Dum Wheel Aerodynamics Q

[skuke]

On 19 Jun 2004 12:11:56 -0700, spock wrote:

>
> Lets say the cross wind is coming at you at a 45 degree angle from the

Major snipping!

Wow Spock, thanks for the lesson! I have a little better understanding now.

Part of what caused me confusion is the definition of "drag". I guess in
the world of aerodynamics, it is used a bit differently.





> I
> really appreciate the fact that you are not gullible.

That's probably because I'm cynical!

--
Skuke
Reverse the domain name to send email

 

[DirtRoadie]

"Andy Birko" <[email protected]> wrote in message news:<[email protected]>...

> ... But, when rotating, wouldn't a high spoke wheel still be more
> like a disc than a low spoke count?

Try this simple experiment. Get 3 wheels, a disk, one with high spoke
count and one with a low spoke count. Set them up so you can rotate
them at a fast riding speed, say 30 mph. As they rotate at speed, poke
your fingers into each one from the side.

I'm not certain, but I would guess that the results with the spoked
wheels will be somewhat similar and that the disk wheel will create a
substantially different results. So I would assume that a
high-spoke-count wheel is not similar to a disk.

But be sure to let us know what you find out.

DR

 

[[email protected]]

Andy Birko writes:

> All else being equal, what is it about fluid dynamics which makes a
> low spoke wheel more aerodynamic than a high spoke count wheel? Is
> this truly the case when rotating?

Each spoke has drag as it moves through air. Often people claim they
draft each other, which they do ineffectively being more than ten
diameters apart, a distance that in effect makes them NOT draft each
other as bicyclists stretched out in a line spaced about 10m apart.
As has been mentioned, disks have surface drag while spokes create
turbulence for each spoke. Therefore the more spokes, the more drag.

The reason for deep section rims is to give them a teardrop shape so
that air can close behind with minimal turbulence. That is to say,
shapes like aircraft wings and rudders allow streamline closure with
low turbulence and therefore low losses to passing air. The measure
of drag can be seen in what is left behind. I'm sure drafting another
rider who has the best aerodynamic wheels is imperceptibly different
from the same rider with conventional wheels. That should be a clue.

The main effect that makes this trivial is that rider air drag is
enormously greater than any of the wheel effects for which riders
spend so much money and concern... especially when they are not
setting world records anyway.

Jobst Brandt
[email protected]

 

[dvt]

[email protected] wrote:
> The reason for deep section rims is to give them a teardrop shape so
> that air can close behind with minimal turbulence.

Isn't there another reason? The deep section rims shorten the length of
spoke that is exposed. The end that is removed is the end that is
travelling the fastest (and thereby generating the greatest drag). I
don't have any measurements to back me up, but I would expect that this
would be an additional aerodynamic improvement over a box-section rim.

Any reason that this effect would be negligible compared to the effect
that you describe?

--
Dave
dvt at psu dot edu

 

[beerco]

Jobst Brandt [/i]
Each spoke has drag as it moves through air. Often people
claim they draft each other, which they do ineffectively
being more than ten diameters apart,
[/QUOTE]

Excellent point. This pretty much clears it up.

Originally posted by Jobst Brandt wrote:
> The main effect that makes this trivial is that rider air drag is
> enormously greater than any of the wheel effects for which riders spend
> so much money and concern... especially when they are not setting world
> records anyway.
> Jobst Brandt [email protected]



I guess that depends on what your goals are. If you've lost a TT by less
than a second (I missed out on a 3rd place this way) then all of a
sudden, a one second time savings is not so trivial.

Finishing a century 1 minute faster is hardly worth $1000 for
fancy wheels.



--

 

[Booker C. Bense]

-----BEGIN PGP SIGNED MESSAGE-----

In article <40d24556$1@darkstar>,
Benjamin Weiner <[email protected]> wrote:
>Andy Birko <[email protected]> wrote:
>
>> I understand what you're getting at, but why doesn't this turbulant air turn
>> into an "air disk" so to speak? I.e., as you've mentiones, there is less
>> friction between laminar air and turbulant air than there is between laminar
>> air and a solid (hence the dimples on golf balls and some of the Zipps), so
>> why doesn't this "disk" of turbulent air created by the spokes act in a
>> similar fashion?
>
>There is no "disk" of turbulent air created by the spokes.
>There is a lot of airspace between each spoke and the next,

_ The rule of thumb is 3x the characteristic length, which
in this case is the width of the spoke. So if you had a spoke
every 6mm or so, then one could "draft" the others so to
speak. Hmm, thats roughly 350 spokes on each side, or 700
altogether. Might be kind of heavy...

_ Booker C. Bense


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