Just got this in an email from Wheelbuilders.com for those interested (edit - I only carried over the summary image, but it looks like just one set of data) -

"Wheelbuilder spent a day in the San Diego Low Speed Wind Tunnel with the objective of comparing our disc covers to uncovered wheels and disc wheels. The obvious advantage of racing with a disc cover is cost, however this data quantifies the performance advantage.

Wheels tested:

Uncovered 30mm aluminum rim
Uncovered Zipp 404 tubular
Zipp 900 Tubular Disc (flat)
Zipp Sub-9 (toroidal bulge)
Aerojacket on Zipp 808 tubular
Aerojacket on 30mm aluminum rim
Conclusions

A covered wheel has dramatically lower aerodynamic drag than an uncovered wheel.


Disc cover performance is virtually identical to disc wheels through the entire range of yaw angles: 0-30 degrees.


A covered 81mm deep wheel outperformed flat disc wheels in every wind condition. The covered Zipp 808 has less drag than the fastest disc wheel tested beyond 10 degrees of yaw.

A typical 30mm training wheel with disc covers approaches the performance of a flat disc at 7.5 degrees of yaw, but exceeds performance of all disc wheels tested at wind angles greater than 12.5 degrees.


The lens-shape on a disc-cover's non-drive side causes wind to pass around it like a wing, producing forward thrust. This effect is more pronounced on covered shallow wheels beyond 20 degrees of yaw.



Discussion

Drag on flat discs is nearly identical with wind traveling from rider's left or right sides. Aerojacket disc covers are asymmetric, with a flat drive-side and lens-shaped non-drive side, so drag differs depending on wind direction. The most favorable direction is with the wind traveling over the rider's right shoulder. For this reason we tested in both wind directions i.e., we ran the test forward and backwards and averaged the data. We ran the tests on the 30mm covered wheel (DT Swiss RR 585) twice on the advice of the wind tunnel staff who were, frankly, astonished by the results."

So, they made a frisbee. Is this relevant at all with the rest of the bike and rider attached?

Every other piece of testing says it is.

So disk wheels and/or covered wheels are lower in drag.

This is not news. We've known this since the 1984 olympics, maybe even longer.

What's the point?

I think the point is that a covered wheel has less drag than a disc wheel and only costs $100. The surprising part to me is WB told me that it would be better to cover a shallow rim. Their chart shows that covering an 80mm rim, I have an 88mm, has less drag when compared to the 30mm up to about 17 degrees of Beta Angle. Interesting.

This is the point. I find this really interesting and somewhat unexpected. Especially that the disc creates forward thrust beyond 20 degs of yaw, kinda like sailing into the wind. I know Zipp & Hed have been trying to accomplish this with their toroidal shaped rims. Of course, I'm not so sure you're going to want that kind of crosswind with a full disc on your wheel.

What I'm not 100% clear on, and I'd be curious to see more data on, is wind direction. They mentioned that they averaged the data, but I'd be very curious to see the effects broken out based on positive yaw (over the right shoulder) and negative yaw (over the left shoulder), assuming 0 degs is wind directly head-on.

This is all well and good, but the uncovered wheels were different than the covered ones, considering they were using wheel covers, wouldn't it make sense to use the same wheel as a control? Obviously an 808 is going to be faster than a 404 regardless of whether or not its covered, but a covered 808 versus an uncovered 808?

They do. They use a 30mm rim as a control and then test it covered. The covered 808 is to show the comparison to Zipp's disc wheels, I believe.

Wheel covers and drag?

Great. Now I have an image of some ugly guy trying to keep his dress out of the spokes. Thanks.

I was happy to read these test results. I also note that the cover as tested was mounted using the plastic screws. Not aero! ;-)

I use the wheelbuilder cover on my HED Belgium wheel. I tape it. The increase in overall bike weight is noticeable -- I don't like that -- but otherwise all good.

Makes me glad I got a wheelcover for my powertap wheel and not my deep carbon one. Though now I need to start using that covered wheel for the super windy stuff too.