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I wonder why wheels don't come with a drag coefficient. It's not hard to calculate.
Wow, you really don't understand either wheels or aerodynamics do you?
It turns out that calculating the drag coefficient of anything is tremendously difficult, and measuring it isn't any easier. You need either a really big supercomputer or a big expensive machine, and you still only get the answer for the particular circumstances you've calculated/measured. Wheels are almost the worst kind of thing, since they roll along the ground. For example, a glider once it is out of ground effect and in coordinated flight is a pretty trivial problem compared with a bicycle wheel.
The total aerodynamic drag of a wheel (disregard rolling resistance and bearing friction for the time being, they are actually much easier to evaluate anyway) is the sum of the force needed to spin the wheel in a given flow stream plus the force needed to hold it still in the given stream. The characteristics of the stream will depend on what else is in the locale, typically the rest of the bicycle with its rider and a ground plane. The characteristics of the bicycle can have quite a large effect on the amount of drag added to the whole by the wheels, and the effect will vary with different wheels, so the lowest drag wheel will be different for different bicycles.
Even after you have found the drag for the condition of zero bulk stream velocity relative to the ground plane and zero yaw, you haven't really found a good way of ranking bicycle wheels, sine we seldom ride in such conditions. Even on a velodrome, a bicycle experiences yaw during cornering, and outdoors we have wind, which can add a fair amount of shear in the stream, since the boundary layer attached to the ground plane is right next to the wheel.
Even beyond mere drag parallel to the direction of travel, we have lift (aerodynamic loads which tend to push the bike sideways) and torque (loads unevenly distributed about the wheel centre which tend to steer it), both of which are significant factors in wheel selection.
gbj_tester-
My bad, didn't mean an absolute final coefficient true for all conditions. Just a compromise to easy rank them. A rolling drag? Wouldn't in most cases, wheels spinning for longer on a work bench after being launched at some considerable velocity, like twice as fast as they would on the road, or causing least drag in a wind tunnel give the same kind of results if pitched against each other? Just a twisted thought. Btw thanks for your time tester. And one more question pls. Why many carbon spoked wheels have drop shaped spokes' sections rather than knife sharp? Has it got to do with crosswinds or maybe safety? Tnx again
Bugno
In reply to @mdcc_tester
Tnx tester. I wonder why wheels don't come with a drag coefficient. It's not hard to calculate. Put them on a work bench at a constant temperature, rotate them at a given speed and see when they stop.