Home Made Aero Spin Down Test of 2 Top Wheels

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ergott
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by ergott

Again, no.

Wind resistance isn't linear. Also, rim shapes are designed to reduce any effects of spoke drag. Also, an ovalized spoke like the CX-Ray is not only aerodynamic is has a much smaller frontal (when positioned at the top of the wheel) area than those fat carbon spokes. That's why the most aerodynamic wheels out there don't have fat carbon spokes.
Last edited by ergott on Sat Aug 27, 2016 6:23 pm, edited 1 time in total.

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cyclespeed
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by cyclespeed

davidalone wrote: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.


Again, why use bladed spokes then? Much easier to just use big fat ones?

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ergott
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by ergott

Sorry, duplicate post

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cyclespeed
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by cyclespeed

davidalone wrote:
cyclespeed wrote:
ergott wrote: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.


That's why I referred to their SPEED and not their VELOCITY, which means your physics is confused.

davidalone
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by davidalone

cyclespeed wrote:
davidalone wrote: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?


Mostly? from en engineers point of view? sortof. product managers need to sell new products every year, right?

Bladed spokes have a minor advantage in resisting flex.

If you want to know why aero spokes don't work: here. I dug this up from some old literature:

In wind tunnel tests, bladed (and ovalized - hereafter "profiled") spokes have proven to be very effective. The caveat here is that they show their greatest advantages vis-a-vis round spokes when the airflow is from straight ahead. As soon as you introduce even a small component of side wind (yaw angle as low as 2 degrees), the larger surface area of profiled spokes acts either as a lifting surface, or develops airflow separation (with related generation of vortices), or both. The bottom line - creation of lift cannot be accomplished without corresponding increases in drag.

Simply stated, a profiled spoke in a static or head-on air stream can be very effective. However, a profiled spoke in a side wind is a propeller.

A round spoke always presents the same small profile to the air stream, whether the angle of attack is head on or from the side. When you consider the wind direction, speed of the bicycle, and rotational speed of the wheel, it becomes apparent that there is an alphabet soup of aerodynamic considerations happening throughout the course of the ride. For this reason, we believe that swaged (butted) spokes present the best real-world aerodynamic profile.

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cyclespeed
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by cyclespeed

ergott wrote:Again, no.

Wind resistance isn't linear. Also, rim shapes are designed to reduce any effects of spoke drag. Also, an ovalized spoke like the CX-Ray is not only aerodynamic is has a much smaller frontal (when positioned at the top of the wheel) area than those fat carbon spokes. That's why the most aerodynamic wheels out there don't have fat carbon spokes.


Of course wind resistance isn't linear; it's been clearly stated that is is a function of the square of your speed.

So it seems we're agreeing that a bladed spoke is 'a good thing' in that it reduces aero drag.

Lightweight have done a much better job than Corima in that their spokes are quite thin (comparable to a steel spoke), whilst the Corima's are fat and roundish. Therefore, it is no surprise to me that the LW performed far better in the test.

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cyclespeed
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by cyclespeed

davidalone wrote:
cyclespeed wrote:
davidalone wrote: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?


Mostly? from en engineers point of view? sortof. product managers need to sell new products every year, right?

Bladed spokes have a minor advantage in resisting flex.

If you want to know why aero spokes don't work: here. I dug this up from some old literature:

In wind tunnel tests, bladed (and ovalized - hereafter "profiled") spokes have proven to be very effective. The caveat here is that they show their greatest advantages vis-a-vis round spokes when the airflow is from straight ahead. As soon as you introduce even a small component of side wind (yaw angle as low as 2 degrees), the larger surface area of profiled spokes acts either as a lifting surface, or develops airflow separation (with related generation of vortices), or both. The bottom line - creation of lift cannot be accomplished without corresponding increases in drag.

Simply stated, a profiled spoke in a static or head-on air stream can be very effective. However, a profiled spoke in a side wind is a propeller.

A round spoke always presents the same small profile to the air stream, whether the angle of attack is head on or from the side. When you consider the wind direction, speed of the bicycle, and rotational speed of the wheel, it becomes apparent that there is an alphabet soup of aerodynamic considerations happening throughout the course of the ride. For this reason, we believe that swaged (butted) spokes present the best real-world aerodynamic profile.


I guess Lightweight, Corima, CX Ray, Zipp, etc. must all have it wrong then. Bladed spokes are for numpties.

davidalone
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by davidalone

cyclespeed wrote:
davidalone wrote:
cyclespeed wrote:
ergott wrote: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.


That's why I referred to their SPEED and not their VELOCITY, which means your physics is confused.


Err. the spokes will have the same speed as well. What are you trying to get at? ou didn;t actaully say anything in that last post about speed or velocity anyway.

Anyway, you're obviously very convinced about the validity of your test despite lots of knowledgeable people trying to convince you otherwise. I'm an engineer. albeit not an aero one, but I conduct FEA and CFD analysis for a living, so to trying to insult my knowledge of physics is pretty laughable.

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cyclespeed
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by cyclespeed

davidalone wrote:
That's why I referred to their SPEED and not their VELOCITY, which means your physics is confused.

Err. the spokes will have the same speed as well. What are you trying to get at? ou didn;t actaully say anything in that last post about speed or velocity anyway.

Anyway, you're obviously very convinced about the validity of your test despite lots of knowledgeable people trying to convince you otherwise. I'm an engineer. albeit not an aero one, but I conduct FEA and CFD analysis for a living, so to trying to insult my knowledge of physics is pretty laughable.


Then you must surely know that speed and velocity are different.

"Speed, being a scalar quantity, is the rate at which an object covers distance. The average speed is the distance (a scalar quantity) per time ratio. Speed is ignorant of direction. On the other hand, velocity is a vector quantity; it is direction-aware."

wingguy
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by wingguy

cyclespeed wrote:I addressed the question of inertia in the video. The two wheels are very similar in weight, and although the LW is a touch lighter, most of that is in the hub. The tyres are exactly the same, even the same pressure.

So we can rule out the effects of inertia for the purposes of this exercise.

Ok great. So you're testing how aero the spokes are and how much drag the bearings have on two sets of wheels. Fascinating.

The test is meaningless and dumb.

davidalone
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by davidalone

Apologise, I should have said it more clearly: All the spokes have the same velocity WITH RESPECT to the tangential directions of rotation. and hence they have the same speed.

wingguy
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by wingguy

davidalone wrote: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.

Oh good lord no. His test is dumb, your objection is dumber :lol:

davidalone
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by davidalone

wingguy wrote:
davidalone wrote: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.

Oh good lord no. His test is dumb, your objection is dumber :lol:


Care to explain? I'm speaking wrt rotational drag.

wingguy
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by wingguy

davidalone wrote: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? ...

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.

Stop talking. Like, seriously, just stop it now.

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wingguy
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by wingguy

davidalone wrote:
wingguy wrote:
davidalone wrote: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.

Oh good lord no. His test is dumb, your objection is dumber :lol:


Care to explain? I'm speaking wrt rotational drag.

Drag caused from spokes going up is not cancelled by drag caused from spokes going down. Drag caused from spoke going forwards is not cancelled by drag caused from spokes going backwards. Drag from every spoke adds up. Drag from every spoke resists the momentum of the wheel. How on earth did you confuse yourself to the point where that needed to be explained? :smartass:

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