How Brakes Work

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The typical brake system consists of disc brakes in front and either disc or drum brakes in the rear connected by a system of tubes and hoses that link the brake at each wheel to the master cylinder.  Other systems that are connected with the brake system include the parking brakes, power brake booster and the anti-lock system.

Brake System DiagramWhen you push on the brake pedal, you are actually pushing against a plunger in the master cylinder, which forces hydraulic oil (brake fluid) through a series of tubes and hoses to the braking unit at each wheel. Since hydraulic fluid (or any fluid for that matter) cannot be compressed, pushing fluid through a pipe is just like pushing a steel bar through a pipe.  Unlike a steel bar, however, fluid can be directed through many twists and turns on its way to its destination, arriving with the exact same motion and pressure that it started with.  Actual applied pressure to the calipers is according to a preset front to rear ratio or "bias" (see Brake Balance or Bias section).

On a disc brake, the fluid from the master cylinder is forced into a caliper where it presses against a piston. The piston, in-turn, squeezes two brake pads against the disk (rotor), which is attached to the wheel, forcing it to slow down or stop. With drum brakes, fluid is forced into the wheel cylinder, which pushes the brake shoes out so that the friction linings are pressed against the drum, which is attached to the wheel, causing the wheel to stop.

In either case, the friction surfaces of the pads on a disk brake system, or the shoes on a drum brake convert the forward motion of the vehicle into heat. Heat is what causes the friction surfaces (linings) of the pads and shoes to eventually wear out and require replacement.

On cars with disc brakes front and rear (such as the Subaru WRX and Mitsubishi Lancer Evo), brake pressure is also split into two separate circuits, the idea being that in the event of a brake system failure half the brake system will still work. On cars such as the Subaru WRX, brake pressure is split into two diagonally split systems. The front left wheel is connected to the rear right wheel to make one circuit, with the remaining front right and rear left wheel making up the other.

Disc Brakes

Disc brakes are used to stop everything from cars to locomotives and jumbo jets.  Disc brakes wear longer, are less affected by water, are self adjusting, self cleaning, less prone to grabbing or pulling and stop better than any other system around. The main components of a disc brake are the Brake Pads, Rotor, Caliper and Caliper Support.

Disc Brake Diagram

Brake Pads

There are two brake pads on each caliper. They are constructed of a metal "shoe" with the lining riveted or bonded to it.   The pads are mounted in the caliper, one on each side of the rotor.  Brake linings used to be made primarily of asbestos because of its heat absorbing properties and quiet operation; however, due to health risks, asbestos has been outlawed, so new materials are now being used.  Brake pads wear out with use and must be replaced periodically. There are many types and qualities of pads available. The differences have to do with brake life (how long the new pads will last) and noise (how quiet they are when you step on the brake). Harder linings tend to last longer and stop better under heavy use but they may produce an irritating squeal when they are applied. Technicians that work on brakes usually have a favorite pad that gives a good compromise that their customers can live with.

Brake pads should be checked for wear periodically. If the lining wears down to the metal brake shoe, then you will have a "Metal-to-Metal" condition where the shoe rubs directly against the rotor causing severe damage and loss of braking efficiency. Some brake pads come with a "brake warning sensor" that will emit a squealing noise when the pads are worn to a point where they should be changed.  This noise will usually be heard when your foot is off the brake and disappear when you step on the brake.  If you hear this noise, have your brakes checked as soon as possible.


Rotor

The disc rotor is made of iron with highly machined surfaces where the brake pads contact it.  Just as the brake pads wear out over time, the rotor also undergoes some wear, usually in the form of ridges and groves where the brake pad rubs against it.  This wear pattern exactly matches the wear pattern of the pads as they seat themselves to the rotor.  When the pads are replaced, the rotor must be machined smooth to allow the new pads to have an even contact surface to work with.  Only a small amount of material can be machined off of a rotor before it becomes unusable and must be replaced.  A minimum thickness measurement is stamped on every rotor and the technician doing the brake job will measure the rotor before and after machining it to make sure it doesn't go below the legal minimum.  If a rotor is cut below the minimum, it will not be able  to handle the high heat that brakes normally generate.  This will cause the brakes to "fade," greatly reducing their effectiveness to a point where you may not be able to stop!

Caliper & Support

There are two main types of calipers: Floating calipers and fixed calipers. There are other configurations but these are the most popular.  Calipers must be rebuilt or replaced if they show signs of leaking brake fluid.

Single Piston Floating Calipers are the most popular and also least costly to manufacture and service. A floating caliper "floats" or moves in a track in its support so that it can center itself over the rotor. As you apply brake pressure, the hydraulic fluid pushes in two directions. It forces the piston against the inner pad, which in turn pushes against the rotor. It also pushes the caliper in the opposite direction against the outer pad, pressing it against the other side of the rotor. Floating calipers are also available on some vehicles with two pistons mounted on the same side. Two piston floating calipers are found on more expensive cars and can provide an improved braking "feel".

