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Originally Posted by emccallum
I have only done one autocross (at BMW plant in SC) and I found the brakes adequate for the short duration, and actually feel better when driven hard. I do not like the modulation of the pedal for daily driving. I wish it had a harder pedal, that required more effort. It almost feels like the vaccum assist comes and goes?? Touchy pedal. Anyone else feel this? I cant see how upgrading calipers would make this go away, as it feels like it is vaccum or master cylinder related.
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For autocross, a tiny 11" front rotor and 9" rear rotor would provide about equal performance to stock setup!! As for pedal force required, this can be altered to taste using different pads, brake booster, master and/or caliper piston diameters. The rotor diameter is a very secondary issue. Anyone who's driven a minivan or other vehicle with modest-sized brake system knows that brake effort is not proportional to brake performance. Most pedestrian cars require very little pedal pressure to reach threshold/lockup.
For a duration of 1 minute of severe duty, the stock M5 brake system barely reaches optimal temperature for performance pads. Rotors are not breathing hard, for sure. Such duty generally places less stress on the brakes than a single hot lap in road racing. Weaknesses in the stock M5 brake system don't surface until about 4-6 quick laps on a brake-intensive race course. I have seen nationally-ranked champion autocrossers take advantage of update-backdate rules to install
smaller brakes for less unsprung weight, better cool stop performance and better lap times.
Stainless Steel hoses may reduce ballooning under pressure to provide a slightly firmer pedal. Pad compound has tremendous influence on brake pedal effort and pedal feel under hard use. Some pads will almost throw you through the windshield with very little effort....other's require a strong leg. Mixing compounds front and rear must be done with some intelligent planning and experience or the ABS on the front or rear will be engaging all the time due to poor bias.
When I swapped stock BMW Pagid pads for HT-10, the required pedal pressure dropped by orders of magnitude. The friction coefficient for pads varies widely by model and it's also a function of temperature. Less pedal and line pressure creates a genuine sensation of firmness. With HT-10, the ABS engages at any speed with only moderate pressure. You can literally engage ABS at triple digits under panic or evasive situation. It really takes some getting used to. I am not recommending HT-10 for street, just offering example. They squeak, generate lots of dust and rotors get chewed a good bit. There are many other aftermarket pads that provide gains for particular use....some investigation is in order.
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