So far I know that E39 540i and M5 share the same front caliper piston dia and pads, based upon service part#'s.
M5 brake torque leverage on the front rotor will be a bit higher due to larger diameter rotor, but this difference will not be huge. I assume that the simple T = r x F formula can be applied to the "center" of the swept areas to come up with a rough percentage torque difference between the two systems. I will likely come out to something in the range 5-10%.
I find my OEM 540i brake system has too much front bias. The front ABS always engages first, even with matching pads f->r. On the track with HT10 race pads in front and premium street pads in the rear, the rear brakes do almost nothing and the fronts become grossly overworked. A bit more rear brake function on my 540i will improve bias and reduce heat buildup in the front.
In conclusion, it sounds like a conversion to M5 rear brakes is feasible for use with OEM 540i fronts. I realize heat buildup in front brakes will still be a limiting factor for severe duty. The next logical step for track use will be BBK for the front only.
I did the upgrade to M5 brakes in two stages because UPS lost my front calipers, and with just the M5 rears on the car, I barely noticed a difference. Granted, I don't track the car or threshold brake very often, but I did notice an improvement when I put the M5 fronts in. back-to-back driving of that car and a stock 540 there was a marked improvement in initial bite, but not a whole lot else. I did it because the car needed pads and rotors anyhow so "while I was at it...."
of course, I ended up having to replace my beloved Style 19s to do the upgrade...so much for a cheap brake upgrade.
also, the ETK shows different part #s for the 540 pads and the M5 ones...perhaps some aftermarket manufacturers use the same pad for both.
-f
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