A lot of wheel and bike companies like to put a lot of emphasis on their wheel performance in the cross winds. They show you a pretty graph that looks something like this:
What you’re generally seeing is the dip due to decreased drag then a steep increase when the flow finally and fully separates from the wheel.
The wind tunnel is a perfect environment while the real world is a very imperfect environment (duh right?). What wind tunnels don’t capture from the real world is something called boundary layer, due to the way that they operate.
Think about this, when you’re riding at 20 mph, how fast is the ground moving? Obviously it’s stationary, but relative to the rider and wheels it’s traveling at 20 mph backward (the same speed as the wind). Why does this matter? If you get down to the smallest levels air speed at the ground is going to be the same speed as the ground (zero). This effect is called the boundary layer, where on the surface of everything that’s moving through a fluid there is a very short transition area where stream velocity goes from zero right at the surface of an object to free stream. To remedy this problem and make the wheel “see” wind speed right at the ground level special platforms are used in wind tunnels:
As you can see on the velocity profile on left, there is a gradient next to the floor, the platform sucks in this low speed air and reestablishes what is the ground (there is inevitably a small BL but it is negligible)
This works great for head on velocity, however it doesn’t capture effects of the BL of the cross wind flow (which IS moving at a velocity different than the ground thus has a boundary layer).
Not only is there a simple ground boundary layer present, there is a larger velocity gradient that is a part of the atmospheric boundary layer (ABL). When the weather stations measure wind velocity it’s typically 10 meters (32 ft).
This is a good illustration of ABL. While most of the gradient is quite high, the steepest gradients are right at the bottom next to the ground (where we’re hopefully riding bikes). This means your 10 meter weather station is reading a significantly higher wind speed than your bike is seeing. This also shows the difference in gradients between different types of terrain
And anyone can tell you this, if you’re riding in the city rarely is it very consistently windy, while in Indiana it’s always windy. However the wheels are the closest component to the ground (again, hopefully).
Alright so now circle back to the neat drag image we first looked at. Zipp in particular likes to emphasize their performance at 15 degrees AoA. This is probably the point where the wheel drag data varies the most, we have also observed this divergence in wind tunnel and CFD results. However let’s think about this for a minute, if you’re traveling at 25 mph (pretty quick, but a good race speed) a 15 degree AoA means that you have almost a 7 mph of direct cross wind. A 7 mph wind seems pretty reasonable and average right?
Remember the wind table essentially eliminates the BL in both components of wind velocity, forward speed (which we want to eliminate) and cross winds (which we’d like to not eliminate but is not physically possible).
So when Zipp says the wheel is saying a 7 mph wind, what does the REAL wind speed have to be to get a 15 degree AoA?
I used this calculator: http://es.ucsc.edu/~jnoble/wind/extrap/index.html
The second set of calculations are applicable to heights under 100 meters, given that the height of a wheel is at the lower limit of this equations usefulness so take this with a grain of salt. To get the magical 7 mph cross wind, the weather station is going to have to measure a 20 mph wind (assuming Agricultural fields as your surface roughness, obviously a town, forest or city will be much higher).
So 20 mph is a VERY windy day without doubt. All I’m suggesting here is that bike companies have determined that 15 degrees AoA is a mythical regime that we’re all riding in all the time, but it’s not QUITE the case.
So a real world measurement of this is what Mavic actually did with respect to the Kona Ironman course:
Kona is a notoriously windy place to ride and more importantly it’s this desolate lava field with nothing to protect you from the wind. They go into this exhaustive breakdown of each portion of the course (OMG triathletes YAWN) A more realistic distribution of wind speeds is observed when they take their setup to France:
A much more real world riding area in France…almost no cross wind!!!!!
Given this is a low wind day and they don’t actually tell you what the real wind speed is so I guess it’s not a perfect example. I also am VERY aware that the times at which you’re pinned in the gutter are usually the hardest parts of races.
However, bottom line is manufacturers like to play up their drag at high angles of attack, I think, because pretty much everything below 5 degrees is very closely packed together and it’s hard to really differentiate your brand. Is <2 grams of drag worth an extra $1000 of proprietary technology?