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While warming up on the trainer for a race, I noticed that my chainrings have several millimeters of side to side movement when pedaling. It doesn't feel like the bottom bracket is 'loose,' but I'm worried that something is wrong with it and that perhaps it needs to be serviced and/or replaced. It's a Ultegra BB, with 6k miles on it. Am I overdue for an overhaul on the BB?

Thanks for any insight. Wanted to check here before taking it to the LBS.

mike
 

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maybe not be the BB...

You can't overhaul an Ultegra BB, just replace it, if it's worn out.

If you're noticing that the chainrings don't maintain exacty the same gap between the big ring and the derailleur cage as the cranks are rotated, it may just be a case of chainring runout due to a slightly bent chainring. Even the best new rings aren't perfect. Pay close attention and see if the same spot on the chainring is always closest to the deraileur. If a ring is actually bent, it can be straightened. There are special tools for the purpose, but an crescent wrench may work just as well.
 

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You were probably just observing trainer wag which is due to the frame being fixed in an unnatural fashion. It's normal but a big part of why I don't like riding a trainer.
 

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...or maybe the spider arm of the crank.

I've had two DA cranks replaced in the past due to a 2mm to 3mm wow in the chainring path caused by the spiders not staying "true".

Bryan
 
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