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Old 09-17-2005, 05:09 PM   #84
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Join Date: Apr 2003
Location: Maplewood, NJ
Posts: 446
My Ride: 2001 540i 6-Speed
Originally Posted by rnitti
Hawh HPS pads are NOT designed for track use. The HP are.
HP Plus pads are not appropriate for the race track. They are ideal autocross pads with incredible cold bite - better than almost any other street pad. Their tendency to squeal under light braking and their moderately aggressive rotor wear and dust prevents many people from running them all the time on the street. But you should not take them to the track. The problem with HP Plus is not a tendency to fade. Their MOT is higher than HPS and most other street pads. However, once you reach MOT, they fade almost completely and without warning. If you look at the friction vs temperature curve of these pads, they seem to fall off a cliff when you reach MOT. That's not what you want on the track. You want a pad that gradually loses friction after reaching MOT, giving you enough warning to manage your brakes. With HP Plus, you could have fantastic brakes for laps 1-4 and then have nothing in lap 5. No warning. You're off the track, hopefully into an area with lots of runout. I've had at least four customers report to me that this is exactly what happened to them when running HP Plus. And while at a brake friction seminar last July, taught by Rob Nelson - former President of Hawk's friction division, he told us to NEVER let our customers run HPS or HP Plus at the track for this very reason. If the president of the company that makes the pads tells me they aren't appropriate for the track, I tend to believe him. And his information was corroborated by my customers.
Slotted rotors are nice. They will help disapate gasses between the pad and rotor which does decrease stopping distance, but you're only talking a few feet at 60mph. Slotting also helps to disapate water off of the disc in the rain. Drilled rotors are more for looks than real benefit IMO. They are also more prone to cracking.
Slotted rotors offer improved initial bite over plain rotors. Drilled can be shown on a brake dyno to offer slightly more bite, but it's barely noticeable by a human. Drilled rotors also shave off 1/4 to 1/2 pound of weight per rotor, which can make a difference in a close race. The cracking problem is limited to track use. For track, I strongly suggest slotted rotors. For the street, it's up to you. Unless you have counterfeit Brembo rotors from eBay, you shouldn't run into any problems with drilled rotors cracking under typical street driving.
Larger brake kits decrease stopping distance in two ways: First, the amount of brake pad and rotor area is increased which increase the amount of area that the pads and rotors contact each other, hence, more friction.
The coefficient of friction between a pad and a rotor is independent of the amount of surface area. Otherwise, pad vendors would need to specify a different Cf for each pad shape! Similarly, the area of contact between a pad and a rotor does not enter into the amount of torque generated by the brakes. The brake torque equation is: Torque = Effective Radius x Clamping Force x Coefficient of Friction

Since coefficient of friction is not a function of pad surface area, then brake torque is not a function of pad surface area.

Big brakes reduce stopping distance the following ways:

In a fade limited situation (i.e., the race track), the higher thermal capacity of a big brake system allows you to brake consistently from lap one to lap thirty.

In a panic stop, a big brake system with fixed, multi-piston calipers can reduce your reaction time. The delay from when you hit the brake pedal to when actual braking starts to occur can be reduced and the reduction in stopping distance is a function of that reduction in delay and the initial speed when you hit the brakes. For example, your car travels 21 feet in 2 tenths of a second at 70 mph.
Dave Zeckhausen
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