Here in Sydney, Australia our cross racing scene is growing but still nascent. I tend to get my fix by devising courses in local parks and by taking my crosser on trail rides. Even though most local trails are pretty rocky, the crosser doesn't seem much slower than my mountain bike. As I am tempted to race the crosser in the upcoming Otway Odyssey 100km MTB race, I decided to perform some testing. I intended to do a variety of tests, but recent torrential rains have limited my riding time.

The process and results are decidedly non-scientific, limited to what instrumentation and bicycles I have on hand, but with Sven Nys apparently considering riding a modified crosser in the Olympics, I thought I'd share anyway:

The Bikes
Cross bike:
2003 Lemond Poprad, 853 frame, Shimano 105 wheels, Ritchey Excavader 700x35 tires (50 psi)

Mountain Bike:
Zion 737 EBB 29er singlespeed, White Brothers Rock Solid fork, Stan's Arch rims, Crossmark rear, 2.2 Karma front, tubeless with goop (25psi), 32x18 gearing

The Test
The course:
16.2km, very hilly, out and partway back, including 1 steep paved climb (1.1km, 140m vertical) at the start, 3 km of road to the trailhead, a loose, rocky fire road descent into the valley, a 2.5km climb up the other side on more occasionally rutted fire road, a coupole of km of flat fire road to the turnaround, descend back into the valley and 2km climb back up the first fire trail descent.
Trail conditions were similar for both tests, damp Sydney grinding paste with the occasional puddle.

The Result
Mountain Bike:
16.2km in 56:10, 17.3km/hr average
147bpm average HR (77% max)
final climb time: 9:51

Cross Bike:
16.2km in 52:15, 18.6km/hr average
162bpm average HR (84% max)
final climb time: 9:00

Tentative Conclusion
So, the crosser was faster, but I seem to have worked substantially harder. Both times I went at something like a 100km race pace, and felt reasonably strong. The descents definitely required more attention on the cross bike, but it also felt reassuringly stable, perhaps because of the lower BB drop on the Poprad. Also I cleaned all of the climbs on the crosser, but had to dismount on one loose steep pinch I have never made yet on the 29er.

Anyway, I'll probably ride the crosser in the Otway race, if only for reasons of senseless bravado.

I'm not surprised. You are basically talking about two rigid 700C bikes, one of which has much lighter wheels and a faster top speed. If the CX tires can handle the terrain at speed. . .

If you search around the web a bit you might find some info on a guy named Rudy Nadler who rides endurance MTB events on a fixed gear CX bike. Does quite well in fact.

What part of Sydney are you in? I lived in Edgecliff and Paddington many years ago.

You may want to consider lower tire pressure on the cx bike. Don't know about you, but I weight 150lbs (don't know what that is in that metric stuff the rest of the world uses) and run my Mud2 clinchers at about 38psi. I've been riding some dirt/gravel roads and single-rack lately and have been amazed (knock on wood) that I haven't pinch-flatted yet. I think that running lower pressure will be more comfortable and offer better handling--that's why all the pro-types are running tubulars.

You may want to consider lower tire pressure on the cx bike. Don't know about you, but I weight 150lbs (don't know what that is in that metric stuff the rest of the world uses) and run my Mud2 clinchers at about 38psi. I've been riding some dirt/gravel roads and single-rack lately and have been amazed (knock on wood) that I haven't pinch-flatted yet. I think that running lower pressure will be more comfortable and offer better handling--that's why all the pro-types are running tubulars.

I'm sure a good set of tubulars would make the cross bike even faster, but I'm not sure I'd want to risk ripping a sidewall given how rocky the trails are around here. I'm still tempted to give tubbies a try though as I've always been attracted to arcane bike parts.

I've found 50psi is about as low as I'm willing to go, having pinch flatted enough to learn the hard way. That said, I'll probably be running Mud IIs in the Otway race and given the total absence of rocks from that course I might risk lower pressure (I'm about 165lbs).

Crosser - I'm not surprised at your results. I have a favorite 10 mile dirt loop that I have done numerous times on my old, full-suspension mtb and a variety of my cross bikes. The loop climbs 5 miles up a mtn and 5 miles down. My highest top speed was always the mtb, but I always had a faster total time on the cross bikes. About 1mph faster.

I think there are several reasons for this, one being the cross bike is lighter and climbs far faster. Even though I was faster on the descent using the mtb, how long are you descending compared with climbing? Give me a bike that climbs any day; descents are over in a blink.

Second reason, and this may be a stretch, the cross bike is not geared nearly as low as the mtb and it forces you to maintain a minimum speed that is higher than a mtb. The lower gears on a mtb are seductive. Face it: if you are struggling on a climb and you have a lower gear, you'll use it. Mtb's have plenty of lower gears. I noticed when I did this loop that, on the cross bike, I'd drop down to around 8mph on the hardest section. I was working hard because, any slower, and I wouldn't be able to tick the pedals over. On the mtb, I'd be pedalling easy at maybe 6mph on this same section. Yes, easier, but slower. I was seduced!

I see this same thing every year on Iron Cross: I am blowing past the mtbs on the climbs, and they dust me down the hill. And when I pass them on the climbs, they are spinning easy while I'm mashing a larger gear.

Thor

I definitely agree that the tendency to take it too easy on big hills is a real tendency with geared mountain bikes. They make you feel sluggish. It's the main reason I gave up my duallie and started racing the rigid singlespeed -- no choice but to turn over the pedals or walk. Fly or blow up, this is what bike racing is all about.

The only surprising thing in my result is that the singlespeed is actually quite a bit lighter than the crosser (at the moment) and my personal best times on the fire road climbs on the course were set on the singlespeed. Of course I was shredded afterwards, more so than on the crosser. It would be interesting to do the same test with a geared MTB, full suspension and hardtail.

I'm glad you mentioned the Iron Cross. I intend to turn the Otway Odyssey into my own personal Southern Hemisphere version of it. I will post a race report.