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Originally Posted by UncleBob
very well said johan, although I would argue about the F1 cars. They use large diameter rotors more for the cooling capacity advantage.....well, we could argue which is more crucial. Both benefits are very important. More leverage and more surface to cool off the brakes.
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When it comes to thermal properties of the brake, there isn't any real benefit of making discs with a larger diameter. Instead there are two factors that will determine the thermal properties of the brakes.
First we have heat capacity. This is related to the material used, and the mass of the brake rotor. Increase the heat capacity of the material and/or the mass of the rotor and we can store more energy without overheating. Naturally, we also want a material that allows the use of high temperatures without a reduction in friction coefficient.
The second factor is cooling of the disc. This is generally related to airflow in the cooling passages of the disc, the airflow over the disc surface and any use of water as a coolant, either in the caliper or as sprayed on the rotor. In a racing car like a F1 car, air is fed from an intake to the center of the disc and out through internal passages. There can also be "nozzles" directing air against the surface of the disc. To provide good cooling the wheel must also be open enough for the air to flow out. Cars fitted with fairings (the wheels covered up on the outside), as some group C cars duing the eighties and early nineties, this was often a cause for brake related heat problems.
When the brakes are used, kinetic energy is converted to heat energy temporarily stored in the disc. Some of it is cooled when the discs are braking, but most of the heat is actually tranfered from the disc between their use.
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Originally Posted by JustSayGo
The reason disc brakes require more force is because they do not self energize like shoes are designed to. That is why caliper pistons are so much larger than wheel cylinders.
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A brake can't self energize, that is a physical impossebility. There are really only two options availible; either the brake system generate a larger force at the cost of distance (for example making pedal travel longer) or we add energy from outside the system using a vacuum brake booster, mechanical springs or whatever that can be a source of energy. The brakes will NOT boost themselves.