Road Crank on MTB?
 

Is there anything inherently wrong with putting a road triple on my MTB? I use the bike pretty much exclusively on the road to do chores with and commute. The gearing on the Stronglight Oxale Avtiv is 30T-39T-51T, which will be a little different than my present 22-32-42, but I don't have any extreme gearing demands.

Paul

Potential issues:

- Front derailleur: The current MTB FD won't fit the curvature of the road crank (with larger chainrings well). If you put a road FD on, its cable pull ratio won't be compatible with the MTB front shifter. A friction shifter eliminates the second concern.

- Chain line: Standard chainline (measured at middle ring) for road triple is 45mm, whereas for MTB it's 47.5mm (or 50mm for most newer cranks and for frames with oversized tubing). The road crank needs to be spaced out far enough for the chainrings to clear the chainstay. The chainline can be moved out by using a bottom bracket with a longer spindle (though for newer cranks with an integrated spindle, this is impossible).

Another potential problem will depend on the width of your bottom bracket shell. Many newer MTB's have 73 mm shell width while the road standard is 68 mm. A road bottom bracket and/or integrated spindle crank won't work with the wider MTB shell.

and, Given the trend is to smaller gears , high, on the hub, cassette,
It doesnt take much for a pretty high gear Ratio
just by changing a couple chainrings to a bigger tooth count,
and retaining the crank arms and BB
you have..

44:11 = 48:12 = 52:13

IF your bike has a Free Hub, a cassette change may be MUCH simpler/cheaper.

What bike do you have (be specific) and what are the tooth counts of the rear?

I'm not really trying to change the gearing from my present setup, Bill. What I am doing is upgrading from my Shimano FC-M440 to something lighter, stronger (7075 alloy), not too expensive and silver.
The Stronglight came to mind, but it apparently won't work. That ok, cause I am not thrilled about having a 30T chainring as my smallest one, up front.
My mechanic suggested the Shimano XT, but I just don't want the black chain rings. Also, my tech was not able to tell me what kind of alloy Shimano uses on the XT. I can't find any info on it. Preferably, I want 7075 in both the rings and the crank arms.
The bike is a Raleigh M80. I don't know the teeth count on the 9-speed SRAM cassette.

Paul

IF you go from a 22-32-42 to a 30-39-50, you're making a MAJOR change in your gearing.
That MIGHT actually be good, depending on your riding conditions (flat?) and current cassette.
You'd also need a longer chain.

Your current cassette is probably something like an 11-32.

The rear stays on a MTB bike are spaced wider than a road bike to accommodate a wider tire. That also impacts available clearance for larger chain rings on the inside smallest ring. I can't myself on my set-up. If you don't currently have clearance with a MTB crankset to go to a larger size, moving to a road crankset won't change that. Otherwise I personally don't see an issue or an advantage either for that matter.

Do you really think you can push a 51T on that bike to the point that it'll make any real difference over swapping just a few chainrings and running a 22-36-48T?

The XT and LX are both availible with silver cranks. The silver LX also comes with silver chainrings while the XT comes with black chainrings.

all good advice...

and having personal experience in trying to mount and turn a nice old road crank (shimano 600, IIRC) even WITHOUT a chainring on it, the spider arms hit the driveside chainstay on my steel '97 gary fisher tassajara. even if i could get that to clear there was no getting a chainring, that i owned, on there so as to clear the stay. wasn't even close. i lwas shocked :eek: and disappointed :(, and more than a little upset :notamused:...

Fat tire clearance has it's costs..

+1

I'm running a 44T big on my commuter, and I'm off power for maybe 30 seconds during my regular commute. That's for a 14-15 MPH average and a one hour ride, half of it through a city center.