They weren't custom - at least I didn't have them custom built. I think SRAM got together with its subsidiary Zipp and had some of these wheels made with the SRAM Rise hubs in order to provide an offering for CX disc riders - which would make sense since more and more CX frames are coming out with disc brakes and Zipp does not make a disc brake hub. Aaaand I gotta say, as little as I know about going fast at CX, I would have to say that the Zipp 303s are about as perfect of cross wheels as I could imagine, given that they are SO WIDE and provide a lot of area to keep the rim and tire glued together, even at low pressure.

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Amateur recreational hobbyist cyclist
https://www.strava.com/athletes/337152
https://vimeo.com/user11846099