Since the EPA's smaller minicompact category is not as commonly used by the general public, A-segment cars are sometimes called subcompacts in the United States. The first car with Teetor's "speedostat" system was the 1958 Chrysler Imperial (called "auto-pilot"), using a speed control dial on the dashboard. 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. 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. In 1920 the French automobile and aircraft pioneer Gabriel Voisin experimented with systems that modulated the hydraulic braking pressure on his aircraft brakes to reduce the risk of tire slippage, as threshold braking on aircraft is nearly impossible. The vacuum is only an assist and the brakes can still function, requiring greater force, if the booster vacuum is used up.
They can also be from steel or carbon ceramic for particular applications. Torsion bars use the twisting properties of a steel bar to provide coil-spring-like performance. Stainless steel is preferred due to its anti-rust properties. In 1953, 50 aluminum-bodied Austin-Healey 100S (Sebring) models, built primarily for racing, were the first European cars sold to the public to have disc brakes, fitted to all four wheels. 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. Disc brakes offer better stopping performance than drum brakes because the disc is more readily cooled. This is done for better heat dissipation, to aid surface-water dispersal, to reduce noise, to reduce mass, or for marketing cosmetics.
The "ventilated" disc design helps to dissipate the generated heat and is commonly used on the more heavily loaded front discs. Disc brakes were most popular on sports cars when they were first introduced since these vehicles are more demanding about brake performance. 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. This model went on to sell 1.5 million units over 20 years with the same brake setup. In 1969, Honda introduced the more affordable CB750, which had a single hydraulically actuated front disc brake (and a rear drum brake), and which sold in huge numbers. The discs spread apart to create friction against the inner drum surface through the action of standard wheel cylinders. 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. 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.
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. The result is that the thicker-hotter sections receive even more pad material than the thinner-cooler sections, contributing to a further increase in the variation in the disc's thickness. Drums are also prone to "bell mouthing" and trap worn lining material within the assembly, causing various braking problems. The rider then brakes harder, forcing the pads onto the disc much more aggressively than standard braking. In 1963, the Studebaker Avanti was factory-equipped with front disc brakes as standard equipment. This Bendix system licensed from Dunlop was also optional on some of the other Studebaker models. In 1921, the Douglas motorcycle company introduced a form of disc brake on the front wheel of their overhead-valve sports models. Modern sport bikes typically have twin large front discs, with a much smaller single rear disc. In Germany, the StVZO (Road Traffic Licensing Regulations) calls for a different function provided by these lights: with the vehicle's ignition switched off, the operator may activate a low-intensity white light at the front and red light at the rear on either the left or right side.
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