Using a MTB group on RB
 

http://www.bikeforums.net/showthread...riple-crankset

After reading this previous thread (I didn't want to thread jack), I was wondering if you can use a MTB group on a RB? I read about possible alignment issues and chain widths, but if you're using an entire MTB group, wouldn't these problems be eliminated? Just curious, I saw some MTB groups for cheap on ebay (I don't know how reliable they are though.

MTB chainrings tend to be smaller than those installed on road bikes, so chainstay clearance will probably only be an issue going the other way. The only problem you may encounter is the front derailleur. 'Road' and 'mountain' FDs have different cable pull ratios, so you'll struggle to get the front shifting reliably if you use an indexed shifter. Using a road FD avoids the indexing problem, but then the shifting won't be great as it's designed for larger chainrings. Of course, using a friction front shifter with a mountain FD removes the issues.

My touring bike has an Alivio 48-36-26 crankset and all Deore shifters, derailleurs, and brakes. Everything works great and with an 11-28 rear cassette gives me a great range of gearing for everything short of flat out road racing. The only alignment issue I had was quickly solved with a change of spindle length. MTB components tend to be pretty beefy which is great for reliability, but can add some weight compared to high end road components.

If you are using road shifters and indexed systems, everything from a mountain bike group plays nice with everything from a road group except the front derailer as Monster Pete says. Some people have reported that they can make them play together but it hasn't worked in my experience. I've never had any problems with making a front road derailer shift quite well on a mountain bike crank, however. Shifts are crisp and quick for cranks with outer ring teeth ranging from 44 to 48.

You may have to adjust the chain line a little to get the derailer to shift over the whole range. Mountain bike cranks are a bit wider and may need to be pulled in a little for proper alignment. The pedals on a mountain bike crank are a little further apart than a road bike crank too. I actually prefer the wider stance over road bike cranks. My bikes with road cranks feel like my feet are under the frame.

road FD on road bike with MTB crankset can lead to the FD bottoming out on the chainstay, or not having enough reach to shift to the big ring.

Again, not in my experience. I have, or have had, Tiagra and Ultegra fronts on touring bikes with 44 tooth outers and have never had any bottoming out problems. You may have to adjust the chainline to get the shift to the large ring, however. That's not hard to do.

yeah, bottoming out on the chainstay is rare, but it is still possible.

but some combinations of seat tubes, FDs and cranksets simply don't work, because the FD doesn't have the motion range. An extreme example would be a surly pugsly with 100mm BB which requires a special fitting to bring the FD closer to the chainrings.

Mostly, and this is just my own limited, personal experience, MTB stuff is designed
for much wider ranges than your average road (non touring).

So I guess I'd want to know why, exactly, you'd want to do this (other than cheap
on e-bay ......and just so you know, I loathe e-bay)?

Again, from personal, limited experience only, the smaller range stuff will
shift and work better on a road built up for fast riding.YMMV

Many a loaded touring rig uses an MTB drive train.. state which period/era, ,
since the cranks fitted to MTBs have changed over the years ,
I've run 80s .. 110 74bcd stuff ever since .. back then.

Gets somewhat a flexible term until you begin to state real ratios and tooth counts.

as I made multiple trips with gearing around 13t -34 t rear hub clusters,
a 50t outer, and a 24t inner, and a 40t middle can be done, also
with a newer Campagnolo 'race triple' , shipping as a 50/40/30, the 3rd is also a 74.

so if anyone needs more 30t 74 chainrings . let me know, I've got several..

I am surprised noone mentioned this... the rear hub form a mtb group will likely be too wide for most true roadbike frames... a touring or CX bike might have the 135mm (or between sizes 132.5 mm spacing), but not a pure road bike which will generally have 130mm spacing.

The front mtb derailleur will likely work ok if you have MTB shifters but not so well with indexed road shifters.

The chainline on a mtb crank is also a couple mm further outboard than a road bike, so it might be a little tricky getting hte front shifting to work perfectly without rubbing... but it will likely be ok.

the 5mm is a spacer added to the left axle end.. EZ to Mod.

The wide range would be nice

So what about using a MTB crank with a rb bb? That would eliminate the fittment of a wider MTB bb right? The wider pedal range would be good for me as well.

Basically because it's cheaper, but also because I haven't heard/read anything about this being done and was curious if it could be done and if it would be an advantage. Major speed isn't an issue to me (yet) because I am going to be riding to/from work for the exercise.


I would be using the rear casette, not the hub (unless they are the same thing?) and the crank from a MTB. As well as the front/rear deraillures so that there aren't any fittment issues (hopefully).

The bottom bracket depends on the crankset. For the most part, a bottom bracket is a bottom bracket and they really don't fall into a road bike/mountain bike dichotomy. You might need a different spindle length to move the crank inboard or outboard as needed if you are using a traditional bottom bracket. If you are using an external bearing crank, you change the chainline with spacers.