So the creation of the ‘Rule of 105’ actually happened at a wind tunnel test with Lance Armstrong following a conversation I had with Johan Bruyneel. Johan was keen to move the team entirely to 23mm tire including in the TT’s, but the tunnel test was being run with 19mm Conti’s on the Hed3. After showing a significant change in performance with that tire change in that test, I went back to the tunnel later that year and ran dozens of tests with different width tires on a bunch of different wheels and found that at a relationship of 101% or less, the drag was more or less entirely tire dominated… the rim in these scenarios is essentially only acting as a spoke shortener moreso than an actual aerodynamic surface. Beginning at around 102% the rims begin to see flow attachment that allows them to see notable drag reductions from ~1-2 degrees and upward, and at 105% flows could remain attached to 10+ degrees (many modern wheels can maintain attachment as high as 17.5deg).
It’s notable that this is NOT only true for toroidal wheels, they just do it better, but something like a Hed3, old Campagnolo Shamal, or a lot of the pointy Reynolds stuff can do pretty well if 105 is observed.
I made a video on this during early covid that uses some CAD models to better describe this: https://www.youtube.com/watch?v=mDZ61aMegnQ
I can’t share the data, but I attended a tunnel test at Silverstone last week with one of our world tour teams and won myself a bottle of wine in a bet about this exact topic. The wheel sponsor delivered a new wheel designed around a 28mm tire that could achieve ~102% of tire width (tubeless setup), the wheel they had previously was only at 99% with a 26mm tubeless tire and the new setup with 28 was indeed faster… everybody was thrilled about this new faster 28mm setup, so I bet them that the new wheel would be even faster still with the 26mm tire… and of course, it was.
thanks for the additional color on this. as i try to remember the timeline, this would have been when not long into your zipp work tenure. i would imagine that this testing you did post-bruyneel must have made you consider the ramifications of the toroidal rim shape, which was a shape at that time solely used by HED, tho the patent was jointly-owned by steve hed and his then-partner. that must be a fascinating, and untold to my knowledge, story.