Tech Focus: Brakes Everything you’ve ever wanted to know about brakes but were afraid to ask… We take a closer at performance disc brakes, the way they work, and how and why different upgrades will improve your BMW’s braking abilities… Words: Jamie King. Photography: Michael Whitestone.
It’s fairly well-known to car enthusiasts that when it comes to performance brakes it really is a case of bigger is better. But have you ever thought to ask why? How do bigger brakes offer improved performance? What advantages do they have over standard brakes? And what makes for a performance brake setup compared to a standard road-going system? Well, that’s what we’re going to take a closer look at here…
But before we start looking at the specifics, let’s just take a minute to remind ourselves how a simple disc brake system works. The easiest way to understand it is to think of it as a plate and a clamp – the disc is the plate, and the calliper is the clamp. When you put your foot on the brake pedal, the clamp squeezes onto the plate and the friction created causes the car to slow down. As the car slows, it is essentially converting kinetic energy into thermal energy – or motion into heat.
Heavier vehicles and higher speeds generate higher temperatures. The braking system needs to evacuate that heat in order to avoid fade and deteriorated performance. It really is as simple and straightforward as that.
But there are many variables – such as the materials used, the positioning of the calliper, the size of the disc, and so on – that all have an effect on how a particular brake setup will perform… and it’s those variables we’ll take a closer look at now.
The heavier the components in a brake system, the more heat they can absorb. That heat needs to be managed to keep the system functioning at its best. So while heavier components may allow a vehicle to slow down more quickly, the components need to be able to disperse that heat efficiently, too. In a single stop, it is mainly the disc that handles all of the heat. Repeated stops can lead to the warming of other components in the system, such as the brake pads, calliper, pistons, brake fluid and so on. But for a single stop, it is the mass of the disc that most greatly affects system performance.
A secondary job of the brake disc is to provide a suitable area for the calliper to clamp onto; therefore alterations to the disc’s size and material will have a huge impact on the braking performance. Disc diameter is perhaps the most notable difference between performance brake kits and standard factory equipment. Besides allowing the use of a larger pad that can increase friction and pad life, it’s also a matter of leverage – or more accurately, brake torque.
If you imagine a 10p piece spinning at 1000rpm it will take a lot of clamping force to stop it, because the distance between the centre to the outer edge, and therefore the leverage effect, isn’t that great. Now imagine a dustbin lid also spinning at 1000rpm. The distance between the centre and the outer edge is significantly greater, so too is the leverage effect, and therefore it will take a lot less clamping force to stop it from spinning.
This leverage effect is known as brake torque. As a simple equation torque = force x distance. Therefore if you place a calliper 100mm away from the centre of the disc, it would require twice as much effort to stop the disc spinning as if the same calliper was placed 200mm away from the centre of the disc. So, you can see why performance brake upgrades tend to use much larger discs than the standard factory-fitted items.
The width of the disc also plays a part in terms of brake performance – not in terms of the amount friction produced, but instead by keeping the brakes cool. Heat build up is one the biggest concerns with a performance brake system; excessive amounts of heat can cause all manner of operating issues and in extreme cases can render the brakes almost non-existent, so keeping the temperatures under control is crucial. This is not so much of a concern for normal road use where hard and frequent braking isn’t so common, but if you take your BMW onto a track you will be braking from three-figure speeds three or four times a minute so brake cooling becomes much more of an issue. Most performance discs will be of a vented design, which are basically two friction surfaces separated by series of vents. These vents help dissipate the heat build up caused by the friction of the brakes pads clamping against the brake discs when the brakes are applied. They run from the centre of the disc to the outer edge, acting like an in-built fan pumping cooling air through the disc as it spins. And the wider this gap the greater the volume of air available to dissipate any heat build up.
Iron Alloy Discs
The material the discs are made from has a huge effect on the braking performance. Most aftermarket and performance brake kits use a highgrade iron alloy composite disc. The exact makeup of the composites depends on the manufacturer, and most are closely guarded secrets, but iron alloy is frequently used because it has the ability to resist distortion and cracking, even after repeated heat cycles, and because it is fairly cheap and easy to work with. Despite persistent misinformation, iron discs cannot warp, but that’s a matter for another column. Different composites have slightly different frictional properties, but most are all fairly similar.
There are two main types of disc: one-piece and two-piece. Onepiece discs are, as the name suggests, made from a single piece of material, and are what you’d expect to find on the vast majority of standard road cars.
Two-piece discs, however, are made from separate bells and rotors. The bell, usually made from an aluminium alloy, is the centre part that allows the disc to be bolted to the hub; and the rotor is the iron alloy or carbonceramic ‘ring’ that the calliper clamps on to. This means the overall weight of the disc can be reduced, while special iron alloys or carbon ceramics can be used for their frictional properties on the rotor.
