Disc Brakes

another article adapted from an article that I don't remember where I got it from!)

Without a doubt the most important point to get sorted at the outset is just what makes brakes do their thing - FRICTION. The sole purpose of the brake set-up on any vehicle is to convert kinetic energy into thermal energy. In English that’s motion into heat - friction. The motion of the disc/drum across the surface of the pad/shoe produces a prodigious amount of heat. The contact of these components and the heat generated creates friction - making continued motion very difficult. Try gently applying hand pressure to a drill chuck when it’s slowing down. Apart from speeding the slowing down process, you’ll notice your hand getting considerably warmer - friction. Excessive heat can be counter-productive. The correct operating temperatures are a relatively fine line so this also needs consideration.

The amount of friction developed is dependent on a collection of components, but ultimately occurs at the disc/drum end of things, so the rest of the system and components should be built up around that. 

Swept area
This is the total surface area of the disc that the pad operates on, so generally the bigger diameter the disc, the bigger the pad area can be, the more friction can be generated. Bit like the bigger a box full of something is, the harder it is to push it along. There are limits though - the wheel size and type will dictate what will fit. 

The main limitations center around the ability of the brakes to over come the tire grip and vice versa. The first tends to lock the wheels up, vastly reducing the grip on the tire on the pavement - therefore also severely reducing retardation. The second develops excessive heat, building up to a point where the pads and discs become ineffective - otherwise known as ‘fade’. Again retardation is greatly reduced.

Driver input
Applying ones foot to the pedal is the start of the retardation process. This component is very variable, and sometimes each vehicle sees a number of different ones. The human body being the amazing piece
of machinery that it is, allows each to have a totally variable input independent of the sum of the other components, combining thought with feel - i.e. ‘I want to slow down quicker’ so the pedal is pressed harder. That sort of thing.

The only direct contact the driver has with the braking system is the pedal. The pedal has a lever ratio all of it’s own, being calculated by the difference in length of the pedal to the pivot pin, and from there to the master cylinder cotter pin. Increasing the length between the pedal and pivot pin will give more pedal travel, but reduce the effort needed to apply the brakes. Reducing it has the opposite effect. Therefore an increase in ratio gives a softer pedal with more ‘feel’, reducing it the opposite. Changing this particular component is a bit involved though, and is generally unnecessary as it suits most combinations of the other components.

The master cylinder translates the pedal movement into fluid movement. The bore size dictates how much fluid is moved for any given pedal pressure, and is directly related to the main brake component - the caliper. Bigger diameter or multiple pistons will need more fluid displacement to make them work properly. A smaller bore means higher line pressure for any given pedal pressure - less driver input for same braking effort. Conversely a larger bore master cylinder means lower line pressure for a given peddle pressure - more pressure for the same braking effort

'Feel', incidentally, is the sensation of what the brakes are up to, and is an important part of our thought process as to how much pressure we need to apply to the pedal in any given situation.

When trying to get the best master cylinder bore size for your application, you need to remember that the amount of hydraulic pressure produced at the pedal is INVERSELY proportional to the master cylinder bore. So if you are locking the brakes up too easily, you need to INCREASE the bore size. Consequently if you are standing on the pedal, pulling of the steering wheel and gritting your teeth together to lock the brakes, a smaller bore is the order of the day.

Disc Mods 
Improving the performance of the disc itself has seen three types of modification - venting, slotting and drilling.

Vented discs have become pretty much a standard item on modern cars as a more efficient disc temperature wise can be fitted into a smaller area. This contradicts the 'bigger is better' principle, but modern technology has seen improvements in pad materials, so small cars that are fairly heavy can have good brakes without going to huge wheels to fit them in. A smaller vented disc does have slight advantages over a bigger solid disc in the effects of inertia stakes.

Slotting discs has been pretty much misunderstood by many. It is generally believed that the slots are there to improve cooling. They are not. They are there to wipe the pad surface. In operation, the heat creates debris and gases between the disc and pad surfaces - reducing their effectiveness. The slots clear this away. To be totally effective though they need regular cleaning as the debris fills the slots up. Now, it has become fashionable to have loads and loads of slots in discs. Bear in mind that friction area is needed to make the brakes effective - lots of slots markedly reduce the surface area of the disc and thus the available
friction area...

Drilling discs is open to the same misunderstanding that slotting is. The same actual reasons apply, except that holes are more effective over time as they are more or less self-cleaning. The only major draw back (apart from going mental on the number of holes - friction area reduction again) is that in discs with insufficient mass - too small in diameter or too thin - they tend to crack and fall apart.