History
Experiments with brakes began in England in the 1890s; the first ever automobile disc brakes were patented by Frederick William Lanchester in his Birmingham factory in 1902, though it took another half century for his innovation to be widely adopted. The first designs resembling modern disc brakes began to appear in Britain in the late 1940s and early 1950s. They offered much greater stopping performance than comparable drum brakes, including much greater resistance to "brake fade" (caused by the overheating of brake components), and were unaffected by immersion (drum brakes were ineffective for some time after a water crossing, an important factor in off-road vehicles). Disc brakes are also more reliable than drum brakes due to the simplicity of their mechanics, the low number of parts compared to the drum brake, and ease of adjustment.
Drum Brake
Drum brakes have a drum attached to the wheel hub, and braking occurs by means of brake shoes, expanding against the inside of the drum.
A drum brake is a brake in which the friction is caused by a set of shoes or pads that press against the inner surface of a rotating drum. The drum is connected to a rotating wheel.
The modern automobile drum brake was invented in 1902 by Louis Renault, though a less-sophisticated drum brake had been used by Maybach a year earlier. In the first drum brakes, the shoes were mechanically operated with levers and rods or cables. From the mid-1930s the shoes were operated with oil pressure in a small wheel cylinder and pistons (as in the picture), though some vehicles continued with purely mechanical systems for decades. Some designs have two wheel cylinders.
The shoes in drum brakes are subject to wear and the brakes needed to be adjusted regularly until the 1950's introduction of self adjusting drum brakes. Self adjusting brakes operate by a ratchet mechanism engaged as the hand brakes is applied. If the travel of the handbrake actuator lever exceeds a certain amount, the rachet turns an adjuster screw that moves the brake shoes toward the drum. In the 1960s and 1970s brake drums on the front wheels of cars were gradually replaced with disc brakes and now many cars uses disc brakes on all wheels.
Another type of drum brake is where a friction belt is wrapped around the outside of the drum and tightened. This type predated the modern drum brake, and was later often used for the parking brake on the central drive shaft. This type of band brake is also used in automatic transmissions and aerobic exercise cycling equipment.
Drum brakes with internal shoes have a particular disadvantage; when the drums are heated by hard braking the diameter of the drum increases due to the expansion of the material and the brakes must be further depressed to obtain effective braking action. This increase of pedal motion is known as brake fade and can lead to brake failure in extreme circumstances. For this reason drum brakes have been superseded in most modern automobiles and light trucks with at least front wheel (often now four wheel) disc brakes.
Drum brakes are still used in some modern cars owing to weight and cost advantages. An advanced technology hybrid car using drum rear brakes is the Toyota Prius. (Hybrid vehicles greatly reduce everyday wear on braking systems owing to their energy recovery motor-generators.)
Early type brake shoes contained asbestos. When working on brake systems of older cars, care must be taken not to inhale any dust present in the brake assembly.
Disc Brake
With disc brakes, a disc attached to the wheel hub maybe clamped between 2 brake pads.
On light vehicles, both of these systems are hydraulically operated. The brake pedal operates a master cylinder. Hydraulic lines and hoses connect the master cylinder to brake cylinders at the wheels.
Most modern light vehicles have either disc brakes on the front wheels and drum brakes on the rear, or, disc brakes on all 4 wheels.
Disc brakes require greater forces to operate them. A brake booster assists the driver by increasing the force applied to the master cylinder, when the brake is operated.
The disc brake is a device for slowing or stopping the rotation of a wheel. A braking disc (or rotor in US English), usually of steel, is connected to the wheel or the axle. To stop the wheel, the braking pads (mounted in a device called a brake caliper) are squeezed mechanically or hydraulically against the disc on both sides. Friction causes the disc and attached wheel to slow or stop.
Automobile cars, motorcycles, and some bicycles use disc brakes.
Disc brakes were most popular on sports cars when they were first introduced, since these vehicles are more demanding about brake performance. Many early implementations located the brake disc inboard, near the differential, but most discs today are located inside the wheels. (An inboard location reduces the unsprung weight and eliminates a source of heat transfer to the tires, important in Formula One racing.) Discs have now become standard in most passenger vehicles, though some retain the use of drum brakes on the rear wheels to keep costs and weight down as well as to simplify the provisions for a parking brake or emergency brake. As the front brakes perform most of the braking effort, this can be a reasonable compromise.
Antilock Braking System (ABS)
An anti-lock braking system (commonly known as ABS, from the German name "Antiblockiersystem" given to it by its inventors at Bosch) is a system on motor vehicles which prevents the wheels from locking while braking. The purpose of this is twofold:
History
The German firm of Bosch had been developing anti-lock braking technology since the 1930s, but the first production cars using Bosch's electronic system became available in 1978. They first appeared in trucks and German limousines from Mercedes-Benz. Systems were later introduced on motorcycles.
Operation
The anti-lock brake controller is also known as the CAB (Controller Anti-lock Brake).
