What is a disc brake? PH Explains
The disc brake has become an almost ubiquitous component of the modern car. There's a good reason for that...
You'll find disc brakes on the front axle of all modern cars, while many more expensive or higher-performance cars will have disc brakes fitted on the rear axle as well.
How does a disc brake work?
There are three main components in a simple disc brake assembly. Firstly, the brake disc itself - which sits between the wheel and the wheel hub. The other two key components are the brake caliper, which contains hydraulically actuated pistons, and the brake pads.
The brake caliper is mounted to the car's axle housing, suspension upright or trailing arm, and the brake pads sit on either side of the disc within the caliper. When you press on the brake pedal, hydraulic fluid in the brake system is pressurised and pushes the pistons out of the caliper. This presses the brake pads, which are metal plates covered with friction material on the disc side, against the surface of the disc. This generates friction and slows the car down.
Cooling is provided by the ambient air flowing over the disc brake's components, which helps prevent them from overheating. When the brake overheats, usually after several hard stops or after a long hill descent while braking, its stopping power reduces - a condition known as 'brake fade'. Many disc brake set-ups feature ventilated discs and air ducts in order to improve cooling and reduce the chance of brake fade.
Are all disc brake set-ups the same?
There are two key elements that vary from disc brake to disc brake. Many cars, for example, use a 'floating' caliper. These feature a caliper support, bolted to the suspension or axle, which locates the caliper over the disc. The caliper has one piston and 'floats' on pins and bushings, which allows both brake pads to evenly the clamp the disc when the brake is applied. Sliding calipers are similar in design but the caliper body rides in machined slots in the support instead of on pins.
Otherwise, the brake system will rely on fixed calipers that are rigidly bolted to their attachment point. These fixed calipers typically have two pistons, one for each side, but can feature more. Increasing the number of pistons grants several benefits, such as permitting bigger brake pads to be used.
You may also encounter high-performance 'floating' brake discs. These have a separate brake rotor, clamped by the pads, which bolts to a 'hat' or 'bell' that then attaches to the hub. This arrangement allows the disc to expand easily when it gets hot, reducing the chance of warping or cracking.
The discs themselves are made from cast iron in most applications but some high-performance set-ups may use carbon-ceramic discs. These are lighter, more durable and resistant to corrosion. They are, however, far more expensive.
What are inboard disc brakes?
This refers to a braking set-up where the disc brakes are mounted inside the car's chassis or body, instead of behind the wheels. These offer several benefits, including being easier to package and improved ride and handling - because the brakes are no longer bolted to and part of the suspension system, reducing unsprung mass.
Inboard disc brakes can be far harder to service, however, and are typically only found in classic racing or road car applications.
Are disc brakes better than drum brakes?
Drum brakes rely on moving brake shoes into contact with the interior of a rotating drum to slow a wheel, unlike a disc brake that clamps pads to a rotating disc.
While drum brakes can provide plenty of stopping power, they are prone to overheating quickly during heavy use. A disc brake, which is not enclosed like a drum brake, is far better at dissipating the heat generated during braking.
Disc brakes are also far easier to service and require little maintenance - whereas many drum brakes can require intermittent adjustment to deliver proper function.
A brief history of disc brakes
The concept of disc brakes has been around for a long time, with British manufacturer Lanchester trialling a system in 1902. Problems with materials led that particular concept to go no further, however.
As cars got faster, manufacturers began seeking improved and more reliable stopping power. Consequently, in the late 1940s and early 1950s, disc brake systems began being developed and pressed into action - most prominently in Jaguar's C-Type race car, from 1952 onwards.
The first mass-produced car with disc brakes arrived not long after, in the form of the 1955 Citroen DS.
This refers to a braking set-up where the disc brakes are mounted inside the car's chassis or body, instead of behind the wheels.
It's not necessarily wrong. It's just terribly written. The person that wrote it might have known what they were talking about, or they might not have, it's hard to tell, but they certainly haven't explained it very well at all.
