Join Date: Jan 2008
Location: Beaverton, OR
Bikes: 2013 Kona Jake, 2015 Kona Jake the Snake, 2008 Kona Major Jake, 2013 Kona Jake the Snake, 2006 Kona Kula, 2012 Ridley Excalibur, 2001 LeMond Buenos Aires, 1984 Pinarello Gran Turismo, 1982 Trek 614, 1978 Austro-Daimler, 1987 Pinarello Montello
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I'm not really sure there are any SS cross bikes that are not able to take more of a beating than your body in MTB racing conditions. The wheelset is the biggest point of concern. A steel fork may be marginally better than a carbon fork.
But let's talk about using a SS cross bike in a mountain bike race. It seems to me that one of the primary differences between CX racing and XC racing is that CX racing is more about picking the right lines while XC racing is more about riding over stuff. If you use a CX bike in an XC race, you're likely to find a lot of places where you are trying to hit a precise line between a rock and a tree root, while the guys with suspension just roll right over it. You're probably also going to find places where you have to roll over the tree root and the rock. A CX bike can do that without breaking, but it's going to slam you hard and slow you down. Of course, relaxed knees and elbows will help with that to some extent.
This summer, I used a rigid 29er with drop bars to do some short track mountain bike racing. Watching videos, it looked just like cyclocross without barriers, but when I actually tried it I found that even with the extra air of 29x2.1 tires I got beat around a whole lot more than I do in CX races with the skinnier CX tires. That said, I think there are distinct advantages to having a bike without suspension, not the least of which is the lower weight, and I'm currently kicking around the idea of doing the STXC races next year on a rigid 26-inch bike with V-brakes or maybe even on my CX bike.
Now back to your original question... You might want to look at the Surly Cross Check. It's heavier than most CX bikes (built up as a singlespeed, mine came in just over 21 pounds), but it also takes wider tires than many CX bikes, which would help a bit with the MTB side of things.
On the other hand, if you want something really fast and light (but still tough), check out the Kona Major One -- no ENO hub necessary. Kona discontinued these, but you should still be able to find one either as a complete bike or as frameset.