A disc brake is a type of brake that uses calipers to squeeze pairs of pads against a disc (sometimes called a brake rotor) to create 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. 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.
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. 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.
There are two basic types of brake pad friction mechanisms: abrasive friction and adherent friction. The disc is usually made of cast iron, but 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.
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 in the casting process). Discs for motorcycles, bicycles, and many cars often have holes or slots cut through the disc. Slotted discs have shallow channels machined into the disc to aid in removing dust and gas. Some discs are both drilled and slotted.
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 anodized 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. 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.
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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 (hat). Both fixed and floating options have their drawbacks and advantages.
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. 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.
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. At Germany's Argus Motoren, Hermann Klaue (1912-2001) had patented disc brakes in 1940. Argus supplied wheels fitted with disc brakes.
The American Crosley Hot Shot had four-wheel disc brakes in 1949 and 1950. 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.
The Citroën DS was the first sustained mass production use of modern automotive disc brakes, in 1955. The car featured caliper-type front disc brakes among its many innovations. These discs were mounted inboard near the transmission and were powered by the vehicle's central hydraulic system. 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. However, many (lightweight 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.
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Many early implementations for automobiles located the brakes on the inboard side of the driveshaft, near the differential, while most brakes today are located inside the wheels. Unlike cars, disc brakes that are located within the wheel, bike disc brakes are in the airstream and have optimum cooling. Modern motorcycle discs tend to have a floating design whereby the disc "floats" on bobbins and can move slightly, allowing better disc centering with a fixed caliper. A floating disc also avoids disc warping and reduces heat transfer to the wheel hub. Calipers have evolved from simple single-piston units to two-, four- and even six-piston items.
One problem with motorcycle disc brakes is that when a motorcycle gets into a violent tank-slapper (high-speed oscillation of the front wheel) the brake pads in the calipers are forced away from the discs, so when the rider applies the brake lever, the caliper pistons push the pads towards the discs without actually making contact. Bike disc brakes may range from simple, mechanical (cable) systems, to expensive and powerful, multi-piston hydraulic disc systems, commonly used on downhill racing bikes. Improved technology has seen the creation of vented discs for use on mountain bikes, similar to those on cars, introduced to help avoid heat fade on fast alpine descents. Discs are also used on road bicycles for all-weather cycling with predictable braking. By 2024, almost all road bikes are equipped with disc brakes, just like Mountain bikes.
Disc brakes are increasingly used on very large and heavy road vehicles, where previously large drum brakes were nearly universal. One reason is that the disc's lack of self-assist makes brake force much more predictable, so peak brake force can be raised without more risk of braking-induced steering or jackknifing on articulated vehicles. Another is disc brakes fade less when hot, and in a heavy vehicle air and rolling drag and engine braking are small parts of total braking force, so brakes are used harder than on lighter vehicles, and drum brake fade can occur in a single stop. In Europe, stopping distance regulations essentially require disc brakes for heavy vehicles.
Still-larger discs are used for railroad cars, trams, and some airplanes. Passenger rail cars and light rail vehicles often use disc brakes outboard of the wheels, which helps ensure a free flow of cooling air. Some modern passenger rail cars, such as the Amfleet II cars, use inboard disc brakes. This reduces wear from debris and provides protection from rain and snow, which would make the discs slippery and unreliable. Some airplanes have the brake mounted with very little cooling, and the brake gets hot when stopping. This is acceptable as there is sufficient time for cooling, where the maximum braking energy is very predictable.
For automotive use, disc brake discs are commonly made of grey iron. The SAE maintains a specification for the manufacture of grey iron for various applications. For normal car and light-truck applications, SAE specification J431 G3000 (superseded to G10) dictates the correct range of hardness, chemical composition, tensile strength, and other properties necessary for the intended use.
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Some racing cars and airplanes use brakes with carbon fiber discs and carbon fiber pads to reduce weight. Reinforced carbon discs and pads inspired by aircraft braking systems such as those used on Concorde were introduced in Formula One by Brabham in conjunction with Dunlop in 1976. Carbon-carbon braking is now used in most top-level motorsport worldwide, reducing unsprung weight, giving better frictional performance and improved structural properties at high temperatures, compared to cast iron. Carbon brakes have occasionally been applied to road cars, by the French Venturi sports car manufacturer in the mid-1990s for example, but need to reach a very high operating temperature before becoming truly effective and so are not well suited to road use.
The first development of the modern ceramic brake was made by British engineers for TGV applications in 1988. The objective was to reduce weight, and the number of brakes per axle, as well as provide stable friction from high speeds and all temperatures. Due to the high heat tolerance and mechanical strength of ceramic composite discs, they are often used on exotic vehicles where the cost is not prohibitive. Porsche's Composite Ceramic Brakes (PCCB) are siliconized carbon fiber, with high-temperature capability, a 50% weight reduction over iron discs (hence reducing the vehicle's unsprung weight), a significant reduction in dust generation, substantially extended maintenance intervals, and enhanced durability in corrosive environments.
Discs are usually damaged in one of four ways: scarring, cracking, warping, or excessive rusting. Service shops will sometimes respond to any disc problem by changing out the discs entirely. This is done mainly where the cost of a new disc may be lower than the cost of labor to resurface the old disc. Mechanically this is unnecessary unless the discs have reached the manufacturer's minimum recommended thickness.
Testing Different Braking Systems
How Do Disc Brakes Actually Work?
Large-diameter 12- and 13-inch rotors are the norm for the latest performance cars because they offer more leverage and a larger brake swept area. High performance is as much about stopping as it is about paint-peeling acceleration and high-rate handling. Baer Racing offers a wide variety of bolt-on systems for popular performance cars such as the Fox-platform Mustang, the late-model Chevy pickup, '64-72 Chevelles, the G-body Monte Carlo, and others. Most Baer street kits feature the PBR/Girlock-made, Corvette-style calipers with 12- and 13-inch rotors.