So why were drums fitted to passenger cars for so long, and why were Ford still fitting them as recently as the last decade?
So why were drums fitted to passenger cars for so long, and why were Ford still fitting them as recently as the last decade?
They are still standard fit on a number of new cars now, on the rear axle at least - IIRC the latest base-spec VW Polo is one such example, and that's not even a particularly cheap car.
So why were drums fitted to passenger cars for so long, and why were Ford still fitting them as recently as the last decade?
Drums can also very easily be made 'self-servoing', where the friction lining of one or both of the shoes is 'grabbed' by the spinning drum as they come into contact and that energy forces the shoe harder against the drum, and so on. This is what produces the distinct 'biting' feel of twin-leading-shoe drum brakes on a classic car - press the pedal, nothing happens for a moment then you get braking force which runs ahead of the pedal movement. Not particularly progressive or controllable but a decent way of achieving extra stopping power.
A lot of the early disc brakes were pretty woeful - the much-vaunted 4-disc system on the Jaguar XK150 was little better than the older all-drum system, that on the Jag Mk2 was downright lousy and the original front discs on the Mini Cooper just fried themselves because they were too small. Without the self-servoing effect of drums many drivers found early discs needed to be worked very hard. The Citroen DS was the exception because it's high-pressure centralised hydraulic system was essentially one big servo, giving huge stopping power with a single squeeze of the brake button.
As for why they're used today - cheap, works well with mechancial handbrakes, and the stopping power of discs aren't needed on the back wheels of a low-performance hatchback. Not so much a problem these days but plenty of early all-disc cars had problems with the rear discs rusting up because they never worked hard enough.
Drums can also very easily be made 'self-servoing', where the friction lining of one or both of the shoes is 'grabbed' by the spinning drum as they come into contact and that energy forces the shoe harder against the drum, and so on. This is what produces the distinct 'biting' feel of twin-leading-shoe drum brakes on a classic car - press the pedal, nothing happens for a moment then you get braking force which runs ahead of the pedal movement. Not particularly progressive or controllable but a decent way of achieving extra stopping power.
A lot of the early disc brakes were pretty woeful - the much-vaunted 4-disc system on the Jaguar XK150 was little better than the older all-drum system, that on the Jag Mk2 was downright lousy and the original front discs on the Mini Cooper just fried themselves because they were too small. Without the self-servoing effect of drums many drivers found early discs needed to be worked very hard. The Citroen DS was the exception because it's high-pressure centralised hydraulic system was essentially one big servo, giving huge stopping power with a single squeeze of the brake button.
As for why they're used today - cheap, works well with mechancial handbrakes, and the stopping power of discs aren't needed on the back wheels of a low-performance hatchback. Not so much a problem these days but plenty of early all-disc cars had problems with the rear discs rusting up because they never worked hard enough.
The servo effect of twin (or 4) leading shoe front drums is very pronounced on a motorcycle.
Usual disclaimers - not an automotive engineer and E&OE...
How do brakes, er, "brake"
Braking is all about turning kinetic energy into heat.The way brake pad material interacts with the disc is complicated and is worthy of a scientific branch all of it's own similar to that of tire dynamics.
However it is useful to look at the mechanics as three phases:
Cold
When pads and discs are cold the only way a brake can generate friction is by good 'ole fashioned surface to surface contact. Microscopic imperfections of both the pad and disc interlock as pressure is applied and generate a useful amount of friction and heat. As there are now two metallic based surfaces in direct contact you can also get squealing noises. This is especially apparent with "Sporty" braking systems containing more elaborate compounds (usually have something ceramic).
Whilst slowing the car down this isn't most effective or efficient use of the braking system. Things get more interesting once conventional surface friction generates some meaningful heat into the system.
Hot
With the brakes now hot, the material in the pads starts to become more "elastic"... to the point where small amounts of the pad material will jump to the disc surface. This reaction also generates heat which in turn can dramatically improve braking performance. The compounds in the pads can also stick to each other far easier than other materials further increasing the maximum friction available.
