Bicycle Mechanics - bottom bracket length?

I'll soon be starting my first build. The frame I've bought comes with a 68 x 110mm bottom bracket. I would like to use this crank: xd500 tripple (http://www.velo-orange.com/suoldloxdtrc.html) which says, "We recommend a 113mm bottom bracket for this crank if used on a road bike, but bikes made for wide tires, such as loaded touring bikes or mountain bikes, might need a 115mm BB."

So... which do I need: 113mm or 115mm? I'm thinking 115. Frame is the Thorn Sherpa and I'll be running a 9 speed 11-34 cassette and 26x1.5 tires. My riding will be a combination of daily commute, long weekend rides, camping several times a year, and eventually (if all goes as planned) a few 1-2 week tours.

If anyone cares to throw in a recommendation for front and rear derailleur I'm very open to suggestions. Currently I am planning to go for 105 fd and xt shadow long rd but some friends tell me to go for a mountain fd instead of the 105.

i would go with what your crank says

I'll soon be starting my first build. The frame I've bought comes with a 68 x 110mm bottom bracket. I would like to use this crank: xd500 tripple (http://www.velo-orange.com/suoldloxdtrc.html) which says, "We recommend a 113mm bottom bracket for this crank if used on a road bike, but bikes made for wide tires, such as loaded touring bikes or mountain bikes, might need a 115mm BB."

So... which do I need: 113mm or 115mm? I'm thinking 115. Frame is the Thorn Sherpa and I'll be running a 9 speed 11-34 cassette and 26x1.5 tires. My riding will be a combination of daily commute, long weekend rides, camping several times a year, and eventually (if all goes as planned) a few 1-2 week tours.


I agree on the 115. I suspect that what they're really saying when they talk about road bikes vs "bikes made for wide tires" is the difference between a 130mm (or less) road rear hub, and a 135mm mountain hub. In your case you likely have a 135mm hub, so a slightly longer BB will move your chainline out a bit to match the wider hub. Either way it's only a 1mm difference in chainline so I wouldn't shy away from trying a 113 if I couldn't get a 115 for some reason. I use a 113 on my Surly Cross-Check with an XT rear hub and a Sugino XD300 crank, though that's a 700C frame so it might be a little different.



If anyone cares to throw in a recommendation for front and rear derailleur I'm very open to suggestions. Currently I am planning to go for 105 fd and xt shadow long rd but some friends tell me to go for a mountain fd instead of the 105.

I use a Sora triple fd on my bike and it works great (running a 46-36-24 crank). Some Shimano FDs don't work well with the 10 tooth difference between the large and middle chainring, but the crank you're looking at has a 12T difference so it should be okay.

A mountain FD would be good if you're planning on dropping down to a 44T big ring, or you want to change your crank to something that has a really wide chainline, like the Shimano external bearing MTB cranks with 50mm chainline. In that case road FD's usually can't move the cage out far enough to reach the big ring. I tried a Shimano mountain FD on my cross-check and it was so bulky that it interfered with my rear fender. With your 26" wheels you might have more clearance behind the seat tube though.

As for your rear derailleur choice, you can't really go wrong with XT. I have an M760 long cage low-normal (aka Rapid Rise) rd, and it's been great.

Thanks for the info! I've ordered a 115mm bb.

Thanks for the info! I've ordered a 115mm bb.

That's what I would have done.
Less chance of the CR hitting the stay.
Since cog spacing is about 5MM or a bit less, depending on what you have on the back, you sre still talking about less than 1/2 cog thickness difference. Since 1MM of that is on the NDS side, it's really insignificant.
Better to be a bit caustious.