Braking systems
- Brake type - principles
- Brake types
- Air brakes
- Exhaust brakes
- Electric brakes
- Parking brakes
- Engine brakes
Brake type - principles
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.
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.
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: to allow the driver to maintain steering control; and to shorten braking distances.
A typical ABS is composed of:
a central electronic unit, four speed sensors (one for each wheel), and two or more hydraulic valves on the brake circuit.
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.Brake types
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:
to allow the driver to maintain steering control; and to shorten braking distances.
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:
a central electronic unit, four speed sensors (one for each wheel), and two or more hydraulic valves on the brake circuit.
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:
Brake pedal, Master cylinder, Wheel speed sensors, Electronic control unit or ECU, and Hydraulic control unit (also referred to as a Hydraulic Modulator).
Mechanism
Pressurized brake fluid travels along the brake line to the caliper. The pressurized fluid pushes the piston and inner brake pad against the disc . Pressure against the disc pushes the caliper away from the piston, pulling the outer brake pad against the disc. As the brake pads clamp together, friction slows the rotation of the disc and wheel.
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:
warping, scarring,and cracking.
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;
to remove warps (restore planarity to the discs), to remove scoring, and to remove a glazed surface when new pads are installed.
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.
Air brakes
Air-operated braking systems are used on heavy vehicles. Compressed air, operating on large-diameter diaphragms, provides the large forces at the brake assembly that are needed.
An air compressor pumps air to storage tanks. Driver-controlled valves then direct the compressed air to different wheel units, to operate the friction brakes.
On articulated vehicles, any delays in applying the trailer brakes should be minimized. This is achieved using a relay valve, and a separate reservoir on the trailer.
This arrangement also applies the brakes if the trailer becomes disconnected from the prime-mover. Exhaust brakes
Heavy goods vehicles can often require increased braking, in situations where friction brakes could overheat and fail. This is achieved by using an exhaust brake.
An exhaust brake works by restricting the flow of exhaust gases through the engine.
It achieves this by closing a butterfly valve located in the exhaust manifold. This maintains high pressures in the exhaust manifold, and the engine cylinders, which in turn, acts as a brake against the engine rotating. This then slows the road wheels through the transmission, or power train.
Other heavy goods vehicles use an engine brake that operates by altering valve timing, and stopping fuel being injected into the engine.
Since diesel engines lack an intake manifold, there is no intake vacuum when the engine is not fueling. The intake vacuum create the slowing effect felt in gasoline engines when they are going down a hill with the foot off the gas. Many different strategies are used on diesels, the least expensive (and also one of the least powerful) of which is the exhaust brake.
Operation
Exhaust brakes (otherwise known as exhaust retarders) are manufactured by many different companies, including competitors Pacbrake and Jacobs. The brakes vary in design, but essentially operate by closing off the exhaust path from the engine, causing the exhaust gasses to be compressed in the exhaust manifold, and in the cylinder. Since the exhaust is being compressed, and there is no fuel being applied, the engine works backwards, slowing down the vehicle. The amount of negative torque generated is usually directly proportional to the back pressure of the engine.
Performance
Some new innovations increase the exhaust back-pressure by various means, leading to more torque at the flywheel, and therefore more braking power. Braking power is generally measured in horsepower, and hovers around 60%-80% of the engine's maximum power output. More performance is usually easily had by down shifting the vehicle (increasing the leverage , or gear ratio of the engine over the wheels).
Related Items
Exhaust brakes should not be confused with engine brake, which work by holding the exhaust valves closed, although the basic principle of operation is similar.
Legal implications
Compression braking (also jake brake), a form of engine brake, produces extreme amounts of noise pollution in comparison to exhaust brakes. For this reason, some vehicle original equipment manufacturers prefer to use exhaust brakes, even when the performance is not as good, just because of the noise issues. This is particularly common for delivery vehicles.
Numerous cities, municipalities, states, and provinces banned the use of unmuffled compression brakes.Electric brakes
Electric brakes are commonly used in conjunction with RV trailers. In this application, they are normally match with standard automotive braking component construction.These components can perform to the manufacturer's specifications provided that the followng points are adhered to:
if properly installed, correctly wired and maintained to manufacturers recommendations.
In some instances these brakes could cause a problem when used in conjunction with boating trailers, particularly using the wet launch boat trailer application, particularly in salt water launching.
