Home Made Aero Spin Down Test of 2 Top Wheels

[cyclespeed]

Because the wheel spins in a 360 degree circle, the minuscule amount of drag caused by the rotating spokes in the tangent direction of rotation is cancelled out by the exact same drag generated on the opposite side of the wheel. The amount of air drag that actually stops the wheel spinning is going to be extremely minusucule. the difference in bearing seals and wheel balance is the factor making up rotational losses.

This makes no sense to me. You are saying that the top spoke's drag is cancelled out by the bottom's spoke drag. This implies that there is no aero loss whatsoever. By the same token I could spin a domestic fan, and it would consume no electrical energy (other than that lost in the bearings). The fan's blades are being pushed through the air to create air movement. This costs energy.

[cyclespeed]

THe test is interesting, and probably has some relation to bearing quality, seal drag, and spoke drag;
But it is not really indicative of anything associated with the aerodynamic drag of a wheel in real use.

When spinning a wheel in place, it is rotating in the steady state air 'field' of its own vortex.
When riding, the top of the wheel is moving at twice the speed of the bike, while the bottom of the wheel has a near zero velocity; and it is always moving into fresh, undisturbed air. There is also the question of loaded bearings, but that isn't part of the "aero" analysis.

{EDIT: So at the bottom of the rotation, the spoke aerodynamics is almost completely irrelevant, while at the top of the rotation, the spoke is moving into the 'shadow' of the tire/rim. In your test, the spokes are creating almost all the drag, while the rim has been almost eliminated, because it is moving into its own vortex.....all WAY different.}

Summary: Sort of a very flawed attempt. Not completely meaningless, but not conclusive of anything either.
So, if the goal was to "show" or "prove" the superiority of one wheel over another, that is a "failure".

I agree that the results would be different if I were blowing a 55km/h wind across the wheel, but unfortunately, I don't have one.

In my test, it could be argued that the spokes' aero drag is diminished because each spoke is 'drafting' the one before it. In a normal riding situation, this drafting effect is reduced.

But I still maintain, that by reducing the variables down to the minimum like this at least shows spoke drag to be a significant factor at high speeds.

[ergott]

By the same token I could spin a domestic fan, and it would consume no electrical energy (other than that lost in the bearings). The fan's blades are being pushed through the air to create air movement. This costs energy.

No. You are confusing the physics pertaining to a stationary spinning object and one that has a velocity perpendicular to it's spinning axis.

If you were to give any of those wheels an initial velocity comparable to the rotational velocity you used and allowed it to travel down the road it wouldn't continue to roll anywhere near as long. The wheel has to resist the friction due to the air it's traveling through. That's why rim shape is very important.

[cyclespeed]

But for crying out loud, you're testing wind resistance, WITHOUT WIND! Really, just sit down for a moment and contemplate yaw, even zero yaw. Yes you're test might have some validity in a 54kmh tail wind (but not really even then, not even by a long shot)... Otherwise, these wheels really aren't facing any actual wind, and the rims might be moving thru air, which is already circulating/rotating in the rims moving direction, but the wheels themselves as a whole are not by any means being pushed thru the wind.

No, and that point is made in the video. However, work has to be done to move those spokes through the air, and the more aero they are, the easier that will be.

Although the aerodynamics would change were there a 50km/h wind blowing at these, it doesn't change the fact that you have to provide work into the system to spin a wheel. And the faster you spin it, the more aero drag the spokes will create.

[cyclespeed]

By the same token I could spin a domestic fan, and it would consume no electrical energy (other than that lost in the bearings). The fan's blades are being pushed through the air to create air movement. This costs energy.

No. You are confusing the physics pertaining to a stationary spinning object and one that has a velocity perpendicular to it's spinning axis.

If you were to give any of those wheels an initial velocity comparable to the rotational velocity you used and allowed it to travel down the road it wouldn't continue to roll anywhere near as long. The wheel has to resist the friction due to the air it's traveling through. That's why rim shape is very important.

No I'm not. I'm fully aware that the wheel has no linear velocity, and that it if had, the results would be altered. Clearly if the wheel was really rolling down the road at that speed with tyre resistance, etc. it would spin a much shorter time. Rim shape is indeed important when cutting through the air at a linear 50km/h. But what I'm looking at here is purely rotational, to isolate other factors, and see what effect spoke aero drag has.

[ergott]

at least shows spoke drag to be a significant factor at high speeds.

No. Rim shape is far more important. You would have had the same results you did even if the rims has a flat outer edge like a railroad wheel or car tire instead of rounded like a motorcycle. Spinning by itself it would just continue so spin freely just like your experiment. However put it in a wind tunnel the CdA would be drastically different due to the leading edge profile.

[cyclespeed]

Hmmm... I watched, I listened, I went away... I thought. And I come to the same conclusion as a few others here... Completely meaningless. If anything, it is much more about bearing drag than anything else.

...
If I was to rerun the test, but at a lower top speed (eg. 25km/h), then yes, bearing efficiency would play a much larger part.

What average speed do you think the wheel was spinning at during the entire test, including the back and forth pendulum swinging due to an unbalanced wheel. Why even include that phase?

How about this? Stick a CD to your hand. Now stick your hand out of a car window at 54km/h. You think maybe you might feel some force on your hand? That is the equivalent surface area of the spokes on the Corima.

The deceleration of the wheel is not linear. It loses speed very quickly in the first 30 seconds of the test. Why? Because this is the time when speed is high and drag (the square of the speed, remember) is also very high. As the wheel slows, deceleration also slows.

