to use disc brakes were racing vehicles. The first mass-produced road-going motorcycle to sport a disc-brake was the 1969 Honda CB750. Disc brakes are now common on motorcycles, mopeds and even mountain bikes.
Historically, brake discs were manufactured throughout the world with a strong concentration in Europe and America. Between 1989 and 2005, manufacturing of brake discs migrated predominantly to China.
