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Discussion Starter · #1 ·
I recently put a 203mm rotor on my 3 year old shimano XT 4 piston rear brake. I was happy with the 160mm rotors, but occassionaly wanted less fade on long downhills.

I paid about $35 each for the rotor and the adaptor and they went straight on. The first ride of about 2 hours was fine: more power, modulation was fine and they were quiet. But from the next ride they started to groan almost all the time and send so much vibration through the frame that the strip of rubber that keeps the rear hose in its guide vibrated out.

I tried cleaning and lightly scouring the rotor and re-torquing the bolts, but the groan was the same. Then I noticed that the adapter mounting bolts did not come with any washers so last weekend I used some spare caliper shims as washers between each bolt and the adapter and it has fixed it. So far (three rides) it is as quiet as can be; I hope it stays that way.

Wombat
 

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Very interesting I’ve wondered why the adapters never came up with any washers; always thought it was kind of odd. Ill do the same as well and get washers and report in. Are caliber shims the same as washers? I was thinking about this while I was stripping the paint off the rear mounts while installing the brakes. I have a 203 on the front as you do and its making a noise as well; the 160 in the back is whisper quiet.

Looking at my bike just about every bolt on my bike has a washer on it; most of the high stress ones. I’m just wondering from an engineering perspective what the purpose of the washer bolt combo is?

SLX :)
 

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Discussion Starter · #3 ·
SLX said:
Are caliber shims the same as washers? ... I'm just wondering from an engineering perspective what the purpose of the washer bolt combo is? SLX :)
Yes, shims are really just thin washers. I would have used washers if I had then, and thought of a spring washer that was the right size, but decided that it would just gouge the frame. I was surprised that the shimano bolts didn't have them.

I don't know what the "real" purpose of the washers is, but I suspect it is to provide a degree of separation (other than just vibration) between the bolt and whatever it attaches presses against. You are right that they are used in most circumstances on a bike or car.

Wombat
 
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