The last two times I raced Ironcross, I've used my Ridley CX bike with knobby 35mm tires pumped up super hard. The course has been tamed since the first couple years. There is less bony ATV track and more gravel road. An aero position with drop bars can be a real bonus on many sections of the course. I managed to podium the large masters field in 2009, the last time I raced here.
So why mess with success? The risk of pinch flatting on this course is very high. I run 60-70psi in my CX tires to help mitigate that risk. But that increases other risks, like losing control on a loose, washboardy turn at 40mph. There's also that two mile singletrack descent. I've never cleaned the bottom portion on a CX bike. I don't even try. It is all ledgy drops and loose rocks. With my skills, I'd go over the bars and need dental reconstruction. On a MTB however, I can fly through that section.
I now have a carbon 29er hardtail. It is a little lighter than my 26" titanium Dean that I raced in 2003, but much heavier than my cross bike. The tires are also about 60% wider and can only go half as high in pressure. So I'd increase the rolling resistance in most places, have no aero position, carry an extra five pounds around, for what? Minimal time is usually lost or gained on descents, about the only opportunities for a mountain bike to excel. The MTB comes with lower gears, which can be useful when cramping occurs in the 4th hour. Both bikes have Schwalbe Racing Ralph tires, my new favorite tire that I have on three bikes now.
I'm still curious though, curious enough to risk experimenting. The Ironcross organizers threaten to heckle anybody that brings a MTB. I saw lots of them last time. I don't think any of the podium finishers were on MTBs, unlike the first couple years. The race could certainly be more fun on a MTB. Winning once and a while is fun too. Help me out here. For once, I might get to race in my definition of IDEAL conditions!