It's all very well having a car that goes well, but stopping well is equally important. Hence, the engineers that designed your vehicle will have put a lot of thought into developing a braking system that will suit your requirements and ensure that you can slow down and stop safely in a wide variety of situations.
Of course, each brake setup has its differences and in turn, its specific strengths as well as its own set of drawbacks, which means that it is a good idea to understand the type of brakes your car is fitted with. Keeping this in mind, as well as having an idea of the types of parts and upgrades available to you will mean that you can ensure that you don't push your car beyond its ability to safely and effectively slow you down.
What are brake pads?
Brake pads are essentially a metal backing plate that holds an abrasive, friction-generating material that works by rubbing against the spinning rotor to provide resistance and thus slow the rotation of your wheels. They are the part of the brake system that is most commonly replaced, and the reason for this is that by their very design, they are a perishable component. Pads are held inside calipers that are actuated by hydraulic fluid and so will clamp down on the rotor when you depress the brake pedal. Most cars will be fitted with two brake pads per wheel, though some high performance setups will indeed utilise more - often with different friction properties that allow progressive braking.
What are brake pads made of?
Brake pads have historically contained a combo of nasty materials such as asbestos, though this has been phased out in favour of safer materials in recent years. That said, older pads may still contain the stuff, so care should be taken when removing the old pads for replacement, that you don't expose your skin to the dust.
Nowadays, brake pad material falls within one of four categories:
- Fully-metallic - These are incredibly long-lasting pads that are not usually found on road-going vehicles as they are noisy and require a lot of force to operate. They are most often used in racing as they will provide a much longer operating lifespan.
- Semi-metallic - These are synthetic pads that have an amount of metal mixed into the compound. Because of this, they tend to last a lot longer, and resist brake-fade better than non-metallic pads, though will often be harder to operate and can cause wear to the rotor surface itself.
- Non-metallic - These are the most common type of pad found on modern cars. They are usually made from a mixture of cellulose, aramid and glass, and have superseded asbestos-based pads as the industry standard. They offer an all-round performance, though will wear quicker than other, more specialised materials.
- Ceramic - These pads are usually made from a mixture of clay and porcelain, with some copper flakes throughout. They tend to fall somewhere between the metallic and non-metallic types, though are more prone to warping due to overheating. Usually, ceramic brake pads also have the added bonus of being the quietest type of pad available, since the noise generated by them is far above the frequency that humans can perceive.
Brake pad applications
Depending on the type of driving you intend to do, as well as the type of vehicle you'll be driving, the type of brake pads you'll require will be different. The most obvious separation lies between road and race cars. Road cars need to have pads that will survive everyday use, but without making a terrible racket, whereas race brakes only really need to be focussed around stopping ability and durability. Even amongst road-going cars though, the type of pad that you will need may differ based on the type of driving you do.
Furthermore, the heavier your vehicle, the heavier-duty you'll need your brake pads to be for them to be up to the task of stopping all that weight reliably. The pads' ability to dissipate heat is something that should be carefully considered when selecting pads for off-road 4x4s, as usually, you won't be travelling at a high enough speed for airflow to cool the brakes properly, but will use the brakes an awful lot when rock-crawling or descending ridges etc. Premium brake pads are also available that have been specifically formulated to meet the exacting standards of European luxury car manufacturers. These pads will usually be supplied with all of the clips, sensors and retaining equipment as well - making them a sound investment for owners of luxury Euro cars.
What are Brake pad sensors?
More modern cars (and high-specced older cars too) are often fitted with helpful sensors that will measure the amount of friction material left in your pads. They'll let you know when it's time to replace your pads and should therefore never be ignored. It is unusual for these sensors to fail, but as with everything, they still can - if they do, then it is easy to figure it out as you'll usually be warned that your brake pads are worn when they aren't. If your car doesn't have sensors, then usually wear will be indicated by either a groove in the friction material, that will fade away as the pad wears down. This indicates that they need to be changed, or a series of small metal strips that will cause a horrible screeching sound once the material around them has worn down enough to allow them to contact your rotors.
What are brake rotors?
Brake rotors are discs of metal that are sandwiched between the hub and the wheel. The brake pads are held within calipers that squeeze the pads onto the surface of the rotors to slow their spin, and therefore the spin of the wheels. Most disc brakes are forged from iron and will tend to vary somewhat in their design, depending on the application that they're being used for. Lower-performance setups will usually utilise simple discs of metal that may or may not have cooling vanes added during the forging process, whereas the higher-end offerings will have a variety of machined slots, holes or both, to further assist with cooling and to allow water, dust and other contaminants to be dispersed. When looking to buy brake rotors, bear in mind that cross-drilled or slotted rotors certainly do increase your brake system's ability to keep cool, and to operate more effectively, but will tend to wear down your brake pads a bit quicker.
Traditionally, most older cars were equipped with drum brakes, but over time there has been a shift to the popular use of discs, due to cheaper manufacturing and better materials being available. A fun fact is that one of the first mass-produced vehicles that were equipped with disc brakes was the infamous German Tiger I tank - during WW2.
What are drum brakes?
