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True cx bike vs 29 inch mtn bike with drop bars?(4 posts)

True cx bike vs 29 inch mtn bike with drop bars?ralbert
Jul 2, 2002 9:41 AM
Any thoughts on this? i know i'm getting something new in a month, but still can't decide whether to go with a "real" cross bike or a 29 incher (with rigid fork) with drops. Both bikes would allow me to swap out tires for the road. Both would be able to have pretty darned big tires (ie, the 700X45's described by glowboy below). What about geometry?--can you fake "cross" geometry on a 29 incher? Any input would be helpful.

re: True cx bike vs 29 inch mtn bike with drop bars?Jan Gerrit Klok
Jul 2, 2002 12:25 PM
I take you're looking at having one made custom?

If you want to run both 23mm road tires and 52mm mtb tires, it's important to get the fork stack height and BB drop just right. Make sure to have at least the needed BB height with 23mm tires.

After reading dozens of reviews on 29ers, I get the impression that, for tight singletrack, the short stems on Gary Fishers really seem to work well with the larger, heavier wheels.
Gary Fishers have a claimed chainstay length of 433mm efective, which I think translates into a 439mm measured the old way. Just a tad longer than a typical CX, isn't it? Not too much mud clearance of course, but it should not be much of an problem.
So you want a rigid fork, that allows you to get one custom and as low as you like. A 413mm high, 26"-designed, only fork is told to work swell with a 50mm IRC tire. I have the pics to prove it even, so I'm now getting one to convert a 26" bike to 29/26" soon.
A low fork gives you the option to keep you handle bar down, 29" bikes tend to have very high handlebars, at least to my liking, as fork are 475mm of 485mm, and the half wheel under the front axle is of course huge.

For a geometry, your favorite CX geometry should be ok, though watch out for too overlap with the biggest tires. Another reason for a longer toptube, shorter stem and meaybe wider handlebar. Also, don't make the seattube angle so slack for the used chainstaylength, so at least the 29" tire will fit.

Man, it should be great to have a bike that does it all : road, CX and XC29"! With the long toptube/short stem setup for a dropbar, the bike would also work with a longer stem and flatbar, yet another option!! A second flat bar setup can be pretty cheap it you know where to get the deals or even have stuff still laying around...

I have yet to ride a CX bike with over 34mm tires, maybe I should get me the widest that'll fit my bikes, just to feel it in training once.

Why not join and ask other 29" maniacs about their ideas, use their knowlegde and add your own?

Good luck with the project!

Happy trails,

I'll take a stabgreg n
Jul 2, 2002 12:26 PM
I don't have any personal experience with 29ers, but to compare a regular mt. bike to a cross bike there might be some similarities.

You might be able to get away with drops and a rigid fork on a 29er to make it "cross-esque". But if you plan on doing any CX racing, you'll probably be pretty miserable. You have to consider the higher bottom bracket of mountain bikes. I've done CX races on a mountain bike and it was like jumping on and off of a horse. I would only imagine a 29er being worse. Plus, unless you go custom on the 29er, it's going to have suspension geometry, which means your rigid fork would need to compensate for that and probably custom. (I can't image there's too many rigid forks to accommodate 29ers in production yet.) Otherwise, you'll be set too far forward with too much of your weight on the front end.

I don't know what else to tell you. It sounds good in theory, but I wouldn't mess with it unless it was tried and true.
I'll take a stabJan Gerrit Klok
Jul 2, 2002 12:41 PM
A mountainbike typically needs 30mm BB height. A CX bike typically 28cm or so, road 27-27,5cm.
When you put CX wheels in a 26" mtb (it mostly fits just fine), the BB will indeed rise even further. Mine was, a VooDoo Bokor on 30mm Michelins, 31,8cm. Pretty high, steering became balancing, although in a strange way I liked it, gave me back the feeling of my FS bike somehow I guess. It worked for me, or at least it seemed to, being a total CX novice taking part in the Elite Nationals just for kicks.

Using a bike just with 700c rims, the BB height issue seems to fade away, as MTB (phat tire) requires the highest BB, and road (tiny layer of rubber around a rim) works best with 27cm. It goes perfectly just by itself!

I already have access to 44cm 700c tandem forks that will work with the WTB 29" tires, and even the Surly 40cm fork with it's square crown setup is reported to do so.
Should he like to use an Avid discbrake up front, a non-suspension corrected 29" frame could well be perfect with a 413mm Dimension disc-only fork, with any type of tire he might like. Just have the frame angles be based on such a 413mm fork and be happy.
Maybe this is even something for manufacturers to consider making standard issue on their models : just that tiny little bit (if any) of extra rear tire clearance and spec it stock with a fork that will take 29" tires without a problem. No-one in CX really needs a 39cm fork to get the handlebar down, especially now with integrated headsets. Look at how many riders have piles of spacers under their riser stems...