Anyone use M3 Competition rotors?

[TiAgX5]

Originally Posted by racingbmwm3

.......how come F1 doesn't use cross drilled? From Red Bulls RB9 (2013)

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.

[racingbmwm3]

Originally Posted by ///Bruce

better stopping, cooling, lower dust, no noise and stops good cold.

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.

[mrbmwx5]

Why not look into M6/M5 rotors?

[PropellerHead]

Originally Posted by racingbmwm3

how come F1 doesn't use cross drilled? From Red Bulls RB9 (2013)

+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.

[racingbmwm3]

Originally Posted by mrbmwx5

Why not look into M6/M5 rotors?

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.

[J.Belknap]

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

[PropellerHead]

Originally Posted by J.Belknap

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.

[TiAgX5]

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.

[racingbmwm3]

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.