BB Size

Twostage

When I changed from a double to a triple I went for a 113mm BB as specified in the shimano tech docs. Having had to fiddle with my front derailleur recently I noticed two things :-

a. The mech hits the seat tube when in the small ring (adjusting the low screw so it doesn't hit means it won't drop into the granny ring) and
b. when I pull the cable hard the mech swings up at the very end of the travel.

I'm wondering if I should have gone for a 115 or 118mm BB as this would prevent 'a' and mean I could lower the DR as it would lift itself up for the biggest cog.

Opinions - 115, 118 or leave it ?

Monty Dog

Measure the chainline of the chainrings i.e. from the frame tube centreline to the centreline of the middle chainring - should be 45mm:

http://www.sheldonbrown.com/chainline.html

If less, then you need to fit longer BB. If OK, then you need to be wary that increasing your BB length might incur drivetrain problems at the other end of the gear range.

Twostage

Interesting, good link. Measured the distance to the middle ring and it's more like 48mm so that makes it look like it's too far out as it is. The thing is it can't be otherwise I wouldn't have to have the cable completely slack with the low screw fully wound back for it to drop into the small ring (plus the mech hits the frame). I note from the Sheldon-Brown link that for MTBs the recommendation is for 47.5 as they have fatter tubes. Wonder if there is something unusual about my frame in that respect. I also note he says you can use sprocket spacers between the bb and the frame to push the bb further out.

Might try simply loosening off the bb so it sticks out a few mill on the drive side and see what effect it has. If it makes it better I could get a suitably sized spacer (or multiple), if it makes it worse I'll just tighten it back.

Berk Bonebonce

Chain line is not life or death and if you have to increase it to get the set up working properly, then so be it.

Monty Dog

Agree, sticking a spacer on the drive side is OK as long as the crank isn't hitting the chainstay on the other side.

Twostage

Finally got round to ordering some sprocket spacers from sjscycles. Fitted them (2) to the right hand side of the bb (pushing it out by 3.2mm). A quick adjustment of the top and bottom limit screws and cable tension and it now goes into the granny ring a lot easier and the big ring still works .
The other crank still has plenty of space and the crank magnets are not a problem either.