Cyclocross frame sizing

jpembroke

I built a cross bike this year using a Planet X Uncle John frame, which looks the business, and did my first 'cross race a couple of week ago (9th place - not bad). However, now I'm starting to think that my 'cross frame is too big. Despite already having a 51cm Planet X Superlight (alloy) road bike (which fits perfectly), I was advised by Planet X to go for the 54cm Uncle John. This was because of the shorter top tubes of the Uncle John frame (54cm Uncle John has a similar top tube length to the 51cm Superlight). However, I have since read that 'cross frames are supposed to have top tubes a few cms shorter than road frames. Anyone know why this is? And should I have bought the smaller frame?

maddog 2

yep that's right. I posted a similar question recently and the advice was to go a tad smaller than you would on a road bike.

My guess is that it makes it more nimble/upright, with your weight further back?

Can you fit a shorter stem?

jpembroke

Yes, just fitted a shorter stem but yet to test it. Have another race this weekend so i'll see if it makes any difference. To be honest, the bike feels fine, it just looks massive in comparison to my road bike. My only issue I can see with having a shorter top tube is that if may result in front wheel lift on steep sections. Having said that, I can see that having a smaller frame would make it lighter and more nimble and thus quicker. Maybe i'll talk to those in the know on Sunday.

pete.whelan

you want a frame 1 - 2cm lower on seat tube, maybe 1cm shorter on top tube. Standover height is the more important

jpembroke

Thanks Pete, that's helpful. I think the top tube is a touch shorter (1cm or less) but it is a longer seat tube so less standover clearnace. I may think about changing the frame at some point.

Monty Dog

Standover height - many cross frames also have the BB higher in comparison to a road frames, combined with fatter tyres makes it higher off the ground - so you need a shorter seatube to give you the same standover - made worse by rough ground too. Shorter toptube and stem also helps to keep the handling quick and easy to lift over obstacles, log-jumps etc.

jpembroke

Hmm, this is all rather frustrating. If i'd gone with my original choice of the small (51cm c-t) frame then the carbon cross forks from my old kaffenback would have fitted (i.e. the two frames would have a similar head tube length). However, because I took the advice and got the medium (54cm) frame I also had to get a new set of forks as the head tube length was so much longer and so forks from Kaffenback were cut too short.