bottom bracket and front deraillure
 

Hi,

I recently replaced the bottom bracket and chainset on my mountain bike. The old bottom bracket had a 122 square taper spindle, while the new one has a 118 octalink spindle.

The problem is, now everything is reassembled, the front derailleur won't move far enough to shift the chain onto the largest gear. I've tried adjusting the screws to give it more movement, and even when I pull the cable by hand rather than using the shifters, it doesn't go far enough. Is my only option getting a new bottom bracket with a shorter spindle?

Thanks,

Ben

Probably you need to replace the bottom bracket with a shorter one.

How much clearance do you have between the driveside chainstay and the chainrings? Do you have enough that a shorter spindle will let them still clear?

Yes, it looks like there's plenty of space between the chainring and chainstay.

Just to make sure we're all on the same page, your FD a is "high normal", sprung to move out, and moved in by cable? I want to confirm that because it's the opposite of normal.

If you have good chainstay/chainring clearance it should be able to move in far enough. Take a careful look at what's limiting it's travel and make sure there's nothing wrong.

If you only need a wee bit more travel, you might be able to get it with some judicious filing. Also check that it's properly squared up (I expect you probably did). You can sometimes gain a bit more inboard travel without a loss of performance by bringing the heel in a few degrees.

Otherwise it's either a new longer BB or new FD.

One last option, depending on how the left side of the BB fits is to put a 2mm shim between the right cup and frame, moving the entire BB to the right.

I didn't interpret the OP as having a high normal fd but that the new crank, despite the shorter spindle, sits further out from the frame and the fd's travel isn't enough to reach the big ring. He said pulling the cable by hand still didn't move it far enough which implies a standard low normal fd. He also said adjusting the limit screw didn't provide enough travel but I'd check to be sure he was adjusting the correct screw.

If this is a MTB crank, other than XTR, Octalink V-2 bottom brackets are available in 113 mm as well as 118 mm and 121 mm in both 68 and 73 mm bb shell widths. If it's a road crank, the V-1 Octalinks are available only in 109.5 (double) and 118.5 (triple) mm lenghts for 68 mm bb shells. If he has an MTB crank and the front derailleur really can't be made to reach the big chainring, it sounds like a 113 mm spindle is needed.

BB length is crankset specific, rather than bike specific. So getting the length right can be a little bit of trial and error. This is one reason buying bb at your LBS can make a lot of sense, even if they charge more. Returning bb bought on line can be a PITA.

Thanks for your help.

This is correct.

I tried adjusting both limit screws just to make sure.

I've just noticed I may have made an octalink version error. The bottom bracket is v1 (this one) while the cranks are mountain bike ones (but they don't specify that they're V2 here).

Could this miss-match cause the chainset to sit further from the frame than it should?

I find when the 3rd ring is sitting over the edge of fixed cup, then all works well.

Yes, see pic
http://bikeforums.net/attachment.php...hmentid=234310

Unless your crank is the top-of-the-line (read expensive) XTR, it requiires a V-2 Octalink and, yes, V-2 and V-1 octalinks are incompatible. The V-2 has longer and wider splines so a V-2 crank will not go onto a V-1 bb far enough. If you have a V-2 crank on a V-1 bottom bracket, that certainly could explain your problem. Look here for a description and pictures of the two types: http://www.sheldonbrown.com/gloss_n-o.html#octalink Scroll down to "Octalink"

Different cranks can require very different spindle lengths, even with the same spindle interface!

I recently replaced a sq. taper crank on my old RockHopper.
Old crank took a 122 mm spindle, while the new crank took 113 mm spindle. Even the 113 was a mm or 2 longer than the original combination.