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Why must you change brake rotors when you replace pads on a Z4MC?
To make more money for BMW and their dealers! it's a common thing to have a dealer call you and say pads and rotors need changing. Then when you push they say they 'may' not make it to the next service.

Seriously, there is no good reason to change rotors unless they have worn down and even then the specs given by BMW are certainly leaning in their favor.
 

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Some Say...he apexes late
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You can check the rotor thickness with calipers. No need to change them unless they're close to minimum spec.

HOWEVER the rotors and pads develop matching grooves over time. A second set of pads will not match up the same way, so you (may or may not) have more noise, vibration, etc. while your second set of pads wear into the rotors.

There ARE sales on rotor sets a few times a year, but DONT buy them from a dealer, you are just tossing money away. Try an independent BMW parts reseller, ask the google, know your part numbers and shop around.
 

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S54=Living on the Edge
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I ran through multiple sets of track pads on my OEM rotors before the thickness of the fronts dropped just past minimum. As long as you bed the pads in right and don't shock the rotors, I don't see why multiple pads can be used on the same rotor.
 

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CSUchemgrad
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totally agree, no need to change the rotors when the pads are low. Not sure how well the sensor works, but once that sensor light comes on or if you hear squeaking when u brake it is time to change the pads. sometimes it can even be rust, but I am a brake freak. When i hear squeaking , I take my car in and if you have a trustly mechanic, they will say it is rust and you drive away. But there is NO need to change the rotors everytime. When the wheel starts shaking when you brake, then you got an issue. You just ****ed your rotors. You can either get then grinded down (if the mechanic wants to or has the ability to) i find that this just buys you time. However, most mechanics will want you to get new rotors. Bottom line, you hear squeaking take it in the worst case it will be rust. Saves you some loot.
 

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When the wheel starts shaking when you brake, then you got an issue. You just ****ed your rotors. You can either get then grinded down (if the mechanic wants to or has the ability to) i find that this just buys you time.
BMW is a bit more exacting than "replace them when they vibrate" The rotors should be replaced when they reach the minimum thickness stamped on the rotor hat - 28.4mm for Z4M front rotors (1.6mm of total wear from the full thickness of 30mm). Z4M OEM rotors can't be re-surfaced at all, as they are floating rotors that have no rigid connection between the aluminium hat and cast iron braking surface.
 

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Some Say...he apexes late
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Just re-interating Mr Stygar's post, since the pic (?) did not appear for me...perhaps it got moved /deleted?

So:
Z4M minimum front rotor thickness ( aka MIN TH), 26.4mm per BMW TIS (8/07)
Z4M minimum rear rotor thickness 18.4mm

You need calipers to measure the thickness accurately.
Nifty pocket caliper-pen here.
 

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Replaced Rotors

When I replaced the pads I had the same question. I had my rotors touched up with virtually no loss of material and well within BMW specs. It may have been me, but after a few days I decided to do the rotor's also - yes, I did feel a difference, but I don't think that I would have loss sleep over the original set.
 

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When I replaced the pads I had the same question. I had my rotors touched up with virtually no loss of material and well within BMW specs. It may have been me, but after a few days I decided to do the rotor's also - yes, I did feel a difference, but I don't think that I would have loss sleep over the original set.
Why did you go ahead with the rotors if they felt fine after resurfacing? Just curious. I swapped to ceramic pads around 10K on mine. No issues so far(5K on the new pads.)
 

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I decided to do the rotors for 2 probably stupid reasons. 1) doubting myself - I had never worked on this car myself until doing the brakes, and since I have a habit of being a bit aggressive behind the wheel I didn't want to find our if they had a good reason. 2) Bored to death - prior to this marriage I always had atleast one project going that's why 2 vehicles are over 1,000,000 and the other 2 "play cars" have over 100K, moving and not having the garage right in back (my last home was purchased with a real 4 bay shop) I don't get to work on the cars as much.
 

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floating rotors???

