To summarize our findings:
(1) At zero degrees yaw and 30mph wind from straight-ahead, most top wheels are very similar. As long as spokes are minimal in number and profiled, and rims are aero, the drag numbers are tightly bunched. We find drag forces in the 120 to 150 gram range for all the top wheels, those with deep rims, minimal spoke number, and no serious aerodynamic flaws.
(2) As yaw increases, the order shuffles. With toroidal shapes, for example, drag gradually decreases, out to 15º, then steadily rises. The broad shape pays this benefit. For Mad Fiber, drag remains steady from 0º to 8º, rises from 8º to 10º, and then begins to decrease. At 15º, where all others are still increasing, Mads continue to drop. By 20º, the decrease steepens and they drop below any other spoked wheels. And this trend continues out to 30º, where our testing stopped.
For example, in late 2009, we tested a number of well known wheels. At 22.5º yaw, one of the best known samples generated 26% more drag than Mad Fiber. At 25º, its drag was 50% greater. At 30º, this competitor's drag was 109% greater, over double. Granted, these are extreme yaws, but this is what was measured.
My guess is that the overall design concept of the wheels came first (i.e. the lightest way to construct the deep section and join the spokes to the rim and hub) and the aerodynamic assessment came as a secondary consideration. This is pure speculation of course, but it would explain why they came up with a rim section that goes against most of the current wisdom on rim aerodynamics.
For example, ENVE made rims saying "lighter is bestest" for several years, before then introducing their "aero" and extremely heavy (relatively speaking) new wheels.
Now they say that aero is what matters and lightest isn't bestest.
Light, durable, aero - pick two.
Madfiber picks light and durable.
Too bad they also got a double dose of the ugly stick or else Lightweight would be in real trouble.
A 3rd-party CFD analysis is available here.
Here is another 3rd-party test.
If you do further googling you'll find more references.
It's fairly evident that from reading the literature that only Zipp and Bontranger are truly aerodynamic wheels. All the rest just 'look aerodynamic' but then don't work very well, especially at slight yaw angles.
The Bontranger data doesn't state if the wheels were rotating in the wind tunnel or not. If they were static tests I'm rather skeptical of the numbers produced from their tests as representative of the real world. However, from a point of view of the fundamental fluid dynamics, the Bontranger and Zipp designs are superior to anything else on the market at the moment.
The idea with a rim that is wider than the tire is that if the flow separates on the rear side of the tire (where the bead meets the rim) the change in contour of the rim accelerates the flow again. This positive pressure gradient will help re-attach the boundary layer if it has separated off the tire.
The other feature that Zipp and Bontranger have is a large radius on the trailing edge. Again, this helps at slight yaw angles on both sides of the rim (i.e. the tire-leading side and the tire-trailing side).
Enve has their "SMART" line of wheels using a similar shape with a blunt nose and wider shape.
Hed, despite your assertions has been using this type of shape for about 3-4 years on their Stinger line and have transitioned it over to their Jets. I remember a Velonews test from about 2 years ago where the Stinger just DESTROYED the competition because they had a blunter nose, deeper tire well and wider rim; Firecrest was in the works at the time and the FC808 apparently edged out the Stinger 9 at the time, but wasn't published because it was yet to be released. The Jets are not quite as refined, but a step up over older toroidal designs.
Others are starting to produce similar rims. 3T has shown some limited wheels with this type of shape. Mavic has begun experimenting with wider wheels. Rolf has also started going this way.
I thought I should answer your question. Lots of wheel makers don't design specifically for aerodynamics. As you point out, it really shouldn't be all that complicated now that the toroidal rim shape patent has expired, they could look at whats out there, tweak it and run some CFD as confirmation.
The best way to think about what we now know is the "best" rim shape is to remember that a tire is going to be put on it, most likely a 23mm. Next, remember that the wind will flow over the tire and back; over the "nose" of the rim and back and over the tire; and finally, some wind will flow over the tire, along the rim and back over the tire.
The key to integrating a 23mm tire into the tire is to make sure that the rim to tire transition is smoothe. This means wider rims.
The key to better wind flow over all parts of the rim seems to be a blunter rim on the inside. (Imagine Trek's kamm tail concept)
Narrow V shape rims have been shown to be pretty outdated at this point. For instance, in Tour's recent data a blunt nosed wide 23mm deep Corima Winium + wheel was more aero than the Mavic Cosmic Carbone SLR at 52mm wide.
NGMN wrote:Hed, despite your assertions has been using this type of shape for about 3-4 years on their Stinger line and have transitioned it over to their Jets. I remember a Velonews test from about 2 years ago where the Stinger just DESTROYED the competition because they had a blunter nose, deeper tire well and wider rim; Firecrest was in the works at the time and the FC808 apparently edged out the Stinger 9 at the time, but wasn't published because it was yet to be released. The Jets are not quite as refined, but a step up over older toroidal designs.
I went back and looked at the profiles on the HED website and looked at the Stinger/Jet lines. My mistake. I wonder if they will ever make an Ardennes with a similar profile that will mimic a Zipp 101.
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