Quote:
Originally Posted by lakefront
The front/ rear bias question is one I have been wondering about too. Have seen other folks (on other platforms) use a 2 piece rotor and more aggressive pad in the rear to help balance the front to rear with a front BBK. That would probably be a question for the BBK manufacturer (Stoptech, Racing Brake).
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The factors affecting the bias are:
1) effective piston area BBK vs OE. If the replacement front caliper has more piston area than OE it will shift the bias forward. This is by no means a certain thing, because the pistons could be (in fact should be) smaller, but the math is simple if you know the piston diameters.
2) effective rotor radius BBK vs OE. If the radius to the center of the pad is increased, so too is the front bias. Since we are talking a BBK, this can be assumed to be true. Again the math is simple, but you need to know both the rotor diameter and the pad shape.
3) friction coefficient of the front pads vs rear pads. Typically, but not always the same. The common Hawk DTC 70/60 Fr/Rr combination increases front bias, which is not inappropriate within reason when using sticky tires, because higher decel rates produce more forward weight transfer.
4) front vs rear hydraulic pressure, which is typically controlled by a mechanical brake proportioning valve. That said, the 135i appears to have electronic brake force proportioning, which within unknown limits should handle some changes in the inherent system bias. James Walker Jr. describes the whole matter pretty well here:
http://stoptech.com/technical-suppor...tioning-valves
As a last resort ABS will modulate the brakes to fix a severe bias problem, but this is not optimal. Ideally your Big Front Brakes should have less total effective piston area proportional to their greater effective rotor radius so that you don't muck up the balance.