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Discussion Starter · #1 ·
A while back I sold an XL mountain bike frame to a guy who said he was going to turn it into a cross machine. It sorta made me wonder whether or not he was going to put some 700C wheels on it. It had pretty decent tire clearance, and a 26" mountain bike tire probably comes pretty close to what a 700c cross tire would be in terms of overall diameter right? Is this something that's commonly done? If so I may start looking for a cheapo mountain bike frame. I also figure I should look for a frame that slightly larger than what I'd normally ride on a mountain bike right? I guess I could also just buy a 29er mountain hardtail but they aren't as abundant.
 

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It might be possible.....
BUT
You will run into problems with the brakes ( if they are cantilever)
It will probably handle funky.

I have seen people do it in the past using old Onza brakes...good luck finding those.

But building a Frankenbike usually ends badly.
Either get a cross frame/bike....or get a mountain bike.
You can race either in cross
 

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Discussion Starter · #4 ·
I just figured it'd be interesting, since I have so many mountain bike parts laying around. I would have to find a killer deal on a hardtail frame to make it worthwhile.
 

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Discussion Starter · #5 ·
One thing I didn't think of though was that the dropout spacing would be different, so I'd have to frankenbike the wheels as well, or I guess I could run a MTB 29er wheel with cross tires.
 

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I have done it several times...

Brakes are the biggest challenge. I have engineered some adapters that work great on Onza's, cheap set and easy to find.

Other than that, it handles fine but raises you more than an inch. Enough to feel it, but never been a problem with handling. The larger the frame, usually the less slope on the top tube and the better it looks.

The fork is the next challenge. You got to get a rigid with the same geometry as the suspension fork you will replace. That's the bigger challenge. You can go with a 700c cross fork, but if it is shorter than you suspension fork, you will not like the turning.

I'd advice trying ONLY if you have enough parts to try.
 

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I built up a Ibis Mojo with a Ti CX fork last year and ran it SS. The only issue was the canti rear brake but the bike rides/handles great and has plenty of tire clearance.
 

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Discussion Starter · #9 ·
I would probably run disc brakes, I've a set of Avid BB7s sitting around.
 

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Discussion Starter · #11 ·
Good question. I'm just kicking around a stupid idea I guess. I don't race CX at all anyway, I was just thinking it might be a fun project.
 

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My SS is set up this way (look in the show me your SS thread from last year, it's the pink one). It is a rigid frame from the 80s, so no fork problems. I have some older, probably also 80s, Tektro cantilevers on there. Maxed out they just reach enough to work. The dropout size is the same as a normal 700c wheel and I've been able to switch wheels easily. It does handle a little strangely, but if you just want to throw something together, why not?
That's what cross is all about, and it's too bad more people don't see this and just want you to "buy another bike".
 

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Another option:

Schwalbe makes the CX Pro in 26x1.35", which I'm going to use on my old DeKerf to race cross this year.

I thought about the other low-cost options and figure this is one of the easiest ways to get the advantages of a cross bike (fast tires) with minimal expense while avoiding the hassles and disappointment of frankenbiking something together.
 

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With disc brakes it is an easy conversion, though I'd just get the Schwalbe 26x1.35s and (assuming a 100m travel fork) a 410ish a-c fork and call it good.

Remember that there are tons of older road bikes with generous clearance out there, too. I really think that road geometry+skinny tires makes for a lot of fun. I was using an '86 (or so) Bianchi Portofino until recently and it managed well enough.
 

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some may recall the Mavic Speedcity wheelset which many of my friends have used for exactly this purpose. of course you could use a 29er disc wheelset or build some road rims on disc hubs too.

I recently tried to fit a 700x35 ritchey speedmax on a road wheel into the Reba on my MTB and it did Not fit. However, Fox forks have much higher arches on the lowers, which might fit such a thing.

regarding handling, increased wheel diameter increases trail, which would "slow" or "stabilize" the steering feel, but only very slightly.
 

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I decided to do just that. I had all parts except for the wheels and I bought those off a friend for $100 (and the fork, but I was looking to go rigid on this MTB anyway...it was like $50 on ebay...search for MOSSO). My main reason for even doing it was for something different for motivation. Sometimes it's just fun to ride something different!

I've tried to attach two picks to show tire clearance. Clearance in the rear is almost as much as the fork has. I've ridden in some nasty stuff and never had any problems and these tires are 35's.
 

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c-lo said:
why would you not just run a 1.9 tire?
Or, better yet... a 1.5 tire. Hutchinson used to make one (maybe they still do) that was designed to turn a 26" mtb into a 'crosser. I happen to have a pair if anyone wants 'em.
 

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The answer to the OP's question is YES you can do it -- even with rim brakes! For the last 3 summers I've been commuting on an old 26"er converted to 700c, using drop bars and Tiagra brifters. At least my bike has clearance for up to 700x38-40 tires front and rear.

Front: Dimension 1" disc fork, Avid BB7 Road disc brakes. Perfect match for the road-oriented brake levers. If you're absolutely set on rim brakes front and rear, you could do the same thing in the front that I've done in the rear. Read on ...

Rear: Paul MotoBMX brake attached to the original 26" canti posts. Yes, that's right: the MotoBMX is an unusual V-brake with tubular arms that allow the pads to be continually adjusted anywhere between the "normal" V-brake position (which would work with 26" rims and would require the use of V-brake levers) and the top of the arms. On a bike designed for 26" wheels, aligning the pads next to a 700c rim puts the pads about twice as far away from the pivot point as "normal". The beautiful thing is that this changes the mechanical advantage needed so that it is perfect with a road lever. Works flawlessly. The MotoBMX is expensive, but this setup works fantastically and this V-brake is easier to set up and adjust than any other, to boot.

Three geometric concerns if you're converting a 26" bike to 700c:
  • You end up with a high BB compared to a lot of cross bikes. The diameter of a 700x32 is about exactly the same as a 26x2.0, so you're going to have the same BB height that the rig did originally as a mountain bike. Personally, I like a really high BB for pedal clearance in turns, so I find it perfect.
  • If you keep the same fork you will increase trail, and slow down your handling. I swapped out a 38mm-offset MTB fork for a 45mm-offset 'cross fork of approximately the same height, which more or less evened things out.
  • Toe overlap could be a problem. My MTB was already sized a bit long for my height so I have had no issues.
 
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