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I think its the fact I dont want 30mm+ of spacers. The head tube is a small 155mm on a 56 Caad10. I dont bend that well.
Josh-
Don't measure the headtube, measure the Stack and Reach.
Headtube size are usually determined by the fork length and headset, a CX bicycle (Genesis Croix de Fer 54cm) with 135mm headtube have about the same height as a Cannondale CAAD10 with 155mm in 56cm.
Also bottom bracket height can make a difference, a lower BB would make the headtube feel taller, whether a higher BB will make it feel smaller.
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According to the geometry chart, it look like you're better off going for the 58cm as it's only 5mm longer, can be remedy by using a different handlebar with a shorter reach (says a compact with a 75mm reach, or 70mm reach).
Cannondale's measurement are fairly traditional, if you got a 56cm frame, expect a 56cm fit.
Having spacers is fine, you'll only notice it if you don't ride the CAAD10.
edscoble
I don't see what's the problem here.