help with chainset/BB issue

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PeteMadoc
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help with chainset/BB issue

Postby PeteMadoc » Mon Nov 19, 2012 14:02 pm

OK so a friend bought a second hand bike and I offered to change the chainset and cassette over for her (needed easier gears)

It's shimano 2300 or sora stuff

Problem: After adjusting the front mech, I spin the cranks and the chain will rub against the mech, then not rub, then rub, then not etc etc. It's as if the chainset is bent. I know it's not the chainset because the last one did the same thing. I'm stuck on this one, is the frame bent or a misalignment in the BB? Can it be fixed?

careful
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Re: help with chainset/BB issue

Postby careful » Mon Nov 19, 2012 14:41 pm

If you are sure that the chainset is not buckled then the only other thing that I can think of is end float in the BB bearings. Did you tighten the end-float screw correctly (in the centre of the L/H crank) to eliminate float before tightening the crank bolts? I dont think that misalignment of the bearings themselves, or of the frame threads would cause these symptoms.

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nicklouse
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Re: help with chainset/BB issue

Postby nicklouse » Mon Nov 19, 2012 14:56 pm

adjust the mech a bit more. most rings have some movement on them.
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John.T
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Re: help with chainset/BB issue

Postby John.T » Mon Nov 19, 2012 15:08 pm

If it is a square taper BB the axle could be bent or the taper damaged.

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PeteMadoc
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Re: help with chainset/BB issue

Postby PeteMadoc » Mon Nov 19, 2012 15:52 pm

I've adjusted the mech as much as possible and she'll just have to accept some chain rub when in the big ring in two lowest gears and ever so slightly in the top gear, not a massive problem. There's no chain rub in when in the little ring which makes me think it's some sort of BB damage/bend

Maybe the bike has been dropped or crashed. :(

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nicklouse
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Re: help with chainset/BB issue

Postby nicklouse » Mon Nov 19, 2012 15:57 pm

PeteMadoc wrote:I've adjusted the mech as much as possible and she'll just have to accept some chain rub when in the big ring in two lowest gears and ever so slightly in the top gear, not a massive problem. There's no chain rub in when in the little ring which makes me think it's some sort of BB damage/bend

Maybe the bike has been dropped or crashed. :(

it will not be the BB manz Square teper cranks does have a little wobble in them.

and chain rub in crossed chain conditions happens and should be ignored as you should not use the gears in that condition. you should be able to set it up so no rub in Big and small both ways.
"Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."

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PeteMadoc
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Re: help with chainset/BB issue

Postby PeteMadoc » Mon Nov 19, 2012 16:03 pm

nicklouse wrote:
PeteMadoc wrote:I've adjusted the mech as much as possible and she'll just have to accept some chain rub when in the big ring in two lowest gears and ever so slightly in the top gear, not a massive problem. There's no chain rub in when in the little ring which makes me think it's some sort of BB damage/bend

Maybe the bike has been dropped or crashed. :(

it will not be the BB manz Square teper cranks does have a little wobble in them.

and chain rub in crossed chain conditions happens and should be ignored as you should not use the gears in that condition. you should be able to set it up so no rub in Big and small both ways.


I know some chain rub will always be there, I've got it to acceptable levels but there's definitely a wobble which is applified in the big ring. Are you saying a wobble is normal on these square BBs?

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John.T
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Re: help with chainset/BB issue

Postby John.T » Mon Nov 19, 2012 17:32 pm

Square taper is the least accurate method of fitting cranks. As nicklouse said you should be able to adjust the mech to remove most of the rub. If it is excessive in only one place you can always try a little 'adjustment' of the ring itself with a suitable adjustable spanner.


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