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MTB drivetrain...(3 posts)

MTB drivetrain...Matno
Apr 27, 2002 8:17 PM
If anyone has experience or knowledge in this area, please share!

I know this has probably been covered before, but I am not sure of the details. What I propose is to upgrade my road bike drivetrain to 9-speed using a mountain bike rear derailleur and cassette (11-32) with a road crank and front derailleur.

I think this should be no problem (?) but I'm wondering if I'll have to go with a triple front crank to make it work. (I think I'd rather have a double since the extra big cogs in the back ought to give me more than enough low gearing. My current setup's lowest gear is 42/26 which is quite a bit higher than a 39/32 combo would be. I want lower gears, but don't need them all that much lower).

Also wondering about the need for a long cage vs. medium cage rear derailleur (e.g. Shimano XT). Would I need the longer one?

I assume that the rear gear spacing will be the same for both road and MTB as long as I stick with Shimano 9-speed. Correct? (I'm thinking downtube shifters on a Profile Swift Shift - which I already have in 6 speed). This bike is used primarily for long rides - lots of centuries, all day rides, and even some light touring (hence the aerobars. I'm no racer, but they sure are nice to take strain off my back on long rides!)
It'll work fine.Spoke Wrench
Apr 29, 2002 5:41 AM
You need about 35 links of chain wrap capacity to fully cover the combination you are considering. An XT GS rear derailleur (the short cage one) has 34. The chain is going to be a little slack in the little/little combination but you don't normally use that gear anyway. I do think that it's important to have a chain that is long enough to cover the big/big combination in order to avoid possible drive train and frame damage.
What Spoke Wrench said andjw25
Apr 30, 2002 9:43 AM
and I'd think the GS rear derailleur should be fine. Remember that the mtb rears are made for three front rings, with a jump of 22 teeth from small to large. With 2 rings, even a 39-53, that's only 14 teeth, leaving a lot more capacity for the rear cluster.
Do be sure to set the chain length correctly - big/big plus one link has worked fine for me, and as people say, you shouldn't be in that combo anyway. Just in case, this leaves enough length to do it, but not so much that the small/small combo droops. Of course, that's one to avoid, too.
Gear spacing is the same, so a 9 speed shifter should work fine. The only problem I can think of is the rear wheel and cassette having issues. A friend did this with some early Spox wheels, and their drive-side flange was large enough to hit an XT cassette spider. I think the solution was a spacer between the freehub and cassette.