At this stage, the brakes will feel alive as the layer where the pad material is transferring creates a surface that responds well to pedal input whilst providing solid feedback to the driver.
One advantage of this process is that when the braking force is removed, pad material remains on the disc surface ready for the next brake application. With pad material pre-loaded on to the discs it takes considerably less time to return to operating conditions even if the brakes have cooled somewhat.
However, if the brakes are cold and then used overly aggressive, the direct surface contact can scrub this layer off and the whole process needs to start again.
Overheated
As the braking process is achieved through more of a chemical reaction than anything else, it is very sensitive to the operating temperatures encountered. Continual high speed application of the brakes can overwhelm the system's ability to expel the heat until a threshold point is reached causing the reaction to perform badly. Aside from the common and obvious side effect of boiling the brake fluid, high heat levels stop the pad material transferring properly and brake down the bonds to the point where the material no longer provides a good, consistent layer between disc and pad.
Brake sizing
With the pads interacting with the disc, the brakes use the wheels contact with the road to keep the disc turning generating heat, slowing the car.So how much force can the brakes actually apply? It's a common misconception to say that "my brakes can lock the wheels at any speed so they are up to the job".
Pretty much any modern disc brake system should be able to grab and hold a wheel from turning - that is relatively easy. However if the disc isn't turning then the brakes are contribution a big fat zero to slowing the car. All the slowing down is achieved by the locked tires sliding over the road surface.
It varies between tire types, manufacturers, compounds but most road tires give peak traction when they slide a small amount (between 5%-15%) over the road. So for braking, the ideal is for the brakes to keep the wheel speed slightly slower than the vehicles' road speed. Operating at this edge of the envelope is what puts a large amount of heat into the brake system and can overwhelm undersized setups.
The pad material process discussed above works well based on two factors - one already covered is heat however another is pressure. It is possible to have the brake system press so hard on the pads that they effectively push through the material transfer layer and return to contact friction. Too little pressure doesn't get sufficient uniform contact between the pads and disc. As with most things there's a Goldilocks zone that's just right.
Brake rotors are essentially a simple lever - the bigger the disc, the further from the centre the pads operate then more force can be applied to the axle for a given amount of force. So being simplistic, if a disc is scaled up by 10% in diameter then the same amount of braking force applied to the disc by the pads will give about 10% more braking effort to the wheel. Similarly, the extra size of the disc can also mean the same braking force can be obtained with less pad pressure. By carefully manipulating the disc size the required level of braking can be achieved with the optimum pad pressure and size.
This is a problem for fast road cars however. As a mechanical system this careful consideration of component sizings only work for a specified and quite narrow range. If the car is on track it'll work flawlessly but when trapped in stop start commuter traffic it'll never go anywhere near it's optimum zone.
In these cases it is actually very easy to cause damage to the discs by under using them which would require replacement or re-skimming the rotor surface. It feels contrary to common sense that not using the brakes hard causes more wear and damage than using them in a spirited manner.
Consider a reasonably heavy, fast saloon car with a large brake package - VXR8 or RS4 for example.
The brakes fitted are monstrous in size - 365mm on the VXR8 and similar to what you would find on a Lamborghini.
From the above, this allows a large amount of braking force to be applied with a firm, consistent pressure on the pads... But what if you don't need threshold braking emergency stops? Then the sheer lever size of the rotor only requires a tiny amount of pressure between pad and disc to get normal day-to-day braking effort.
At a microscopic level, the disc is full of peaks and valleys and insufficient pad pressure results in only the peaks being in contact with the pad. The peaks starts to heat up and enters the pad material transfer process but the valleys do not. The peaks collect more and more pad material making them even more proud of the disc. The situation gets worse and worse until the effect can be felt through the brake pedal as vibration under braking with the assumption that the disc has warped.
For cars with big impressive brakes, a good rule of thumb is that if you don't need to slow down more than what you can achieve by downshifting then you don't need to use the brakes. If you do need to use them then don't feather or baby them.
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