Originally Posted by
msu2001la
This.
The 105% rule is pretty well documented and accepted as a design guideline for aero wheels. This was developed in the early 2000's by Josh Poertner while doing wind tunnel testing for Zipp. He's spoken about it many times on his Marginal Gains podcast. You can read about it here: https://silca.cc/blogs/silca/part-5-...d-aerodynamics
The Rule of 105 states that the rim must be at least 105% the width of the tire if you have any chance of re-capturing airflow from the tire and controlling it or smoothing it.
“The critical point is that subtle variations in rim shape can and will change aerodynamic drag as well as handling, but none of it is possible unless the rim is at least 105% of the tire width.”
In order for a wheel to provide aero benefits with a 40mm tire, it would need to have a 42mm wide external dimension.
Knobby tires also disrupt air flow - and would further negate any aero benefits.
Josh Poertner isn't the only voice in aero. HED and FLO have done their own CFD and wind tunnel tests that show
some benefits of deeper section wheels with gravel tires, but these are far lower than the numbers you see posted for 25-28mm road tire sizes. Enve says that once you get above 33mm tire size, there is no aero benefit on their SES wheels.
This article on Pro's Closet website seems to sum up the conversation pretty well:
https://www.theproscloset.com/blogs/...-carbon-wheels
I suspect Poertner’s 105 rule was formulated in a very particular context, and one totally specific to road bikes, and so is not the same thing as saying there is no aero aero benefit for gravel tires. It’s something that needs to be specifically tested, particularly since those who did do gravel-specific testing, e.g. Flo, did find benefit.
Interestingly, Flo claim to have found that when testing wider 28mm tires which broke the 105 rule, that their combined drag number (i.e. Cd and Crr) was lower than it was for 25mm tires that met the 105% rule on that same rim:
https://blog.flocycling.com/aero-whe...ls%20at%20Zipp.
At the very least, I find there are no easy answers; it’s all quite complicated!
Anyway, I’m not getting too deep in the murk myself, as I usually run slick 35mm gravel rubber, the
exception being winter slop, when I move up to 42mm knobbed. Winter aero I’m not worried about at all, as I pretty much look like the Michelin Man when in winter gear.