And while we've a little primer on some of that on the site aleady a post seems useful since new people discover the fun of ultracross every year.
First, you can use any bike you like. Cross, 29er, 26er, 650ber, fatbike, unicycle, frankenbike - they're all welcome, just make sure they're working well before you put them through this wringer.
You'll absolutely hear us heckle mountain bikers about needing the fat tires since we designed this thing for 'cross bikes 10 years back (and they've been doing something similar over in England for 50 years at the 3 Peaks Cyclocross and they outlawed mtbs a long time ago). But we get it. Not everyone has a 'cross bike in their stable. Bring what you got you'll have a great time regardless of the bike you're on.
Though to be honest we absolutely think that doing Iron Cross in the seat of a 'cross bike brings something special to the experince.
Riding the cross bike in places you probably wouldn't normally brings something like the feeling the 14 year old kid had in 1959 when he set out to conquor what had to that point only been thought of as a hiking and running contest, the Three Peaks Challenge through the Yorkshire hills.
"He rode, pushed and carried his bicycle 30 miles to the summit cairns of Whernside (2,419ft.), Ingleborough (2,373ft.), and Penyghent (2,273ft.). And he complied with the accepted condition of the walk by finishing at his starting point." From the 3peakscyclocross.org site.
And of course we have fond memories of sorting out the original Iron Cross course by cross bike. That course was rougher than today's (and included an early run-up we keep trying to figure out how to re-integrate) but totally cross-able. The course was just a little too mtb friendly at that time and cyclocross racing was just starting to capture the cycling world in the states.
"I think you guys have a course that is better suited to a cyclocross bike now. If I had one I'd have used it instead of my mountain bike." 2011's third place men's open finisher Mike Keefer, a multi-time podium finisher at Iron Cross and one of only two riders in the top 10 overall on mtb.
"Yea right, a 'cross bike," about 1/3 of the field.
It has a lot to do with your comfort level on a 'cross bike in technical sections. If you can ride those sections well on a 'cross bike then that's the best choice. If you're feeling a little intimidated on a 'cross bike then a mountain bike might be better. (Unless you want that "core" experience of course...see, there we go again!)
While we love and promote the 'cross bike, let's face it, 29ers barely existed when we set out Iron Cross and a rigid 29er with a slightly narrower tire certainly gives up little to a 'cross bike on the fast surfaces of the course while being more suited to the technical bits. So again, grab what is hanging in the garage, maybe modify a little to make it a little better ultracrosser if you want, and then get ready for a wicked fun day on the bike.
And then there is this: "I'm more comfortable on my mountain bike, but I'll run 'cross because you'd make fun of me if I didn't," multi-time Iron Cross finisher Rob Lichtenwalner.
Whatever it takes.