Sure, looking at headtube length is a bad idea these days. But looking at headtube length and trying to guess at how the fork length is going to compensate for it is an even worse idea. Both geometry charts have stack in them, just look at that instead.c60rider wrote: ↑Thu Dec 26, 2019 10:17 pmIn hindsight I should perhaps have gone with the 50s as the front end is a fraction high for me on the 52s but it's ok. Historically I've always had it in my head to go with the smallest frame you can get to fit you. So looking at the CAAD13 the 54cm seems to be a very close match to the 52s. So I'm going to go with a 51. I'm 1.79m (5ft 10 1/2) and the Cannondale guide suggests I should be looking at a 56! The seatpost is plenty long enough to cope and the front can be tweaked with a longer stem or small spacer. Although the head tube is much smaller on the CAAD that's a bit of a red herring because there's more clearance for bigger tyres so I always think head tube length is a poor measurement to compare. The reach difference between the 52s and 51 CAAD is just 6mm.
In this case the 54 is 1cm lower than your 52s. The 51 is 3cm lower than the 52s and 12mm lower even than the 50s.