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  #11  
Old 03-03-2014, 12:19 PM
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Just because they are from a m3 doesn't mean they are better
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  #12  
Old 03-03-2014, 01:09 PM
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Bruce, you still haven't mentioned what your goal is. And why cross drilled?

Repeated stops doesn't mean much, as that could apply to rush hour traffic in any large city.
I'm assuming you mean repeated stops from high speed. If so, larger is better, and like J.Belknap mentions, more mass is better. Unless you want to spend a few grand on a BBK from Brembo or Stoptech as was mentioned before, the 4.6 stock rotors are the best deal you can get.
Your limiting factor with the stock 4.6 brakes are the brake fluid and the pad compound.
You need at a minimum, ATE Superblue or Typ200 brake fluid to get a higher boiling point than stock brake fluid. If you happen to be supercharged like J.B., then I'd suggest some better brake fluids like Castrol SRF, or Motul RBF.
Then for brake compounds, OEM pads are horrible for repeated high speed braking, they begin fading after the first couple of triple digit slowdowns. You'll need proper high performance pads, or if you are doing actual lapping days, I'd go as far as suggesting racing pads. Cool Carbon, Pagid, Performance Friction, Hawk, etc. all make racing pads or high performance pads. Disclaimer, racing pads tend to not offer good initial braking when cold and therefore tend to make driving on the street more dangerous because of increased reaction time and reduced braking capability. This can also apply to many of the high performance pads.

And once you've upgraded your brake fluid and put on some racing pads, you'll find that the stock 4.6 brakes are perfectly capable and your tires are now your limiting factor on your stopping (evidenced by ABS coming on more often). So with the money you saved not buying a big brake kit, you should also get yourself a set of 18x10 wheels with Michelin Pilot Super Sports in 315/18 all around, then when you've exhausted the limits of those tires, upgrade to some R compounds

Wait, why are we doing this to an X5? Those M3 floating rotors you alluded to earlier also happen to come preinstalled on the M3 which is an extremely capable car designed to be driven on the track all weekend...the X5, not so much.
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  #13  
Old 03-03-2014, 01:25 PM
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Quote:
Originally Posted by THE VEIN View Post
That's true if the car is skidding
Dynamic and static friction are both dependent on the normal force. And since the contact area is dependent on vehicle weight, which is static (kinda), only the shape of the contact area changes based on the characteristics of the tire (width, diameter, materials, tire pressure). And the applied friction is dependent on the shape of the contact area and the behavior of the tire compound while it is in that state (shape and braking).
Stopping distance is more about the tire compound. Then throw in some weight transfer characteristics, brake bias, wheel alignment, and track surface to top off the list of items contributing to braking performance.

Also, ABS tends to prevent the tires from ever slipping while braking. Jamming the brakes and going straight to ABS is not the fastest possible braking though.
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Last edited by racingbmwm3; 03-03-2014 at 01:33 PM.
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  #14  
Old 03-04-2014, 02:18 AM
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Quote:
Originally Posted by THE VEIN View Post
Just because they are from a m3 doesn't mean they are better
This is coming from a guy that since 1995 has owned FOUR M3's: Yes. That does make it better.
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  #15  
Old 03-04-2014, 02:21 AM
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And, I'll say that with a caveat: everything, for the most part, in an M3 is over-built for hard use. Of the four I owned, aside from clutches and a blown water pump on my 95, my cars never saw the inside of a shop because something broke.

I can't say that about my previous 2002 4.6is and my 2006 4.8is.
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  #16  
Old 03-04-2014, 02:27 AM
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racingbmwm3, I hear what you're saying. I would normally go with an aftermarket cross-drilled rotor. There's plenty out there. Why, you say, do I want them? Because of the insane way I drive. And, with about 6500 pounds, I want the odds in my favor: better stopping, cooling, lower dust, no noise and stops good cold. I just installed EBC Red Stuff all around. Had this on my Shelby and the wife has them on her C63 AMG. But, I want more in the way of steel brake lines ATE Blue or Green and cross-drilled rotors to release hot gases when running around Houston in 110 degree heat this summer.
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  #17  
Old 03-04-2014, 02:38 AM
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Well, you could buy modern pads that don't have any significant outgassing.
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  #18  
Old 03-04-2014, 11:13 AM
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Why not just buy a big brake kit and be done with it. You won't get any significant increase in brake power by changing rotors. Maybe your "insane" driving is just too much for an OEM gigantic engineered rotor and pad.
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  #19  
Old 03-04-2014, 12:04 PM
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Quote:
Originally Posted by JCL View Post
Well, you could buy modern pads that don't have any significant outgassing.


You noted that you've owned 4 M3's since 95 and those are overengineered. They didn't start using cross drilled rotors until the E92 M3, which is also when they switched to fixed calipers and their braking performance saw zero improvement over the E46 (Car and Driver 70-0, 156ft both cars).

I was disappointed that BMW finally gave in to the fad of 4/6/8 piston fixed calipers and cross drilled rotors. Up until that point, they continually had the best braking performance in every comparison with their single (or double) piston floating calipers and solid (vented) discs vs. cars with multipiston fixed calipers and cross drilled. But I guess the old tech that worked awesome doesn't look cool enough...

quick summary about cross drilled here:
Signal to Noise - cross-drilled rotors

how come F1 doesn't use cross drilled? From Red Bulls RB9 (2013)
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  #20  
Old 03-04-2014, 12:08 PM
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Switch out the 4.6is rotors for M3 comp rotors and you lose 3 things critical to stopping a heavy vehicle..........

1) Total pad swept area of rotor (less surface area=higher surface temps and increased rotor wear for any given pad compound).

2) Distance of clamping force from center of wheel rotation (if clamping force is moved closer to rotation center, less braking force will be transfered to the tire contact patch, at any given caliper clamping force short of ABS intervention. Think simple lever, longer bar=more force generated vs force exerted).

3) Mass (rotor mass acts as a heat sink, less mass=higher swept area temps and increased fade).
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