When it comes to cars, the first things that come to mind are attributes like size, styling, color, seating capacity, functionality, acceleration, power, and fuel efficiency. Very little consideration is given to the one feature that everyone takes for granted but without which driving would be perilous and deadly. We are talking about the brakes.

It is the car’s brakes that allow it to decelerate and stop immediately if needed. In this article, we examine the different types of braking systems, how brakes work, the major components of a car’s braking system, and the proper maintenance of those components. A complete guide to everything you need to know about brakes so that you can take care of them, and they, in turn, will take care of you.

Even if you prefer to leave car maintenance and repairs to the professionals, it is prudent to know how the most critical safety feature on your car works. More importantly, having a general familiarity with your car’s brakes will enable you to recognize when they are not operating properly or when it is time to have them serviced or repaired.

Important Braking System Concepts

Before we get started, there are a few key braking systems that you need to understand.

Friction

Regardless of the type, car brakes rely upon friction to slow or stop a vehicle’s forward or backward motion. Basically, friction is the resistance that one surface encounters when sliding laterally against another surface. In the case of brakes, one surface is the wheel (more specifically the drum or rotor), and the other is the brake (brake shoe or brake pad). The greater the friction, the more the resistance.

Hydraulic Pressure

Modern car brakes also depend on hydraulic pressure to create the force needed to press the braking surface against the wheel surface hard enough to create the necessary friction to slow or stop the vehicle. Hydraulics are what enable the simple action of pressing down on the brake pedal to be converted into a force powerful enough to bring a several thousand pound vehicle to a screeching halt.

In simple terms, hydraulic pressure is the conversion of a small amount of force on a small area of liquid into a correspondingly larger amount of force on a larger area. In the case of drum brakes, the small force is the brake pedal being depressed, the liquid is the brake fluid and the larger force is the action of the brake pressing against the wheel.

Shifts in the Center of Gravity

Another concept that is important to understand is that as a car slows down, its momentum and center of gravity carries forward, and instead of resting equally on all four wheels, a greater portion of the vehicle’s weight transfers to the front axle and wheels. Sitting at rest the front of most cars is roughly ten percent heavier than the rear to begin with, due to the weight of the engine.

As such, when a car’s brakes are applied, the front brakes typically bear a greater burden in slowing the vehicle than the rear brakes. When a car rapidly decelerates, the weight distribution differential can be as much as 70% on the front and 30% on the rear. This is why front brake pads require more frequent replacement than rear ones.

The Different Types of Braking Systems and How They Work

The first gas powered automobiles were built in the 1880s and since that time, there have been major design and engineering advances both in the way they are powered and how they are stopped. Through the years, car brakes have become less costly to manufacture with greater stopping power. Today’s vehicles employ disc brakes, drum brakes or a combination of both.

Drum Brakes

The first drum braking system was used on automobiles around 1900. Since then, materials have changed, and design improvements have been made but the concept remains the same. Because of their lower production cost, drum brakes are still used on automobiles today, although they are typically found on economy cars and installed on rear wheels where the stress of braking is less than the front of the vehicle.

How Drum Brakes Work

Each wheel of a drum braking system has a drum-shaped cylinder that is attached to the hub and rotates with the tire. Inside the drum is a hydraulic cylinder that pushes two brake shoes outward when the brake pedal is pressed down. It is the friction that is created when the brake shoes press against the inner wall of the drum that slows or stops the vehicle’s motion.

These are the primary components of a drum braking system:

Backing Plate – this is a rigid, circular structure (commonly made of steel) to which the major components of a drum brake are attached. This is the portion of the drum brake assembly that is bolted to the hub of the wheel.
Brake Drum – the brake drum is cylindrical in shape and commonly made from steel, cast iron, or in some cases aluminum. It is the inner wall of the drum that provides the opposing surface to which the brake shoes apply pressure. Together they create the friction that slows or stops the vehicle.
Wheel Cylinder and Piston – the wheel cylinder is affixed to the top portion of the backing plate. When the brake pedal is pressed, this cylinder converts the incoming brake fluid pressure into a larger force which pushes two pistons outward on opposite ends of the cylinder. Each piston is attached to a brake shoe and the outward motion presses the brake lining against the drum wall.
Brake Shoe – each brake drum contains two brake shoes, which are curved, crescent shaped metallic components that conform in shape to the inner wall of the brake drum (in other words, they are convex to the concave structure of the inner drum wall). Each brake shoe is fitted with a brake lining which is the actual surface that rubs against the drum wall.

