The Disc Brake

The Disc Brake

Believe it or not but the disc brake or at least the basic function of a disc brake was developed in the 1890's in the UK but it was over 60 years before the material technology could support this interesting device and make it a truly reliable mass market part. Dunlop made a race-application disc brake for a Jaguar C-Type race car in 1953 and was the precursor to all modern day disc brake systems. It still was not until the 1980's that the disc brake made it from the exotic side to the ubitiquous everyday device that it is today.

In essence it is a heat generating device and the source of energy is the kinetic energy of the moving car. Through a simple process of rubbing friction at the interface of the brake pad and brake disc, movement of the car and thus the rotating disc is slowed significantly as kinetic energy is rapidly converted into heat energy by a simple application of pressure to the brake pads that act on the surface of the disc.

As energy is lost, the resevoir of energy, i.e. kinetic energy(speed x mass) is lowered and since the mass cannot lowered (except for miniscule amounts as brake dust) speed is reduced significantly. The drum brakes also managed this quite reliably but as performance and fade resistance became lmore important considerations as cars power outputs rose over the 100 bhp mark.

Whilst 100 bhp sounds like a laughable figure, back in the 70s and 80s it was quite something to have such "high" outputs. But eventually as even the most mundane of family compacts exceeded this figure, the advantages of the more expensive disc brake system became necessary. And as mass production ramped up, disc brake unit prices fell and is today the defacto standard.

Brakes always have more ability to decelerate the car than it has horsepower to accelerate it. However  what it does not have is a huge capacity to dissipate the heat generated so prolonged brake application will cause temperatures to rise and will overheat the friction material and the brake fluids. The symptoms of which are a soft brake pedal that goes right to the floor and the stopping ability is significantly diminished, this is called brake fade.

Small brakes heat up fast and easily overheat components, larger brakes can soak up more heat before temperatures rise over the material limits. they also offer a larger surface area over which to allow heat to dissipate.

Today one of the biggest selling "mod" items are big brakes. These are only necessary for trackday sessions or any situation where one will repeatedly apply the brakes to full effect. Any OE brake system has the capacity to generate full stops at least a few times before fade sets in. So in daily operation one has no real need for huge brakes as their advantage only comes when heat capacity and stamina is required. However there is no denying the street cred it gives the happy owners.

Some oft heard brake-speak:

Competition Pads: Usually refers to high temperature pads that will be both more effective at high temperatures and also resist the heat well. Used in racing or competition or just trackday sessions. The effectiveness of a high temp pad is lower when cold and thus for daily use needs warming up before it delivers its full effectiveness.

Brake Pad Material: Refers to the composite matrix that develops the friction in the brakes. It has adherent friction material, metallic or ceramic bits and a binder. The formula for normal and competition pads are different.

Six pistion calipers/six-pot brakes: Is a reference to the number of hydraulic chambers used to apply the hydraulic pressure from the master cylinder. The more the pistons the more it can spread the pressure evenly over a large brake pad. However it is also increasingly effective as surface area of all the pistions when added up are greater than the original. This can cause over effectiveness and upsets brake balance.

Monobloc caliper:  Refers to the construction of the caliper itself. Monobloc is a single casting or forging of aluminium alloy into a caliper and many argue that this is the strongest type of caliper as it is made of a single piece. The other camp use a two piece construction and with the latest in metallurgical technology has overcome the problems associated with the inherently weaker two piece construction.

Cross Drilled Rotors: Holes are drilled through the discs in a spiral pattern to allow the hot gasses to escape the pad-disc interface that might cause the pad to be lifted from the disc. It also is said to help ventilation and cooling but has the effect of weakening the rotors when heat cycled severely. Many drilled rotors develop hairline cracks around the drilled holes after severe use.

Slotted Rotors: Similarin  concept to drilled rotors with slots or grooves milled into the rotor's surface but the grooves are typically 3-4mm deep and not through the entire width of the rotor. This supposedly does not weaken the rotors as much but manage to vent the hot gasses that are formed when the pads get red hot. The milled grooves also expose numerous "leading edges" where the pressure builds up and provides slightly more retardation at the cost of more wear.

Floating Rotors: Most rotors are single piece cast items but for high performance where there will be a lot of heat and heat-cycles generated, the continuous expansion and contraction of the rotors distort the entire structure as the cooler center hat area does not expand as much. A solution for this is the independant rotor and hat also called the floating rotor. By attaching the rotor to the hat in a manner that only allows radial expansion reduces the problem. Some use rivets and precision milled joints that allow for radial expansion but no movement during torque transfer.

Dual Cast Rotors: An extremely innovative way to get around the complex floating rotor. Brembo casts the aluminium hat into the cast iron disc and thanks to its greater thermal expansion coefficient, the cooler hat area keeps up with the hotter rotor in terms of expansion.

Floating Caliper: Not to be mistaken for floating rotors. This is the less rigid but more adaptable and less costly disc brake design where the caliper is not rigidly fixed and the piston and pad clamps the rotor by a sliding caliper. Not meant for high performance or racing cars.

Fixed Caliper: This is a more rigid design where the inboard part of the caliper is fixed rigidly to part of the wheel hub. The pistons are arranged to face each other and in this manner take up the slack. Costly and complex but this rigidity is needed in racing. 

Carbon Ceramic Brakes: The search for ever higher temperature tolerance has resulted in ceramics entering brake design. Used in racing when not specifically banned, has been pioneered in high performance cars by Porsche. Now almost any serious car manufacturer offers this type of brake system. It is not significantly more effective in developing friction but its thermal tolerance is huge and it is very lightweight and saves several kilos each wheel compared to steel discs. However it must be used and tuned as a system and cannot be just bolted on as its modulation characteristics at low pressures is unlike the friendlier steel disc variety. If you want smooth stops choose the steel discs. If you are a trackday demon perhaps the CCBs are for you but be prepared to pay. They are hugely expensive.

 

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