Accepting that I’m not an expert in aerodynamics, it’s quite possible that I’m wrong here (please be gentle, aerodynamic experts). That said, based on my lay understanding of wheel aerodynamics, I cannot see how (except in very limited circumstances) one would benefit aerodynamically from an “aero” gravel wheel. That’s not to say that there aren’t other benefits to a deeper wheel, CX racers will know that it’s quite common for so much mud to collect on the wheel that it rolls over the rim and starts to gum things up exponentially. Having a deeper rim could make it harder for that to happen – in theory anyway.
Getting back to aerodynamics; it’s my understanding that the wider the leading edge (tire), the longer the trailing edge (rim) must be to gently escort the air from the front to the back. Which means that a 30mm x 45mm rim is not as aerodynamically efficient as a 28mm x 45mm. Put another way, a 30mm wide rim needs to be significantly deeper to get the same benefit in saved watts as a 28mm (or narrower) rim. Also, if the air detaches, say because the tire is too wide for the rim aerodynamically (i.e. 105% rule), I’m not confident that adding depth will do anything but pile weight into the system.
Both of these concepts tear at the basic idea of the “aero gravel” wheel; which are very wide, not terribly deep, and will almost always have tires mounted that are wider than the rim. Basically, everything that could be wrong, is.
I’ve been meaning to write this post for quite a while now, but I never gotten around to actually doing it until stumbling on this “white paper” from Hunt. In it, they make some bold claims around their “48 Limitless Aero Disc” wheelset. Running through the specs for the rim, it’s exactly the kind of thing that seems unrealistic given what I understand about tire/wheel aerodynamics (35mm wide, 48mm deep, testing done with a 38c tire). So while I’m not confident enough in my expertise on this subject to call it BS, it definitely smells funny.
I’ve been wondering about the same thing, especially since I am planning to buy gravel wheels. There are probably some limited circumstances when aero do make some sense – primarily mixed paved + decent gravel races like rough road 100 in mostly dry conditions. I was fine with 32mm tires with tubes in 2019 (cheap alloy wheels), 30mm tubeless would have done the job that day. While 45/30 are less aero than 45/28, probably still better than shallow. But most of the time they probably don’t make sense (e.g. riding DPRT for me). I’ve been looking at FAR 30/30 and… Read more »
When done right though you have hell of a versatile wheel-set. Look at Zipp’s latest 303S and 303 Firecrest Disc Tubeless wheel-sets. Both are super light and are stronger than previous wheels. Pair them with 40mm gravel tires or switch them to 28mm road tires and reap the wind cheating advantages. It really is great having a single bike and wheelset that can essentially do it all. Yes the aero benefits are lost on wider tires but perhaps the deep sections can help you maintain a straight line though a patch of mud.
It’s my understanding that the rim would need to be 50+mm deep to have an aero benefit at 30mm wide.
These Hunt’s look purely road oriented rather a mixture of either road or gravel. After looking at the spec’s of these wheels I certainly would not use them for my definition of gravel. As far as the aerodynamic’s I’m not an expert but it seems like the key here is real world performance. It’s easy to create the perfect aerofoil shape for wind in a single direction (think raindrop falling from the sky) and another thing entirely to account for wind coming from different directions. Reading a little bit of the white paper their design is an approximation of a… Read more »