Just came across this thread while waiting for the football games to start. Happy Thanksgiving to all in the US.
I interviewed product managers and engineers from Bontrager, Campagnolo, ENVE, and Zipp a couple of years ago for research on how wide wheels and tires can make you faster. None of those companies considers the so-called “rule of 105” when they design wheels. In short, it may have been a finding or guideline coming out of wind tunnel testing for Zipp Firecrest wheels of 2001 (the one Josh based his rule on), but so much has changed in the last 20+ years with rims, tires, and how they are combined to make them more aero and faster that this finding or guideline is not considered in the modern wheel design process.
Seems like 105 is merely a starting point that has been made much more complicated with the increasing growth of tire widths. I myself was at one point only concerned with internal width but have learned that for low drag and stability in crosswinds that the relationship of internal and external width is critical with ratios lower than 75% being the most beneficial. Had no idea how much this mattered until I read through this.
While I realize this is just one article, it is quite methodical in its approach and does seem to make sense.
I’ve only skimmed it but where is this 75% rule explained?
It doesn’t really make sense to me logically, and I fear it’s one of those things chucked out there on YouTube that develops a life of its own and becomes fact.
There is a table they show with the ratio of inner to outer width. Wheels that had a ratio 75% or lower tested much better in crosswind stability. It generally works well from what I can tell if the delta is about 10mm. That gets in the ballpark.
There is such a large amount of data on this subject. My first wider carbon wheelset was a 2001 vintage set of Zipp 404 Firecrest that were purported to be more stable in crosswinds, they were not.
I think I may have misquoted the report and I found the table in another report that also spoke about rim width and relationship to inner width as well as tire width. My appologies for misleading anyone. What I was hoping to convey is that there is much more going on than simply the 105% rule as there are several other factors in play. A close friend bought a pair of CRW’s earlier this year. Those wheels.are supposed to be very up to date, yada, yada but he has found them to be somewhat sensitive in crosswinds and the same has been reported by others,
re: the “rule of 75” lol.