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      06-22-2022, 11:55 PM   #1
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What's the Deal with the "Grooves" in Z4M Rotors

I had noticed these "grooves" in my rotors before, but never thought too much of it. I'm in the middle of replacing my original brakes with oem bmw rotors/pads and noticed something - the replacement rotors have grooves that span very little of the diameter compared to my original ones (see pics). So does anyone know what the design intent of these grooves are, and why bmw would change the design over time? My car is MY2007. More a curiosity than anything. Thanks in advance!
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      06-23-2022, 12:20 AM   #2
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If you're referring to the "grooves" in the center venting vanes, I believed it's for balancing the rotor. It's a method for them to remove materials in order to properly balance it. Each rotor should be slightly different, if you look closely.
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      06-23-2022, 12:28 AM   #3
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Ah, interesting. Yes, in the center venting area. The new ones have material removed too, just for roughly 10 degrees of the diameter. My originals are like 120 degrees. It’s a bad pic - should have snapped one of the other side that’s not on the car yet.
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      06-23-2022, 12:39 AM   #4
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Our Audi's rotors have similar grooves, but runs shorter (only about 3 to 4 inches long) compared to what's currently on the Z4. I'm currently running Zimmermann front rotors on the Z4 and it has similar groove as the OE rotors that it replaced...similar to your old rotor pictured in the original post.
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      06-23-2022, 07:57 AM   #5
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Quote:
Originally Posted by XMetal View Post
If you're referring to the "grooves" in the center venting vanes, I believed it's for balancing the rotor. It's a method for them to remove materials in order to properly balance it. Each rotor should be slightly different, if you look closely.
Makes sense.

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      06-23-2022, 08:18 AM   #6
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This is great, thanks guys. Since I’ve got 8 rotors, I’ll pay attention and post up how much they all differ if anyone is curious. Will take me a bit more time though - not sure when I’ll finish the job.
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      06-30-2022, 09:39 AM   #7
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Ok so wrapped the job up. Basically all 4 fronts had about a 40 degree cutout. The old rears had about a 120 degree cut out and the new ones had a 10 degree cutout. It’s probably the case that each manufacturing lot will have similar cutouts.
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      07-08-2022, 01:04 PM   #8
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40 fronts with 40 degree each
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      07-08-2022, 01:06 PM   #9
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40 fronts for 40 degrees each
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