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-   -   BMW brake rotor facts and deductions (http://forum.e46fanatics.com/showthread.php?t=456412)

jpr 03-24-2007 04:15 PM

BMW brake rotor facts and deductions
 
Here's the real answers to what seem to be the FAQ's. The facts are taken direct from the TIS, the deductions come from my engineering background. As always, please do not hesitate to identify and correct any errors I may have made.

Rotor sizes (in mm)323 - 286x22 front, 276x19 rear
325/328 - 300x22 front, 294x19 rear
330 - 325x25 front, 320x22 rear

Minimum Acceptable Thickness for reuse -
323/325/328 - 20.4mm front, 17.4mm rear
330 - 23.4mm front, 20.4mm rear (deduced)

Maximum Allowable Runout
Disc on it's own - 0.04mm
Disc installed - 0.2mm

Is it allowed to machine (turn) BMW rotors?
Yes, there is an actual BMW procedure in the TIS describing how to machine the rotors. You can machine them down to the minimum thickness if you need to. You do however, need to ensure that both sides are evenly machined and the appropiate runout and surface finish specifications are maintained.

Should you machine your BMW rotors?
Generally speaking, BMW rotors are cheap enough where it does not make economic sense to turn them. Plus, replacing them guarantees you will have a rotor with the correct runout and surface finish criteria.

When do you have to replace your rotors?
When they are below the specified minimum thickness.

What if my rotors are at, but not below the minimum thickness?
Then you can use them again, but only for one more set of pads, assuming you are using OEM pads or pads with equal or less rotor wear than the OEM pads.

What's the theory behind the minimum thickness criteria?
When BMW selected the pads and designed the rotor thickness, it was not done randomly. The expected range of rotor and pad wear is known and designed on purpose.

Your pads have a wear sensor which goes off when you have 3mm or less pad material. It is a safe assumption that the pad wear sensor is designed to go off before you wear your rotors down so thin that they are hazardous. So the deduction from this is that the wear rate of rotors with OEM pads is 1.6mm or less per set of pads. Which in turn means the absolute safety limit of rotor thickness is probably about 18.8mm front and 15.8mm rear (for 323/325/328 - add 3mm for 330). To put it another way, the BMW rotors and pads are designed in such a way that if you reuse the rotors at minimum thickness, your pad wear sensor will go off before your rotors get too thin.

So do should I replace the rotors everytime I replace the pads?
Assuming they are at or above the minimum thickness for reuse and are otherwise within spec, it's up to you to strike the best balance of tradeoffs between performance and money. It is an absolute certainty that you will get better braking performance out of new rotors than reused rotors. But depending upon the condition of the rotors, the difference in performance may be slight.

The one thing you do NOT want to do however, is reuse rotors with a non-OEM brake pad of unknown wear rate. By doing so, you run a very serious risk of having a higher rotor wear rate than was intended by design. As a result, your rotors may become dangerously thin and even fail before the pad wear sensor goes off.

jbeurotech 03-24-2007 06:51 PM

You forgot to have on there DO NOT TURN M rotors!

jpr 03-24-2007 07:00 PM

Didn't forget it, didn't know it.

Are you specifically refering to the ones with the floating disc design? I can see where that wouldn't work out very well.

jbeurotech 03-24-2007 07:03 PM

Look up M3 rotors it says right in TIS do not machine M rotors, weird huh I never rec machine any BMW rotors, I used two and have changed my mind after years of real world results with turning BMW rotors. Do a search on a thread where me and Alex323i had this discussion

flyinj3x 03-24-2007 07:22 PM

nice job jpr! great info as usual. i like how you included "BMW", "brake", and "rotor" in the title, shouldnt be missed on the search.

jpr 03-24-2007 07:22 PM

Wierd -I'm running a somewhat elderly version (Dec 2000) of the TIS, which only covers the E46 M3 up to 2001, largely prior to it's USA debut. There is no such disclaimer on my version.

But back checking to the E36 M3, it is exactly there where you said it would be - "(M models may not be machined)"

So the two questions are -
(1) did the non-machining direction get included for the E46 M3 in a later version of the TIS?
(2) Why not? As noted above, my first guess would be that the real issue is the compound floating rotor design prevents accurate machining, but are there other considerations? And for that matter, did the E36 even use that design?

As I recall the thread you're refering to, I believe the conclusion was, yes you can machine the rotors, but it's to imagine the circumstances where it would be worth the bother (and expense).

flyinj3x 03-24-2007 07:31 PM

my TIS version 3.4.1 (NOV-2003) states the front or rear M3 rotors not be turned.

jpr 03-25-2007 10:01 AM

Thanks for settling question 1.

