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Thread: chainrig rub

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    chainrig rub

    I posted this in the road bike forum will here to see about another question. I have an FSA mgeo expo carbon/light 39/53 crank on my Motobecane Immortal Spirit. It has dura ace brifters and shifts beautifully from big to small. In the big ring I get all 10 rear cogs although I avoid cross chaining but it can work in a pinch. I did notice in the small ring on the 12, and even a little on the 13 cog I get some chain rub on the chainring itself on the bottom as I it returns to the rear.

    I realize this is cross chaining so I avoid this, but is this normal? From other responses some road bikes do this, I know my other bike with a compact crank does not so that is what I have to go on. Is this common and here is another question.

    It has a 68mm bottom bracket english treads and I notice an Italian is 70mm. This might make things better but can I use and Italian BB with this bike. It seems the tread count is different and of course it is not reversed threaded?

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    Senior Member Retro Grouch's Avatar
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    That's fairly common.

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    No you cannot use an Italion BB. The 68 mm is the shell width not the spindle length. Conventional BB's can be shifted a few mm's with a spacer, don't know about yours though.
    Last edited by Al1943; 01-01-11 at 11:09 AM.

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    Chain rib in the small chainring and smallest one or two cogs is common, particularly with a significant difference in chainring sizes; 53/39 chainrings often have this problem while 52/42 chainring sets usually do not.

    You cannot substitute an Italian bottom bracket for your English one. First, as Al1943 noted, the width difference is in the bottom bracket shell, not in the spindle and second, the threading diameter and drive-side cup thread direction are both different.

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    Thanks I hate to be stupid about this but someone has to be, this really helped.

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    Senior member Dan Burkhart's Avatar
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    Quote Originally Posted by HillRider View Post
    Chain rib in the small chainring and smallest one or two cogs is common, particularly with a significant difference in chainring sizes; 53/39 chainrings often have this problem while 52/42 chainring sets usually do not.

    You cannot substitute an Italian bottom bracket for your English one. First, as Al1943 noted, the width difference is in the bottom bracket shell, not in the spindle and second, the threading diameter and drive-side cup thread direction are both different.
    Actually, you can, but it's a fix usually reserved for rescuing a valued frame when the original BB threads are totally shot.
    Involves reaming, tapping, and 2 1mm spacers.
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    Quote Originally Posted by deacon mark View Post
    Thanks I hate to be stupid about this but someone has to be, this really helped.
    Not stupid at all. I don't worry about chainrub on the side of the big ring if the chain is fully cross-chained, as you pointed out, this combination should not be used anyway. I like to have the option of using my 39t inner ring with the second smallest cog though I try to avoid this too. If it is important to you to use the second smallest cog then you may want to research the possibility of a bottom bracket spacer to move your drive-side cup and spindle to the right 1.5 or 2 mm. This has been a common and easy fix for your problem, but I don't know if you can do that with your particular bottom bracket if it is an outboard bearing type.

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    You can eliminate the chainring rub when crossed outer rear to inner front by increasing the chainring separation a bit. There are 0.6mm spacers sold for this purpose. Before you run out and order spacers, consider that the cure might be worse than the problem because excess separation can cause a narrow chain to hang up between the rings on down shifts.

    If it's minor, and you don't expect to use the problem combinations, you're best off living with it. If it bothers you, try the spacers. Worst case scenario, it won't shift right and you'll remove them and be back where you started except out a few bucks and some time.
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