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I'm getting a cx bike soon, mostly for commuting but i may end up racing it in my local series.
I was wondering what size i ought to be looking at. I ride a 58cm road road bike at the mo and i've heard it said that it is usually the case to go down a size for CX. However when i compare the geometry of 58cm road bikes & cross bikes, it seems that the cx bikes come with shorter top tubes anyway?
Make sure you have good stand over clearance on the cross bike, if you actually intend to be trying a few races and going off road. You can compensate for reach with bar choice and stem length.
+ Maybe run a wider bar. In short, go for roughly the same the ETT and don't be concerned about dropping down a size if it it gives you the stand over you're looking for.
Same reach and drop as your road bike. Vertical size makes no odds really.
Typically you go for the same size as your road bike which should mean approx 1cm shorter top tube. As I'm sure you know though, seattube size and top tube length aren't directly linked so I'd use the 1cm shorter top tube as a rough guide.