Four Piston Fixed Calipers are mounted rigidly to the support and are not allowed to move. Instead, there are two pistons on each side that press the pads against the rotor.  Four piston calipers have a better feel and are more efficient, but are more expensive to produce and cost more to service.  This type of caliper is usually found on more expensive luxury and high performance cars.

Drum Brakes

Drum brakes consist of a backing plate, brake shoes, brake drum, wheel cylinder, return springs and an automatic or self-adjusting system. When you apply the brakes, brake fluid is forced under pressure into the wheel cylinder, which in turn pushes the brake shoes into contact with the machined surface on the inside of the drum.  When the pressure is released, return springs pull the shoes back to their rest position.   As the brake linings wear, the shoes must travel a greater distance to reach the drum.  When the distance reaches a certain point, a self-adjusting mechanism automatically reacts by adjusting the rest position of  the shoes so that they are closer to the drum.

Drum Brake Diagram

Brake Shoes

Like the disc pads, brake shoes consist of a steel shoe with the friction material or lining riveted or bonded to it.  Also like disc pads, the linings eventually wear out and must be replaced.  If the linings are allowed to wear through to the bare metal shoe, they will cause severe damage to the brake drum.

Backing Plate

The backing plate is what holds everything together.  It attaches to the axle and forms a solid surface for the wheel cylinder, brake shoes and assorted hardware.  It rarely causes any problems.

Brake Drum

Brake drums are made of iron and have a machined surface on the inside where the shoes make contact.   Just as with disc rotors, brake drums will show signs of wear as the brake linings seat themselves against the machined surface of the drum.  When new shoes are installed, the brake drum should be machined smooth. Brake drums have a maximum diameter specification that is stamped on the outside of the drum. When a drum is machined, it must never exceed that measurement.   If the surface cannot be machined within that limit, the drum must be replaced.

Wheel Cylinder

The wheel cylinder consists of a cylinder that has two pistons, one on each side. Each piston has a rubber seal and a shaft that connects the piston with a brake shoe. When brake pressure is applied, the pistons are forced out pushing the shoes into contact with the drum.  Wheel cylinders must be rebuilt or replaced if they show signs of leaking.

Return Springs

Return springs pull the brake shoes back to their rest position after the pressure is released from the wheel cylinder.  If the springs are weak and do not return the shoes all the way, it will cause premature lining wear because the linings will remain in contact with the drum.  A good technician will examine the springs during a brake job and recommend their replacement if they show signs of fatigue.  On certain vehicles, the technician may recommend replacing them even if they look good as inexpensive insurance.

Self Adjusting System

The parts of a self adjusting system should be clean and move freely to insure that the brakes maintain their adjustment over the life of the linings.  If the self adjusters stop working, you will notice that you will have to step down further and further on the brake pedal before you feel the brakes begin to engage.  Disc brakes are self adjusting by nature and do not require any type of mechanism.

Examples of Brake Master Cylinders

Master Cylinder

The master cylinder is located in the engine compartment on the firewall, directly in front of the driver's seat.  A typical master cylinder is actually two completely separate master cylinders in one housing, each handling two wheels. This way if one side fails, you will still be able to stop the car. The brake warning light on the dash will light if either side fails, alerting you to the problem.

Master cylinders have become very reliable and rarely malfunction; however, the most common problem that they experience is an internal leak. This will cause the brake pedal to slowly sink to the floor when your foot applies steady pressure. Letting go of the pedal and immediately stepping on it again brings the pedal back to normal height.

Power Brake Booster

Power Brake BoosterThe power brake booster is mounted on the firewall directly behind the master cylinder and, along with the master cylinder, is directly connected with the brake pedal.  Its purpose is to amplify the available foot pressure applied to the brake pedal so that the amount of foot pressure required to stop even the largest vehicle is minimal.  Power for the booster comes from engine vacuum. The automobile engine produces vacuum as a by-product of normal operation and is freely available for use in powering accessories such as the power brake booster.  Vacuum enters the booster through a check valve on the booster. The check valve is connected to the engine with a rubber hose and acts as a one-way valve that allows vacuum to enter the booster but does not let it escape. The booster is an empty shell that is divided into two chambers by a rubber diaphragm. There is a valve in the diaphragm that remains open while your foot is off the brake pedal so that vacuum is allowed to fill both chambers.  When you step on the brake pedal, the valve in the diaphragm closes, separating the two chambers and another valve opens to allow air in the chamber on the brake pedal side.  This is what provides the power assist.   Power boosters are very reliable and cause few problems of their own, however, other things can contribute to a loss of power assist. In order to have power assist, the engine must be running. If the engine stalls or shuts off while you are driving, you will have a small reserve of power assist for two or three pedal applications but, after that, the brakes will be extremely hard to apply and you must put as much pressure as you can to bring the vehicle to a stop.

