All components must be structural, not specifically to aid aerodynamics. Same rule which prevents fairings.http://www.uci.ch/Modules/BUILTIN/getOb ... M&LangId=1
USA Cycling specifically allowed them, viewing it as an economic fairness issue, although most fatty masters end up buying full disks anyway, even if they're no better.
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To me it's an area where there's some conflict between different sections, and it seems not 100% clear to me which components are intended to be covered by which parts.
Reg 1.3.024 says (paraphrasing as it won't let me copy/paste now) ANY device added or blended into the structure to decrease aero drag is prohibited.
Then, later on, the same reg specifies how to make a fairing that IS allowed - a fairing encloses a moving part and is legal if a credit card can be passed between the moving and fixed parts of the bike.
I don't see a strict requirement in terms of being "structural", maybe because that is very difficult to define/determine - Can't bolt on wheelcovers, but you can tape them? Or can't tape them but you can glue them? Or gluing, but only if done by the original manufacturer?
Not that they have gone completely away from the"difficult to define" - see that you can only tape valve holes in discs with tape supplied by the wheel manufacturer