I work with rolling road wind tunnels in my career, and the balance concern is addressed in my post.

Tom, real world testing as you say just has too much variation. It could be a poor man's version, but if funding is a non issue, I'd test in a rolling road tunnel for confidence in wind conditions and consistent crr

IIRC

I work with rolling road wind tunnels in my career, and the balance concern is addressed in my post.

Tom, real world testing as you say just has too much variation. It could be a poor man's version, but if funding is a non issue, I'd test in a rolling road tunnel for confidence in wind conditions and consistent crr

You aren't RC.

It isn't, it doesn't have to be.

The roller apparatus sits on the load cell, not next to it.
The load cell measures where the aero drag vector acts on the spinning wheel (in isolation).
From there it's trivial to back-calculate the true longitudinal force acting on the wheel hub and compare it to the measured aero drag scalar value.
The difference gives you 'the torque to spin", and thus 'the power to spin'.

All of this was discussed and finalized what, 6 years ago?

I'm not following. Isn't part of that "torque to spin" being used to overcome the tire rolling resistance? Eliminating that would require a hub motor, no?

Ahh, I get the ambiguous phrasing now, and how that messes with the discussion. The "power/torque to spin" the wheel in the tunnel, on the load cell platform is not of interest here. That "drive power" is only there to make the wheel spin at the exact rotational speed to match the air stream velocity.

No, the property of interest is the aero drag vector location, direction and magnitude of the spinning wheel. This is measured by the 6-axis load cell (via a transformation matrix), but only if the wheel is by itself in the tunnel.
Now do a basic free body torque equilibrium study of the wheel as it pivots around the contact patch. This is where most folks get confused, me thinks... The wheel does NOT rotate around the hub axle, it rotates around the contact patch/point. Anyone wants to argue different should think about it until they get it...
Anyway, the torque equilibrium equation gives you the required horizontal force at the hub, which is
1. what is resisting forward motion
2. NOT equal to the measured horizontal force at the hub by the x-component of the load cell.

How good "real world" testing is depends on how you do it. Is THIS good enough? :-)
http://bikeblather.blogspot.com/...ng-chung-method.html

Using a wind tunnel has the advantage of being able to control yaw. The chung method is more difficult if you want to know drag at exactly 10 degrees of yaw.

Then you didn't get it...

It's very difficult to explain lines of reasoning over the internet.

Ahh, I get the ambiguous phrasing now, and how that messes with the discussion. The "power/torque to spin" the wheel in the tunnel, on the load cell platform is not of interest here. That "drive power" is only there to make the wheel spin at the exact rotational speed to match the air stream velocity.

No, the property of interest is the aero drag vector location, direction and magnitude of the spinning wheel. This is measured by the 6-axis load cell (via a transformation matrix), but only if the wheel is by itself in the tunnel.
Now do a basic free body torque equilibrium study of the wheel as it pivots around the contact patch. This is where most folks get confused, me thinks... The wheel does NOT rotate around the hub axle, it rotates around the contact patch/point. Anyone wants to argue different should think about it until they get it...
Anyway, the torque equilibrium equation gives you the required horizontal force at the hub, which is
1. what is resisting forward motion
2. NOT equal to the measured horizontal force at the hub by the x-component of the load cell.

The roller and motor is 'inside' the free body. It can not show up any equations...
The free body is the entire platform and everything mounted to it.
The only forces acting on it is the aero pressure distribution and the reaction on/from the load cell. Now apply equilibrium conditions.

I will go on to say that this is one of the dumber arguments that pops up on this forum every 6 months. If deep section wheels were faster, and disc wheels were just a gimmick, then you sure the hell wouldn't see the ProTour teams riding non-sponsor correct disc wheels, or all the hour record and track guys riding double discs.

Wait...didn't you say to do the free-body torque equilibrium around the contact patch? How can the motor and roller be inside the free body then?

Ummm...

First figure out the aero drag vector location, direction, magnitude, using the 6-axis load cell. The free body (#1) here is the entire platform and everything mounted to it (after proper tare, of course).

Then use said vector in the free body (#2) that is the wheel rolling down the road. That free body (#2) has the aero drag vector and the front fork mount horizontal force as external forces and has the contact patch/point as reference for the torque equilibrium.

Trivial ;-)

The vertical force from the tire interaction with the road is superimposed with the whole aero drag stuff we're discussing here.
Not forgotten and an order of magnitude more demanding... but not correlated.

Edit for clarity: the vertical force would create another horizontal force at the front fork mount. Add the bearing friction. They all belong in free body #3. Superimpose.

BTW, if one wants to measure the "power to spin" in the tunnel with the roller motor (or even better, the motors reaction torque), one would of course apply the minimum tire loading possible, just shy of it slipping. My WAG is that the load would be <1% of the typical "road load". So maybe insignificant/in the noise in relation to the aero drag?