Can a rotor naturally go back into true?...- Mtbr.com
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  1. #1
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    Can a rotor naturally go back into true?...

    Calling all braking experts...
    Have a Avid Cleansweep...I know on the back of my 29er. Don't believe I ever hit the rotor on anything but is went out of lateral true which made adjusting my BB7 caliper a bit nighmarish because there really wasn't enough clearance to compensate for amount of run out. But...over time...after riding, it seems my out of true rotor went back into true allowing me to adjust my caliper a bit tighter. The question is...can a rotor go out of true and then back into true as easily for the same reason, i.e. the rotor heats up and spot loading the rotor can cause either out and back in true?

    Curious of what the experience of others has been. I didn't want to mechanically try to straighten the rotor by bending it...but it seems to now have straightened out...perhaps due to heating an prolonged braking while descending.

    Thanks for any advice..

  2. #2
    What could go wrong ...
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    " I didn't want to mechanically try to straighten the rotor by bending it."

    why not ? ... Park even makes a tool just for that purpose
    I used to ride to Win ... Now I ride to Grin

    While my guitar gently weeps, my bike sits there mocking me

  3. #3
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    It was hard to see exactly where the wobble was and if not bending in the right place, would make matters worse. It did come back into true however...which I presume occurred by descending which heated the rotor and prolonged brake application.

    Question remains if anybody knows.

    PS. Park has a tool for everything. I own a few of them but believe most use a large crescent wrench for straightening rotors.

  4. #4
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    I use the Park Tool for straightening rotors all the time. Its much easier to use the Park Tool due to the slimmer size. But an adjustable wrench works ok in a pinch too.

    Just watch where it makes contact on the brake pads and easily tweak the rotor in the opposite direction until the noise goes away.

    It seems like every 2 out of 3 disc brake equipped bikes I build at the LBS I work for, need this done.

    Its not an issue.

  5. #5
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    Quote Originally Posted by frdfandc
    I use the Park Tool for straightening rotors all the time. Its much easier to use the Park Tool due to the slimmer size. But an adjustable wrench works ok in a pinch too.

    Just watch where it makes contact on the brake pads and easily tweak the rotor in the opposite direction until the noise goes away.

    It seems like every 2 out of 3 disc brake equipped bikes I build at the LBS I work for, need this done.

    Its not an issue.
    Pretty remarkable frequency of out of true rotors on mtb's. No substitute for the experience of working on a lot of bikes for the best understanding.
    Thanks for sharing your experience.

  6. #6
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    Quote Originally Posted by dirtrider7
    Calling all braking experts...
    Have a Avid Cleansweep...I know on the back of my 29er. Don't believe I ever hit the rotor on anything but is went out of lateral true which made adjusting my BB7 caliper a bit nighmarish because there really wasn't enough clearance to compensate for amount of run out. But...over time...after riding, it seems my out of true rotor went back into true allowing me to adjust my caliper a bit tighter. The question is...can a rotor go out of true and then back into true as easily for the same reason, i.e. the rotor heats up and spot loading the rotor can cause either out and back in true?

    Curious of what the experience of others has been. I didn't want to mechanically try to straighten the rotor by bending it...but it seems to now have straightened out...perhaps due to heating an prolonged braking while descending.

    Thanks for any advice..
    Possible, but not likely. In order to heat the rotor hot enough for it to become malleable, you'd likely destroy the pads and some other plastic or rubber bits associated with the brakes.

    But the spot loading of the rotor is quite likely what forced the rotor more or less back into true. It likely isn't perfect but forcing the rotor through the pads under braking certainly can smooth out minor wobbles in the braking surface. How well it works depends on how you run your brakes, i.e. where in the lever pull the pads make contact and how badly out of true the rotor is. This determines how "ture" your rotor needs to be. I have a customer at the shop that prefers almost instant pad contact on his bb7s. The levers move only about 3mm before the pads make contact. His brakes are the biggest pain to set up as the rotors have to be true within .05mm or they rub. That kind of precision requires a truing stand and dial run out caliper to achieve.

    It think that a combination of a fairly small spot on the rotor and spot loading of that spot during use is likely what smoothed out your rotor. However I wouldn't count on it working for larger out of true sections. Learning to true a rotor is something that I would highly recommend anyone that uses disc brakes learn to do. It can be done with a small cresent wrench or a Park Truing fork or similar tool, and it isn't that difficult.

    Anyway, glad it worked out for you.

    Good Dirt
    "I do whatever my Rice Cripsies tell me to!"

  7. #7
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    Thanks for your thoughtful reply Squash...all makes sense. Good to learn that rotors going out of true is not uncommon. I will indeed heed this as something I need to practice.
    As to your customer wanting perfection with BB7's...I give you a lot of credit for doing this.
    I would say most shops wouldn't and does indeed sound like a PITA with little return. Better to have the custom learn to live with more practical runnout. BB7's aren't the most forgiving there in my experience but great brakes if the rotors are close to straight.
    For my 29er in back...a Moto Ti Fly I went to an organic pad because I couldn't tune out the squeal in back. I have a Windcutter rotor on deck which likely will work with metallic pads for longer life...after I wear the organic pads out....but that's another subject.

    Thanks again for sharing your experience.

  8. #8
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    No problem dirt. As far as not being able to tune the squeal out of the rear, sometimes it happens. I had a ridged commuter with a Surly 1x1 fork on it that I couldn't completely tune the squeal out of the front brake. The only time it squealed was at low speed with the rotors warm. But I couldn't for the life of me get it to quit. Just something about the fork/frame and brake combination that set it up I guess. Put the fork on a different frame and no problem. So you will occasionally find a frame, fork, or brake combination that just wants to make noise. That's the only bike set up I've ever had that I couldn't get a BB7 brake to be quiet on. Not real common but it happens.

    Good Dirt
    "I do whatever my Rice Cripsies tell me to!"

  9. #9
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    Quote Originally Posted by Squash
    No problem dirt. As far as not being able to tune the squeal out of the rear, sometimes it happens. I had a ridged commuter with a Surly 1x1 fork on it that I couldn't completely tune the squeal out of the front brake. The only time it squealed was at low speed with the rotors warm. But I couldn't for the life of me get it to quit. Just something about the fork/frame and brake combination that set it up I guess. Put the fork on a different frame and no problem. So you will occasionally find a frame, fork, or brake combination that just wants to make noise. That's the only bike set up I've ever had that I couldn't get a BB7 brake to be quiet on. Not real common but it happens.

    Good Dirt
    Yeah...it happens as you say. Resonant frequency of combined components....a lottery. In my experience even the peskiest of squealing can be tuned out with either change to organic pads, different rotor or last resort...different brakes.
    Avid's have a rep for squealing to be sure but I sure love the simplicity of the BB7's.
    My Moto Ti Fly 29er is pretty quiet now by just changing the rear pads to organin BB7's.
    The rear triangle of this bike seems sensitive to squeal which can be quieted by changing a few things out.
    Alligator windcutters are reported to solve the issue on the back of the Ti Moto even with metallic pads.
    Thanks for the advice Squash.
    Ride safe...

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