A typical ABS is composed of:
The electronic unit constantly monitors the rotation speed of each wheel. When it senses that one or more wheel is rotating slower than the others (a condition that will bring it to lock) it moves the valves to decrease the pressure on the braking circuit, effectively reducing the braking force on that wheel.
Effectiveness
On high-traction surfaces such as bitumen, whether wet or dry, most ABS-equipped cars are able to attain braking distances better (i.e. shorter) than those that would be possible without the benefit of ABS. A moderately-skilled driver without ABS might be able, through the use of cadence-braking, to match the performance of a novice driver with an ABS-equipped vehicle. However, for a significant number of drivers, ABS will improve their braking distances in a wide variety of conditions. The recommended technique for non-expert drivers in an ABS-equipped car, in a typical full-braking emergency, is to press the brake pedal as firmly as possible and, where appropriate, to steer around obstructions. In such situations, ABS will significantly reduce the chances of a skid and subsequent loss of control—particularly with heavy vehicles.
In gravel and snow, ABS tends to increase braking distances. On these surfaces, locked wheels dig in and stop the vehicle more quickly. ABS prevents this from occurring. Some ABS controllers reduce this problem by slowing the cycling time, thus letting the wheels repeatedly briefly lock and unlock. The primary benefit of ABS on such surfaces is to increase the ability of the driver to maintain control of the car rather than go into a skid—though loss of control remains more likely on soft surfaces like gravel or slippery surfaces like snow or ice.
When activated, the ABS causes the brake pedal to pulse noticeably. As most drivers rarely or never brake hard enough to cause brake lockup, and a significant number rarely bother to read the car's manual, this may not be discovered until an emergency. When drivers do encounter an emergency that causes them to brake hard and thus encounter this pulsing for the first time, many are believed to reduce pedal pressure and thus lengthen braking distances, contributing to a higher level of accidents than the superior emergency stopping capabilities of ABS would otherwise promise. Some manufacturers have therefore implemented "brake assist" systems that determine the driver is attempting a crash stop and maintain braking force in this situation. Nevertheless, ABS significantly improves safety and control for drivers in on-road situations if they know not to release the brakes when they feel the pulsing of ABS.
It is worth noting that the heavier a vehicle is, the more it will benefit from ABS. This is particularly true of vehicles with less-sophisticated hydraulic braking systems where fine control is not as easy as with the more developed braking systems. Conversely, lighter vehicles, especially sports cars with highly-developed braking systems without ABS can outbrake comparable vehicles even with ABS.
Traction control
The ABS equipment may also be used to implement traction control on acceleration of the vehicle. If, when accelerating, the tire loses traction with the ground, the ABS controller can detect the situation and apply the brakes to reduce the acceleration so that traction is regained. Manufacturers often offer this as a separately priced option even though the infrastructure is largely shared with ABS. More sophisticated versions of this can also control throttle levels and brakes simultaneously, leading to what Continental Teves terms Electronic Stability Control or what Bosch terms the "Electronic Stability Program" (ESP).
Summary
The antilock braking system prevents wheel-lock or skidding, no matter how hard brakes are applied, or how slippery the road surface. Steering stays under control and stopping distances are generally reduced.
It consists of:
Mechanism
Discs
The design of the disc varies somewhat. Some are simply solid steel, but others are hollowed out with fins joining together the disc's two contact surfaces (usually included as part of a casting process). This "ventilated" disc design helps to dissipate the generated heat. Many motorcycle and sports car brakes instead have many small holes drilled through them for the same purpose. Additionally, the holes aid the pads in wiping water from the braking surface. Other designs include "slots" - shallow channels machined into the disc to aid in removing used brake material from the brake pads. Slotted discs are generally not used on road cars because they quickly wear down brake pads. However this removal of material is beneficial to race cars since it keeps the pads soft and avoids vitrification of their surfaces. Some discs are both drilled and slotted.
Disc damage modes
Discs are usually damaged in one of three ways:
In addition, the useful life of the discs may be greatly reduced by excessive machining.
Warping
Warping is caused by excessive heat build up, which softens the metal and can allow it to be disfigured. This can result in wheel shimmy during braking. The likelihood of warping can be reduced if the car is being driven down a long grade by several techniques. Use of a lower gear to obtain engine braking will reduce the brake loading. Also, operating the brakes intermittently - braking to a slower than cruising speed for a brief time then coasting will allow the brakes to cool between applications. The suitability of this is of course, dependent upon traffic conditions. Riding the brakes lightly will generate a great amount of heat with little braking effect and should be avoided. The wheel shimmy during braking is caused by thickness variation of the disc. Tests have shown that high temperature does not permanently warp discs.
Scarring
Scarring can occur if brake pads are not changed promptly, all the friction material will wear away and the caliper will be pressed against the metal backing, reducing braking power and making scratches on the disc. If not excessive, this can be repaired by machining off a layer of the disc's surface. This can only be done a limited number of times as the disc has a minimum safe thickness. For this reason it is prudent to periodically inspect the brake pads for wear (this is done simply on a vehicle lift when the tires are rotated without disassembly of the components). When practical they should be replaced before the pad is completely worn.
