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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?

user44903
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3 Answers3

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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.

ojs
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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 '19 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 '19 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 '19 at 17:29
  • Not sure I fully understand the chainstay issue. Can anyone elaborate? Thanks! – user44903 Aug 15 '19 at 17:31
  • Sure, go ahead then. – ojs Aug 15 '19 at 17:32
  • I'm not opposed to using an MTB groupset. Problem is all the new MTB hubs are 135mm (or more) and they all have hubs designed for discs! I guess I could use 130mm road hubs and an MTB crankset. The derailleurs would just need to work with the 10 or 11 speeds I could get onto the road hub. Sound ok? – user44903 Aug 15 '19 at 17:48
  • The chainstay issue is the gap between chainrings and frame. MTB frames tend to have wider chainstay spacing to make room for wide tires and road cranks tend to be narrower because. Back when square taper bottom brackets were used you could just install a bottom bracket with longer spindle, but this trick doesn't work with 2-piece cranks. You may be lucky, though. – ojs Aug 15 '19 at 19:08
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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 '19 at 19:13
  • Steel frames can be easily stretched to accommodate slightly wider hub. Besides, the standard switched to 135mm in the 1990s. Non-disc hubs are still available and you can always use disc hub without rotor. 10-speed road cassettes are compatible with MTB hubs and I really wouldn't go for 11 speed on project like this. Regarding brakes, cantilever and V fit the same mount. 1990s MTB cantilevers were awful, and replacing them is a good idea. Brakes aren't usually considered as part of frame, I thought you were replacing them anyway. – ojs Aug 15 '19 at 19:15
  • @MikeBaranczak road discs tend to be flat mount or post mount. If old MTB has disc mounts, it's more likely to be IS or some weird proprietary thing. – ojs Aug 15 '19 at 19:19
  • @ojs, Got it, the big chainring would probably hit the chainstay. That makes sense. So you'd recommend cold-setting the frame, using a 135mm MTB rim brake hub (examples of this?) and MTB BB and crankset? How would I get the high gearing from this kind of setup? Yeah, I'd update the brakes (either with V's or new cantis). What does the road cassette get me in this setup? I'm interested in also having higher gearing available (~100 gear inches or so) – user44903 Aug 15 '19 at 19:57
  • @ojs - there are still plenty of square taper or octalink 3 piece cranks available that are quite “modern” and allow for changing spindle length to accommodate a road crank on an MTB frame. One minor problem with using an MTB group is that the road shifters have different cable pull from MTB. With road brifters, you need to use a road front derailleur to have correct indexing. – Andrew Aug 15 '19 at 20:06
  • Also wanted to add that you can get good canti brakes so you don’t have to switch to v and worry about cable pull differences. Tektro CR720s for example are inexpensive and easier to set up than the 90s ones. – Andrew Aug 15 '19 at 20:08
  • @user44903 I wouldn't bother with cold setting the frame, 5mm difference works just fine with steel frames. Shimano Deore or LX hubs are just fine and available without disc mount if you want. Road cassette removes some gears too low for road use and gives more options at usable range. I would use a 28-38-48 crank, because that's what these frames were often designed for and give tall enough gears for road use. Non-indexed front is another reason why I would use bar end shifters. You can find gear calculator here: https://www.sheldonbrown.com/gear-calc.html – ojs Aug 15 '19 at 20:44
  • Cool, I'm down with bar-end shifters. Any good 2x setups that you'd recommend or should I avoid those as well? I'm looking into the Deore XT T8000 groupset but I only see a 3x10 crankset there. Like 30-48 would be ideal I think. Seems like that groupset has a 135mm rim brake freehub too which is cool. – user44903 Aug 15 '19 at 20:49
  • I don't really know. MTB 2x cranks have too small rings for road use and road 2x has the risk of clearance issues. 30-48 is a huge gap, I have 28-38-48 on my commuter bike and wouldn't imagine using it without middle ring (actually, I haven't used the small ring in this decade). – ojs Aug 15 '19 at 20:55
  • @ojs, Also interested in specific bar-end shifters that would work well for this setup? I see the Microshift BS-M10 but not sure if that would work with Deore 10speed derailleurs. Also, seems like the front is indexed and the rear is index/friction which is strange to me. Shouldn't front be index/friction? – user44903 Aug 15 '19 at 20:56
  • The first Google hits I get for BS-M10 says it's Shimano MTB (including Deore) compatible, front is friction and rear index/friction. With these shifters, index/friction means there's indexing but you can switch to friction mode if the adjustment is off and you don't want to stop to adjust, you want to use it with incompatible derailleur or some other reason. – ojs Aug 15 '19 at 21:07
  • @ojs Their website (https://www.microshift.com/en/product/bs-m10/) says front shifting is micro-index only. That's what I was referring to. – user44903 Aug 15 '19 at 21:17
  • I don't know, but I guess micro indexing means that front has enough steps to be virtually non-indexed. Campagnolo used to have 12-step front indexing that works as if it was friction shifting. – ojs Aug 15 '19 at 21:23
  • @ojs I have a 7-year-old CX bike with drop bars,of course, Shimano 105 10-speed road shifters AND mechanical disc brakes (Avid BB-7, if I remember correctly) They work together very nicely without any compatibility issues. – Carel Aug 16 '19 at 18:55
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    Comments are not for extended discussion; this conversation has been moved to chat. – jimchristie Aug 17 '19 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.

Argenti Apparatus
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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 '19 at 19:59
  • I fixed the link – Argenti Apparatus Aug 15 '19 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 '19 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 '19 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 '19 at 21:13
  • @ArgentiApparatus "105 5700 and R7000 cranks both work with 68 and 73mm shells". I followed the link as this comment surprised me. I don't see anywhere mentioned compatibility with 73mm shells and that sounds way too wide for the spindle to work. Was this a typo for 70mm shells? If not, which of the three documents lists the 73mm compatibility? – Superman.Lopez Aug 31 '22 at 18:18
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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).

Adam Rice
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