There are several different styles of disc face available. Some are plain, some are grooved, some are cross-drilled, and some are both grooved and cross-drilled. Far from just looking pretty these different face designs all have their unique advantages and disadvantages, and choosing the right one will depend on the application it will be used for.
Plain discs, for example, are commonly used on road cars because of the low noise levels they produce, and because of they are cheaper and easier to mass-produce. However, they used to have issues with clearing gases and debris from the rotor face, so performance road and track cars often swap to a cross-drilled disc design. The little holes machined into the rotor allowed hot gases trapped between the pad and disc face to escape, therefore keeping everything cooler. The holes also help clean the pad of brake dust, ensuring a clean surface between pad and disc. The down side to cross-drilled discs is that by their very nature, machining a hole into the disc weakens it, and if the disc is going to crack you can bet it starts from one of these holes. They’re also quite outdated nowadays – cross-drilled discs were first introduced to be used with asbestosbased brake pads.
As modern brake pads don’t contain any asbestos they tend to work better with grooved discs. These offer better initial ‘bite’ than plain or drilled discs, as the leading edge of the grooves help clean the brake pad of any debris, allowing for a cleaner initial contact between the pad and the disc. There are different styles of groove including straight grooves from the centre to the outside edge, curved grooves which run from the centre of the discs to the outer edge, and a ‘J hook’ design, which looks like little hooks across the disc face. The different styles of groove have different characteristics: some increase initial bite, some have better release qualities, and some affect the overall friction between the disc and the pad. Therefore the best solution is not necessarily the same for each application. Grooved and crossdrilled discs combine the advantages and disadvantages of both styles. They tend to be noisier than the other styles and are mainly chosen for aesthetic reasons and not recommended for vehicles that are going to see any track time.
There are two methods of fixing the bells to the rotors: fixed, or floating. Fixed bells and rotors are when the two parts are simply bolted directly together. This is sometimes referred to as semi-floating setup because there is some allowance for movement between the bell and rotor due to the two parts being constructed from different materials and therefore expanding and contracting with heat at different rates. In this sense a one-piece disc that has the bell and rotor cast as a single piece would be referred to as a true fixed disc.
However, where the bell and rotor of a two-piece disc expand and contract at different rates can fatigue the materials and potentially cause the disc to distort or crack. To prevent this a fully floating disc as found on the E34 and E36 M3s is used. Rather than being directly bolted to one another, floating discs utilise a series of bobbins that allow slight movement between the bell and rotor. This is particularly advantageous in motorsport applications where the brakes reach incredible temperatures but has performance benefits for road use, too, such keeping the rotor perfectly in line with the calliper and pads to ensure the maximum contact area between pad and disc, and therefore maximum braking efficiency at all times.
Carbon Ceramic Discs
The difference in frictional properties of the disc becomes most noticeable when you start looking at alternative materials to iron alloy, such as carbon-ceramic or full carbon discs.
Carbon-ceramic discs as available as optional upgrades on recent M3/4/5/6 models are, as the name suggests, a combination of ceramics and carbon. The ceramic part gives the disc its strength and rigidity, whilst the carbon gives its frictional properties. The advantages of carbon-ceramic discs include: being lightweight – notably lighter than an equivalent size iron alloy disc; longevity – as carbon-ceramic discs wear more slowly than an equivalent iron alloy item; and having the ability to work form cold. All of which make them perfect for fast road use, if not a bit pricey.
Full carbon brake systems are the pinnacle of braking technology. Carbon-ceramic discs may be significantly lighter than iron alloy items, but they are still quite weighty compared to full carbon discs as used in Formula One technology and other top-end motorsport applications. Full carbon brakes (known as carbon/carbon because both the disc and pad are 100 per cent carbon) offer even greater resistance to brake fade, are even lighter, and have even higher frictional properties. However, they are ludicrously expensive, don’t work unless they are already up to temperature, and require specialist wear monitors – as both the disc and pad are carbon there is very little loss in performance with wear and the brakes will work perfectly well until the point that either the pad or disc has completely worn through, at which point you have no brakes at all!
The calliper is the clamp that forces the brake pad onto the disc. The friction this causes is what slows the car down. There are two types of calliper available: either a one-piece monobloc item, or a two-piece split calliper. Monobloc callipers are usually reserved for use in topflight motorsport series where weight is crucial. Typically, these will be made from lightweight material such as billet alloys. They may be incredible light but monobloc callipers are actually more susceptible to flex than the cheaper, easier-to-produce split callipers. Flex in a calliper is usually discouraged because any system compliance caused by flexing will affect performance and feel. However the brake line pressures involved with stopping a lightweight race car are significantly less than they would be with a two-ton road car, so flex within the calliper is less of an issue with race cars, which is where monobloc callipers are most commonly used.
