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I'm thinking of building up a fat-tire commuter bike. I need fenders, racks and wide tires since I live in the northwest (potholes, rain, more rain, hills). I also need it to be fast since I want to be fast on my commute.

I was thinking about buying an old MTB and replacing the groupset with something like the Shimano 105. There's plenty of nice old steel MTBs on Craigslist for ~$100. I would essentially only want the frame.

The dropout spacing on the old MTBs is 130mm, just like the 105 hubs. I'd have new 26" wheels built around those hubs and use the 105 crankset, rear and front derailleurs. I'd also put in a drop bar and I think I should be able to use the 105 brifters with the MTB cantilever brakes. Then I'd finish it off with tires like gatorskins or marathons.

Does that sound reasonable? Anything I forgot to think of?

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  • V brakes and mechanical MTB discs aren't compatible with normal road levers. You'll need either V-brake specific drop bar levers or cantilever brakes.
  • Road cranks have narrower chainline and larger chainrings and MTB chainstays are wider. You may run into compatibility issues here, too.
  • You probably won't be using the highest gears anyway, since Marathons or Gatorskins have huge rolling resistance compared to proper road tires
  • Fast, potholes and rack don't fit together. Get a messenger bag or backpack instead.
  • Drop bars and road components aren't going to make you that much faster. The most difference is in ergonomics.

What I would do would be MTB groupset with road cassette, V-brake compatible levers and bar end shifters.

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    He mentioned canti brakes, so there’s no brake lever issue. The only issue I foresee is the chainring chainstay clearance you mentioned. Easily solvable with a wider BB spindle, but certainly something to be aware of. I disagree on the backpack. Well attached panniers do fine with potholes, especially with fat tires that absorb some of the bouncing.
    – Andrew
    Aug 15, 2019 at 17:14
  • Modern road crankset have fixed bottom bracket spindles. It is true that panniers may survive, but unsprung mass isn't a good thing for speed or handling.
    – ojs
    Aug 15, 2019 at 17:23
  • -The MTBs I'm looking at have cantilevers, not V brakes and certainly not discs. -Really don't like backpacks for commuting, been doing it for a while and getting sick of it. -I have gatorskins on my current (road) bike and use all the gears. Granted, with a fat tire resistance would be higher but I'd rather have gears I don't use than not have gears I need to use. -Yeah, I really like the ergonomics of dropbars. Great for high winds that we get.
    – user44903
    Aug 15, 2019 at 17:29
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    There are mechanical disk brakes which are designed to work with normal road levers. In fact, a lot of newer road bikes come with disks. So that shouldn't be an issue. And if you ride in the rain a lot, then disks are a good investment.
    – Mike Baranczak
    Aug 15, 2019 at 19:13
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    Comments are not for extended discussion; this conversation has been moved to chat.
    – jimchristie ♦
    Aug 17, 2019 at 17:15
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There are a couple of problems using a road group on an MTB frame.

If you have a frame with 130mm rear spacing and a 68 or 73mm BB shell, you can fit a external BB, road crank and rim brake hub. (I checked: 105 5700 and R7000 cranks both work with 68 and 73mm shells. Shimano specs are here, including a link to archive specs in PDF form.)

First problem is clearance between the chainstay and the chainrings. Old MTBs were designed for triple cranks with a 40 tooth big ring and a 47.5mm chainline as opposed to a 50 tooth big ring and a 43.5mm chainline.

You might have a problem with clearance between a wide tire and the chain, due to the reduced 43.5mm chainline.

Also:

You'll want to look into what effective gear ratios you'll get. Your gear-inch (or gear meter) values with 26" MTB rims will be lower than regular road bike with 700c rims .

You'll probably need quite a short stem as drop bars extend the hand position forward compared to flat bars.

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  • Thanks, that makes sense. I don't see any links though. Did you forget them or do I need to look somewhere specific?
    – user44903
    Aug 15, 2019 at 19:59
  • I fixed the link
    – Argenti Apparatus
    Aug 15, 2019 at 20:24
  • Cool. So given the clearance issue, would you also recommend an MTB groupset? What about something like the Deore XT T8000 trekking groupset? I'd really prefer a double over a triple though...
    – user44903
    Aug 15, 2019 at 20:46
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    MTB derailleurs won't work with road shifters. You could build a flat-bar commuter, or perhaps a drop bar bike based on an old cyclocross frame.
    – Argenti Apparatus
    Aug 15, 2019 at 21:04
  • A lot of modern MTB groups use a 142 or 148mm rear spacing and wider chainline, possibly wider BB shell as well.
    – Argenti Apparatus
    Aug 15, 2019 at 21:13
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You don't say what your budget is, but there are plenty of new drop-bar bikes that take fat or fat-ish tires these days, sold as gravel bikes or all-road bikes. The bike I'm riding as my commuter will take 34-mm tires, for example.

One thing to consider if you buy an old MTB with 26" wheels is that (as I understand it) 26" wheels aren't that popular on MTBs this day, so your tire choices will get more and more limited as time passes. Both mountain bikes and gravel bikes these days are using either 650B wheels (called 27.5" on MTBs, or ETRTO 584) or 700C wheels (called 29" on MTBs, or ETRTO 622).

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