Chain rub.....front derailleur.....

[stoatsngroats]

This has been hapening for about 6 months, when the left pedal was descending under load, and I thought it was a twisted chain.

I've just replaced my BB, and the problem has stopped...

So, my question is, If I had asked the question "Why does my chain rub when the left pedal descends under load?" would anyone have suggested the BB?

Thanks!

 

[Globalti]

As long as the BB isn't slack with worn bearings allowing movement of the crankset it will have been due to tiny amounts of flexing in the frame and crankset, probably mostly in the BB axle and square tapers if that's what you had.

I fitted a Shimano Hollowtech crankset to a bike last year and was amazed at how much stiffer it felt than the old square taper crankset.

 

[accountantpete]

Possibly worn bearings within the BB allowing more play than they should or the BB wasn't tightened sufficiently.

 

[Mr Pig]

I may have nothing to do with the bottom bracket. By far the most likely cause of chain rub is that the derailleur is not aligned properly. You could get chain rub if the bottom bracket is worn allowing the chain rings to rock from side to side or the chain ring could be bent a little, which is not that unusual.

It is very possible that when the chain rings were fitted to the new bottom bracket, assuming it's a square taper, that they weren't pushed into exactly the same position. So they may be sitting slightly further in or out and are now missing the derailleur. Even if the original bottom bracket was not worn the frame will flex under load and that could cause the rub. Without having seen it it's impossible to say.

 

[Globalti]

Wot i shud hav sed

 

[stoatsngroats]

Thanks for those - I'm happy to know that I seem to have no problem now, and I did it all myself too!