Mondo734

I am looking at buying a deore xt crankset for my 2007 Jamis Nova. The nova has a 68mm bb and the crankset is the m780, and has an external bb. I need to know if this will work. Any other related info would also be appreciated.

digger531

I tried this a couple years ago and had problems getting it to shift properly. Now, i was using a 9spd crank with brifters but the problem was that I couldn't get a road derailleur to reach out far enough. I could have offset the crank to the non-drive side but my OCD wouldn't allow that. I went with a "cross double (I believe its a 46-36) and a mountain cassette (12-36) and rear derailleur. That probably doesn't help much and I will certainly be watching to see what others say.

Retro Grouch 

36/36 is a pretty low gear. Way back when that 1:1 ratio was what we used to shoot for. If that's still not enough I'm thinking you could probably find a 34t chainring that will fit and give you a little easier hill climb gear.

Mondo734

I am planning on running an all mtb drive train. I am going flatbar with either alivio or deore shifters.

bikemig 

I haven't done this particular set up but there is no reason why it can't work. People have been using mtb cranks on road bikes for a long time. My soma double cross is currently set up with an mtb crank but I am running bar ends and the left is friction. The BB is a standard english threaded bb so that is not the problem. The FD might give you issues and so might the shifters. Indexing for the crank is kind of a dumb idea, IMHO. If you're running 9s on the back, I'd get 9 speed bar ends (either micro shift or shimano).

Mondo734

I guess the real question I have is if I will need to space it differently since mtbs typically have a different bb size than a road frame. (73mm vs 68mm if I recall). The install of the crankset is the focus.

cyccommute 

No, there are no real issues. You do have to get the chainline set properly. See below.

Your OCD is leading you astray. Mountain bike cranks are slightly wider than road cranks because of the wider chain stays on mountain bikes. You are supposed to offset the crank to the non-drive side to get the proper chainline when using a mountain bike crank on a road bike. If you had a 3 piece crankset, you'd use a bottom bracket with a shorter spindle. With an external bottom bracket, you use spacers to achieve the same thing because the spindle length is fixed.

It's not about how the bike looks but about the way the bike works.

Most mountain bikes use a 68mm bottom bracket shell. Only rarely do you see a 73mm. The spacers will let you get the proper chain line.

Mondo734

can you please elaborate more about spacers. Do they come with the crankset or do you purchase them separately?
also is there a chart or something that tells you how much space is needed for the chain to line up?

cyccommute 

An external bottom bracket comes with one or 2 spacers but you can get them from many bike shops in various widths. Look here for information on the chain lines. Just to be clear you want the middle ring to be set at the distance for a road chain line.

Mondo734

@cyccommute thanks a lot you have been really helpful.

hueyhoolihan

i have attempted, unsuccessfully, in the past to use a '85ish road (Sakae) crank on a '95 Gary Fisher MTB frame. it didn't work. more or less the opposite of what you are describing.

the road crank spider is not dished enough and the 40t chainring hit the chainstays. wasn't even close. if you reverse the situation, as you are describing, i think, i wouldn't be surprised if alignment tolerances might be exceeded and create shifting problems no matter what.

OTOH, it's pretty easy to test and may well be worth the attempt.

FastJake

Did I travel back in time? I didn't realize this was 1994

Almost every MTB I've come across since the late 90s has had a 73mm BB shell. Except for the stupid new ones with the press fit nonsense.

Kimmo

Yuck, I'd be inclined to increase the OLD as much as the frame would tolerate up to 135mm first... seen me mention my old Giant CFR-1 that had a 135mm OLD?

Or just disregard chainline in favour of symmetrical Q factor if it'll work without the chain snagging on lift pins etc... chainline is NBD with modern chains IMO, and if the OCD kicks in, you can reverse the chain when you clean it to equalise wear.

Also with HTII cranks there's a bit of leeway with how far onto the spline the left crank is clamped; even more if you ditch the locating pin. If you can add a couple of mm to the OLD and clamp the crank a couple of mm narrower, you're home free with no compromise - it's only a 2.5mm discrepancy to correct; I'm sure 2mm is enough (a 2mm OLD increase will improve your chainline by 1mm, and narrowing the cranks 2mm will do the same, assuming symmetrical spacing).

cyccommute 

Since 1994, I've owned 12 mountain bikes. Most of them were Specialized but not all of them. Not one of them has had a 73mm bottom bracket shell, including the current crop of mountain bikes that I have. That includes a Specialized Epic (2003), a Specialized Rock Hopper (2009), a Specialized Stumpjumper Pro (2003), a Moots YBB (1998) and a Nashbar Flashback (1998 cruiser/mountain bike).

I also volunteer at a coop every Saturday and see bikes of every vintage imaginable. I have yet to see a 73mm bottom bracket there. By the law of the Bike Depot, however, every bike I see this Saturday will probably have a 73mm bottom bracket that needs replacement and we don't have any

Dan Burkhart 

While it's usually fairly straight forward to make a MTB drive train play nice with a road frame, the reverse does not hold true as you have found. Those dang chain stays always get in the way.

Dan Burkhart 

Most MTB external BBs I've dealt with come with 3 spacers. You use from none of them to all three depending on whether you have 73 or 68mm shell, and whether or not there is an E type derailleur.

surreal

Bingo. Nailed it. Your crank will have 3 spacers along with the BB. The install should be supremely easy; just make sure your BB shell is faced nicely before proceeding.