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Re: Disk brake design Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Thu, 16 Jan 2003 13:40:22
In Reply to: Disk brake design, JJ, Wed, 15 Jan 2003 20:24:45 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The pads have a certain surface area (square inches - in^2). The brake piston applies a certain pressure (pounds). Press the pedal, and you get a certain pressure in pounds/in^2.
Increase the pad size, and the pressure per square inch DROPS. Not as much 'squeeze'. To actually increase braking, you need to increase the PRESSURE and the brake swept area.
But wait, there's more. Why not make the rotor smaller? When you stop a car, you're turning that forward momentum (energy) into heat. All that heat goes into the rotor. Make the rotor smaller, and it can't throw off heat as well - it'll get hotter. Bigger radiators throw heat off better than small ones.
OK, so make the rotor bigger. Ooops - the rotor disk has to fit inside the wheel rim, and leave room for the brake caliper.
So, look at the biggest rotor you can fit into the wheel rim. Figure out how hot it can get without warping or frying the brake pads or caliper. Figure out the energy from stopping the car (you know the weight) a few times. That defines how big the brake pads are.
You want more braking? Either find a way to throw away more heat (slotted rotors, vented rotors), make the brake components live at hotter temperatures (ceramic rotors - see Ferrari), or make the rotors bigger. To do that you need bigger rims. You can do that with bigger rims (16", 17") with lower profile tires. All of those changes are possible, and are upgrades.
This is just the tip of the iceberg when it comes to the design tradeoffs. There are Federal requirements on braking and brake holding, including parking brake holding. There are Saab design practices. There are cost and weight trade-offs - the brakes cost more, and the car will cost more to buy, and more to repair. People don't like that. Bigger brakes weigh more, and weight added to the sprung suspension (wheels, A-arms) adversely impacts handling. That's why some performance cars (Alfas, for example) have in-board brakes - the brake pads are at the transmission, not at the wheel. Better handling, but (1) more expensive, and (2) a real pain to service.
And a bunch of other things I either forgot or never knew.
posted by 192.249....
, Thu, 16 Jan 2003 13:40:22 <-- Viewing This Message, Thu, 16 Jan 2003 15:15:40
, Thu, 16 Jan 2003 11:48:10
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