Anyone use M3 Competition rotors?
 

There is a 1/2" difference in diameter. Think its 14" stock for a 4.6is and 13.5" for the two-piece Euro CSL comp rotors.

Anyone know something?

Why would you want to put less braking surface area rotors on your car? If it was me, I would want larger rotors, not smaller.

Okay... Anyone else have any opinions about running a cross-drilled two-piece rotor that can provide repeated stops without any brake fade?

If the outer diameter is less with the same pad then you will have less braking surface area, so therefore less braking. Also, are the two piece rotors the same offset? If you want improved rotors just go with a two piece 14" that fits the stock calipers.
Most upgrade to the 4pot stop tech or 8 pot Brembos and don't stick with the stock calipers.

I've yet to do the swap, but comparing parts from www.realoem.com I've found that the E38 740 and E53 X5 use the same brakes. I've went ahead and bought 750iL brakes from a junkyard (calipers, rotors, caliper hangers, lines just in case, pads) and plan on swapping them into the X5. Front brakes dual piston w/ larger rotors and pads, rear gets upgraded from a solid disk to a vented disk.

I'll give you my opinion. Unless you plan on racing your X, it's all just for show. Based on your lack of information in your post, one can only guess that you intend to replace rotors on a 4.6. If that's the case, you are throwing money away. The OEM rotors for a 4.6/4.8 are insanely huge and will stop these big SUV repeatedly with no fade. Save the money for airbags and suspension bushings.

I think that's a step backward.

With those rotors you have less swept area, less mass because they are smaller, and additional reduction in mass because they are cross drilled.

All things being equal they will reach threshold temps faster.

Just curious, in what situations do you feel the stock 4.6is brakes need improvement? I think they do a great job in anything I will ever need to do in the city or highway. I have not tracked one yet though.

Here's an interesting tidbit...

A 2002 X5 4.6is took one foot LESS to stop from 70mph than a 2013 BMW M5.

Pretty impressive considering the M5 weighs almost a thousand pounds less AND has 6 pot brakes in the front.

BMW X5 4.6is - Short Take Road Test - Car Reviews - Car and Driver

2013 BMW M5 Road Test – Review – Car and Driver

Stopping distance is a function of the tire's contact area.

Repeated stops.... go with more mass. 4.6 rotors.

That's true if the car is skidding

Just because they are from a m3 doesn't mean they are better

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.

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.

This is coming from a guy that since 1995 has owned FOUR M3's: Yes. That does make it better.

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.

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.

Well, you could buy modern pads that don't have any significant outgassing.

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.

:iagree:

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)
http://www.racecar-engineering.com/w.../05/uprb9a.jpg

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).

Drilled rotors are on their way in F1. Several teams were testing them pre-race at Curcuit of the Americas, TX last year.

These are carbon/carbon brakes though, hardly similar to cast iron rotors.

Unless you upgrade to ceramic discs, you can't have all these characteristics together.
The best stopping brakes (racing pads) have very high dust, are extremely noisy, and only stop ok when cold with sluggish response.
If you want less dust, noise, and good cold stopping, those are characteristics of street pads which don't enjoy repetitive high speed stopping.

Cooling is a trait of how the vanes inside the rotor are designed. Race cars tend to run cooling ducts to the back of the hub to help force air through the rotor. Maybe you should consider this if racing pads still aren't sufficient for you. Super Truck Racing in Australia, they use water cooling for their brakes. This would actually be really awesome to see someone implement in the US.

I'm noticing that none of us have asked what your actual symptoms are that you are trying to correct. Is it brake fade, fluid boiling, pedal feel? Are you having these problems while towing, autocross, lapping days? The fix needs to be tailored to the problem. Look at WRC. They have a few different styles/sizes of brakes they install on their cars to match the racing conditions.

Why not look into M6/M5 rotors?

+1

I've used vented in my SC'ed 2000 E39 540 for 10 years. Never had a fade issue. I don't suspect that I'd ever have seen one with the stock 4.8is brakes. They're over built as it is. BMW style.

As for size, note that the StopTech kit (that I just installed on my 4.8is) is actually a 14" rotor up front as opposed to the 4.8is 14.1". Do you think (really think) that this is a downgrade? Surface area is *part* of the equation in getting rid of the heat- that's the main symptom of fade- but it's not even close to the *only* thing that makes an effective heat sync.

The main thing I would be concerned about on the proposed upgrade for the OP is the change in unsprung weight between the two choices. I am nearly betting that the M3 parts will weigh less than the stock components. The difference in weight on the StopTech kit was about 12lbs per rotor and 13lbs per caliper. That's 50lbs on the front alone. The rears are lighter (and larger at 14" rotors) still. After a couple weeks with the kit, the difference in ride quality and acceleration from the reduction remain both consistent and very, very easy to feel.