Trailers and caravans towed by light vehicles including SUV/Pick-up must have a braking system if the trailer gross mass exceeds a certain value as determined by some legislative bodies/Government. An electric braking system is commonly used to activate the drum-type friction brakes on the trailer.
Braking effect can be increased or reduced by the driver, adjusting a control unit to suit the load on the trailer.
When the brakes in the towing vehicle are applied, the brake-light circuit sends the signal to the control unit. The control unit then sends an appropriate current to the trailer brake actuators, to operate the trailer brakes, at the level selected. This should be appropriate for the loaded conditions of the trailer and the relative speed.
Legislative Background
Many legislative jursidictions around the world have varying requirements for auxilliary braking for trailers and so forth.For instance, one jurisdiction has the following legislative requirement enacted:
"All trailers with a Gross Trailer Mass (GTM) exceeding 750kg (1650 lbs) and first registered after 30 June 1990 are required to be fitted with an efficient braking system. For trailers with a GTM of up to 2 tonnes (2.2 ton),this braking system can be either an overrun sysytem or one that is operated from the driver's seat.
For trailers with a GTM exceeding 2 tonnes (2.2 ton), the braking system must be capable of being applied from the driver’s seat - overrun brakes are not acceptable. In addition, the braking system must be such that if the trailer accidentally breaks away (from the towing vehicle), the trailer brakes will apply automatically and remain applied for at least 15 minutes''."
Selection of Braking System
When selecting the appropriate electric brake components for your trailer, consideration must be given to:
the weight of trailer fully loaded with all the desired cargo requirements and equipment.
For multi-axle trailers, consideration to the number of axles that will be equipped with brakes to meet the vehicle and its combination's stopping requirements.
Electric brake systems consist of components mounted both on the tow vehicle and the trailer. An electric brake controller must be properly mounted in aconvenient location within the vehicle. The trailer side of an electric brake system consists of left & right electic brake components, drum & hub assemblies, EMERGENCY BREAKAWAY KIT-(battery w/box, breakaway switch, trickle charger) and end plug connector.
Basic Componentry
Brake Control Unit
All electric brake systems require a brake controller to apply power to the brake system. With the majority of electric brakes, as power is applied to the electromagnets in the brake cluster, the magnets are pulled to the interior flat surface of the hub drum. As the wheel and drum turn during highway travel, this attraction of the magnet to the flat drum face pulls the lever arm of the brake actuating cam, rotating the cam and forcing the brake shoes out to contact the drum braking surface. The more power applied to the magnets the greater the leverage on the came resulting in greater braking response.
As a safety overview, most installers locate the electric brake controllers on or near the dash allowing the driver access to the manual override button. As with any auxilliary braking component, it must have a manual override button allows the driver to activate the trailer brakes without operating the tow vehicle braking system. This can be important if your trailer is prone to sway. Some compoent manufacturers have suggested that, by bumping the electric brakes on the trailer during sway, the driver can dampen and control sway problems should they occur.
Major Disadvantage for Electric Auxilliary Brakes
If you use a dash mounted brake controller, you must have a controller on every vehicle that you tow the electric brake equipped trailer with.
Electric brake controllers come in a variety of types:
Simple power adjustment controllers Dash mounted inertial deceleration controllers Trailer mounted inertial deceleration controllers Hydraulic over electric controllers. Parking brakes
All vehicles must have at least two independent systems. They were once called the service brake and emergency brake. Now they are usually referred to as the foot brake and the park brake.
Most light vehicles use a foot-brake that operates through a hydraulic system on all wheels, and a hand-operated brake that acts mechanically on the rear wheels only.
One common use of the hand-brake system is to hold the vehicle when it is parked.
The systems are designed to be independent so that if one fails, the other is still available.
Basic Systems
An emergency brake is a brake system that is generally only to be used in emergency situations to slow or stop a machine. The most well-known emergency brakes are those in trains and automobiles. Many people shorten emergency and call the devices e-brakes. Additionally, in the automobile context, they are also known as parking brakes and hand brakes.
Automotive
In cars, the emergency brake is a supplementary system that can be used if the vehicle's primary brake system (usually hydraulic brakes) has a failure. Automobile e-brakes usually consist of a cable (usually adjustable for length) directly connected to the brake mechanism on one end and to some type of lever that can be actuated by the driver on the other end.
The lever is:
most commonly a handle on the floor between the driver and front passenger (hence the hand brake name), or a pedal in the foot well in front of the driver.