Z4M OEM rotors can't be re-surfaced at all, as they are floating rotors that have no rigid connection between the aluminium hat and cast iron braking surface.
Can you confirm that the Z4M's rotors are floating? As you stated, the rotors are two piece with aluminum hats and drilled cast iron disks, but I believe the disks are solidly bolted to the hats and are not floating.

It does not make sense to me to have floaters on a street car. The floating connectors wear way too much for a street application, don't you think? I feel they even wear too much for race applications and run rigidly bolted hat and disks. I think floaters have too much risk of failure for the slight amount of benefit. IMHO

A two piece rotor can be re-surfaced by being disassembled and then placing the disks on a flat grinder. Drilled or slotted rotors can not be turned due to the holes/slots.

http://www.hygrade.com/flat_grinding_lapping.html
 

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Can you confirm that the Z4M's rotors are floating? As you stated, the rotors are two piece with aluminum hats and drilled cast iron disks, but I believe the disks are solidly bolted to the hats and are not floating.

It does not make sense to me to have floaters on a street car. The floating connectors wear way too much for a street application, don't you think? I feel they even wear too much for race applications and run rigidly bolted hat and disks. I think floaters have too much risk of failure for the slight amount of benefit. IMHO

A two piece rotor can be re-surfaced by being disassembled and then placing the disks on a flat grinder. Drilled or slotted rotors can not be turned due to the holes/slots.

http://www.hygrade.com/flat_grinding_lapping.html
You can see the connecting pins in the OEM floating rotors, the hat and disc are not solidly bolted or riveted together, and yes, it is unusual to find this in a street application. They were originally designed for the European M3 CSL, but made it through the parts bin to the Z4M:

 

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I think we are having a minor disagreement on the term "floating rotor" vs a two piece rotor. A true floating rotor has slight movement between the rotor and hat to allow for expansion and centering of the rotor in the caliper at speed.

http://www.braketech.com/index.php?...is-full-floating&catid=17:tech-talk&Itemid=69

"True full-floaters move on the carriers, this allows them to self-center in the caliper for reduced brake drag and "float" unimpeded for unrestricted expansion and contraction during repeated thermal cycling. The only serious down side is a bit of rattle that reminds you these are indeed full-floaters."

The M rotors are rigidly connected to the hats via the pins, are they not? ie, they do not rattle.

See the third picture of these motorcycle rotors with a movable connection.

http://www.oppracing.com/product_di...rformance-brembo-hp-front-floating-rotor-kit/

The M rotors are a two piece design with an air gap to isolate the hat from the disk but they do not rattle.
 

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I think we are having a minor disagreement on the term "floating rotor" vs a two piece rotor. A true floating rotor has slight movement between the rotor and hat to allow for expansion and centering of the rotor in the caliper at speed.

http://www.braketech.com/index.php?...is-full-floating&catid=17:tech-talk&Itemid=69

"True full-floaters move on the carriers, this allows them to self-center in the caliper for reduced brake drag and "float" unimpeded for unrestricted expansion and contraction during repeated thermal cycling. The only serious down side is a bit of rattle that reminds you these are indeed full-floaters."

The M rotors are rigidly connected to the hats via the pins, are they not? ie, they do not rattle.

See the third picture of these motorcycle rotors with a movable connection.

http://www.oppracing.com/product_di...rformance-brembo-hp-front-floating-rotor-kit/

The M rotors are a two piece design with an air gap to isolate the hat from the disk but they do not rattle.
BMW defines them as fully floating, the current M3 uses the same design (http://www.bmw.com/com/en/newvehicles/mseries/m3sedan/2007/allfacts/ergonomics/compound_brake.html). The stainless steel pins are held rigidly in the aluminium hat and locate into the slots in the cast iron disc/rotor (they seem to also have small bushings between the slots in the rotor and the pins, looking at this picture: http://www.s14.net/photopost/data/500/555E46-M3-disc-to-bell-spokes-1.JPG). You will notice that sometimes you can get quite a bit of pad push back leading to more pedal travel than expected after prolonged periods without braking, due to the rotors needing to align themselves. Perhaps BMW runs tighter tolerances between the pins and the rotor than the average motorcycle manufacturer?
 
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