Drum brakes are a brake setup that uses pads (or shoes) to push outwards against the inner surface of the housing, or drum, to slow the spin of the wheel. They used to be the most commonly found type of brake, however now they are usually only found on the rear axles of commercial vehicles, trailers and cheaper, economy cars. Modern drum brakes are auto-adjusting, whereas older setups required manual adjustment of the tensioners as the shoes wore down. Drum brakes are also more prone to brake-fade as they tend not to be able to dissipate heat as well as disc brakes can. They are also more likely to face issues when soaked since they can't dry as rapidly. It's not all bad news, however, if your vehicle has drum brakes - drum brakes allow for a simple mechanical handbrake assembly, and also offer better braking than a disc brake of the same diameter. This is due to the fact that the friction pad makes contact with (almost) the entire circumference of the drum - an added benefit of which is less wear on the pads, and therefore less maintenance required.
What is brake fluid?
Almost all modern braking systems utilise hydraulic fluid to operate the brakes. The hydraulic fluid isn't particularly compressible, so the force enacted upon it by the brake pedal being pushed (via the master cylinder) will build up pressure that will be transferred through the fluid and will then make the brake calipers or piston within the drum brakes push the pads against the rotor or drum respectively. Most brake fluids are based on a chemical compound called glycol-ether though older fluids are based on castor-oil, and newer fluids are based on silicone. Brake fluid is usually referred to by a DOT rating. DOT stands for Department of Transport in the USA, and this rating generally denotes boiling points, with the higher number referring to a higher resistance to overheating, as well as a resistance to boiling after moisture has been absorbed.
The advantage of silicone-based (DOT 5) fluid over glycol-ether based fluids (DOT 3 and DOT 4) is that silicone is hydrophobic, meaning that the fluid is unlikely to absorb any moisture from the atmosphere and therefore less likely to decompose. Unfortunately, silicone is far more compressible than the glycol-ether compound, and so DOT 5 fluid tends to feel spongier than the lower-rated fluids, and therefore shouldn't be used in systems with ABS. Any glycol-ether based fluid should always be stored in an airtight container and used quickly after opening since it tends to absorb moisture from the environment (and the air in a half-used bottle) at ambient temperatures. DOT 3 and DOT 4 Brake fluid should be changed at a minimum of 1-2 years since it will leech moisture through the rubber hoses, and therefore degrade. DOT 5 fluid, being silicone based, is immune from this requirement, however, should not be used in a system that has previously run glycol-ether based fluids without proper flushing. It is, however, recommended in freezing climates for non-ABS systems, and generally only used in specific vehicles from America, such as Harley-Davidsons and Hummers.
To further complicate things, the newest type of fluid is rated as DOT 5.1, though it too is a glycol-ether based fluid! Brake fluid based on glycol-ether is extremely damaging to paint since glycol ether is a strong solvent. It is therefore imperative to clean any paintwork that is exposed to a spilled fluid. Care should also be taken to avoid spillage onto any hot components, such as your exhaust, as these fluids are also very combustible.
What is a brake cylinder?
A brake cylinder converts the kinetic energy of your pedal being depressed into hydraulic force, or back again to actuate the brakes themselves - with the master cylinder being responsible for the former, and the slave cylinder responsible for the latter. Each cylinder is attached to the brake system at either end of the lines that carry the hydraulic fluid, and they work in unison. The master cylinder operates by moving a piston through a barrel and compressing the brake fluid when you push the pedal. This hydraulic pressure pushes a similar piston setup in the slave cylinder, which then either clamps the calipers together or pushes the brake shoes outward so that in each case, the friction pads can contact the rotor or drum's surface and bring you to a halt. There is usually a proportioning valve in the mix, that controls how much pressure is put on the fronts and rears separately. Some setups have brake bias adjusters, though in most cars this is set from the factory. Almost all cars that are equipped with hydraulic brakes are also equipped with a brake-booster (also called a vacuum servo, or power-booster) which utilises the vacuum generated by your intake, to assist in the operation of your brakes.
It is quite a clever device, invented by a Belgian guy called Albert, all the way back in 1927. The way it works is by using a large diaphragm of rubber, and a small air intake valve, to create a pressure differential within the housing. This pressure - caused by the intake vacuum being allowed through the intake valve when you push the brake pedal - will work on the piston in the master cylinder, thus making it both easier for you to push the brake pedal, and also amplifying the amount of force enacted upon the master cylinder's piston. It is uncommon to find a car that's equipped with hydraulic brakes, that doesn't also have a brake booster, though they do exist - because of America.
Common tools used when working on brakes
If you'd like to carry out work on your brake system, the good news it that most of the jobs involved are fairly easy to do. From changing pads or shoes to draining, flushing or replacing the fluid, most jobs only require a handful of different tools, which we'll outline below. For initial disassembly of any brake system component, you'll require a set of sockets and spanners, though often it's a good idea to have some long-nosed pliers on hand to remove any clips and grabby-bits in the caliper assembly, or to put tension on springs that retain components in drum brakes. You'll need open-ended spanners if you need to disconnect brake lines too. A great option that will allow you to use the ratcheting action of your socket driver is crow-foot spanner attachments.
If you are going to be changing your brake fluid then there are a variety of brake bleeding kits that will make this job a breeze - some will use vacuum pumps, and others can be attached to a compressor to do all the work for you. Either way, they are a better option than enlisting a friend to pump the pedal for you while you open and close the bleed valve! More intensive work on your brake calipers may require the use of a piston press tool, or brake caliper spreader, which will aid in forcing sticky pistons back and therefore allow new pads to fit over your calipers. bIf the pistons are particularly sticky, then using a brake cylinder hone to clean up the internal surface of the cylinder will set them right in no time.
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