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  1. #1
    It's MY mountain DiabloScott's Avatar
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    Bottom Bracket Loosening

    What can cause bottom bracket loosening? This is an ISIS cartidge BB, Italian threaded, right/drive side "cup" that somehow in normal riding unscrewed almost all the way out. I didn't notice it until my crank moved so far out I couldn't get it into the big ring. Note that the unscrewing motion is opposite the pedaling direction.

    Some kind of precessional motion from pedalling?

    And how did it unscrew that far without pushing my crank off the spindle?
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  2. #2
    DOS
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    Righthand threaded fixed cups (French and Italian) have a tendency to loosen unless securely tighened. I'm sure there is a physics text book explanation about centrifugal (centripetal?) force but since I am a simpleton, when pedaling forward the cartridge body wants to roll forward so cup will want to work in opposite direction.. counterclockwise.

    Edit: Forget that crap above; read this thing about Precession instead :http://www.sheldonbrown.com/brandt/left.html
    Last edited by DOS; 02-18-09 at 02:42 PM.

  3. #3
    cab horn
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    Quote Originally Posted by DiabloScott View Post
    What can cause bottom bracket loosening? This is an ISIS cartidge BB, Italian threaded, right/drive side "cup" that somehow in normal riding unscrewed almost all the way out. I didn't notice it until my crank moved so far out I couldn't get it into the big ring. Note that the unscrewing motion is opposite the pedaling direction.

    Some kind of precessional motion from pedalling?

    And how did it unscrew that far without pushing my crank off the spindle?
    Go read post #2. The simple answer is - reinstall with proper torque. Check that the thread aren't ****ed as well, if the right cup looseining problem is not due to bb shell anomaly/proper torque then you can try reinstalling with blue loctite. You really shoudln't need to do this. There is a reason why nearly every single modern bike comes with english threaded bb shells (reverse threaded on the drive side)
    Mes compaingnons cui j'amoie et cui j'aim,... Me di, chanson.

  4. #4
    It's MY mountain DiabloScott's Avatar
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    Quote Originally Posted by operator View Post
    Go read post #2. The simple answer is - reinstall with proper torque. Check that the thread aren't ****ed as well, if the right cup looseining problem is not due to bb shell anomaly/proper torque then you can try reinstalling with blue loctite. You really shoudln't need to do this. There is a reason why nearly every single modern bike comes with english threaded bb shells (reverse threaded on the drive side)
    Good answer, not sure why I didn't find that before, it explains the problem. BUT...

    reason some right BB cups have used right hand threads and some with left hand threads have loosened.

    So it appears to be a problem with English threaded BBs as well, although less so. AND...

    Precession forces are large enough that no manner of thread locking glues, short of welding, will arrest them.

    So locktite appears not to be a solution.

    I don't ever remember a real fixed cup coming loose, but if it started to (or you just wanted to check), you could put a wrench on it without taking off the crank... cartridge types you can't.
    http://diabloscott.blogspot.com/

  5. #5
    DOS
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    Quote Originally Posted by DiabloScott View Post
    reason some right BB cups have used right hand threads and some with left hand threads have loosened.
    So it appears to be a problem with English threaded BBs as well, although less so. AND...


    I think the point is that chain tension and pedaling force counteract precession and supports the case that a properly torqued cup regardless of threading should not loosen as a result of precession. That is, the act of pedaling puts competing forces on the cups. A right had thread cup that is not torqued fully can succumb to precession; less common, but possible, a left hand thread cup installed incorrectly can succumb to the opposite force from the chain and pedaling forces. A properly torqued cup will stay tight because the forces working on it largely cancel each other out.

    See how much I have learned in the last 24 hours. Thank You Mr. Brandt.

    Precession forces are large enough that no manner of thread locking glues, short of welding, will arrest them.

    So locktite appears not to be a solution.
    .
    The point about locktite not withstanding precession forces regards freewheels, not BBs.

  6. #6
    It's MY mountain DiabloScott's Avatar
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    Quote Originally Posted by DOS View Post
    The point about locktite not withstanding precession forces regards freewheels, not BBs.
    Nope, BBs and pedals. Locktite is generally for things that vibrate loose.

    Precession forces are large enough that no manner of thread locking glues, short of welding, will arrest them. Mechanical fretting, the micro-motion of tightly fitting parts moving against one another, is the mechanism of this motion. Motion in these joints causes visible fretting rouge, red iron oxide, on the shoulder of the BB cup and on the face of the pedal spindle.
    Last edited by DiabloScott; 02-19-09 at 12:45 PM.
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  7. #7
    DOS
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    Quote Originally Posted by DiabloScott View Post
    Nope, BBs and pedals. Locktite is generally for things that vibrate loose.

    Precession forces are large enough that no manner of thread locking glues, short of welding, will arrest them. Mechanical fretting, the micro-motion of tightly fitting parts moving against one another, is the mechanism of this motion. Motion in these joints causes visible fretting rouge, red iron oxide, on the shoulder of the BB cup and on the face of the pedal spindle.
    Hmm, but I found another citation online that quoted Brandt thusly "bottom brackets are not subject to nearly as much precession- inducing load as are pedals. As the cranks rotate during riding, the pressure (which would induce precession) being exerted by the act of pedaling is being counteracted by the tensions being exerted by the chain which is being pulled by the pedaling motion the two loads all but cancel each other out. Thus the fixed cups of standard bottom brackets are reverse-threaded more on principle than for any practical purpose."

    Thus it would seam logical that locktite might help with BB cups but not with pedals or freewheels because precession is not countered by some other force. That said, my original point remains, with proper torque, BB cups should stay put, locktite or no.

    Look at me arguing about something that, until yesterday, I knew nothing about.

  8. #8
    It's MY mountain DiabloScott's Avatar
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    Quote Originally Posted by DOS View Post

    Look at me arguing about something that, until yesterday, I knew nothing about.
    This is not arguing - this is investigation and discovery!


    Anyway... I had the bike shop fix it because I couldn't even get my crank off, so hopefully they torqued it down good whether they Locktited it or not.
    http://diabloscott.blogspot.com/

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