MTB to Cross/Gravel Conversion.

The spirit of Grav-lo-cross. No but seriously, cyclocross and gravel go here!

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airwise
Posts: 1018
Joined: Tue Jul 27, 2010 8:31 pm

by airwise

I have an old Seven Sola which I don't ride anymore as I've stopped MTB'ing.

I'm hatching a plan to convert it to a tourer/gravel bike.

I've worked out that the 26" wheels with 2.1" knobblies are the same diameter as a 700c wheel with 25c's on them and the stack and reach will be fine with a 90mm stem and a 450mm fork like it's current Fox 80RLC or a Ritchey rigid offering.

The only issue I have is the old 73mm threaded BB and fitting a double crankset - preferrably with 110bcd spider. Also looking to run this with Ultegra Di2 Disc gruppo.

Does anyone have suggestions for a 50/34 chainset solution that would work?

by Weenie


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rasmussloth
Posts: 314
Joined: Mon Dec 09, 2013 3:19 pm
Location: Denmark

by rasmussloth

Are you sure that 50t chainring will fit and not hit the chainstay?

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breezerboy
Shop Owner
Posts: 279
Joined: Fri Jun 22, 2007 8:14 pm
Location: Bristol, UK

by breezerboy

We have managed this conversion successfully in the past, but it's more dependant on the frame than the crank (crank/ring clearance with chainstay).

Best suggestion is trial fit a right hand crank with rings as is to check clearance. Pretty much any road crank will do as the chainline is pretty universal for this purpose. If you have plenty of clearance then the BB shell can be machined down to 68/70mm to work with hollowtech/gxp cranks. If using an older square taper crank you just use a 73mm specific BB.
If clearance is tight, try a road triple instead. These generally run a spacer each side on hollowtech that can be removed/machined down if you don't want to machine the shell. They also have a slightly higher Q factor if crank arm clearance is an issue. Downside is Di2 compatibility.

Last option is MTB crank with custom spider/rings, but again, you'll need to check ring/stay clearance carefully.

Last problem - If the chainline has to move out for clearance the front mech will not be in the optimum position.

Marin
Posts: 4035
Joined: Wed Jan 22, 2014 11:48 am
Location: Vienna Austria

by Marin

You could go 46/34 if the 50 is a problem, 46/11 is plenty fast.

jooo
Posts: 1510
Joined: Wed Feb 02, 2011 3:48 am

by jooo

If you don't want to hack down the BB shell to use a road crankset or the chainstays are too wide regardless, you could come up with a few options using a Q156 XX1 crankset and a 104 or 110 BCD spider.

airwise
Posts: 1018
Joined: Tue Jul 27, 2010 8:31 pm

by airwise

Many thanks for the suggestions -really helpful and just what I was looking for. I'm going to check the chain stay clearance today but had (rather stupidly!) forgotten about that aspect.

Once I have that I can look at the alternatives more closely. I really need something bigger than a 46 as I ride a lot of Alpine downhills and regularly spin the 50/11 on those. I might manage a 48/11 but again, clearance is the key as you point out.

shadwell
Posts: 575
Joined: Fri Oct 16, 2009 1:25 am
Location: Gold Coast Australia

by shadwell

Also i'd question the suitability of 25mm tyres.. way too small for gravel.. 28-35 is better.... perhaps check clearance for this..

And re cyclo-cross, the front triangle is too small on most mtb to shoulder easily... means lifting under the saddle / down tube and not actually shouldering....

All the best with it tho..

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LeDuke
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Joined: Sun Oct 07, 2012 2:39 am
Location: Front Range, CO

by LeDuke

I'm struggling to understand spinning out a 50x11 on gravel downhills on 25mm tires.

I hope you have great insurance.

by Weenie


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