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MTB to Rd Crank and Drive Train Conversion RE: Building up Cross Bike from MTB

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MTB to Rd Crank and Drive Train Conversion RE: Building up Cross Bike from MTB

Old 10-28-13, 01:00 PM
  #1  
dacelsen
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MTB to Rd Crank and Drive Train Conversion RE: Building up Cross Bike from MTB

I am building up my Mamba Gary Fisher frame to use as a cross bike. This was my first mtb and I want it to ride on. I have run into a couple issues with the drive train. I am working with parts I have from another rd bike as funds are limited on this project. So far everything is solid. handling of the bike brakes tires etc except shifting on the front chainring. Also the old existing mountain crank needs to be replaced and prob the front derailleur. I am not sure how to proceed.

I need help what direction to go.

Below is the present components.

Sora STI 8spd rd Shifter (have both a double and triple version)
8spd cassette and chain
Square taper Shimano Alivio 7spd 42/34/24 crank
Shimano BB-LP26 73x113

Problems
The Sora shifter paired with mountain triple derailleur does not shift the mountain triple. The crank and derailleur needs to be replaced anyways. I am not sure what Crank to get that will work and if I should replace the BB and if so what combo will work and work with rear cassette system.

I have a double shimano rd derailleur to work with as well.

Any advice on the simplest solution??

Will a double 8spd square taper crank fit and solve the problem?? Length of Crank are suggests 170mm or 175mm for cross??

Any other problems I will run into??

Thanks for any thoughts and help.
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Old 10-28-13, 01:50 PM
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Bill Kapaun
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You need a "road" FDER.
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Old 10-28-13, 02:11 PM
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dacelsen
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Originally Posted by Bill Kapaun View Post
You need a "road" FDER.
Good to know. Now any suggestions on a crank and or BB??
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Old 10-28-13, 02:19 PM
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go with a double or triple crank depending on what you'd prefer. I think most cyclocross bikes do double cranks b/c they'll just go on foot if a hill is ugly.
170mm or 175mm cranks are down to your height and your preference.

Whatever the case, you'll need a road front derailer.

You could just use the 42/34 crankset as a double, and either remove the granny ring on the triple, or just don't use it.
If you go for a double crank, you'll want chainline a bit more inward than if you had a triple.
But if you go for a road double crank, remember that your mtb frame probably has 135mm rear spacing (where a road bike has 130mm spacing) so you'd want a bottom braket that puts chainline 2.5mm further out to correspond with the alignment of the cassette.
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Old 10-28-13, 02:31 PM
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Chainstays for fat tire clearance will limit the size of chainrings

as if .. too big & the teeth will make contact, with the chainstay tube.

If you go to a double , the space between the 2 chainrings is the center of your chainline.
so a little shorter BB axle than a triple. .

triple, the middle chainring should be on the chainline center. (center of the rear Cassette cluster)

Last edited by fietsbob; 10-28-13 at 02:37 PM.
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Old 10-28-13, 02:32 PM
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dacelsen
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Originally Posted by TallRider View Post
go with a double or triple crank depending on what you'd prefer. I think most cyclocross bikes do double cranks b/c they'll just go on foot if a hill is ugly.
170mm or 175mm cranks are down to your height and your preference.

Whatever the case, you'll need a road front derailer.

You could just use the 42/34 crankset as a double, and either remove the granny ring on the triple, or just don't use it.
If you go for a double crank, you'll want chainline a bit more inward than if you had a triple.
But if you go for a road double crank, remember that your mtb frame probably has 135mm rear spacing (where a road bike has 130mm spacing) so you'd want a bottom braket that puts chainline 2.5mm further out to correspond with the alignment of the cassette.
Thanks this is awesome info. Its the chainline issue that confuses me and the mtb frame and rd crank.

Will the rd derailleur shift the mtb crank??

I have a rd cassette but it is a frame issue with chainline??

Also I have no idea how to tell what bottom bracket and crank to get related to his issue. Any help for the rookie here??
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Old 10-28-13, 02:39 PM
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Here are the specs for the Mamba if that helps
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Old 10-28-13, 02:43 PM
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Will the rd derailleur shift the mtb crank??
the arc of the cage will be matched for a much larger Chainring than your largest one .

But as a thing to shove the chain sideways the FD is a pretty crude , simple piece .

the crisp precision as a hand skill works better with a friction lever , IMO.
but if you want Shimano Brifters , you Cope.. somehow.
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Old 10-28-13, 06:51 PM
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a road derailer will work fine on a 42/34 double mtb crank. Obviously the road is designed for a larger-diameter chainring (typically 53/39) but that is mostly an aesthetic issue; it'll shift fine. As fietsbob says, FD is a pretty crude/simple technology.

Chainline is a function of frame and rear hub, not cassette. A typical mtb is 135mm between the rear dropouts, a typical road frame is 130mm (note that these are changing somewhat, but your frame is almost certainly 135mm). So road vs. mtb frames take differently-spaced hubs.
An 8-speed cassette will fit on a hub that is spaced for 135mm and 130mm; it will just be 2.5mm further from the center of the bike on a wider-spaced hub (2.5mm added on both sides of the hub; the cassette is on the right side of the hub).

The bottom bracket that came with your 42/34/24 triple crank is probably optimized for 3 chainrings, to match with a 135mm-spaced rear wheel. For perfect chainline if you're just using the outer 2 chainrings, you'll want a narrower bottom bracket, probably a 73x107mm. This isn't the most urgent thing on your list, so if you don't have the money you can stick with the 113mm bottom bracket.
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Old 10-29-13, 02:35 AM
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alter / misuse the cable clamp on your current mtb front derailer to reduce leverage. better odds of shifting via road shifter
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Old 10-30-13, 09:25 AM
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Thanks everyone for all your input. Very helpful in making this project successful!
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