Last time, “We looked at Brake Fading", a phenomenon arising out of the brakes getting overheated. If a brake rotor was a single cast chunk of steel, it would have terrible heat dissipation properties and leave nowhere for the vaporised gas to go. Because of this, brake rotors or discs (as some people prefer to call them) are typically modified with all manner of extra design features to help them cool down as quickly as possible as well as dissipate any gas from between the pads and rotors. In the olden vehicles, you may notice that, these rotors are smooth chunks; in such vehicles, the brakes are highly inefficient! Basically, the grooves give more bite and thus more friction as they pass between the brake pads. They also allow gas to vent from between the pads and the rotor. Grooved, drilled rotor - the drilled holes again give more bite, but also allow air currents (eddies) to blow through the brake disc to assist cooling and ventilating gas. Dual ventilated rotors - same as before but now with two rot
ors instead of one, and with vanes in between them to generate a vortex which will cool the rotors even further whilst trying to actually 'suck' any gas away from the pads.
A number of race cars bear drilled rotors, these are typically found (and to be used on) race cars. The drilling weakens the rotors and may result in micro-fractures to the rotor. On race cars this isn't a problem - the brakes are changed after each race or weekend. But on a road car, this can eventually lead to brake rotor failure - not what we would want to occur. I only mention this because of a lot of performance suppliers may supply you with drilled rotors for street cars without mentioning this little fact. Sports cars and race bikes typically have much bigger discs or rotors than the average car. A bigger rotor has more material in it so it can absorb more heat. More material also means a larger surface area for the pads to generate friction with, and better heat dissipation. Larger rotors also put the point of contact with the pads further away from the axle of rotation. This provides a larger mechanical advantage to resist the turning of the rotor itself.
All brakes work by friction; friction causes heat which is part of the kinetic energy conversion process. How they create friction is down to the various designs. Basically, the functioning of brakes is very simple and out-in-the-open. A pair of rubber blocks is attached to a pair of callipers which are pivoted on the frame. When you pull the brake cable, the pads are pressed against the side or inner edge of the bicycle wheel rim. The rubber creates friction, which creates heat, which is the transfer of kinetic energy that slows you down. Like In Bicycles, there are only really two types of brake - those on which each brake shoe shares the same pivot point, and those with two pivot points. The next, more complicated type of brake is a drum brake. The concept here is simple. Two semicircular brake shoes sit inside a spinning drum which is attached to the wheel. When you apply the brakes, the shoes are expanded outwards to press against the inside of the drum. This creates friction, which creates heat, which t
ransfers kinetic energy, which slows you down.
Because the brake shoe pivots at one end, simple geometry means that the entire brake pad cannot contact the brake drum. The leading edge is the term given to the part of the brake pad which does contact the drum, and in the case of a single leading edge system, it's the part of the pad closest to the actuator. The action of the drum spinning actually helps to draw the brake pad outwards because of friction, which causes the brakes to "bite". The trailing edge of the brake shoe makes virtually no contact with the drum at all. This simple geometry explains why it's really difficult to stop a vehicle rolling backwards if it's equipped only with single leading edge drum brakes. As the drum spins backwards, the leading edge of the shoe becomes the trailing edge and thus doesn't bite.