In the central handle configuration, the brake can be activated either by the driver or passenger (if the driver were to become unconscious, for instance). Traditionally, American car manufacturers (who more often sold cars with bench seats, thus requiring this configuration) were equipped with pedal activated emergency brakes, while imported cars, which often had bucket seats, were equipped with a lever between the seats. While either configuration serves for parking brakes, for use as an emergency brake, or for initiating handbrake turns, the lever operated brake is preferred, as the release button can be held down to prevent the brake from latching; this is very difficult with the pedal operated configuration.
However, the most common use for an automobile emergency brake is to keep the vehicle motionless when it is parked, thus the alternative name, parking brake. Car emergency brakes have a ratchet locking mechanism that will keep them engaged until a release button is pressed. On vehicles with automatic transmissions, this is usually used in concert with a parking pawl in the transmission. Automotive safety experts recommend the use of both systems to immobilize a parked car, though many individuals use only the Park position on the transmission and not the parking brake.
A parking brake cable which is unused for a long period of time may rust and seize, so that the brake will not be able to be actuated when it is eventually desired to do so. Conversely, in cold climates, a parking brake which is applied when there is some amount of water in the cable housing or in the mechanism may freeze when left for several hours, particularly overnight when temperatures drop, immobilizing the car when it is desired to restart it. It is recommended for this reason that when conditions are such as to make this a possibility, the parking brake be only partially applied, as it is relatively easy to break free of the ice by pulling the lever or pressing the pedal further, then releasing the brake, whereas the return/release spring does not have enough strength to do so by itself and there is no way to aid it in the release direction.
Historically, some cars with automatic transmissions were fitted with automatically releasing parking brakes. The parking brake would be released if the gear selector was placed in a forward or reverse gear. This automatic release system was eventually discontinued as a safety hazard, since there would be no protection against accidentally knocking the transmission into gear. Worse still, many North American-market Ford Motor Company cars from the late 1960s had a flaw in which, when the steering-column mounted shifter's bearings wore, the car could jump into reverse from park on its own. This and automatically releasing parking brakes were a deadly combination.
In cars with rear drum brakes, the emergency brake typically uses the same mechanism. In cars with rear disc brakes, the emergency brake most often actuates the same system, but sometimes (in the Mazda RX-5 and its twin the Cosmo, for instance) actuates a small drum brake housed within the hub assembly.
A number of production vehicles have been made with a separate drum brake on the transmission tailshaft. This has an advantage of being completely independent of other braking systems. As long as the drive train is intact (propellor shaft, differential, and axle shafts) this is effective. It is however, particularly dangerous when used in combination with a bumper jack at the rear of the vehicle if wheel block wedges are not used; jacking one rear wheel up will allow the differential to operate and the vehicle can roll off of the jack. This can be particularly dangerous if the wheel has been removed.
Electric parking brake
A popular new technology is the electric parking brake. First installed in the 2003 Lincoln LS, electric brakes have since appeared in a number of vehicles, including the Audi A6 and A8, BMW 7 Series, Jaguar S-Type, and the XJ. The 2006 Volkswagen Passat will also use this system.
Two variations are available: In the more-primitive "cable-pulling" type, an electrical motor simply pulls the emergency brake cable rather than a mechanical handle in the cabin. A more advanced unit uses a computer-controlled motor attached to the brake caliper to activate it.
It is expected that these systems will incorporate other features in the future. BMW already has a system where the emergency brake initiates when the car stops and then goes off as soon as the gas pedal is pressed preventing the car from drifting. The vehicle operator can easily turn off the system.
Park brake on drum brake system
All vehicles must be fitted with at least two independent braking systems.
They were once called the:
service brake, andemergency brake.
Now they are usually referred to as the:
foot brake, andpark brake.
Most light vehicles use a foot-brake that operates through a hydraulic system on all wheels, and a hand-operated brake that acts mechanically on the rear wheels only.
One common use of the hand-brake system is to hold the vehicle when it is parked. The systems are designed to be independent so that if one fails, the other is still available.
This light commercial vehicle (pictured below) uses a single drum brake on the rear of the gearbox as a hand-brake. It’s sometimes called a transmission brake.
On this duo-servo drum brake (pictured below), the cable for the hand brake lever pulls on an actuating lever inside the brake drum assembly. The actuating lever is connected to the secondary brake shoe by a pin, and to the primary shoe by a strut. Movement of the lever forces both shoes against the drum.
