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i would recommend inputting that weight difference into analytic cycling and see if you still care
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404's are reasonably aero, though slightly less so with the extra spokes. Those wheels will definitely be strong and stiff, so cornering will be excellent. They will be HEAVY, though, as they're clinchers, and beefy ones at that. For flat courses it shouldn't be an issue, but on hillier courses, or courses with tons of turns (meaning, lots of braking before turns and then accelerating out of corners) the much lighter tubular version, esp. with fewer spokes, will be faster. I personally wouldn't buy Zipp clinchers due to the added weight issue, and I'm a huge Zipp fan w/3 sets (303, 404, & 808's)
Interesting site, analytic cycling- haven't seen that one before. I didn't plug any numbers into their site for this discussion, but rather base my comments on my personal experience with Zipp wheels of many styles, and many other brands, in time trials, criterium racing, and long climbs. Those experiences have shown me that all things being equal, the lighter wheel will spin up faster, making your bike much more responsive, in sprints, climbing, cornering, etc. The only time they may be slower is on downhills where the added momentum helps you maintain speed.
So since both Zipp 404's are equally deep / same shape one can assume they are equally aero. I'll even say they are likely equally stiff, despite the fact that the clinchers have a bonded brake surface vs. a monocoque carbon construction for the tubulars. So the main difference is weight, and if you go by Zipp's site, the stated weights for a 404 set (current year) is 1278g for tubies, 1658g for clinchers. That doesn't take into account the added spokes for the wheels we're discussing. ~400g of extra rotational mass is a lot in my book, and definitely noticeable. Still good wheels (stiff, aero, quality), but a weight weenie like myself would never spend Zipp-size dollars on wheels that heavy. While I can appreciate the science that may be behind analytic cycling's calculations, I can't imagine the calculations telling me something my legs haven't already learned- lighter is better!