Ride quality/responsive handling was one of the things I lamented about when I first got the 4.8is. It was softer and felt 'heavier' than my 2004 3.0 E53 I'd driven for 7 years prior. Even after I swapped the Eibach sways to the 4.8is, I still felt a bit more weight getting thrown around. Since my 3.0 was an early '04 with the small sunroof, I blamed it on the higher center of gravity from the giant glass roof on my 4.8is.

That weight high up is still there, but I am able to toss the 4.8is around a lot more confidently now with the reduction in outboard, unsprung, and rotating mass from the brake upgrade. The communication or 'numbness' that I felt from the over-assisted drive by wire steering is still there in the 4.8is compared to the 3.0, but I have recaptured that confidence that the X5 will return more calmly than before from interactions with high speed imperfections in the road.

This weight reduction is not much different than the impact when using heavy aftermarket wheels on any car. If you've ever thrown on some cheap, heavy wheels then gone for a ride, you experience might have been close. +25lbs/wheel though would be some silly heavy wheels even on an X5. I never much minded the heavier replicas on my 3.0- it was never going to be a speed demon anyway. But I have not used them in about a year on my 4.8is bc I can get a little funky with all that power. Having less weight- especially rotating mass- to get moving- and to stop moving- is key to calming down that funky.

Would also need the matching calipers, and then engineer a custom bracket to hang the calipers. Also need to consider the depth/size of the rotor hat on how it fits on the hub and around the steering knuckle. Rather than re-engineering a system designed for another vehicle having a different curb weight/weight bias/and suspension design, it would make the most sense to buy a BBK from Stoptech or Brembo where the calipers/rotors have been matched for the vehicle. Like Propellerhead has done.

Less weight is good but NOT at the expense of that much swept area. The M3 rotor is a bad idea. Sorry to be blunt

Depending on whether or not the reader agrees that the stock 4.8is rotor is larger than necessary, this is looking closer to truth for this application. Not only is the M3 comp rotor smaller, it is also not as thick.

Stock 4.8is rotors are 356mmx36mm.
M3 Comp rotors are 325mmx28mm.

Now, for even more consideration, look at the 04-06 front E53 4.4:
332mmx30mm.

Still larger. The M3 braking system was designed for a ~3500lb vehicle. The X5 4.8is was ~5,000. I think this may be a bad idea.

-------
But size isn't everything. Let's consider balance. White papers on the concepts below are available here.

The reasons for my StopTech choice were clear enough, but why the four wheel kit not fronts only? Balance. We can't think of braking without considering what's going on at the rear. ABS systems are designed to apply measured amounts of pressure in the system based on what it 'knows' (was programmed) about the capacity of the system including the stock calipers front and rear.

Of course, the system will adjust. That's what it does. But it could be .5 sec behind while listening' for feedback from the sensors. As it adjusts, pressure is distributed across the system the way it was 'taught'. The system works, but the ABS system does not 'learn' about the new (in)balance of the system. Of course, what you end up with is this awesome 'feel' of more bite. This is bc the system doesn't know that the new, larger fronts have so much more brake TQ capacity and will continue to throw the same old pressure up as it always has to the OE front brakes. The result can be this awesome GRAB! and maybe even a little nose dive as those big binders squeeze with way more pressure than is needed up front- and not enough at the rear. Feels awesome. Doesn't work as well.

But then the rears start squawking. "Hey! Unexpected input! Wheels not doing what we expect! Need! Want! Hey! What ab us?", and ABS responds. ABS effectively evens out the braking pressure until the driver is out of the brakes. This will happen every time.

Of course, this is not GUARANTEED to happen in every front only kit, but it is certainly more possible when the braking equation is changed radically from front to rear. As above, it'll FEEL great. But will it actually be a more efficient system? When you start replacing parts on specs alone without considering the engineering equation you're changing, the chances are that you'll end up with a system that is not properly balanced- and therefore won't work as well as it should- perhaps not even as well as the stock config.

That's a good point.

Changing rotor diameter and caliper piston surface area TOTALLY changes the way ABS functions.

I remember a few yrs ago Car & Driver did a "Battle of the Import Tuners", all the cars showed up with massive rotors and multi-piston calipers. When the testing was completed a common issue came to light, bigger brakes with stock ABS tuning had increased stopping distance on EVERY car in the test. Some by around 50 ft from 70 mph.

Another factor that needs to be considered to prevent the problem TiAgX5 mentions, is the total caliper piston displacement. The master cylinder and ABS/DSC pump/valves are also matched to the stock calipers as part of the bias setting. increasing your total displacement without increasing your master cylinder will require more pedal effort/travel to achieve the same braking pressure. If you only increase the displacement on one axle, that axle will require more effort to achieve optimal braking, and yet because of the bias built into the master cylinder, the stock axle is going to hit peak braking first and cause the system to go into ABS. On 4 channel ABS like ours, that's not as bad as a vehicle with 1/2/3 channel ABS.
I guess this could be good if you are trying to setup the vehicle to have better turn in with trail braking. You'll also get better wear on the upgraded brakes, but worse wear on the stock brakes.