Thanks for the post. I was indeed a little confused by this as I hadn't seen it mentioned in protocols before. Honestly, I have given the FLO guys a lot of crap recently and didn't want to seem like I was being a dick, yet again. If I'm going to be a dick I want to really mean it...I want to be a dick with purpose.
Based on your post it seems reasonable to ask if you have any idea what effect this might have on results? Would this be a significant factor or lost In the noise?
As perhaps the internet's leading voice of "you can't compare one test on one day with one tire to another on a different day with a different tire, even if the other tire is the same make/model/size," there's no way I could quantify it accurately.
In a recent test, a Rail 52 and a Zipp 404, each with a GP4000s II that had been moved from the 404 to the Rail (404 was tested first), at 100 psi, at A2, tested at zero yaw at 167 and 170 grams of drag respectively. That is an average of 2 runs each. The 404's runs were 170 and 170, the Rail's were 167 and 166 (we got screwed by rounding up!). The difference between 167 and 170, practically speaking, is not a difference. Shallower wheels had higher zero yaw drag values - the highest was a 24 spoke Pacenti SL23 at 188 grams of drag. Taking the chart that FLO has on their site, their wheels all start at about 140, 141, 142, except for the disc which starts at roughly 125 grams of drag.
At zero degrees yaw, the struts have their lowest exposed surface area. You can get an idea for this here http://a2wt.com/BikeManufacturers.html
if you let the pictures scroll. I can measure them tomorrow but I will guess that they are maybe 8mm wide by 25mm deep. So as you turn them, they expose more surface area to the wind. They are rounded rectangles, not an airfoil shape. As I said before, the leeward one stays in clear air as it turns with just the struts, but putting a wheel in the struts puts the leeward strut into obstructed air from the wheel. This same principle is why a rear wheel is aerodynamically less significant than a front. So it's a little bit like measuring a rear wheel alone, and a front wheel alone, and saying that the savings you will see on the bike is the sum of the savings from the front and rear wheel's tested individually. We tested that two weeks ago and quantified about a 50% reduction in aerodynamic savings of the rear wheel when it was put into the bike. Same principle applies to the struts as they turn through the sweep.
FLO does not clarify whether the Mavic Open Pro they show on their chart for baseline comparison has had the tare removed. It's not immediately clear whether the values for the Open Pro were taken from tests published otherwise or if it was tested at the same time as the wheels to which it's compared.
So, from a purely factual standpoint, here's what I can say:
1. The differences between the starting zero yaw values of our test of Zipp 404 and Rail 52 to what FLO publishes are on the order of 25 to 28 grams of drag, which is about 3 to 4 watts. Their values exhibit much less variation among the different depths than our test showed, excluding their disc. We didn't test any discs so let's just exclude that as a data point in this comparison. Their suite of measured wheels includes their 30, 60, 90, and disc wheels. Our suite was Rail 52, Rail 34, Enve 3.4 (front rim), Zipp 404, Kinlin XC279, and Pacenti SL23.
2. The struts are rounded rectangles and show their least surface area to the wind at 0 degrees yaw.
Anything beyond that would be conjecture.
We have no desire to run the struts alone when I'm there tomorrow, knowing that A2 considers this to be an error in protocol I don't want to even touch it.