Brake System: Overview
This article looks at one of the most critical parts of your car, from a safety, performance and enjoyment aspect – brakes. We will look at some of the technical aspects of the braking system for road and race cars, and the parts available to increase your braking performance.
The braking system is one of the simplest in the car, but tends to be the source of much confusion. I’m going to explain the brakes from an engineering point of view (stay with me on this….) It might seem complicated but really the best way to understand the system is to understand the energy flow.
The purpose of the brakes is to turn the kinetic energy (motion) of the car into heat energy. It does this by creating a frictional force between the brake pad and disc, which are pressed together by hydraulic pistons in the caliper, ultimately driven by the master cylinder attached to the brake pedal. (Drum brakes are not really worth talking about in detail but they work on the same principle.) The car slows down and the discs and pads get hot, and there is the energy transfer easy to see. This energy is then transferred to the cold air flowing past the brakes through the vanes of the disc.
The total braking force is therefore the RATE at which motion can be converted into heat – the harder you brake, the faster motion energy is turned into heat energy. From the factory, all modern cars produce enough braking force to lock the wheels, and therefore there is little to be gained from increasing this force. The limiting factor in stopping power in a car is always the tyre, not the brakes.
If you were to stop repeatedly however, your brakes might not stop as well as the first couple of times. Most of us have experienced this, and probably know it is referred to as ‘fade’. To understand why, lets think about where that motion energy we have converted into heat energy is – it is in the brake components. If you keep stopping repeatedly, the components don’t have time to cool down and transfer the energy to the surrounding air.
The brake components are very hot and this is now causing the brake pad material to disintegrate and break down, no longer providing as much friction against the disc. The brake fluid is beginning to boil in the calipers, creating bubbles in the hydraulic system. Bubbles of gas (unlike fluid) can be compressed, giving the classic spongy brake pedal and less braking force. The hot brake fluid is now heating the flexible lines running from the hard copper brakelines on the car to the calipers. This makes the rubber softer, and the lines expand more easily. Again the driver experiences a softer, longer pedal and slightly less braking force.
Essentially there are 4 things that we can do to lessen these effects and make the brakes work better stop after stop – increase the cooling (airflow) to the brakes to stop them getting as hot, replace the pads with ones designed to operate at higher temperatures, use racing brake fluid which boils at higher temperatures, and install metal braided lines to prevent the lines expanding under high temperature and pressure.
Increasing the cooling is relatively simple – you can run a brake duct (some air hose to direct cold air toward the brakes). Race cars sometimes use fancy ducts to take the air straight to the vanes in the disc. A simple hose taking air from the front of the car and channeling it towards the brakes is fine. Most cars have a backing plate to reduce brake dust and reduce the effect of dirt and foreign objects getting stuck in the brakes – this can be removed or deformed to allow more air to the disc. Also the wheels the car runs have an enormous effect on brake cooling – a larger more open wheel will generally allow more airflow than a small, closed design.
Racing Brake Fluid tends to be Dot 4 specification, and generally has a dry boiling point of 300 degC or more, compared to 200 degC for a road fluid. Dry boiling point refers to the fact that brake fluid likes to absorb water from the atmosphere (hygroscopic) – the more exposure the fluid has to air (the older it is) the lower the boiling point will be. The wet boiling point is the boiling point after some water has been absorbed (3.7% water by volume to be exact) and is always much lower. Its worth remembering that the wet boiling point is not the minimum boiling point – brake fluid can get to more than 3.7% water by volume and boiling points will plummet!
The moral of the story here is that high temperature Dot 4 racing brake fluid will boil at higher temperatures but only if it is fresh. To make sure your fluid is in top condition, bleed fresh fluid all the way to the calipers every 6 months. In the Civic we bleed fluid after every single on track session, to remove any bubbles (you can still get a little boiling even with good fluid!) and to ensure fresh fluid with the highest possible boiling point is in the caliper. The reason manufacturers don’t use racing brake fluid from the factory is that it is expensive (£30/litre as opposed to £10/litre for standard) and it is much more aggressively hygroscopic – this means it deteriorates much more over time as opposed to normal fluid, and OEMs would rather fit a consistent fluid, especially as few people ever use their brakes enough to need a higher temp fluid.
