Originally Posted by
Chris Wilson
Brake upgrades can set out to try to achieve several objectives.
The commonest are to increase resistance to fade and or increase braking effort for a given pedal effort. IE, the pads are pushed against the discs harder for a given pedal effort than before the upgrade, or the brakes will stop the car from 100 MPH, hard, for more times before fade sets in, than previously. The feel from the pedal, that almost intangible quality, can also be addressed and sometimes improved upon by brake size, or pad material changes, or brake flexi hose upgrades to something less squashy than rubber.
It's easy to get carried away by the thought of brake upgrades. The limitation in most cars as to how short a distance they can stop in is tyre friction. Leaving aside pedal feedback, and fade, it is almost certain that a Supra on stock Jap spec brakes will stop in just as short a distance as one with an AP six pot kit on it, a Brembo kit, an AP Racing kit, or whatever. It may not feel to the driver that it does, but usually such is the case if you just nail the pedal as hard as you can. The fancy kits may *FEEL* to stop the car faster, due to less pedal effort, and a better bite, but in reality, if you hit the pedal as hard as you can with stock Jap spec brakes, UK spec brakes, AP kit, KAD, whatever, the car will stop in the same distance. Repeat this test 10 times and stock Jap brakes may be on fire and long since faded, or the fluid boiled, UK ones may be very hot and bothered, but the upgraded ones will probably still be working within pad, disc and brake fluid temp limits. Add in the intangible "feel" factor, and a desire to brake as hard as possible, using as little skill as possible, but WITHOUT relying on the ABS to take over, and for sure a well set up brake upgrade may well allow more finesse.
Herein though lies the rub.
Upgrade only the fronts and the brake balance of the stock car may well be compromised. Let's take stock brakes. You press smoothly on the brake pedal with (say) 50 pounds force. The car stops fine. 70 pounds, the fronts are just beginning to lock (car makers ALWAYS aim for the fronts to lock first, as rear wheel lock makes the car very unstable and liable to swap ends). The rears are doing as much work as the brake engineers deemed safe to prevent premature rear lock up. The ABS cuts in, and maximum retardation has been reached. Now, take a car with big front discs and calipers. Only 40 pounds pressure now gives a smooth, lock free and powerful retardation. 50 pounds and the new, more powerful, (for the same pedal pressure), fronts are locking. The ABS cuts in. BUT, and this is the crux, those original rear
calipers and discs are still well below the caliper pressure where they are able to achieve maximum retardation without fear of the rears locking.
In other words the FRONT brakes are doing TOO MUCH work, albeit without breaking into a sweat, and the rears are, to exaggerate a bit, just along for the ride. The BEST scenario is to upgrade front AND rear brakes, carefully ensuring the original balance of effort at any given brake pedal pressure remains as designed, but that the more efficient front AND rear brakes stay cooler for more hard stops, and that old intangible "feel" from the brake pedal is improved, at lower rates of driver effort on the pedal. The latter may or may not be good or desirable, and can be engineered out by changing BOTH front and rear caliper piston sizes, or pad areas. In a race car the balance would be adjustable via 2 brake master cylinders, with a driver selectable change in mechanical leverage effort between front and rear brake circuits, one cylinder operating the front brake calipers, the other the rear. This can also be achieved on road cars, but to do so is usually complex and expensive, especially if ABS and brake circuit failure safeguards are to be maintained. It is far easier to calculate the caliper and disc sizes, along with pad area and compound to achieve this, as near as available off the shelf equipment will allow.
Caveat. I said before makers engineer more effort on the front brakes to encourage straight line stopping if the tyres are locked up . They err on the excessive side, as, in the wet, the rear tyres can take a lot more braking effort than in the dry, due to less weight transfer onto the front tyres, as they will lock before as much weight is transferred when the grip of the road surface is reduced. So adding yet more front brake effort worsens this existing imbalance, especially in the wet. If it were not for the ABS the front wheels would be locking up very early.
On cars with basic (1 or 2 channel) ABS, or no ABS at all, a brake upgrade on just one end of the car can be lethal.
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