Primary Brake Shoe

This is the shoe that is in the forward position (closer to the front of the vehicle). Also referred to as the “leading” brake shoe.

Secondary Brake Shoe

This is the shoe that is in the rear position (closer to the back of the vehicle). Also referred to as the “trailing” brake shoe.

Brake Lining – Due to the nature of the job it performs, a brake lining must be highly durable and resistant to the deteriorating effects of friction and high temperature. Commonly used brake lining materials include iron, copper, steel, and graphite, which are combined with resins and other hardening and bonding agents. Asbestos is no longer used due to its carcinogenic qualities.
Return Spring – as the brake pedal is pressed down by the driver, the drum brakes engage. When the brake pedal is released, the return spring returns each brake shoe to its home (disengaged) position. If this spring becomes worn or when a brake shoe is out of adjustment, the brake lining can drag against the drum causing faster wear on both the lining and the drum wall.
Parking Brake – one major advantage of a drum braking system is that the parking brake can be incorporated into the drum apparatus utilizing one of the brake shoes to lock the wheel in place.

Disc Brakes

Disc brakes began appearing on automobiles during the 1950s to provide greater stopping power for vehicles that were becoming larger, heavier, and capable of traveling faster. Disc brakes also offer more effective braking in adverse driving conditions such as wet roads or rainy weather.

Disc braking systems are the most common type of brakes found on cars today with a great many brands and models offering disc brakes on all four wheels as standard factory equipment.

How Disc Brakes Work

In contrast to drum brakes where brake shoes apply outward pressure to the inner wall of a drum on each wheel, disc brakes squeeze a large metal disc affixed to the wheel as it travels between a pair of brake pads mounted inside calipers. The idea behind disc brakes is that the larger surface area afforded by the disc and the squeezing action of the brake pads generate greater friction and stopping power.

Rotor – this is a large metal disc that is affixed to the hub and rotates along with the wheel. Rotors are typically made of steel, cast iron, or a special composite of the two. Aside from the brake pads, the rotor is the most crucial component of a disc braking system as it is the other half of the friction equation.

During braking, a tremendous amount of heat can be generated, as much as 500° F. It is therefore important for the rotors to dissipate heat as efficiently as possible to avoid brake drag, which is a significant loss of braking effectiveness caused by overheating of brake components.

There are several types of rotors:

Solid – this is a solid piece of metal fashioned into the shape of a disc. Because it has no openings, solid rotors dissipate heat less efficiently than the other types.
Perforated – also known as “drilled,” this type of rotor has holes punched all around its surface. These holes encourage airflow, which assists heat dissipation and also allows water to quickly wick away.
Slotted – instead of holes, this type of rotor has slots or channels that are very effective in dissipating heat and clearing away water from the braking surface.
Vented – also referred to as a “ventilated” rotor, the typical vented rotor consists of two discs joined or held together by ribs or spokes; the open space between the discs encourages strong airflow to cool the rotor. Vented rotors are typically used in vehicles with a heavy front weight load, such as large trucks.
Caliper – the caliper assembly is mounted on a bracket which is firmly affixed to the vehicle, and the rotor travels within the caliper. This mechanism supports the brake pads and is responsible for extending the pads against the rotor during braking and retracting them when the brakes are disengaged. There are two primary types of calipers, floating and fixed.

Floating Caliper

Also known as a “sliding” caliper, this is the most common type of caliper used on cars today. When the brake pedal is pressed down, pressurized brake fluid presses the inner brake pad against the inner surface of the rotor, and the continued pressure pulls the caliper which draws in the outer brake pad against the outer surface of the rotor, resulting in a squeezing action.

Fixed Caliper

A fixed caliper does not move at all when the brakes are applied. Typically, there is a piston on each side of the rotor, and when the brakes are engaged, the pressurized brake fluid causes each piston to move toward the rotor forcing the brake pads against the rotating surface from opposing sides.