Brake Fluid

Brake fluid is a special oil that has specific properties. It is designed to withstand cold temperatures without thickening as well as very high temperatures without boiling. (If the brake fluid should boil, it will cause you to have a spongy pedal and the car will be hard to stop.) It is very important that the fluid is pure liquid and that there are no air bubbles in it.  Air can compress, which causes a sponginess to the pedal and severely reduced braking efficiency.  If air is suspected, then the system must be bled to remove the air. There are "bleeder screws" at each wheel cylinder and caliper for this purpose.  

The brake fluid reservoir is on top of the master cylinder. Most cars today have a transparent reservoir so that you can see the level without opening the cover. The brake fluid level will drop slightly as the brake pads wear. This is a normal condition and no cause for concern.  If the level drops noticeably over a short period of time or goes down to about two thirds full, have your brakes checked as soon as possible. Keep the reservoir covered except for the amount of time you need to fill it and never leave a can of brake fluid uncovered. Brake fluid must maintain a high boiling point.  Exposure to air will cause the fluid to absorb moisture, which will lower that boiling point.

Brake Balance or Bias

As a car decelerates under braking, vehicle weight is transferred forward and onto the front axle. This requires brake force to be regulated or "biased" to take into account of weigt transfer, which increases front wheel grip at the expense of the rear. If not for this adjustment in brake force, the rear wheels would lock up and skid, while the fronts still had plenty of excess braking traction available. This biasing is known as brake balance and regular viewers of motor racing will often hear discussion regarding the effect of fuel loads, track conditions and tyre wear on brake balance, and how it is constantly adjusted throughout the race to suit. A rally cars rear bias is higher than normal so that the driver can throw the car sideways if needed by deliberately locking up the rears before the fronts.

Production cars use a complicated pressure regulating valve that adjusts brake balance automatically, without any sort of driver intervention.

The very best types of adjustable brake balance commonly used in motorsport is a simple system where two master cylinders are used, one for the front wheels and one for the rear, operated by a common bar that is, in turn, moved by the brake pedal. The pivot point of the "balance bar" can be adjusted, usually by a cockpit mounted driver adjustable knob. Moving the pivot point towards one master cylinder and away from another changes the amount of pedal force that is converted into brake pressure by both master cylinders, altering the brake balance front to rear. Cost, complexity and incompatiblity with production anti-lock brakes make it impractical for all but serious competition.

Anti-Lock Brakes (ABS)

The point at which maximum brake force is transmitted onto the road surface is the moment just before the wheel stops turning and locks. When you lock up the wheels from excessive pedal pressure, you lose the ability to steer your car and stopping distances increase greatly. Once the wheels lock and start to skid, grip reduces markedly, and control of the vehicle is lost as it no longer responds to steering inputs and continues on travelling in the same direction it was when the wheels first locked.

This means in an emergency situation the driver is unable to steer around obstacles on the road, taking a greater distance to stop the car completely. ABS (Anti-Lock Braking System) monitors wheel speed and regulates brake force applied to each wheel by way of rapidly pulsing hydraulic pressure according to changes in rotational speedto achieve the point of maximum braking. This is the rapid pulsing felt through the brake pedal when ABS is active.

Under a few situations, such as fresh show or loose gravel, ABS actually increases stopping distances. As a wheel never locks completely, it cannot dig through the top layer of loose material and into the harder material below. ABS also stops the tyre from building up a wedge of loose material which can also help slow the car. This is purely a technicality, as in the majority of cases the ability to steer the car outweighs the extra stopping distance. If the driver can brake and steer at the same time, you stand a much better chance of not needing the services of a crash repair shop!

Electronic Brake Distribution (EBD)

ABS system operations can be further augmented with the addition of EBD, which constantly adjusts brake system pressure when in operation. EBD works like a sophisticated brake proportioning valve, optimising brake system performance in accordance with such things as dynamic weight transfer and prevailing road surface grip. Overall stopping distances decrease with EBD, along with a marked increase in stability while braking hard.

Replacing Brake Discs and Pads

Avoid fitting new brake discs and pads at the same time. New pads should always be fitted with old discs and new "green" discs with olds pads. Why?

Because with use, discs form a heat-hardened surface allowing them to bed in new, soft pads. Similarly used pads will also become heat-treated and are the best way of bedding in green discs. However this assumes that all used parts are mechanically sound and in good condition. Badly worn or scored brake discs should not be used to ped in new pads, and pads worn to suit the contours of scored or warped discs cannot be used to bed green discs.Unless stated specifically by the supplier, never "stand" on the brakes after fitting new pads or discs. Overheating the brakes inthe early stages of the bedding process will overcook pad and the disc material and result in reduced longevity.

Sometimes badly worn or damaged brakes will necessitate discs and pads being fitted at the same time. Even more effort should be put into the bedding in process. The longer the time, the more frequent the cycle of heatingand cooling before the big "test", the better most pads and discs will perform. This is one case where stop-start city driving is useful!