A final word on fluid – the DOT ratings are confusing, and higher number does not mean ‘better’. The fluids are of completely different composition and often can’t be mixed. Racing fluid is almost always DOT 4 – make sure you know what your car has already and what is compatible. Google is your friend.
Brakelines are fairly simple – if you go on track often, a set of braided lines will give you better pedal feel as the brakes get hot. Buy quality lines from a reputable manufacturer. Why aren’t they fitted as standard? Cost, and also they need inspecting. The factory rubber lines are extremely durable to the elements, whereas a braided line can be nicked by stones and foreign objects. It is not an issue, and checking once a month or so and keeping an eye on your reservoir level will be perfectly safe, but OEMs can’t trust all their customers to keep on top of these things! All race cars generally run braided lines, and some single seaters (and our Civic) actually run braided flexible lines all the way from the master cylinder to the corners because it is more convenient than hardlines.
Brake Pads are a more complex topic. It is very important to understand the concept of temperature operating windows when selecting which pads to use, as the wrong type fitted can be plain dangerous.
OEM pads are designed to work well from stone cold. That means if it is -5 degC outside, and you have been on the motorway for an hour without touching your brake pedal, your brakes pads will still bite hard. Unfortunately this excellent cold performance requires a certain type of construction and material, which doesn’t work so well when really hot, such as after repeated stops. On a road car its obvious what is more important – most of your braking is from cold.
Now lets go for the same scenario but with a race spec pad. Your brake components are stone cold on the motorway, and you hit the pedal. Unfortunately these pads don’t generate any significant friction under 200 degC. You have crashed before the pads reach half that. Trust me when I say racing pads are not for a daily driven car – its genuinely very dangerous and until you experience how little these pads work when cold you wouldn’t believe it. They need repeated hard stops to get them up to temperature.
On track however this is fine – cold performance doesn’t matter, the hot performance is more important. When fitting these to a track car don’t forget that when you pull out the pits they won’t really work for a lap or two (Accidents happen on track too), but once they are up to temperature you can enjoy the fact the pads will keep working right up through the temperature range.
There is a final option with pads, which is the road/trackday pad. These have a temperature window somewhere between a race and a road pad. They can brake cold safely, but also work to higher temperatures. These are great for a daily driven track car, but its worth remembering an OEM pad will almost always be better on the road, and a race pad will almost always perform better on the track. These pads still require a bit of respect, and in the winter motorway scenario still perform noticeably worse than a road pad.
So what about ABS and proportioning? Modern 4 channel ABS is very good, and worth leaving in a track car unless you really want to remove it. In a road car it is a necessary safety net. The ABS system usually takes care of some of the proportioning (front to rear balance) also, so an ABS light being on may mean you are more liable to lock your rear wheels and spin. If you remove it you may need to install another method of reducing rear line pressure. Leave ABS alone unless you really know what you are doing.
And big brake kits? Bigger brakes take more energy to get hot, and they transfer energy out better as they have more surface area and generally a FAR thicker disc to vent air through. A high quality rigid opposed piston caliper and floating disc reduces brake drag and can give better pedal feel. More pistons (‘pots’) generally spreads the load better on the pad, but lots of pistons give a long pedal in the car. More than 6 is generally excessive – most good quality brake kits for normal sized cars are based on a 4 or 6 pot caliper. Beware bad quality kits, beware super expensive replacement discs and pads, and beware that Civic Cup get on just fine with fluid, pads and lines on OEM brakes.
As usual the message is the same – racing components are for racing cars, and OEM components are for road cars. Don’t fall into the trap!
Much more to say on brakes in future articles – we will explain rotors, grooves, discs, carbon ceramics, pedal boxes, split circuits and brake bias! Let us know what you want to hear about first.
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