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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Suspension | Brakes | Chassis > Issue With Geomet Rotors and Cool Carbon Brake Pads



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      12-05-2012, 05:32 PM   #1
Tim603
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Exclamation Issue With Geomet Rotors and Cool Carbon Brake Pads

Hey Guys so I got the geomet slotted front rotors for my car and installed them about 1500 miles ago. I used the existing cool carbon brake pads on my car since I only installed them like 3000 miles prior to the new rotors so there was plenty of life left on them. Well they have been great thus far and now I have steering wheel wobble when i apply 40% or more brake at speeds around 30-70mph. Someone told me that it could be carbon buildup on the rotors making the surface uneven and that some heavy braking could help burn it off? Any suggestions on what I should do. Ive contemplated buying new brake pads to see if that will change anything. Thanks for any advice.
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      12-06-2012, 09:23 PM   #2
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Quote:
Originally Posted by Tim603 View Post
Hey Guys so I got the geomet slotted front rotors for my car and installed them about 1500 miles ago. I used the existing cool carbon brake pads on my car since I only installed them like 3000 miles prior to the new rotors so there was plenty of life left on them. Well they have been great thus far and now I have steering wheel wobble when i apply 40% or more brake at speeds around 30-70mph. Someone told me that it could be carbon buildup on the rotors making the surface uneven and that some heavy braking could help burn it off? Any suggestions on what I should do. Ive contemplated buying new brake pads to see if that will change anything. Thanks for any advice.
Sounds like you didn't properly bed the pads to the new rotors. Depending on the pads, the procedure may vary a little but the general concept is to do a series of 60 - 5 MPH hard stops with a few minutes of driving (while minimizing the use of the brakes) in between to let the brakes cool down. This ensures an even transfer of pad material to the rotor to prevent brake pulsation at high speeds.
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      12-06-2012, 10:02 PM   #3
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HERE is a great set of articles explaining the theory and procedures for bedding in brakes....
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      12-06-2012, 10:30 PM   #4
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Originally Posted by Tim603 View Post
Hey Guys so I got the geomet slotted front rotors for my car and installed them about 1500 miles ago. I used the existing cool carbon brake pads on my car since I only installed them like 3000 miles prior to the new rotors so there was plenty of life left on them. Well they have been great thus far and now I have steering wheel wobble when i apply 40% or more brake at speeds around 30-70mph. Someone told me that it could be carbon buildup on the rotors making the surface uneven and that some heavy braking could help burn it off? Any suggestions on what I should do. Ive contemplated buying new brake pads to see if that will change anything. Thanks for any advice.
It does sound like you may have a high thickness variation (TV) of the brake material on the rotors (the "transfer layer")that can result from not properly bedding in the pads to the new rotors. This can continue to get worse, as the pads continue to deposit more material on the "high spot" and skip the lower spots, much as a small dip in a road can quickly become a pothole.

The answer is not to brake heavily - the transfer layer is built up when the pads meet the rotors at high temperatures, thus resulting in more pad material being deposited. In light use, when the pads don't heat up and transfer material to the rotors, it's actually possible to "unbed" the pads from the rotors, since the stopping forces are more friction than adhesion, and the friction burns off the transfer layer from the rotor. For street pads this "unbedding" doesn't usually result in a loss of stopping power, and a few hard stops will re-bed that pads.

I suggest a few gradual "rolling stops" from 60 down to about 10, do not let the car stop with the brakes applied. Do this several times allowing the brakes to cool between stops by driving at a steady rate of speed without using the brakes. Then follow a normal bed-in process (see the articles I linked above) and see if that helps.
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      12-07-2012, 09:54 AM   #5
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The cool carbons are your problem.
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      12-07-2012, 11:31 AM   #6
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Quote:
Originally Posted by andrew_b View Post
It does sound like you may have a high thickness variation (TV) of the brake material on the rotors (the "transfer layer")that can result from not properly bedding in the pads to the new rotors. This can continue to get worse, as the pads continue to deposit more material on the "high spot" and skip the lower spots, much as a small dip in a road can quickly become a pothole.

The answer is not to brake heavily - the transfer layer is built up when the pads meet the rotors at high temperatures, thus resulting in more pad material being deposited. In light use, when the pads don't heat up and transfer material to the rotors, it's actually possible to "unbed" the pads from the rotors, since the stopping forces are more friction than adhesion, and the friction burns off the transfer layer from the rotor. For street pads this "unbedding" doesn't usually result in a loss of stopping power, and a few hard stops will re-bed that pads.

I suggest a few gradual "rolling stops" from 60 down to about 10, do not let the car stop with the brakes applied. Do this several times allowing the brakes to cool between stops by driving at a steady rate of speed without using the brakes. Then follow a normal bed-in process (see the articles I linked above) and see if that helps.
I will definately have to try this, I will let you know how it goes. Also I called ECS tuning and they said roughing up the pads with some sandpaper could also help, what do you think about that?

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The cool carbons are your problem.
I think I would have to agree, red stuff or Hawks in my future

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Originally Posted by SmellyGreek View Post
Sounds like you didn't properly bed the pads to the new rotors. Depending on the pads, the procedure may vary a little but the general concept is to do a series of 60 - 5 MPH hard stops with a few minutes of driving (while minimizing the use of the brakes) in between to let the brakes cool down. This ensures an even transfer of pad material to the rotor to prevent brake pulsation at high speeds.
Where would you guys recommend I do this, I need to find a place that doing this would be legal (or not completely obvious that Im breaking the law) and I cant think of any places. Highway is obv not an option b/c of how slow i need to get.

Thanks for all your help guys
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      12-07-2012, 11:50 AM   #7
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Tim - sounds like you need to re-bed the pads.

The idea behind ruffing up the pad a little is to remove it's "bedded face" and help the re-bedding procedure.
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      12-07-2012, 02:47 PM   #8
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Any suggestion on where I should Re-bed the pads?
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      12-07-2012, 03:24 PM   #9
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....I did mine on a nearby subdivision where they're building new houses. There's a 3/4 mile stretch with only 1 traffic light and 1 stop sign. I did 10 65mph to 20mph stops and 5 45mph to 15mph stops and a 2 mile cool down. Try looking for areas where they're building new homes...perhaps an empty parking lot, or long stretch of road with moderate or little traffic, but never the freeway.
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      12-07-2012, 03:25 PM   #10
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Back roads with higher speed limits (45-55 MPH) are typically good locations for bedding brakes. Never bed brakes around busy areas, or where there is a chance of children running about.
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      12-11-2012, 10:49 PM   #11
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Quote:
Originally Posted by andrew_b View Post
It does sound like you may have a high thickness variation (TV) of the brake material on the rotors (the "transfer layer")that can result from not properly bedding in the pads to the new rotors. This can continue to get worse, as the pads continue to deposit more material on the "high spot" and skip the lower spots, much as a small dip in a road can quickly become a pothole.
Bingo. Although, an improper bedding isn't always the root cause of inconsistent pad deposit.

Either way, trying to re-bed the pads/rotors probably won't do anything at this point, it's already too late IMO. It would be like trying to get the flat spot out of a tire by doing a burnout = not gonna happen.

The only way that I've found to get a "high spot" out of a rotor caused by pad transfer is to swap the pads with aggressive race pads THEN do the bed in process with those. The Cool Carbons, or any other street pad for that matter, will not be aggressive enough to "knock down" that high spot given their rotor friendly compound. After the high spot is gone, you can switch back to your street pads. Do this (good idea if you have race pads laying around or can borrow a set), machine the rotors (terrible idea), or buy new rotors (only other option).

Just my $0.02
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