Most performance callipers (and nearly all standard production brake callipers, too) will be of a two-piece design. Many production cars will come fitted with a floating (also known as sliding) type calliper. This system sees the piston(s) on one side of the calliper, which is attached to a sliding mechanism bolted to the hub. Because the calliper isn’t directly fixed to the hub it’s position in relation to the disc can alter, and this is exactly what happens – when the pedal is pressed the piston pushes one pad onto the disc, which causes the calliper to slide across and then pulls the other pad onto the opposing side of the disc face. This type of calliper is fine for most standard road cars, but for performance cars a fixed, multi-piston calliper is preferred. A fixed calliper is easier to picture – the calliper is bolted directly to the hub and has pistons on either side of the disc face. As you press the brake pedal brake fluid is forced into both sides of the calliper (via the bridge pipe you can see joining the two halves of the calliper), which forces the pistons out of the calliper body, which in turn presses the brake pads against the face of the disc. The number of, and size of, these pistons will affect the braking characteristics.
The pistons are the biting force behind the brakes, and are what causes the brake pads to clamp onto the disc. The number of pistons (often referred to as ‘pots’) in a calliper will affect the braking performance, and the exact design of the piston arrangement can get surprisingly complicated, and is often bespoke to a specific application. But do not assume that a given calliper is better than any other merely by piston count. You see, it’s not a simple case of ‘more is better’, as factors such as piston size, piston pressure, and pad wear all need careful consideration.
A larger piston will exert a larger pressure at the pad, but can cause the pad to wear unevenly, whereas a small piston will have less pressure acting on the pad. It’s a balancing act of getting the right number of pistons to control the pad wear as best as possible, and getting the correct size of piston to create the required pressure acting on the pad. Adjusting piston sizes can also greatly affect front to rear brake bias, so balanced systems that maintain the factory bias are preferred. Most four-pot callipers will have pistons of the equal size, but many six- and eight-pot callipers tend to have different size pistons – known as differential bore callipers. In these, the disc will pass a smaller piston first, then a medium size piston(s), and then finally the largest piston. This is to control the pad wear and stop the pad being forced into the calliper body. The size of the pistons depends largely on the weight of the car. As a general rule of thumb a lighter car will use smaller pistons and a heavier car will use bigger pistons.
Brake pads have a huge impact on a car’s braking performance. They are, after all, the component that physically causes the friction used to slow the car. As such, simply replacing the brake pads with a different performance-orientated compound can make notable improvements even when the standard calliper is retained.
The exact blend of a brake pad compound is specific to the manufacturer, and these compounds are closely guarded trade secrets. Different compounds will have different characteristics. For example, some will wear quicker than others, some will work fine when cold, whilst others only work when warm. Some are even designed to produce low amounts of brake dust to save you cleaning your wheels every five minutes! It really is a case of horses for courses, so make sure you choose a compound that suits your needs.
There are two ways in which to mount a calliper to a hub: either lug mounting or radial mounting. Lug mounted callipers are mounted axially onto the hub, which means there is no need for a bracket between the calliper and the hub. However this type of mounting is more susceptible to flex in the calliper under pressure, and means the same calliper can only be used for one specific application – fine for original equipment manufactures if they are massproducing the same design but an expensive exercise for aftermarket upgrades.
Most performance brake kits are radial mounted. An alloy bracket is axially mounted to the hub, and the calliper is then radially mounted to this. Radial mounted callipers are more resistant to calliper flex than lug mounted callipers, and because they usually require a bracket the same calliper design can be used for more than one application. To make the calliper fit a different application you simply fit a different bracket to suit the hub.
While it’s not as exciting as huge callipers, pretty discs, or aggressive compound brake pads, the brake fluid is the lifeline of the entire braking system and shouldn’t be overlooked. Again, the type of fluid you should use will depend on the application. For example, for road use the fluid needs to be resistant to water, not deteriorate over time, have lubricating qualities for any rubber seals, and also be relatively cheap. Also, for road use brake fluid needs to be DOT rated to meet the required standards. Whereas on a race car you won’t care so much about any of that, instead you’ll want a performance fluid that will be capable of handling the enormous temperatures involved – some brake fluids can deal with over 300 degrees Celsius before it boils! As a general guide, a good quality DOT5.1 brake fluid will be ideal for performance road cars and occasional track use.
In the end, upgrading to performance brakes will generally have a positive effect on your car’s stopping performance, but not necessarily in the way you might think. Performance brakes are more about reducing fade, improving feel and allowing better modulation than they are about shorter stopping distances. Upgrading your tyres will have a much more dramatic effect on stopping distances. If you are able to engage your ABS under braking with the stock setup, performance brakes will only reduce the time to do the work to engage ABS. That might result in shorter stopping distances in a single stop, but only a very small difference. Instead, the larger discs, stiffer callipers and more aggressive pads are there to be more precise, to reduce fade by giving protection from heat build-up, to improve the feeling you get through the brake pedal to allow better modulation. After repeated high speed stops, such as at the race track, the performance brake’s ability to manage heat dispersal will mean reduced fade and more consistent stopping performance, giving the driver more confidence. That’s the true advantage that performance brake components and systems provide.