Cracking
Cracking is limited mostly to drilled discs, which get small cracks around the drilled holes. These cannot be repaired.
Unnecessary resurfacing machining
Resurfacing machining has three purposes;
Brake shops will often resurface through a machining operation regardless of the need to do so due to warping or scarring. This can reduce the useful life of the disc in cases where only a light glaze removal (using emery cloth) would suffice. Reducing the life of the discs is of little concern to many brake shops as they can make money on replacing discs worn (or machined) below the manufacturer's minimum specified thickness.
Calipers
The brake caliper is the assembly which houses the brake pads and pistons. The pistons are usually made of aluminum or chrome plated iron. There are two types of calipers: floating or fixed. A fixed caliper does not move relative to the disc. It uses one or more pairs of pistons to clamp from each side of the disc, and is more complex and expensive than a floating caliper. A floating caliper (also called a "sliding caliper") moves with respect to the disc; a piston on one side of the disc pushes the inner brake pad till it makes contact with the braking surface, then pulls the caliper body with the outer brake pad so pressure is applied to both sides of the disc.
Floating caliper (single piston) designs are subject to failure due to sticking. This can occur due to dirt or corrosion if the vehicle is not operated. This can cause the pad attached to the caliper to rub on the disc when the brake is released. This can reduce fuel milage and cause excessive wear on the affected pad.
Pistons & cylinders
The most common caliper design uses a single hydraulically actuated piston within a cylinder, although high performance brakes use as many as 8. Modern cars use different hydraulic circuits to actuate the brakes on each set of wheels as a car safety|safety measure. The hydraulic design also helps multiply braking force.
Failure can occur due to failure of the piston to retract - this is usually a consequence of not operating the vehicle during a time that it is stored outdoors in adverse conditions. For high milage vehicles the piston seals may leak, which must be promptly corrected.
Brake pads
The brake pads are [[design]]ed for high friction with the disc, while wearing evenly. The brake pads must be replaced regularly, and most are equipped with a method of alerting the driver when this needs to take place. Some have a thin piece of soft metal that causes the brakes to squeal when the pads are too thin, while others have a soft metal tab embedded in the pad material that closes an electric circuit and lights a warning light when the brake pad gets thin. More expensive cars may use an electronic sensor.
Early brake pads (and shoes) contained asbestos. When working on older car's brakes, care must be taken not to inhale any dust present in the caliper (or drum).
Parking brakes
Most vehicles include a mechanical parking brake system (also called an "emergency brake") which operates on the rear wheels. These systems are very effective with drum brakes, since these tend to lock. The adoption of rear-wheel disc brakes caused concern that a disc-based parking brake would not effectively hold a vehicle on an incline. Though some early vehicles (like the Toyota 2000GT) did use the disc for the parking brake, others used a tiny drum brake embedded inside the rear disc.
Today, most cars use the disc for parking, though some still rely on separate drums. The advent of electric parking brakes will change the rear caliper configuration substantially.
Materials advances
Recently, carbon-ceramic and carbon-carbon composite brakes have been used in racing, sport car, and even high speed railroad train applications. This should not be confused with ceramic brake pads for use with standard steel discs, which are simply high quality brake pads. Carbon-carbon brake discs are composed of carbon fiber within a carbon matrix, exploiting the excellent thermal conductivity of graphite. Among other things, they have been used in airplane brakes. Moisture can reduce the braking power of carbon-carbon brakes. Another major problem with carbon-carbon lies in its reactivity under high temperature. Additionally, carbon-carbon pads do not perform at their full capacity till they reach 300°C (572°F). Above 500°C (932°F) the carbon will react with the air and burn, and even at normal braking temperatures there will be some burning of the outer layers. This is minimized by coating the disc, sometimes with carbon-ceramic.
Carbon-ceramic brake discs are composed of carbon fiber within a silicon carbide matrix (C/SiC). Carbon ceramic brakes are lightweight and have a very high specific heat and thermal conductivity, making them ideal as brake discs able to withstand over 1600°C (2912°F). They are also very expensive and require special pads, delegating them for use mostly on high end applications such as the Porsche Carrera GT. The lifespan of carbon-ceramic brakes is limited by cracking that occurs because of the different rates of expansion between the carbon and the silicon carbide. These cracks slowly allow air to come in contact with the carbon, resulting in burning.
The early Lotus Elise models came with Aluminium metal matrix composite (MMC) brake discs. These brakes were also lightweight and a cost effective alternative to the carbon/ceramic variations available but they cannot operate at the same temperatures. However, the manufacturer for these discs closed down, and Lotus was forced to switch to a iron disc once again. Brakepads are still available for the MMC discs.
Source: CDX Global & Wikipedia - en.wikipedia.org
