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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 07-19-02, 04:08 PM   #1
john_dun
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Front derailler

I took my bike to the LBS to fix my gears. The front derailler wouldnt move the chain the to top cog. When I went to collect the bike he said he couldnt fix as the wrong size of bottom bracket had been fitted!

A week before that it would change to the 3rd cog.

I had been messing about with the bottom braket before, could I have put it on the wrong way or something?

Any suggestions??

Thanks.
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Old 07-19-02, 05:08 PM   #2
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It is possible to put the bb in wrong. There are two cups on either side of the bb. One of them has a flange on it to position the bb in the frame the other will screw all the way in so you can tighten the bb. Check and make sure they are on the proper sides. If not it will cause the spindle to be off. This may or may not be the prob. but since you had the bb out you might want to check it. There are some bb that do not have the flange, in that case you might want to ask you LBS mech. to take a look at it. Did you adjust any thing else while the bb was out? Good luck
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Old 07-19-02, 05:15 PM   #3
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Ive checked it any it is in the correct way.

Beside the point - i was wondering tight i should make it - should i be able to turn the axle bit without the pedals being on?
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Old 07-19-02, 06:14 PM   #4
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if it's the correct BB installed the right way and it was working,the shop guy should have got it right without resorting to the lame 'wrong BB' story. If it's a cartridge type,there is no adjustment.If it's cup and ball, there should be no play but the spindle should trun easily by hand.
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Old 07-19-02, 06:59 PM   #5
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Also, look down at the crankset from above. The inner face of the inner chainring should be approximately flush with the outer face of the bottom bracket cup. If the gap is more than 3mm or so, the front derailleur may have a hard time getting the chain out to the big ring.

This doesn't exactly answer the question of why the crank would be out too far, since it still could be either an axle-length issue or an installation problem/error. But it's a start.

edit: Let me add that the bottom bracket cup on the drive side should be all the way into the frame, with its flange (assuming it has one) firmly fastened against the frame. If that were not the case, it needs to be addressed first.

Last edited by mechBgon; 07-19-02 at 11:19 PM.
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Old 07-20-02, 02:25 AM   #6
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The gap between the edge of the frame and the first ring of the cogs (whats the proper name for the front 3 cogs) is about 1cm!!
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Old 07-20-02, 09:00 AM   #7
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1cm is definitely too much space, particularly for a triple.

Is your bottom bracket the "traditional" kind, where you can take the spindle out? It's possible to put such spindles in either way, and they're often asymetrical. If you had it apart, or suspect the LBS did for some reason, it could have been reassembled with the spindle reversed. If you have the proper tools and knowledge to adjust your bottom bracket after investigating, maybe you should open it up and see if this could have happened.
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Old 07-20-02, 01:51 PM   #8
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Your cranks are not seating all the way down onto your spindle. This is a common problem with some of the Shimano sealed units, such as the UN-72. (Although there is only one standard taper angle for square-shank crank spindles -- 2 degrees, if I recall correctly -- there are at least two standards for the ultimate diameter of the tapered shaft.) If your spindle is too fat for your cranks, the latter will seat only halfway or so.

I try to set up triples so that the grannie ring just barely clears the chainstay under maximum load/flex.
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