Any ideas on question 2?

328ciNY 03-25-2007 10:21 AM

Hey.. I thought this would be the right thread to ask since its about brakes/rotors. The auto places around here have nothing listed in inventory for brake pads OR rotors for a 2000 328CI (my car) but they DO HAVE brakes/rotors for the 325CI -- will these brake/rotors fit my car? When I search around for "e46 brakes" I see most brands fit ALL (323/325/328/330) but NOT M-series, so I assume the brake pads/rotors will fit from a smaller-engined 3 coupe?

Thanks for any help.

jpr 03-25-2007 10:43 AM

Rotors - the 323 and 330 have uniquely sized rotors, the rotors on the 325 and the 328 are the same.

Pads - the 323, 325, and 328 all use the same calipers and therefore all use the same pads. The 330 has different calipers and use different size pads.

The M3, is of course, it's own animal all the way around.

328ciNY 03-25-2007 10:56 AM

Thanks for your response. So, any E46 328/325 rotors I could use (I guess that means .. 99 - 2005), and any E46 323/325/328 brake pads will work?

Thanks in advance!!

jpr 03-25-2007 11:11 AM

Quote:

Originally Posted by 328ciNY (Post 5731330)
Thanks for your response. So, any E46 328/325 rotors I could use (I guess that means .. 99 - 2005), and any E46 323/325/328 brake pads will work?

Thanks in advance!!

correct

Commander FAT 03-25-2007 11:47 AM

Minimum disc thickness is stamped on the disc. No need to deduce anything.

jpr 03-25-2007 12:03 PM

Quote:

Originally Posted by Commander FAT (Post 5731504)
Minimum disc thickness is stamped on the disc. No need to deduce anything.

Yes- that would be the fact part, not the deduction part. It's also in the TIS and handy thing to know without having to look at your disc, say for example when your shop is on the phone trying to sell you new discs because at they are almost at the minimum thickness.

Commander FAT 03-25-2007 12:45 PM

Then why is deduced in paranteecees ( I can't spell that damn word)?

jpr 03-25-2007 12:56 PM

Quote:

Originally Posted by Commander FAT (Post 5731766)
Then why is deduced in paranteecees ( I can't spell that damn word)?

I listed the 330 rotor minimum thickness as deduced since, my version of the TIS predates the introduction of the 330 and does not have that info. The deduction is that they use the same designed rotor wear rate. IF you can confirm it, that would be helpful.

Rob Levinson * UUC Motorwerks 03-25-2007 09:51 PM

Quote:

Originally Posted by jpr (Post 5730032)
And for that matter, did the E36 even use that design?

Just like the E46 M3, US versions pf the E36 M3 got 1-piece rotors and Euro got the floating design.

- Rob

jpr 03-25-2007 09:59 PM

Quote:

Originally Posted by Rob Levinson * UUC Motorwerks (Post 5734687)
Just like the E46 M3, US versions pf the E36 M3 got 1-piece rotors and Euro got the floating design.

- Rob

Does that include the ZCP rotors? Are the ZCP rotors the Euro/CSL version?

What do you make then of the proscription on machining the "M" rotors? Is the terms "M rotor" BMW code for floating rotor, or is there something special about even the 1 piece rotors that prevents them from being able to be machined?

jpr 03-26-2007 08:57 AM

Just to consolidate info, here's the E46 M3 rotor sizes -
M3 - 325x28 front, 328x20 rear
M3 optional drilled (ZCP) - 345x28 front, 328x20rear

I believe the minimum thickness would then be 26.4mm and 18.4mm, but it would be nice if someone could confirm this.

Rob Levinson * UUC Motorwerks 03-26-2007 09:38 AM

Quote:

Originally Posted by jpr (Post 5734740)
Does that include the ZCP rotors? Are the ZCP rotors the Euro/CSL version?

Yes, the 345mm front rotors are identical to CSL rotors. The 325mm front rotors are standard M3 Euro. The 328mm rear are common to CSL/Competition Package/Euro.

Quote:

What do you make then of the proscription on machining the "M" rotors? Is the terms "M rotor" BMW code for floating rotor, or is there something special about even the 1 piece rotors that prevents them from being able to be machined?
I would think that it has to do with the same "commen sense" concept against machining any BMW rotor, that the allowable wear depth is very small... and considering that the Motorsport applications are expected to be used more aggressively and see higher heat ranges, it's simpy easier to say "Nein, machining verboten" than to say "Das machining ist für Dummköpfe!" and get into a whole discussion about it.

I suspect that the TIS uses "M rotor" to mean floating, but cannot substantiate that beyond a qualified guess.

- Rob


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