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, Fri, 17 Oct 2003 17:36:27
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I would wager that you could talk to a lot of Saab mechanics and not find one who knows squat about heat transfer.
So here goes. Then the brake are applied, the duration is usually short, just a few seconds. The kinetic energy of the vehicle is being turned into heat at the interface of the pads and iron rotors. It does not matter if you stop gently or very quickly, the amount of heat generated at the pad-rotor interfaces is almost invarient. A faster stop, with ABS and EBD going will move more of the heat generation to the front rotors, but thats not material to the point.
So we have all of this heat generated at the interface of a pad and a turning rotor. The iron is very conductive compared to the air and very much more conductive than the pads. The pads are actually a pretty good insulator, and it would be hard to find another material that was a better insulator which could also take the heat and pressures.
So where is the heat going to go? The iron is vastly a better conductor of head than the pad. And the turning rotor ditributes the heat evenly around the rotor. The rotor is heavy and is a large heat sink. So most of the heat goes into the iron and very little into the pad. The ratio of the iron swept area to the pad area is around 10:1 and this makes the iron 'appear' to be about 10 times better of a conductor than the conductivity of cast iron alone would suggest. So the pad is getting very little of the heat in normal braking service. And if one pad is a bit more conductive than another, then perhaps that pad gets a bit more heat than the other, but this is an incremental change to a small fraction. The bottom line... differences in the conductivity of one pad vs another will have very little effect on the thermal response of the iron rotor. In other words, no effect at all.
The heat dissipation from the braking event is first conductive into the heat sink of the mass of the iron rotor. After the braking event is typically over, the bulk of the heat is then conducted to the ambient air by forced convection. The rotor temperatures are low enough that radiation heat transfer contributes very little and this can be ignored for most purposes.
Some racing rotors are light weight composites and are designed to get very hot. They are not heavy and are poor conductors. You can see these rotors get 'white' hot very quickly. These brakes are disipating heat by radiation heat transfer. With repeated braking, iron rotors would not be able to disipate heat sufficiently by forced convection.
posted by 66.139.12...
, Fri, 17 Oct 2003 17:36:27
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