Consistency is paramount. You need to know that every time you stomp on that brake pedal the car will react the same and impart that supreme feeling of confidence. To evaluate braking performance, several Chevelles were fitted with different front brake systems and subjected to 10 consecutive 60-to-zero stops. The stopping distance was measured on each trial and then averaged to see how each system performed.
The testbeds included a drum-equipped '66 Chevelle without power assist, another '66 Chevelle equipped with 11-inch power discs, a '65 El Camino with manual 11-inch discs, and a '65 Chevelle equipped with Baer 13-inch front and 12-inch rear rotors. The first three cars had stock 9½-inch drum rear brakes. Part of the test included different brake pads as well. The No. 2 Chevelle used standard replacement Raybestos pads and shoes while the El Camino employed more aggressive Raybestos Brutestop pads. The Baer-equipped '65 Chevelle was equipped with Performance Friction street performance carbon metallic pads.
Three of the candidates were prior recipients of a disc brake conversion. The No. 2 Chevelle employed a complete 70 Chevelle front disc brake system, including a dual-reservoir master cylinder and a matching combination valve. The El Camino had 70-77 Camaro 11-inch front discs applied with a 1-inch taller spindle and Global West Suspension Components tubular upper control arms. The '65 Chevelle used Baer's complete B-car spindle that came with the rotor mounted and the caliper already in place. The B-car-spindle version also requires either a Global West or Hotchkis tubular upper control arm.
The Baer brake system is composed of an aluminum hub that accommodates a 13-inch front rotor and an '88-'96 Corvette PBR/Girlock caliper. The two-piston, pad-guided caliper limits caliper deflection when the brakes are applied. The rear system uses a 12-inch rotor with a similar PBR/Girlock caliper and a built-in parking brake. The conversion comes with flexible braided steel lines and all the necessary hardware. We equipped the system with a '70s Chevelle master cylinder that doesn't require a power booster.
Each car was subjected to 10 consecutive decels from 60-to-zero mph. With the radar gun, we didn't have to stop and measure the distance after each trial; this, in turn, allowed the luxury of running all tests in quick succession with little or no cooldown time.
The drum-brake Chevelle completed all 10 stops without the total fade anticipated. The first stop was the best: 168 feet. From then on, the distances suffered dramatically, and the last few were well beyond 200 feet. The No. 2 Chevelle, with power-assisted 11-inch front discs and rear drums, equaled the drum brake distances on the first few stomps, but then the rear brakes began locking up. The performance was the worst of the four, exhibiting longer average stopping distances over 10 tries than even the drum brake car. The El Camino was up next, employing 11-inch front discs and rear drums, but equipped with more aggressive Raybestos BruteStop front pads and a Baer adjustable brake proportioning valve. Once tuned, it delivered an excellent 136-foot span and a much shorter average than the first two cars over the 10 stops.
The two factory-style disc brake cars employed an 11-inch diameter rotor and a cast iron, single-piston GM floating caliper. The Baer adjustable proportioning valve allows users to reduce the amount of pressure applied to rear brakes to prevent lock-up under abuse. This leeway improved the stopping distance roughly 20 feet over a standard non-adjustable valve.
After repairing the kink with a section of Classic Tube stainless steel line, the unmerciful bringdowns from 60 and 100 mph were repeated. With the front brakes working in unison, it was possible to haul the 3,550-pound Chevelle down into the 122-foot distance more than once. The 100-to-zero distance of 370 feet is favorable when compared to the supercars that Motor Trend has tested.
The bottom line: For everyday driving, the stock GM-style, floating-caliper, single-piston disc brakes do an admirable job. But if you're looking for consistent performance and you tend to lean heavily on the brakes more than once every hour, the Baer Claws really shine. Hot or cold, the Baer brakes inspired a confidence level and exhibited a solid pedal feel that wasn't evident in the other three systems.
Most cars are equipped with proportioning valves that reduce the pressure to the rear brakes, especially when rear drums are combined with front disc brakes. Under braking, weight transfers from the rear to the front, unloading the rear tires. Combine this with the reduced pressure required by rear drum brakes and it's clear why proportioning valves are necessary.
The graph clearly illustrates how a proportioning valve functions. Initial pressure input to the rear brakes is equal to full master cylinder pressure up to the split point. Above the split point, pressure out of the proportioning valve is reduced by a ratio between 25 and 50 percent of the master cylinder pressure. Adjustable proportioning valves allow you to move the split point (also called the knee point) for pressure reduction. Because of the design of adjustable proportioning valves, changing the split point also simultaneously changes the amount of pressure reduction.
Factory proportioning (or combination) valves are not adjustable, but the aftermarket offers a variety of adjustable valves that allow custom tuning of front-to-rear brake bias. The reason for adjusting the rear brake bias is to prevent rear-brake lock-up. This is a highly unstable condition and can easily cause a loss of control.
Typical disc brake system diagram.
Baer 13-inch front and 12-inch rear brakes installed on a '65 Chevelle.
Table: Braking Performance Comparison
| Vehicle | Brake System | Average Stopping Distance (60-0 mph) |
|---|---|---|
| '66 Chevelle | Drum Brakes | >200 feet |
| '66 Chevelle | 11-inch Power Discs | Worst of the four (rear lock-up) |
| '65 El Camino | 11-inch Manual Discs, BruteStop Pads | 136 feet (tuned) |
| '65 Chevelle | Baer 13-inch Front, 12-inch Rear | 122 feet |
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