Braking systems consist of many components and the system as a whole can be described as a safety component in a vehicle. The full braking system will consist of a means for the driver to apply the brakes, the means to transfer that action to the braking face and the components that make up the braking face. By braking face we mean the components or parts which change the kinetic energy into heat energy. In most cars the first component will consist of a brake pedal, the second component will consist of solid and flexible brake lines and the third component will consist of brake calipers or drums and brake pads.
Brake Fluid and safe operating temperatures
|Brake Fluid Type||Dry boiling Point||Wet Boiling Point|
Dry Boiling Point is the temperature at which a brake fluid will boil in it's brand new state with no moisture.
Wet Boiling Point is the temperature at which a brake fluid will boil when saturated with moisture.
Maximum continuous working temperature of brake pads.
|Brake Pad Type||Maximum C.W.T||Friction Classification|
Brakes for different applications.
Consideration should be given to the application of the brakes, for instance a normal road going family saloon car will need different pads to a track used race car. Other applications can be 4x4s that could be used in dusty and wet conditions or a car that is used for performance driving and the occasional track day. Brakes used on trackdays are commonly mis-understood, sometimes people will install pads that they feel will work for 'fast road/occasional track day'. But it doesn't matter how occasional a car goes on a track day, it matters more how hard the pads are being used while on the track. Of course even if a pad is being used lightly on a track it will more than likely wear out faster than if it was only used for normal road use but what really matters is the temperatures that the pad is experiencing.
Back to school - it's all about changing energy, do you remember from science that you can't create or destroy energy you can only change it. So you have a car that weighs a tonne or more and you need to slow that car down from 70mph down to 30mph. To do that you have to find a way of converting that movement or kinetic energy into something else.
The most direct way is to have some form of friction type application on the wheel, but that would damage the wheel very quickly especially if they are alloy. So the next best thing is to have a drum or better still a disc that is attached to the wheel in some way so that if you slow the disc down it slows the wheel down and so slows the car down. The most efficient way of slowing the disc down is to clamp it with friction material or in other words a set of pads.
But you are still adhering to the basic principle of converting kinetic anergy into something else and obviously when you have friction you have heat, so the more energy you have to convert the hotter the pads will get. So either the faster you go or the heavier the car, the more energy you need to convert.
First of all you need to make sure you have enough friction being applied to actually do the job and that is a combination of the radius at which the pads act to give leverage, the size of the calipers(the pistons give the necessary pressure) and the size of the friction area or size of the pad. Therefore for a big heavy car that is capable of high speed you need big diameter discs to allow for a lot of leverage acting against the rotation of the wheel, you need big calipers to be able to apply a large amount of force to create friction between the pad and the disc and of course you need a large pad to give the biggest frictional area.
So a small Nissan Micra with the braking system off a Bentley Continental GT will be standing on it's nose most of the time and conversely the Bentley with the brakes from a Nissan Micra would take far too long to stop. As well as the length of time it would take to stop, the heat generated would overwhelm the braking system and the heat would be transferred into areas that would be undesirable, such as the brake fluid and the wheel bearings, pretty soon brake fade would take effect and it would be very difficult to even slow the car down. Regardless of the car size and braking system there has to be a way of removing the heat from the system so that it does not become overwhelmed and that is usually the air flow passing through and around the system while the car is in motion.
To illustrate the point imagine the Bentley with the Nissan braking system descending a very long hill and the driver decides not to keep the car in a low gear instead he either allows the auto gearbox to simply run or puts it into neutral and keeps a small amount of pressure on the brake pedal. Eventually the braking system will suffer from what is known as brake fade and this is the effect of the system getting too hot to cope with the amount of heat being generated. This can also happen on a race track when the brakes can be very quickly overused for their capabilities.
The next thing to think about is the pad material. Yes they are all friction materials but everything is relative or so Albert Einstein told me. Remember that there is more energy involved in some applications than others. If we have a car on the motorway, let's say a Ford Mondeo, even at high speed when the car comes off the slip road towards the roundabout the driver will expect there to be good braking and if the car designer has done his job properly there will be ample braking available and the car will come to a reasonably quick stop, this is known as 'good cold bite'. The driver then goes about his normal driving and in a few hundred yards he may use his brakes again and because the brakes have had a bit of time to cool down a little bit and because the heat that had been generated is now dispersing around the system the braking will be as efficient as before.
Imagine a different scenario, the car is now on a race track on a track day. The Mondeo will be accelerating down the straight, brake hard into the corner, accelerating down the straight, brake hard into the corner, accelerating down the straight, brake hard into the corner, you get the picture. Very quickly the pads will get very hot, the discs will get very hot and won't be able to transfer enough heat to the surrounding air and soon the heat will need to be dissipated into the fluid and the hub and anything else which is in contact with the pad and disc either directly or indirectly. Because the car has not been designed with track use in mind it is most likely that the heat will not be able to get away fast enough, so 2 major effects will take place, the pad material will have gone past it's working temperature and will start to disintegrate and the brake fluid will start to boil. These 2 things are known as brake fade and as the name suggests the driver will be pushing on the brake pedal as hard as possible but the system will have no capacity left for changing the energy from kinetic to heat and if the fluid has boiled it is quite likely that the brake pedal will go straight to the floor. This is because as the fluid has boiled there will be gas in the system and gas is compressible, hence the pedal going to the floor. Once the fluid has boiled it needs to be changed.
So when deciding on taking a car onto the track 2 things I always advocate, change the pads for a track material pad such as EBC Yellow Stuff and also change the fluid for a higher grade such as Ate Super Blue Racing brake fluid. Doing both of these will help to maintain braking efficiency at the higher temperatures. Some thought should also be given to directing more air towards the vital parts of the system, generally the disc. and this is why vented discs are more efficient than solid discs, they allow air too pass between the surfaces to help the cooling.
When choosing a higher grade pad you must remember that there is generally a contradiction between a pad with good cold bite and a pad with high temperature or track capability, so a lot of research has gone into creating pads that have the best of both worlds and some manufacturers have been able to come up with suitable pads. It's a bit like creating a tyre that has very good grip but also has low roll resistance for fuel economy.
It is worth remembering that momentum and kinetic energy
is quadrupled when you double the speed. So a braking system that is only
just capable of stopping a 1 tonne car from 100kph within 60metres will
only be able to stop the car within 240metres if it was travelling at
200kph. Worth considering when deciding to 'put your foot down'.
Here is the equation. Energy = 0.5 x mass x (Velocity x Velocity)
Consideration should be given to what application the pads will be used for and the properties that will be required of the pad.
1. COLD STOPPING PERFORMANCE. How well does the pad
grip on the first stop when the system is at ambient temperature?
2. HOT STOPPING PERFORMANCE. How the pad reacts in higher temperature such as on the track?
3. PAD LIFE. How long will the pads last in a given driving environment?
4. ROTOR LIFE. How aggressive is the pad on the rotor, will it groove the surface?
5. NOISE. Does the pad squeal?
6. DUST. How much dust does the pad generate, how easy is it to clean?
7. COST. Is the pad affordable compared to how it performs?
Different pads from different manufacturers will have different combinations of the above properties. Here are some of the manufacturers:
Some of these manufacturers are now under the TMD umbrella, Mintex, Pagid, Textar.