I don't know the answer but here is their web site.

http://www.zipp.com/wheels/index.php

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Train safe & smart
Bob

the straight 9 I believe is the flat disc - non lenticular .... the sub 9 and super 9 also have the aero bump below the brake track for better aero and I think the super 9 is the stiffest? A few other threads on this I think ...

The 900 and Super 9 are flat while the Sub 9 has the toroidal bulge. The Sub 9 and Super 9 are only available in tubular while the 900 is available as a tubular or a clincher. I'm guessing it's only a matter of time before the Super 9 is offered as a carbon clincher.

I'd go with a Super 9 if budget wasn't a concern. You can run a 23mm tire without the aero penalty. Add it all together and you have more comfort, better rolling resistance, and again, no aero penalty.