A disc brake is a type of brake that uses the calipers to squeeze pairs of pads against a disc (sometimes called a [brake] rotor) to create friction. There are two basic types of brake pad friction mechanisms: abrasive friction and adherent friction. This action slows the rotation of a shaft, such as a vehicle axle, either to reduce its rotational speed or to hold it stationary. The energy of motion is converted into heat, which must be dissipated to the environment. Disc brakes are found in most automobiles and are more expensive to manufacture than a drum brake. Disc brakes are also found on some higher-end bicycles.
Hydraulically actuated disc brakes are the most commonly used mechanical braking system for motor vehicles. The principles of a disc brake apply to almost any rotating shaft. The components include the disc, master cylinder, and caliper, which contain at least one cylinder and two brake pads on both sides of the rotating disc.
Design
The development of disc-type brakes began in England in the 1890s. In 1902, the Lanchester Motor Company designed brakes that looked and operated similarly to a modern disc-brake system, even though the disc was thin and a cable activated the brake pad. Other designs were not practical or widely available in cars for another 60 years. Successful applications began in airplanes before World War II. The first vehicle of any sort to be equipped with disc brakes was the German Tiger tank, which needed a type of disc brake on each of its axles to slow it.
After World War II, technological progress began in 1949, with caliper-type four-wheel disc brakes on the Crosley line and a Chrysler non-caliper type. In the 1950s, there was a demonstration of superiority at the 1953 24 Hours of Le Mans race, which required braking from high speeds several times per lap. The Jaguar racing team won, using disc brake-equipped cars, with much of the credit being given to the brakes' superior performance over rivals equipped with drum brakes. Mass production began with the 1949–1950 inclusion in all Crosley production, with sustained mass production starting in 1955 Citroën DS.
Disc brakes provide better stopping performance than drum brakes because they cool more effectively, reducing heat-related brake fade and recovering faster after immersion. Consequently, discs are less prone to the brake fade caused when brake components overheat. Unlike drum brakes, which often exhibit a self-servo effect from the leading shoes, disc brakes have no self-servo action, so braking force is proportional to applied pressure. This improves driver "feel" and helps prevent wheel lockup. Disc brakes also recover more quickly from immersion (wet brakes are less effective than dry ones). Drums are also prone to "bell mouthing" and trap worn lining material within the assembly, causing various braking problems. Although seldom encountered, the drum should be reconditioned if there is any bell-mouthing.
The disc is usually made of cast iron. In some cases, it may be made of composites, such as reinforced carbon–carbon or ceramic matrix composites. This is connected to the wheel and the axle. To slow down the wheel, friction material in the form of brake pads, mounted on the 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.
Operation
The brake disc is the rotating part of a wheel's disc brake assembly, against which the brake pads are applied. The material is typically gray iron, a form of cast iron. The design of the discs varies. Some are solid, but others are hollowed out, with fins or vanes joining the disc's two contact surfaces (usually included during casting). The weight and power of the vehicle determine the need for ventilated discs. The "ventilated" disc design helps to dissipate the generated heat and is commonly used on the more heavily loaded front discs.
Discs for motorcycles, bicycles, and some cars often have holes or slots cut through them. This is done for better heat dissipation, to aid surface-water dispersal, to reduce noise, to reduce mass, or purely for non-functional aesthetics.
Slotted discs have shallow channels machined into the disc to aid in removing dust and gas. Slotting is preferred in most racing environments to remove gas and water and deglaze brake pads. Some discs are both drilled and slotted. Slotted discs are generally not used on standard vehicles because they quickly wear down brake pads; however, removing material is beneficial for race vehicles, as it keeps the pads soft and prevents their surfaces from vitrifying. On the road, drilled or slotted discs still have a positive effect in wet conditions because the holes or slots prevent a water film from forming between the disc and the pads.
Two-piece discs consist of a central section combined with a separately manufactured outer friction ring. The central section is often called a bell or hat because of its shape. It is commonly manufactured from an alloy such as 7075 alloy and hard anodised for a lasting finish. The outer disc ring is usually made of grey iron. They can also be made of steel or carbon ceramic for particular applications. These materials originated from motorsport use and are available in high-performance vehicles and aftermarket upgrades. Two-piece discs can be supplied as a fixed assembly with nuts, bolts, and washers, or as a more complex floating system in which drive bobbins allow the two parts of the brake disc to expand and contract at different rates, thereby reducing the risk of disc warping from overheating. Advantages of a two-piece disc are a reduction of critical unsprung mass and the dissipation of heat from the disc surface through the alloy bell (hat). Both fixed and floating options have their drawbacks and advantages. Floating discs are prone to rattle, the collection of debris, and are best suited to motorsport, whereas fixed discs are best for road use.