Brake Pad – this is the actual surface that comes into direct contact with the rotor to create friction and slow or stop the vehicle’s motion. They are attached to metal backing plates, usually made from steel. Brake pads are composed of many types of materials to suit different types of vehicles and braking scenarios.

All brake pads, regardless of their composition, must strike a balance between creating friction (and stopping power) on the one hand, and absorbing and dissipating heat on the other. Brake pads can be broken down into four main categories:

Fully metallic – usually a mixture composed of steel, iron, copper, and graphite and bonded together. This type of pad is very durable and cost effective, but because it is very hard, it can wear down the rotor over time. Fully metallic pads are typically used on trucks and heavy industrial or commercial vehicles.
Semi-metallic – this type of brake pad is composed of synthetic materials blended with flakes of metal. They are heat efficient, but because of their metal content, they can gradually wear down rotors over time. They also create a moderate amount of brake dust (fine particles that are ground off the brake pad and rotor surfaces during braking).
Non-asbestos (organic) – this is the most common type of brake pad and is usually made from rubber, carbon compounds, glass/fiberglass, Kevlar, and various resins. It is a softer pad and therefore does not tend to wear down rotors as much as fully metallic and semi-metallic pads. A moderate amount of brake dust is created by this type of pad.
Ceramic – this is the most expensive type of brake pad and is usually reserved for high-performance applications such as sport, luxury, and race cars. Ceramic pads are composed of dense clay and porcelain bonded with copper fibers and are very lightweight, quiet and create very little brake dust.

Master Cylinder – acting like a hydraulic pump, the master cylinder compresses the brake fluid via an internal piston and distributes the pressurized fluid throughout the braking system to the individual drum or disc brakes via hydraulic lines (steel tubes) and hoses (flexible rubber).
Vacuum Booster – by multiplying the force used to press the brake pedal down, the vacuum booster effectively serves as a brake assist. An internal pump creates low pressure inside the booster and when the brake pedal is depressed the pressure normalizes, which pushes a diaphragm toward the master cylinder which in turn pressurizes the brake fluid.

Disc brake systems are becoming increasingly prevalent on new automobiles because of their greater stopping power and performance in adverse driving conditions. However, there can be instances where slowing or stopping too suddenly can be extremely dangerous and cause the vehicle’s tires to slip on the surface of the road losing all traction or grip.

Anti-Lock Braking Systems

When brakes are applied too aggressively, one or more of the wheels can freeze up causing tires to lose traction with the road and begin to skid. During a skid, a driver’s control over the vehicle is greatly diminished. Fortunately, technical advances have enabled automobile designers and engineers to incorporate safety features to combat this problem.

An anti-lock braking system (ABS) is now a standard safety feature on many new vehicles. An ABS consists of various sensors throughout the vehicle that can detect when certain driving conditions have gone awry, for example, if the rotational speed of any particular wheel (revolutions per minute or RPM) does not correlate to the speed that the vehicle is traveling, meaning that the car is skidding.

An ABS combats loss of traction by preventing the brakes from locking up or freezing any of the wheels. It accomplishes this by reducing and increasing the hydraulic line pressure repeatedly and in rapid succession. In essence, the brakes are engaged and disengaged at an incredibly high rate of speed – as many as 15 times per second. This prevents the wheels from locking up while still slowing the vehicle’s motion.

To a driver who has just slammed on the brakes, the ABS engaging will feel like the brake pedal pulsing or vibrating very rapidly. The car may shudder as it comes to a sudden stop, but there will be minimal skidding or loss of traction which is what the ABS is designed to do.

Along a similar vein, traction control systems operate similarly to an ABS but can provide variable hydraulic pressure to the front and rear brakes to regain lost traction and steering control. An internal gyroscope can determine that the direction or orientation that the vehicle is traveling does not correspond to the manner in which the driver is steering the car and initiate corrective action to restore traction.

Brake Maintenance and Troubleshooting

Regular inspection and maintenance of your car’s brake system will keep your vehicle’s most important safety feature operating properly and reliably. Even for those who are not familiar with the inner workings of automobiles, being able to recognize certain telltale signs will enable you to identify and address a minor issue before it becomes a bigger and costlier problem.

