The Cannondale site does not do the best job of explaining the diff between CAAD8 and CAA9. They talk about larger seat tube and top bar on the CAAD9. Most of my team mates are riding CAAD9s and I am currently riding a CAAD8. I noticed the CAAD9 top tube is a bit more dropped then the CAAD8. I believe that is one way to get more lateral stiffness. One of the guys "Brad" from Cannondale has a Blog and he puts it this way; "A CAAD 9 is going to be slightly lighter and have less flex “side to side” in the bottom bracket compared to the CAAD8. We only make one frame per model, so a CAAD9 from 2007 is the same as a CAAD9 for 2008." http://www.cannondaletube.iuploadccs.com/ask_brad/ As long as you get the correct size either one will have you screaming down the road with a trail of fire behind you! I hope this helps. May all your roads be downhill ;-)