The Basics of Brakes put simply – BRAKES MAKE HEAT!!
To accelerate your car, you burn fuel and the heat energy accelerates the mass of the vehicle. Brakes reverse that process. By applying your brakes, you are converting kinetic energy into heat, pure and simple, a straight thermodynamic conversion. The amount of heat to be absorbed (and therefore dissipated) by the brakes is a function of the velocity of the car, the amount it weighs and the rate of deceleration.
Acceleration/Deceleration are mathematically “square functions”, meaning to stop twice as hard takes four times as much effort, generates four times as much heat and wears out the brakes four times as quickly. Stopping four times as hard requires 16 times and so on.
It therefore follows that a big heavy vehicle stopping quickly from high speed requires large heat absorbing capability and the ability to dissipate that heat to the atmosphere.
Brake Pads and Disc Rotors make the heat that stops the car
Brake Pads are manufactured from fibres, metals and binders chosen for their friction properties. Essentially, all heat the pads generate has to be passed into the disc rotor. The pad itself has to be able to handle the extreme heat generated but it is not expected to do much towards dissipating the heat, as its all done by the disc rotor.
A larger pad surface area is required for reducing the specific heat load; a very small pad would become extremely hot, even if running against a large rotor. It therefore follows that the key to stopping a heavy car is good ability to absorb and then dissipate heat before being called upon to do it all again. If the system is not sufficiently cooled, fade will occur. The type and quality of the cast iron, the design of the internal cooling vents and the surface friction characteristics of the rotor all have an influence.
Calipers make the forces to push the pads onto the disc. Hydraulic pressure from the master cylinder is converted to force on the pistons and act on the pads to apply friction to the rotors.
Replacing a single piston caliper with a four small piston with the same size pads will have only a small effect. However when fitting a larger rotor, you can get the best from them by having a larger pad; but applying a greater force via a single piston can cause the pad to bend in the middle. Therefore the recent trend to twin piston/four piston calipers which allow greater force and more even loads on the pad.
Master Cylinders Make Pressure
To apply a hydraulic fluid pressure to the brake calipers, we use a master cylinder, in essence small pistons connected to your brake pedal which generates fluid “pressure” according to the force applied to it and the area of the piston by the formula.
Pressure = Force
Therefore a greater pressure is generated by a smaller piston. However there is a lot of elasticity in a brake system and so master cylinder piston diameters are sized according to pedal travel constraints as much as pressure requirements.
A small piston may give nice high pressure and thus reduced efforts, but at the expense of excessive pedal travel. Master cylinder bore sizes are always a balancing game between fluid pressures and pedal travel. Everything has to be balanced including front to rear.
Pedal Ratios and Brake Pedal Leverage
Brake Pedal leverage ratios affect pedal effort, the greater ratio giving the greater force on the master cylinder, but once again the trade off is increased pedal travel. Ratios range from 5-1 up to 7-1. Physical size as well as ratio does come into it. The free play in every system means that the push rod must move a small distance before the brakes begin to apply, therefore a reasonable length from push rod to pivot must be used. A very short pedal length with a good ratio of 5-1 will still move a long way before taking up the slack. In the end, you have to balance getting the pedal position comfortable, the ratios and sizes correct, and still fit the assembly to the car. Car companies rarely make mistakes so if not sure, see how they do it.
Brake Systems for Modified cars
Whenever fitting larger brakes, you have to balance master cylinder, caliper and wheel cylinder hydraulic sizes. Small cars usually have small bore master cylinders (Eg. 13/16-inch and 7/8-inch etc.) and matching small bore calipers and wheel cylinders. Larger cars are usually 1-inch to 1.1/8-inch. Fitting big calipers and rotors to your small car will necessitate fitting a matched bigger bore master and maybe booster, otherwise the car will actually stop with less pedal effort but will have excessive pedal travel. Once again, a good rule of thumb is to see what the car companies do and use matched sets, master, calipers etc. all off the same model or at least of the same bore sizes.
Mixing and Matching Parts
Modified cars are often concoctions of different sources of parts, with one brand of master cylinder with a different brand of booster and so on. Sometimes they appear to bolt straight together but looks can be deceiving.
You must ensure that the output push rod at the booster is correctly set to the master cylinder so that there is no free play, but also that the seals within the master are fully returned past their ports so that the brakes will release. The push rod may need lengthening or shortening to suit. As little as 1mm will make a big difference to initial pedal take up travel. The booster input shaft should not be preloaded but should have minimum free play too. On disc brakes, you can check that the master cylinder is not over adjusted by opening a brake bleeder. It should drip under gravity if all the ports are unobstructed. If not, the brakes will drag, get hot and lock on.
Brake Vacuum Boosters act by using atmospheric air to assist the force on the master cylinder. When not in use, both sides of the diaphragm are under vacuum from the engine. When applied, the central valve closes and the valve closest to the fire wall opens allowing a shot of air in to push on the diaphragm.
Generally, and particularly on heavy cars, disc brakes will require boosting or pedal pressures will be too high. Lighter cars or older types with high friction asbestos type pads were used but with still high pedal efforts.
Drum brakes are often self energising, that is the rotation of the drum acts to pull the shoes on harder. Most modern cars with rear drums are boosted to all four wheels, however if using a single VH40 remote booster, you can compensate for not having a rear booster by fitting larger rear wheel cylinder sizes (Eg. go from 7/8 to 1.1/8 bore for higher forces on the rear shoes), and so on.
Conversely, if your car suffers from rear brake lock up, you can consider smaller bore wheel cylinders. When mixing large rear/small front tyres, the same applies. If space constraints mean remote boosters are needed and you have four wheel disc brakes, then two remote boosters will be needed with the accompanying cost and piping.
ADR 7 requires that automotive brake hoses be fully fatigue tested, manufactured in a way that they cannot be taken apart and pressure tested to 3000 PSI (20,000 KPA). Amongst other things, stainless braided hoses swell less under pressure, particularly when hot, giving better pedal feel, but the types which are assembled at home and not pressure tested are not street legal.
Most automotive brake piping is 3/16-inch O/D or ¼-inch O/D plated steel seamless tube with doubles flares or ball flares (drill point seats). Under no circumstances make automotive brake lines with singles flares. Copper lines are unsuitable due to fatigue hardening. All approved automotive brake fluid connections are based on positive compression seals, flares, flat copper sealing washers and lately, positive o-ring quick connect seals. No new car was ever produced with a taper thread fitting (ie. needing sealing compound or Teflon tape). Some aftermarket calipers are sold with taper thread connections and their acceptability with ADR’s is therefore debatable. Check every connection for brake fluid leaks before driving the car.
A well-designed brake system should always lock the front first. If the rear wheels lock, the car will spin around. The purpose of rear proportioning valves is to ensure that line pressure cannot rise to the point of rear lock up. Up to that point, front and rear line pressures are usually the same. A proportioning valve is not a band-aid for a badly balanced system.