Originally cross bikes were just old road bikes that had canti's added to them so that they could fit in the cross knobbies. The result of this is that it essentially lifted the bottom bracket the extra radius increase of the tire thickness, perhaps an inch or so.
Somewhere the idea became popular that a cross bike needed more clearance. So they lifted the bottom bracket still higher. There's some good reasoning behind this - the ground is not flat where you ride a cross bike and more bottom bracket clearance gives you more clearance for the pedals over broken ground.
In order to retain the same stand-over height this made the seat tube shorter. So in those types of cross bikes the fit is a bit funny.
Remember, you would usually want a slightly less stand-over height because off-road you may have to dismount on the side of a hill or some such and it is nice to have a little more crotch clearance. Add to that the higher bottom bracket and the higher ground clearance and you now are faced with some guy who normally rides a 58 cm road bike is now getting a 53 cm cross bike.
But only for some cross bikes.......
Other cross bike makers had other ideas. Tall bottom bracket bikes set the center of gravity higher and they don't feel as maneuverable as road bikes. They also don't corner as easily and they are clumsier in slow tight stuff.
So another school of thought makes a bike with a lower bottom bracket. When you put knobbies on these bikes it lifts the bottom bracket up to the original road bike height. These bikes feel a lot better to road bike riders. Mountain bike riders are used to the feel of high center-of-gravity bikes and might not even notice it.
Whether or not these bikes are a bit more manageable in close quarters is a matter of opinion more than fact. Let’s face it, feel is not everything and some relatively minor quirk of geometry really doesn't make a great deal of difference in a 20 lb-ish bicycle that you can throw over your shoulder. But people have tastes and some people are particular about one thing while others might not even notice.
Because you STILL want the additional crotch clearance most makers will design the top tubes slightly longer than a road bike would be. That means that if you, for instance, normally ride a 58 you might want a 56 in this sort of design.
A third school says that they want to duplicate a road bike down to the last detail. World Championship ‘cross bikes from the late 70's and early 80's were like that. They were built to race so they didn't give a flick about extra crotch clearance, you'd ride the bike through anything possible and you'd run everything else. On these bikes, they had the low bottom bracket and the normal length top tube so you'd fit your normal road size. Some times the top tube would even be a little short like on a touring bike in order to get you a little more upright for better visibility.
Now let's add some further complications. Are you intending to race or recreational ride or both? It makes a difference to the way the bike will fit.
Cross racers will want shorter chainstays and tighter geometry to make steering lighter in nasty mud and gunk. They want lighter bikes because they spend more time carrying them than a recreational rider. Lighter means more expensive and more fragile. Of course fragile is relative and cross bikes have to be more heavily built to begin with because of the much higher impact loads encountered in racing.
On a recreational cross bike you want it to be a little larger and maybe with longer chainstays since you can then use it for commuting or touring and there will be heel clearance for pannier and more overall comfort. You would normally have a shorter and higher stem so that you aren't tilting your head WAY back to see forward.
Now this does give you a whole lot of confusion over the correct size bike to buy. It makes buying a cross bike you don't know really difficult unless you're standing next to it or have complete specifications and a full knowledge of bike fit. So you should usually buy a cross bike from a dealer where you can sit on it and actually take it for a test ride. And that can be a challenge in this day when most bike shops stock low end mountain bikes and the occasional road bike.
And that's what this forum is for. Ask questions and don't forget to add your crotch measurements and height.
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Originally Posted by nad45
