It is an interesting study that you performed. I have a question for you. You reduced each of your variables in question by 50%. Is there a realistic basis for this, in other words, has it been shown that going from the reference case rim to the most aerodynamic rim possible, results in a Cd reduction of 50%? Is it possible to reduce the mass and inertia of a wheel by 50%? The rolling resistance coefficient?

It makes sense that cutting certain terms in half gives the largest reduction in power, since each term is weighted differently. However, this doesn't tell us if this is practical or not. It may very well be possible to reduce the wheel mass by 75%, but only be able to reduce the coefficient of drag by 10%, therefore it is important to know how aerodynamic wheels change the mass and drag coefficient in order to determine if it is worth the money to buy an "aerodynamic" wheel. So how much reduction in each of these quantities is currently achievable, going from a stock wheel to the various types of race wheels? Going one step further, how much reduction is theoretically achievable?

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"What the mind can conceive and believe, the mind and body can achieve; and those who stay will be champions."

Where is this "sail" effect documented? I'd like to read about it.

FWIW, I think the reduction in drag due to increase in yaw angle on Hed's website is due to a reduction in speed of the oncoming flow. In other words, as the flow is moved off center, it can be broken into two components using vector analysis, one component at zero deg yaw, and one component at 90 deg yaw.

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"What the mind can conceive and believe, the mind and body can achieve; and those who stay will be champions."

I totally agree with wind tunnel testing. Technology would not have progressed to the point it has today if we didn't have the scientific method of performing tests in controlled ways.

However, I am confused by your statement below. Are you suggesting that testing a bike stationary without wheels moving and legs moving is approximately the same as a bike with these parts moving? Certainly the flow is unsteady due to wheel and leg motion. This doesn't detract from the need for a wind tunnel, actually it enhances the need for a wind tunnel. There are currently very high fidelity computational models capable of dealing with steady flow around objects, it is when the flow becomes unsteady that wind tunnels are so important nowadays, since this is a much harder problem to accurately model.

Is the majority of testing in wind tunnels done with the rider and bike motionless?

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"What the mind can conceive and believe, the mind and body can achieve; and those who stay will be champions."

Cobb' s website discusses the sail effect on the frame as far as water bottles and appendages to the bike. The basic idea of any sail effect is that on the leeward side of the wing, there is an area of low pressure compared to the windward side. This effect varies greatly with angle and degree of turbulence. The pressure differential creates lift. Only a small portion of the lift is going to be in the forward direction of the bike.

As far as real-world testing versus windtunnel goes. As I work as a research engineer and conduct experiments everyday, I can tell you that reducing the number of variables in an experiment is the first goal of any researcher when designing a test. Changing more than one thing at a time in an uncontrolled or unmeasurable way is the best way to waste time and money in engineering by producing non-meaningful results.

it sounds like interesting stuff.

As far as your second statement, you are correct that the force in the direction you are trying to move is decreased, however a side force in introduced, which requires power to be taken away from forward motion and applied to overcome the side force and continue in a straight line.

After looking at the Hed website again, I am confused as to what they are showing. They have not defined the direction the drag force is acting- whether it is opposite to forward motion, or it is in the direction of the flow. Therefore, I have no idea what component is being presented in the graph and table. I may be intrepreting it wrong.

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"What the mind can conceive and believe, the mind and body can achieve; and those who stay will be champions."

The drag data should be the net force in the direction opposite the apparent movement of the wheel in the air stream. You can see there is some ideal angle of attack to the wind. Above that angle and the air becomes turbulent on the leeward side and the wheel stalls. The same thing happens in flying a plane or sailing a boat.

As to how the tests are performed, in our case it depends on what we are trying to measure.

If we are examining a part of the bike that is not effected by the rider, we test without a rider. If the test is about an area where the rider does have a significant effect, we test with a rider. If the riders arms or torso are what matters (say you are testing aerobars), we would test with the legs still. If we're testing seatposts or seatstays where the legs have a major impact, we test with pedalling legs. So basically we will test with as little disturbance as possible, and eliminate as much as possible as long as it doesn't have an effect on the test.

The above is a bit of a simplification, but it is correct for about 95% of our testing.

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Gerard Vroomen
3T.bike
OPEN cycle

I had intended to keep quiet about this but I just cannot help myself. I've got to ask: What in the world is a yaw angle?

I keep seeing this pop up in aerodynamics discussions, and my curiosity is getting the better of me.

RP

Interesting stuff.

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"What the mind can conceive and believe, the mind and body can achieve; and those who stay will be champions."

Yaw is a term carried over from the aerospace industry. Any body has six different ways to move, three translations and three rotations. In the aerospace industry, the three rotations are coined pitch, roll, and yaw. Looking straight at the object, pitch is the rotation of the nose up or down. Roll is the rotation just like you use while swimming, and yaw is side to side motion. Yaw is important in wind tunnel testing of bikes since wind can hit you at many different angles.

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"What the mind can conceive and believe, the mind and body can achieve; and those who stay will be champions."