The Issue with “Cross Winds” in Wind Tunnels

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:

Boundary Layer Table

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:

http://www.engineerstalk.mavic.com/yaw-angle-measurement-in-real-conditions-on-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?

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12 Comments

  1. It sounds like your saying, V shaped, U shaped, dimpled, it really doesn’t matter? I’m a crit racer on a budget, about to plunck down some coin on a set of race wheels. Any recommendations?

  2. Well, if you have the expendable cash, the good stuff is known to be good for a reason; Enve, Zipp, and they will back them up. Or you could get a close approximation and save roughly 2/3 the price. There are actually some nice rims on eBay (Chinese of course), from reputable sellers, with modern shapes, very respectable weights and nice features like basalt braking surfaces. I got some of these, a set for less than $300, and using the Reynolds Cryo Blue pads in races and on some pretty long descents (into Roanoke, for example), I’ve had no problems. Tubulars 20/24 built up at ~1200g, 38mm deep, 27mm wide below the brake track.

  3. I agree, especially for criterium racing, where most of your time and energy is spent re-accelerating while in the pack. So the less rotating mass you have to spin up out of every corner, the better off you’ll be.

    Also personally, I feel much more confident dive bombing a corner on a cheap wheel or team supplied wheel that isn’t going to bankrupt me if I end up laying it down.

    • Extra wheel rim mass does not necessarily reduce one’s acceleration: if the extra mass comes with a sufficient improvement in aerodynamics (think low profile light rim vs heavier deep section aero rim) then it may indeed mean you can accelerate more quickly (and don;t slow down as fast either).

      If you can have superior aero *and* lighter weight, then sure, the weight reduction is useful, although vastly over rated in terms of it’s impact on performance. I discuss this in this item where I model accelerations of different wheel and aero scenarios:
      http://alex-cycle.blogspot.com.au/2014/12/the-sum-of-parts-ii.html
      which was an update on this earlier item:
      http://alex-cycle.blogspot.com.au/2013/02/the-sum-of-parts.html

  4. I bought a set of Mavic Aksium’s and really liked them; they are their entry level after market wheel. They felt great coming off a stock wheel. The Ksyrium was pretty awesome as well, a bit more expensive though.

  5. Good analysis, but one thing to consider is stability at much higher speeds on fast descents. I’d rather have a wheel that handles the crosswinds well in this scenario. I’ve used Martindale 60mm deep super wide (25mm) rims that handle crosswinds really well and had no problems with fast, windy descents in Birmingham. But doing those same descents on my older 60mm narrow Reynolds wheels is super sketchy with the front wheel buffetting in the crosswinds.

  6. “So the less rotating mass you have to spin up out of every corner, the better off you’ll be.”

    While that is certainly true, the magnitude of the effect is very very small. In almost all cases if you have the choice of a more aero wheel, or a lighter one, the more aero one will be faster, EVEN when accelerating out of a corner. (math: http://www.slowtwitch.com/Tech/Why_Wheel_Aerodynamics_Can_Outweigh_Wheel_Weight_and_Inertia_2106.html )

    But as this blog post illustrates, yaw angles are often very low, and when they are, a shallower, lighter wheel will tend to have nearly identical aero performance. So you do get that tiny tiny mass advantage. (though really even that may be canceled out by worse rotational aero drag which is not reflected in wind tunnel tests)

    But yeah, a well shaped shallow wheel with a low, aero spoke count, like a mavic cosmic elite, Flo 30, Zipp 101, etc….. as long as it isn’t super windy, isn’t giving up much. Saves ya some money.

  7. To be honest the rotating weight thing was just an opinion I’ve had for a while just as a racer and I kinda took it for granted.
    I took your sheet Jack and tried to find something I could use to prove my point (apparently the solid disk is a great approximation).

    I also wanted to compare the inertia weight savings of a shallow rim to the drag penalty so I did a quick addition with drag data I have for two particular sets of wheels (a 90 mm and 60mm)
    https://docs.google.com/spreadsheets/d/1BR5H_LoC5Fn_XutwFWCwIU8S3c3vfxwxgp8VMb97TQk/edit?usp=sharing
    I went with zero AoA, kinda a worse case scenario for differentiating rims and the difference is still huge. I guess I have to abandon my old school of thought :/

    Also in response to Brians comment that a deep rim with good cross wind characteristics is needed for windy high velocity. I totally agree, I’m just asserting that from a pure performance standpoint the performance at low yaws shouldn’t be disregarded. Also I’m not entirely convinced that a low in line drag value for a rim necessarily means that the rim is more stable. I think that has more to do with the center of pressure of the rim relative to the steering axis.

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