CX Bike geo. has slacker angles (73 ST/72 HT) which I know effects the sizing of what bike I need, but not positive in what way: http://motobecane.com/ftx_geometry.html In addition the 54cm is c-t so it’d be more like a 52 c-c, and the 56 more like a 54c-c.
I donno, ever since high school my head doesn’t deal with numbers as well as some people. Throw me a bone here.
You might want your top tube a bit longer, since your bars will be higher relative to your saddle, probably, closer to level with it. I haven’t checked out the geo of that bike, but that might mean a slightly larger size than you may have bought in other bikes. If you like a 55cm tt on your track bike, a 56cm tt on your cross bike could be about right.
It would be nice if they published the head-tube height as well. But unless it’s shorter on the 54 than say 110 I’m pretty sure you would be much better off on the 54. The slacker headtube will most likely have you running a bit longer stem (or longer reach bars) than on your road bike but that’s much better than trying dismount/remount (or deal with in technical sections) a cross bike that is too big. In many cross bikes you do go a size down from your road bike but in the case of this one they seem pretty true to size.
Keep in mind the 54 being center to top rather than center to center is more a result of the bottom bracket (and thus the entire frame) being higher off the ground than the top tube being shorter. For example, the stand-over of the cross bike you are looking at in 54 is the same as that of a 54cm Cannondale Caad9.
I probably should have said earlier but I’ll be racing it cross by next season, but before that I’m going to do road training / possible road racing with it. I’m poor and can’t afford a real road bike yet haha
So, steepening the seat tube puts the bars / reach farther away while slackening it brings them closer, meaning that a longer top tube on a slacker frame will fit more closely to a slightly shorter top tube on a steeper frame, yes? That’s what I gather. In that case I’m strongly leaning towards the 56.
For what it’s worth I am 5’10 but with longer than average legs (my saddle height is 75cm from bottom bracket to saddle w/ 172.5 cranks). I ride a 54 Cannondale cross.
Top tube 55
Head angle 72
Seat angle 73.5 (.5 steeper than the motobecane)
head tube 150 (integrated)
The standover on my bike is higher than on the 54 motobecane but that is largely because the top tube slopes up to what appears to be a taller headtube. You should have no problem standing over the 56 but unless you have much longer arms and torso than I do I really think the 56 is going to feel either too long with a 120-130 stem or you will find yourself running a shorter stem which REALLY does make the steering worse in my opinion.
I should add that if you are using it on the road the shorter headtube of the 54 might work better because it will allow for a lower road like bar position if you need it. I’m not sure what kind of drop you run on your track but I run about 4cm more drop on my road than on my cross.