Background
In cars, the emergency brake is a supplementary system that can be used if the vehicle's primary brake system (usually hydraulic brakes) has a failure. Automobile emergency or park brakes usually consist of a cable (usually adjustable for length) directly connected to the brake mechanism on one end and to some type of lever that can be actuated by the driver on the other end. The lever is most commonly a handle on the floor between the driver and front passenger (hence the hand brake name), or a pedal in the foot well in front of the driver. In the central handle configuration, the brake can be activated either by the driver or passenger (if the driver were to become unconscious, for instance).
Traditionally, American car manufacturers (who more often sold cars with bench seats, thus requiring this configuration) were equipped with pedal activated emergency brakes, while imported cars, which often had bucket seats, were equipped with a lever between the seats. While either configuration serves for parking brakes, for use as an emergency brake, or for initiating handbrake turns, the lever operated brake is preferred, as the release button can be held down to prevent the brake from latching. This is very difficult with the pedal operated configuration.
Park brake on disc brake system
All vehicles must be fitted with at least two independent systems. They were once called the service brake and emergency brake. Now they are usually referred to as the foot brake and the park brake.
Most light vehicles use a foot-brake that operates through a hydraulic system on all wheels, and a hand-operated brake that acts mechanically on the rear wheels only.
One common use of the hand-brake system is to hold the vehicle when it is parked.
The systems are designed to be independent so that if one fails, the other is still available.
This light commercial vehicle uses a single drum brake on the rear of the gearbox as a hand-brake. It's sometimes called a transmission brake.
Some incorporate a drum brake for the hand-brake, in the centre of the rear disc brake. Others use a mechanical linkage to operate the disc brake from the hand brake system, or separate hand-brake calipers with their own pads.
Some vehicles have the hand brake operating on the front wheels. Engine brakes
Engine braking is the act of using the energy-requiring compression stroke of the internal combustion engine to dissipate energy and slow down a vehicle. Compression brakes is a common legal term for the same mechanism. Large trucks use a device called a jake brake to increase the effectiveness of engine braking.
Design
Most four stroke internal combustion engines require compression of the fuel-air mixture before ignition, in order to extract useful mechanical energy from the expansion. Diesel engines are adiabatic and have no spark plugs and use energy transferred to the fuel-air mixture during compression to directly ignite the mixture.
Regardless of engine type, compression of gas and vapor requires energy as described by theories in physical chemistry and thermodynamics. Compression in an engine is driven by the drive shaft which is driven by the moving wheels of a vehicle (as well as the flywheel). So, the engine ends up converting energy that was formerly kinetic energy of the vehicle into heat in the fuel-air mixture. These hot gasses as well as other heated components of exhaust are eventually ejected from the vehicle.
Advantages
The advantage of using the engine to dissipate energy is this immediate ejection of energy. Hot gasses are ejected from the vehicle very quickly and the gasses also transfer much of their heat directly to engine parts. In addition, friction produced within the engine system also adds heat to the engine parts.
This engine heat is taken away by the engine's integrated cooling system: usually a liquid circulation system and a radiator. Disk or drum brakes have no such energy dissipation mechanisms. They must rely on air flow to remove heat and they use their mass to retain heat without producing temperatures that would deform and damage the brakes.
Placing a vehicle in a low gear causes the engine to have more leverage (mechanical advantage) on the road and the road to have has less leverage on the engine. This is what allows multi-ton trucks to slow down using their relatively flimsy engine parts. The engine maintains a high RPM to dissipate a lot of power without forcing too much strain on the engine.
Applications
Active use of engine braking (shifting into a lower gear) is only advantageous when it is necessary to control speed while driving down very steep and long slopes. It should be applied after regular disk or drum brakes have been used to reduce speed to the desired speed. The desired speed is maintained by using engine braking to counteract the acceleration due to gravity.
Engine braking is otherwise always active in all non-hybrid cars with an internal combustion engine, regardless of transmission type. Engine braking passively reduces wear on brakes and helps a driver maintain control of the car. It is always active when the foot is lifted off the accelerator, the transmission is not in neutral, and the clutch is engaged.
Legal implications
Compression braking (jake brake), a form of engine braking, produces extreme amounts of noise pollution if there is no muffler on the exhaust system of the engine. Anecdotally, it sounds similar to a jackhammer, however the loudness is between 10-20 times the perceived loudness of a jackhammer.
Numerous cities, municipalities, states, and provinces banned the use of unmuffled compression brakes.
