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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Suspension | Brakes | Chassis > Do I need to change rotors?



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      09-27-2012, 06:33 PM   #1
e90_will
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Do I need to change rotors?

So my brake light came on saying its worn down to the minimum thickness on my rear pads. I drive an 07 e90 335i sedan and just got up to 80k miles. Im pretty sure this isnt the first brake job so Im not too sure whether or not the rotors were replaced before. Would I be okay with just replacing the pads only and not the rotor/sensor? I only intend to do regular daily driving in SoCal weather, and dont experience any current vibrations or cant see any visible signs of warping, etc.

Duno if a pic will help, but thanks in advance guys.
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      09-27-2012, 06:43 PM   #2
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don't skimp out on the sensor. They are prone to breaking when doing a brake job. Plus, they're pretty cheap anyway. Rotor looks fine, and if you do not experience any vibrations on the wheel when breaking, then pads should be fine. rotors last a while too.

but then again rotors also have minimum thickness, you should have a caliper or something to measure it. Again, it may look good but it may be too thinned out
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      09-29-2012, 03:49 PM   #3
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agree with previous response on the sensor.
BMW Dealer told me it's not unusual to replace pads without replacing rotors, as long as rotor is still in spec. Said many cars can go through second set of pads before needing new rotors. That said, as previous response states, thickness of rotor being in spec is what matters.
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      09-29-2012, 07:02 PM   #4
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I'd replace the rotors at the same time. You won't ever have to do a brake job again unless you keep the car for a very long time. Also, if the surface of your rotors are out of shape in any way it you will notice it when you put the new pads on bc they are not "formed" to the face of the old rotor like the old pads were. also iirc bmw rotors cannot be turned so new rotors are the only option if that is the case.

"When installing new pads, the rotors should be new or at least resurfaced to remove any transfer film from the previous set of brake pads."
^ taken from a tech article on tirerack.
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      09-30-2012, 03:15 PM   #5
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Alright thanks for the info guys!
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      09-30-2012, 03:41 PM   #6
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Hard to tell from a picture like that, but your current rotors have significant lips, so they probably are lower than spec or very close at this point.
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      10-01-2012, 01:19 AM   #7
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what is the minimum rotor thickness on rear 335i E92?

... going to replace my pads shortly... debating if i want to do rotors at same time...
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      10-01-2012, 07:31 AM   #8
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Quote:
Originally Posted by Frosty View Post
what is the minimum rotor thickness on rear 335i E92?

... going to replace my pads shortly... debating if i want to do rotors at same time...
It is stamped on the hub of the rotor
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      10-01-2012, 08:02 AM   #9
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I don't think any pad manufacturer would recommend putting new pads on without resurfacing your old rotors. It is not a complicated process and can be done at home with a drill and a tool like this:

http://www.brushresearch.com/brushes.php?c1=6

Without breaking the glaze on the rotors and possibly removing friction material transferred to the rotor from the pad (some pads do this, others do not), you are impeding the ability of the new pads to "bed" into the old rotors. The new pads depend on a very specific surface finish to "bed in" and if you never resurface the rotors, this process can't happen. The medium grit (120) should be fine.

As others have said, ensure your rotors are not below the minimum thickness specified on the hub of the rotor.

[Edited for typographical error]
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      10-01-2012, 11:50 AM   #10
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Quote:
Originally Posted by ajm8127 View Post
I don't think any pad manufacturer would recommend putting new pads on without resurfacing your old rotors. It is not a complicated process and can be done at home with a drill and a tool like this:

http://www.brushresearch.com/brushes.php?c1=6

Without breaking the glaze on the rotors and possibly removing friction material transferred to the rotor from the pad (some pads do this, others do not), you are impeding the ability of the new pads to "bed" into the old rotors. The new pads depend on a very specific surface finish to "bed in" and if you never resurface the rotors, this process can't happen. The medium grit (120) should be fine.

As others have said, ensure your rotors are not below the minimum thickness specified on the hub of the rotor.


[Edited for typographical error]

oops i missed the 120 grit last time I read this... Thank you sir! , now I need a caliper to measure rotor..... ~Frost
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      10-01-2012, 12:32 PM   #11
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^ I just went through the whole thing myself on Saturday. I have 100.8k miles on my car... I just decided to do the rotors at the same time. I ended up with power slot rotors on all 4 corners with cool carbon pads. I also ordered new brake sensors too (which was great, since I pretty much broke one taking it off).

If you do plan on doing all 4, it is pretty straight forward, having access to an impact wrench will help greatly when taken the caliper brackets off.

Best of luck!

Here is a picture of one of rotors





I'd like to add that I had a lip on my rotors as well ...
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      10-03-2012, 06:36 AM   #12
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Another one? Measure thickness, and if below minimum spec, replace the rotor.
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      11-22-2012, 01:44 PM   #13
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I have a 09 335 xdrive and am doing the brakes in March of next year. I wanted to see if the car needed new rotors because its a 51k and if I'm already doing the brakes so it'd be easy to swap the rotors out at the same time. I took multiple measurements of my front and rear rotors and came up with these measurements (Fronts: 29.4mm, Rears: 23.8mm). The minimum requirement for the fronts is 28.4mm and I believe the rear is 20.4mm. So my question is, do you guys think the rotors will last another set of brake pads? I looked online and saw that the front factory rotors are 30mm new.
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