Is it possible to shorten the chainline without getting a new BB?
 

I have a 50mm chainline, just got a new FD that requires a 45mm chainline. Is getting a new BB the only way to solve this?

Thanks,


Mot

different crankset..

Yeah, that might actually be a good alternative since my current one isn't the best/is fairly worn.

Thanks for the idea, friend!

Alright, so I went on a ride just expecting to only be able to use the two smallest chain rings. Well it turns out I CAN use the third one, but I have to pedal slowly while holding the shifter all the way as far as it will go.

So I'm guessing it's only like 2-3mm off at most. I am really hoping someone at the bike shop can adjust it better than I did and I won't have to get a new FD just for 2mm... Also it's a fairly new bike and I've heard the cranks can come loose after the first couple hundred miles. Is this true?? Could tightening them possibly make up for those couple mm's??? :twitchy:

If you have long enough chainring bolts, you might be able to use a washer between the carrier and the rings to move them a sufficient amount inboard.

If it's a square taper bb, there is theoretical possibility for some inboard creep. But the rest of the designs shouldn't offer that possibility.

Depending on what bb shell you have, and how finicky you are, it might be possible to remove one spacer from the driveside of the bb shell to shift the bb slightly inboard. It'll leave you with the NDS crank a little further outboard, which shouldn't be an issue IRL.

If it's square taper, tightening the bolt harder should do the trick. If you have a crank puller, pull the crank first and lube the taper. Either way, pull the bolts and lube the threads and under the head first.

Depending on your derailleur a limit stop or cable adjustment may be enough to accomplish what you want. I recently needed a derailleur to go just a little farther outboard and was able to file the internal stop (where the limit screw hits) enough to get it working satisfactorily.

Great way to destroy a crank!

Yep! Definitely a risk of that...

=8-)

It's not a super nice bike, as long as it works and it's still safe to ride that's all that matters right now.

If the outer-limit screw is already unscrewed so the FD stop no longer contacts, then your only alternative is to move the rings in. Most rings have the teeth offset from centreline. Flip middle & outer rings so that the flat surface is on the inside. That will bring the teeth inwards about 3mm. That may be enough in your case.

It sounds like you are using a road derailleur with a MTB crank and BB.

Why are you hesitating to get a new BB? This would be the correct solution and not be too expensive (if you have a squatre taper BB, less cheap if you have a splined BB, and not possible if you have an integrated BB).

+1.

One of the issues with index shifting is all the drivetrain parts need to be designed to work together, Even getting a different bottom bracket or crank, won't solve the issue of the chainline for the cassette now being off; does the bike 135mm dropouts? Regardless, I would get the proper front derailleur or there will always be some problem with shifitng. As far as the SQ crank goes, at some point within the first 10-20 miles you should re-check the torque. It's rare that it will loosen up, but if it does and the result is a damaged beyond use crank. So, either get yourself a torque wrench, or give a really good effort with a 8mm socket and 8" socket wrench..