TECH: Why aren't the OEM rotors cross-drilled? - The Unofficial BMW M5 Messageboard (m5board.com)

B-Pac · 9th May 2002, 03:59

If all the "high-end" aftermarket brake packages have cross-drilled rotors, and you usually see them stock on Ferrari, Porsche, Lambo, etc..., then why aren't they standard on the E39?

It seems BMW spared no cost with every other part of the car, so why not go ahead with the rotors? The brakes seem to be a common complaint from some guys on here who have tracked the car and notice fade problems. Would cross-drilling be the answer? On the other hand, I've read on here results of 60-0 stop times of 1.3 secs or so, and 110 feet, so does it really matter actually? Anyone have a logical answer? Thanks.

M5Ted · 9th May 2002, 04:35

Cross drilled are nice looking and perform well, but their life span is pretty short. Extended heavy braking and just exposure to the sun will lead to cracking over time which hurts the strength of the rotors. I have heard that slotted and vented rotors perform about the same, but last a lot longer. Other than that I have no idea why BMW doesn't use cross-drilled like the other performance cars? Perhaps BMW feels the M5 is more of a luxury car than an all out racer which would need extremely high performance brakes.

greg · 9th May 2002, 04:35

The M5 brakes are more than adequate for street driving - in an emergency stop they are 100% up to the task and the car stops amazingly fast and short - better than any other car in its class.

The track is an entirely different situation. Think "emergency stop" immediately followed by full-throttle acceleration to some speed north of 100mph (which in the M5 doesn't take long at all), and then doing another emergency stop, over and over. THIS is where the heat becomes a problem. don't forget this is a 4000lb car with the engine in the front - together with weight transfer during braking this means the front brakes have a HUGE job.

Cross-drilled rotors do not shed heat better. My understanding is that their purpose is to vent gasses that build up and reduce the braking effect. Another benefit of cross-drilled rotors is reduction of rotational intertia. Slotted rotors do the same thing. I can't answer as to why we don't have them, but I don't think they are the primary problem in our cars, which is immense heat buildup. The solution is better cooling, and pads and fluid designed for higher temps. Pads like this tend to squeak more and are more abraisve - there is no free lunch. cross drilled rotors must have every hole chamfered, and must be properly heat-treated/annealed AFTER the holes are put in - or you have nothing but stress risers where cracks will start and then propagate. (After my last outing on the track even my Porsche developed this - albeait to a very minor degree.) So such rotors are expensive.

In the end - I agree with your assertion - on such a great car, they shoujld have made the brake system more capable of withstanding aggressive track driving. My point is only that cross-drilling probably isn't the place to start.

if you have not read this article yet, it is on my top ten list of "must read" articles - the best, most b.s.-free article on brakes I've ever read, from one of the true elder statesmen of racing, Caroll Smith: The "Warped" Brake Disc and Other Myths of the Braking System

Note - I am generally skeptical of articles like this when they appear on a manufacturer's web site - in this case, StopTech - but because I know who Smith is, and that he is not affiliated with them (even though he may have been payed to write it) I trust the article, and I have a lot of respect for stoptech for putting it there.

B-Pac · 9th May 2002, 05:25

Ted/Greg: thanks for the replies. I think the best answer is that it's a small compromise to the "luxury" side of the car. Greg, I read that article you mentioned when you first posted it under another thread. Excellent, no question about it, I learned a ton. I appreciated his recs on how to break-in a new set of brakes as well, I'll be certain to do that when my Beast arrives. One more point re the current topic.....I always thought that cross-drilling WAS for added ventilation/surface area and better cooling?? What "gases" are you referring to? My impression from Smith's article was that there's a fluid equilibrium at the interface, and any "cook-off" of gases is exactly what you don't want since it causes the deposits and subsequent warped-feel.

greg · 9th May 2002, 05:34

B-pac -

the pads evidently outgas with heat. I read somewhere about a) the gas decreasing braking efficiency and b) the cross-drilling not really helping with cooling much - if you think about it there isn't going to be much, if any airflow through there. As I understand it the heat is radiated mostly from the rotor surfaces and the boundary layer is "scraped off" by the brakes and or proper cooling ducts (see Smith's Engineer to Win). BUT i am not 100% certain about either.

atomic80 · 9th May 2002, 05:46

Since I just had a pair of Stoptech cross drilled brakes installed on my M5, I felt that I should chime in here. I was given the choice of going with either cross drilled or slotted brakes. I chose the former because I'm told that unless I plan to take on a heavy dosage of tracking events where slotted rotors would do a better job in terms of durability; I'd be fine with the cross drilled rotors.

I waited a very long time before finally getting a set of aftermarket brakes because I wanted to go through this with my eyes open. I wanted to be the one who knew what he was doing as opposed to being the one that stuck his foot someplace where it didn't belong. One of the things about Brembo is that it's overpriced and would not even fit using the stock wheels without the use of spacers which in turn has the potential for causing the wheels to shimmy if not done properly. I'd heard other potential issues with the use of Mov-it brakes as well. The only one that I could say with confidence that I trust is the Stoptech's. Folks such as Dave Z have nothing but positive accolades for this setup so their word is good enough for me.

One other observation that I've made while my Stoptech's were being installed is how heavy the stock rotors are! They are HEAVY! What's more...how can BMW possibly even think about putting single piston calipers on all four corners of a 4000+ pound high performance sedan. Maybe BMW has a very good reason for this shortfall. The reason being beyond me.

I have not had the chance to try out the new Stoptech's yet but I am definitely looking forward to doing so!

TCM · 9th May 2002, 06:08

I think one aspect that you have to look at is that the M5 is not a race car. It was not made to be a race car, it purpose in life is high speed cruising on the autobahn. BMW made the brakes adequete(sp) for stopping the M5 quickly in case of an emergency on the highway but does not expect owners to take the car out to the track and beat the crap out of it with continuous high speed braking. We are a rare group of M5 owners, I beleive that most do not track their Beasts and really do not understand how amazing the M5 is. I agree better brakes should have been used but for the R&D and parts costs, it did not make economical sense for the company. But upgraded brakes are a definate mod IMHO, you can never have too much braking performance.

foreman · 9th May 2002, 07:11

I was told that in european M cars they use a two piece rotor design which is apperantly truely outstanding but they would not pass federal crash test standards in N.A. so the were shipped with one piece street adequate rotors...

foreman

Bart Carter · 9th May 2002, 07:34

Quote:

Originally posted by foreman
I was told that in european M cars they use a two piece rotor design which is apperantly truely outstanding but they would not pass federal crash test standards in N.A. so the were shipped with one piece street adequate rotors...

foreman

It was my understanding (you can do a search on this) that they European rotors are not as reliable or as long lasting as the U.S. solid disks, prone to warping and do not perform any better.

BKH · 9th May 2002, 08:13

I believe that BMW has specifically stated that cross-drilled rotors would not be suited in their view for the NA market because they would be unacceptably noisy for the "average" owner.