These are common signs that your car’s brake system needs to be inspected:

Reduced Responsiveness of Brake Pedal

If your car’s brakes do not engage as rapidly as normal when the brake pedal is depressed, or if the brake pedal itself feels spongy when you press down on it then you may be experiencing one or more of the following issues:

Low Brake Fluid – this is easily remedied by refilling the brake fluid reservoir (unless fluid is leaking from somewhere in the system); regular brake fluid checks are recommended
Leak in Brake Line or Brake Hose – a sudden loss of brake pedal responsiveness could indicate a brake line/hose leak (especially if accompanied by stains on your garage floor)
Ruptured Seal in Master Cylinder – this is a serious problem that needs to be addressed immediately, or you run the risk of a complete brake failure
Worn Brake Pads or Brake Shoes – as brake pads and shoes wear you may experience having to press the brake pedal down further to stop the vehicle
Air or Moisture in Master Cylinder – as a general rule when pressed, the brake pedal should stop one to two inches above the floor, if it sinks all the way down you need immediate service

Screeching or Scraping Sound

A screeching or scraping sound during braking is usually a sure sign that your brake pads or brake shoes are wearing down and require replacement. Many brake pad and shoe manufacturers incorporate metal indicators into the pad/shoe material that scrape the rotor or drum surface and make an audible sound to alert the driver that it is time to replace them.

In more serious cases, the pad or shoe has completely worn down, and the sound is created by metal (rotor/drum) on metal (pad/shoe backing) contact. This is a far more serious condition as the backing metal will etch grooves into the rotor. Rotors are far costlier to repair or replace than pads or shoes.

On rare occasions, a grinding noise when the brake pedal is depressed could indicate that some debris (gravel, small stones) has become caught in the brake caliper assembly.

Vibration when Braking

If you experience vibration or shaking of the steering wheel while braking, it may be a good idea to have your brake rotors inspected. Over time slight imperfections in the rotor surfaces can develop, and even a minute variation in thickness can result in strong vibrations or shaking. If left unchecked, this could develop into a more serious deformation of the rotor, which may not be serviceable.

Another possible cause of vibration during braking is faulty caliper alignment. Pistons inside the calipers extend and retract the brake pads, and it is not uncommon for a piston to stick and not fully retract the pad from the rotor’s surface when the brake pedal is released. This is a fairly straightforward repair, but as with other issues, if ignored for a long period of time, it can worsen to a more serious problem.

Car Pulls to One Side

If your car veers or tugs to one side when the brakes are applied, it is possible that one or more of your brake calipers has stuck or failed, and a single caliper is doing all the braking work by itself. Another possibility is that one or more of the braking system’s fluid lines or hoses has ruptured, resulting in a complete loss of pressure on the side where the rupture has occurred.

Either of these scenarios is serious enough to warrant an immediate visit to your mechanic.

Fluid Leaks

Any fresh leaks of fluid you observe on your garage floor are worthy of serious attention, but if it is accompanied by reduced braking performance (brake pedal feels spongy or travels further down than normal), then your problem is likely narrowed down to a brake hose or line. This is a problem requiring immediate attention before complete brake failure occurs.

Burning Smell or Smoke

If you detect a sharp, pronounced odor when braking extensively on steep inclines, then your car’s brakes may be overheating. Brake components that reach high temperatures may cause brake fade which results in very poor braking performance. Even though disc brakes are designed to dissipate heat and encourage airflow around the rotor and pads, it may be necessary to allow them to cool before driving.

It would be a good idea to ensure that the parking brake is not engaged as this would also produce the same burning chemical smell.

Any visible smoke coming from a wheel could be a brake caliper that has become stuck in the extended position, and the incredible amount of friction has ignited combustible components.

Warning Light

Any warning light appearing on your dashboard or display panel should not be ignored. If the warning light pertains to your brakes or anti-lock braking system, it needs to be checked by a professional mechanic immediately. At best, it may be indicating that one or more of the brake system components are due for a routine safety inspection.

At worst, it may be signaling to you that a serious problem with your brake system has been detected by your vehicle’s onboard sensors. Simple prudence dictates that any warning that your car’s most important safety feature may be compromised should be heeded in full and with due haste. Your car’s braking system is a technological marvel, but if you simplify its workings to basic concepts, then its proper care and maintenance can be easily manageable through regular inspection and immediate action when the first sign of trouble presents itself.

Your Car’s Brakes: Know Before You Go