Disc brake
A disc brake is a type of brake that uses the [|calipers] to squeeze pairs of [|pads] against a disc 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 more sophisticated automobiles and are more expensive to manufacture than a drum brake. Disc brakes are also found on a higher end bicycle.
Hydraulically actuated disc brakes are the most commonly used mechanical device for slowing 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 application began in airplanes before World War II. The German Tiger tank was fitted with discs in 1942. After the war, 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 offer better stopping performance than drum brakes because the disc is more readily cooled. Consequently, discs are less prone to the brake fade caused when brake components overheat. Disc brakes also recover more quickly from immersion.
Most drum brake designs have at least one leading shoe, which gives a servo-effect. 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, brake pedal, or lever. This tends to give the driver a better "feel" and helps to avoid impending lockup. Drums are also prone to "bell mouthing" and trap worn lining material within the assembly, causing various braking problems.
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. 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 many cars often have holes or slots cut through the disc. 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 of material is beneficial to race vehicles since it keeps the pads soft and avoids vitrification of their surfaces. On the road, drilled or slotted discs still have a positive effect in wet conditions because the holes or slots prevent a film of water from building up 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 a 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 regular nuts, bolts, and washers or a more complicated floating system where drive bobbins allow the two parts of the brake disc to expand and contract at different rates, therefore reducing the chance of a disc warping from overheating. Key advantages of a two-piece disc are a reduction of critical un-sprung weight and the dissipation of heat from the disc surface through the alloy bell. Both fixed and floating options have their drawbacks and advantages. Floating discs are prone to rattle and collection of debris and are best suited to motorsport, whereas fixed discs are best for road use.
History
Early experiments
The development of disc brakes 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 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, Douglas described the device as a "novel wedge brake" working 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 US, 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 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 had 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 55 cm Argus-Werke disc on each drive shaft.
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 drum brake conversions 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.
Chrysler developed a unique braking system, offered from 1949 until 1953. Instead of the disc with caliper squeezing on it, this system 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. Because of the expense, the brakes were only standard on the Chrysler Crown 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. This four-wheel disc brake system was built by Auto Specialties Manufacturing Company of St. Joseph, Michigan, under patents of inventor H.L. Lambert, and was first tested on a 1939 Plymouth. 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 braking 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. Today's owners consider the Ausco-Lambert very reliable and powerful, but admit its grabbiness and sensitivity.
In 1953, 50 aluminum-bodied Austin-Healey 100S models, built primarily for racing, were the first European cars sold to the public to have disc brakes, fitted to all four wheels.