+ Types of brake disc designs Image Name Description Non-ventilated (solid) smooth brake disc The most basic form of brake rotors uses a solid disc, primarily at the rear wheels of a vehicle, as rear brakes are not subjected to braking forces as high as those at the front due to the vehicle's weight transfer to the front during braking (vehicle dip). Ventilated smooth brake disc This brake rotor is primarily used at the front of the vehicle and features ventilation ducts in the disc design, helping the disc cool faster during and after braking and preventing brake fade. Ventilated straight slotted brake disc) A modified form of ventilated smooth brake rotors. The slots collect and discharge brake dust or debris during and after braking. Ventilated curve slotted brake disc The purpose of the curved slots is to increase the length of each, maximizing the collection of brake dust and debris. The curved design facilitates faster discharge of brake dust and debris as the rotor rotates. Ventilated straight drilled brake disc The drilled design prevents a vacuum from forming between the brake pads and rotor when the braking system comes into contact with water. The drilled holes in the disc facilitate water egress from the contact surface between the brake pads and the brake disc. Ventilated curve drilled brake disc A modification of the straight-drilled rotor design. The purpose is to increase the number of holes per thread and ensure that at least one hole is in contact with the brake pad at any position of the brake pad. Ventilated straight drilled & slotted brake disc This design combines a straight slotted and a straight drilled design to provide collection and discharge of brake dust and debris, while preventing vacuum from being generated at the contact surface between the brake pads and the brake rotor disc. Ventilated curved drilled & slotted brake disc A modified form of straight drilled & slotted rotors, to maximize the collection and discharge of brake dust and debris, along with the prevention of vacuum being generated between brake pads and rotor disc. These are primarily for performance and race cars. Ventilated dragon scale slotted brake disc A custom slot design may include variants such as V Slots, U slots, J hooks, etc. The names of each design may vary by manufacturer, and the slot design depends on brake performance requirements. Ventilated floating 2-piece brake disc The brake rotor is assembled into two parts: the rotor bell (carrier) - shown in black - connecting the brake rotor to the wheel hub, and the disc (rotor blade) - which contacts the brake pads and performs the braking function. These types of brake rotors are used in motorsports and racing to accommodate thermal expansion at high braking temperatures, increasing the reliability and performance of the braking system.
History Early experiments Disc brake development 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 at the time meant he used 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 system impractical.
In 1921, the Douglas motorcycle company introduced a form of disc brake on the front wheel of their overhead-valve sports models. Patented by the British Motorcycle & Cycle-Car Research Association, the device was described by Douglas as a "novel wedge brake" operating on a "beveled hub flange". A Bowden cable operated the brake. Front and rear brakes of this type were fitted to the machine on which Tom Sheard rode to victory in the 1923 Senior TT.
Successful application began on railroad streamliner passenger trains, airplanes, and tanks before and during World War II. In the United States, the Budd Company introduced disc brakes on the General Pershing Zephyr for the Burlington Railroad in 1938. By the early 1950s, disc brakes were regularly applied to new passenger rolling stock. In Britain, the Daimler Company used disc brakes on its Daimler Armoured Car of 1939. The disc brakes, made by the Girling company, were necessary because in that four-wheel drive (4×4) vehicle, the epicyclic final drive was in the wheel hubs and therefore left no room for conventional hub-mounted drum brakes.
At Germany's Argus Motoren, Hermann Klaue (1912-2001) patented disc brakes in 1940. Argus supplied wheels fitted with disc brakes, e.g., for the Arado Ar 96. The German Tiger I heavy tank was introduced in 1942 with a Argus-Werke disc on each drive shaft. They served as the main braking system operated by foot pedal controls as well as an emergency steering system.
The American Crosley "Hot Shot" had four-wheel disc brakes in 1949 and 1950. However, these quickly proved troublesome and were removed. Crosley returned to drum brakes and conversions to drums for Hot Shots were popular. Lack of sufficient research caused reliability problems, such as sticking and corrosion, especially in regions using salt on winter roads. Crosley four-wheel disc brakes made the cars, and Crosley-based specials, popular in SCCA H-Production and H-modified racing in the 1950s. The Crosley disc was a Goodyear-Hawley design, a modern caliper "spot" type with a modern disc, derived from a design from aircraft applications.