Originally Posted by
msu2001la
The 303 Firecrest is 2mm wider than the 303S and has a minimum tire size requirement (and optimized tire size) of 28mm.
404 and 454 have the same width as the 303S (23mm internal), but Zipp says they are optimized for 25mm.
Zipp says what is optimized for 25mm?
Per 303S description on sram's site:
"The key innovation is in the 303 S Series’ rim design. Its 45mm rim depth is the same as its predecessor, the 302 Disc brake. But its rim width is 2mm wider, at 27mm, and its tire bed is a full 7mm wider, at 23mm. That rim platform is designed to be the fastest with a 28mm tire."
I didn't see any ad copy on the 404 page that has any similar language.
I believe the issue is ETRTO. An inflated 25mm tire on a wide rim would probably be about ideal (105% rule) inflated width from aero perspective on a 27mm width external rim. However, ETRTO is going to hamstring manufacturers by indicating that for a 23c rim, the minimum width tire you're "allowed" to run is a 28mm. For a hookless rim, to bring the internal width down to 21mm minimum (says ETRTO), but keeping the 27mm external, they'd need a thicker bead wall.