[davidalone]

Because the wheel spins in a 360 degree circle, the minuscule amount of drag caused by the rotating spokes in the tangent direction of rotation is cancelled out by the exact same drag generated on the opposite side of the wheel. The amount of air drag that actually stops the wheel spinning is going to be extremely minusucule. the difference in bearing seals and wheel balance is the factor making up rotational losses.

This makes no sense to me. You are saying that the top spoke's drag is cancelled out by the bottom's spoke drag. This implies that there is no aero loss whatsoever. By the same token I could spin a domestic fan, and it would consume no electrical energy (other than that lost in the bearings). The fan's blades are being pushed through the air to create air movement. This costs energy.

You and me are talking about entirely different things.
ROTATIONAL drag and FORWARD drag are two entirely different concepts.

Assuming our wheel is entirely symmetrical, then yes. you would get almost essentially zero rotational drag- but rotational drag is not what you are concerned about when testing aero wheels. you want to test forward drag.

Fan blades are an entirely different story. Fan Blades are orders of magnitude larger and heavier, and they spin at a different reynolds number. Have you noticed that almost NO fans are symmetrical- they usually have an odd number of blades? plus they have a different purpose- fan blades are actually designed to push or pull air, in the direction normal to rotation, and are shaped as such. Put your hand to the side of the fan. is there actually much air movement? no. not really. Fan blades INTENTIONALLY create vortex effects. THIS is what costs energy, not the minuscule amount of rotational drag

Spokes are not designed as such, and assuming you had a perfectly symmetrical wheel the contribution of the spinning spokes to rotational drag is negligible and not worth thinking about.

[ergott]

Testing rotational aerodynamic drag without a linear velocity won't be useful since it doesn't represent the conditions a bicycle is exposed to. When a spoke is perpendicular to the road it either has a velocity of twice the bicycle it's attached to or has a velocity of 0.

[cyclespeed]

at least shows spoke drag to be a significant factor at high speeds.

No. Rim shape is far more important. You would have had the same results you did even if the rims has a flat outer edge like a railroad wheel or car tire instead of rounded like a motorcycle. Spinning by itself it would just continue so spin freely just like your experiment. However put it in a wind tunnel the CdA would be drastically different due to the leading edge profile.

The rim shape in this test is mostly negligible. That's the whole point. We're not looking at the rim, which in a stationary spin offers very little drag.

Of course, as soon as you have linear velocity, then yes, the rim becomes much more important.

I'm not saying that rim shape isn't important in bike aerodynamics. I'm simply trying to isolate for the purposes of this test to look purely at spoke aero drag.

Clearly spoke shape and design is an issue, otherwise why do so many wheels use bladed spokes, and why did Corima modify this wheel to introduce semi-bladed front spokes, (from round).

[ergott]

Okay then. You can just wait for all the bicycle companies to come knocking on your door seeking your advice on this.

[davidalone]

THe test is interesting, and probably has some relation to bearing quality, seal drag, and spoke drag;
But it is not really indicative of anything associated with the aerodynamic drag of a wheel in real use.

When spinning a wheel in place, it is rotating in the steady state air 'field' of its own vortex.
When riding, the top of the wheel is moving at twice the speed of the bike, while the bottom of the wheel has a near zero velocity; and it is always moving into fresh, undisturbed air. There is also the question of loaded bearings, but that isn't part of the "aero" analysis.

{EDIT: So at the bottom of the rotation, the spoke aerodynamics is almost completely irrelevant, while at the top of the rotation, the spoke is moving into the 'shadow' of the tire/rim. In your test, the spokes are creating almost all the drag, while the rim has been almost eliminated, because it is moving into its own vortex.....all WAY different.}

Summary: Sort of a very flawed attempt. Not completely meaningless, but not conclusive of anything either.
So, if the goal was to "show" or "prove" the superiority of one wheel over another, that is a "failure".

I agree that the results would be different if I were blowing a 55km/h wind across the wheel, but unfortunately, I don't have one.

In my test, it could be argued that the spokes' aero drag is diminished because each spoke is 'drafting' the one before it. In a normal riding situation, this drafting effect is reduced.

But I still maintain, that by reducing the variables down to the minimum like this at least shows spoke drag to be a significant factor at high speeds.

No. This has been tested in wind tunnels and CFD programs. I could run one if you want. give me a CAD model of a wheel and I'd do it for you.
Given the same rim shape, the contribution of the difference between type of spokes has shown no appreciable difference to drag.

[cyclespeed]

Testing rotational aerodynamic drag without a linear velocity won't be useful since it doesn't represent the conditions a bicycle is exposed to. When a spoke is perpendicular to the road it either has a velocity of twice the bicycle it's attached to or has a velocity of 0.

Which surely evens out then? If the top spoke tip is doing 110km/h, the bottom zero, and left and right 55km/h (90' and 270').

Could we at least agree that if a wheel's spokes were big fat square rods, 1cm wide, it would perform badly in my test?

[cyclespeed]

Given the same rim shape, the contribution of the difference between number of spokes and type of spokes has shown no appreciable difference to drag.

So why do wheel manufacturers use bladed spokes then? For fun?

[davidalone]

Testing rotational aerodynamic drag without a linear velocity won't be useful since it doesn't represent the conditions a bicycle is exposed to. When a spoke is perpendicular to the road it either has a velocity of twice the bicycle it's attached to or has a velocity of 0.

Which surely evens out then? If the top spoke tip is doing 110km/h, the bottom zero, and left and right 55km/h (90' and 270').

Could we at least agree that if a wheel's spokes were big fat square rods, 1cm wide, it would perform badly in my test?

All the spokes are moving at the same linear velocity. your phyiscs is confused.