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bikin' Ed 10-13-2011 09:36 AM

Aero bicycle wheels
 
Bike rims have gotten deeper (and heavier) all based on the aerodynamic profile of the front of the tire into a tapering rim. I understand that can be much more aero, especially at speed.

Here's my question: After the air comes off the front of the rim, gets whipped up by the spokes, it hits the same aero profile BACKWARDS before moving to the rest of the bike. How does the backward teardrop shape fair in this moving air?:confused:

Weather Spotter 10-13-2011 06:58 PM

put smooth covers on the rims and that will help.

Patrick 10-14-2011 12:21 PM

Quote:

Originally Posted by bikin' Ed (Post 265336)
How does the backward teardrop shape fair in this moving air?:confused:

Better than the square section of ordinary rims.

Frank Lee 10-14-2011 03:01 PM

Dunno about that. I'm assuming that even under 0 yaw, and ignoring the effects of the spokes and being in the wake of the leading quadrant of the rim and forks, it only needs a small radius for "attached" flow as it were (or weren't). Figure a wheel, wind coming from 3 o'clock... 1:30-4:30 o'clock is where the rim is a trailing edge, 4:30-7:30 is the bottom and hardly moving in relation to still air; 7:30-10:30 is the following quadrant where the rim would be the "leading edge", and 10:30-1:30 is where the rim is speeding along at 2x vehicle speed in relation to the air. I would think that especially on bikes, due to the low speeds, they're almost always in yaw, perhaps severe... so then would rim profile matter?

jkv357 10-14-2011 05:41 PM

I would think the profile and type of tire would have a bigger effect on aerodynamics than the rim, as it defines the shape of the leading and trailing edge.

The rim and spokes are all in dirty air anyway.

Please correct me if I'm wrong.

SoobieOut 10-14-2011 08:12 PM

Tri bikes use carbon fiber wheels that are lighter and have less wind resistance. Here's a website that sell a few different types.
Wheels | Zipp - Speed Weaponry

I have heard stories that the full carbon disks do not do well in side winds, causing the bikes to move all over the road. Could be a safety hazard.

I use Cane Creek wheels, nice aero shape, not too extreme.

The type of wheels makes a huge difference on top speed.

I would like to see a bike made without spokes, that would just have a rim, open in the center.

That would clean up the airflow considerably.

Frank Lee 10-14-2011 08:59 PM

http://i146.photobucket.com/albums/r...ubless2011.jpg

Patrick 10-14-2011 09:47 PM

1 Attachment(s)
Look at this chart from Cervelo.com and compare the square crossection at the top to the half-oval (fourth item down). 40 times the drag of a 3:1 airfoil as opposed to 6 times for the half-oval. That's a 40/6 = 6.67 times improvement.

Rokeby 10-14-2011 11:28 PM

"Sometimes apparently simple things have an underlying complexity that is astounding.
Take a bicycle wheel for example. It's round, has a rim, a tire and some spokes and
turns in a circle. Simple, right?

"Wrong.


http://media2.hpcwire.com/dmr/bicycl...rodynamics.jpg

"The aerodynamics of a bicycle wheel are so complex that powerful CFD (computational
fluid dynamics) software running on high performance computer (HPC) systems are
required to generate the simulations needed to evaluate different design
approaches...

The aerodynamic flow around a rotating bicycle wheel, including its interaction
with the front fork and frame components, is incredibly complex. For example, in
one recent study, Godo investigated multiple wheels and fork/frame combinations
at 10 different yaw angles. About 3.6 gigabytes of data were generated during the
steady simulation; the unsteady simulation threw off nearly 1.2 terabytes of data.
The most recent work includes high resolution full bike simulations that are generating
exponential data growth."

Read more:
Designing Bicycle Wheels In the Cloud

euromodder 10-15-2011 05:51 AM

Quote:

Originally Posted by Weather Spotter (Post 265417)
put smooth covers on the rims and that will help.

It's being done on many velomobiles where it doesn't hurt wind-stability as the velomobile is already fully enclosed.

Lots of drag is generated by the many fast spinning spokes.
This has led to less spokes (down to 3 on CF wheels)

A high rim improves the length/width ratio of the rim/tube assembly, which is good.
It also reduces the length and thus the drag of the spokes, right where they are spinning the fastest / causing most drag.

A fully covered wheel is the culmination of both : the length/width ratio of the entire wheel is far better and constant, while spoke drag is gone.
The downside is wheel weight and instability in crosswind.

bikin' Ed 10-16-2011 12:05 PM

I've been using 40mm deep rims front and rear. I seem to notice adifference between these and box style rims--although it may all be weight and placebo effect.:confused:

I've been away from racing for acouple years and hope to retun in '12. I've been very good and I'm sure Santa will bring me a new set of wheels,:D I just want to know what I should ask for.

My thought was a deep V in the front and a light box mag ie: American Classic magnesium in the rear. The front gets the clean airflow and it should make a difference. The rear wheel gets a wierd and changing flow dut to everything in front of it and apair of moving legs. Short of a full disc, I don't see anything likely to clean up the exiting air. A full disc weighs about 950-1200 grams the mag wheel about 480.

funbuddy 10-22-2011 02:24 AM

Very creative....Its the start of the future

Patrick 11-10-2011 07:46 PM

Quote:

Originally Posted by bikin' Ed (Post 265792)
I've been using 40mm deep rims front and rear. I seem to notice adifference between these and box style rims--although it may all be weight and placebo effect.:confused:

I've been away from racing for acouple years and hope to retun in '12. I've been very good and I'm sure Santa will bring me a new set of wheels,:D I just want to know what I should ask for.

My thought was a deep V in the front and a light box mag ie: American Classic magnesium in the rear. The front gets the clean airflow and it should make a difference. The rear wheel gets a wierd and changing flow dut to everything in front of it and apair of moving legs. Short of a full disc, I don't see anything likely to clean up the exiting air. A full disc weighs about 950-1200 grams the mag wheel about 480.

From http://en.wikipedia.org/wiki/Racing_bicycle:

Moment of inertia changes result in a decrease in watts of between .004 and .022%, while lower mass provided between .2 and .46%, and better aerodynamics provided between .6 and 1.8% decrease in wattage. Therefore, wheel moment of inertia effects are neither noticeable nor important.

Emphasis added.

Edit - I think this might be the paper the Wiki article took the info from: http://biketechreview.com/reviews/wh...el-performance

Here's a quote from that paper:

So, what do all these numbers mean? It means that when evaluating wheel performance, wheel aerodynamics are the most important, distantly followed by wheel mass. Wheel inertia effects in all cases are so small that they are arguably insignificant.
How can it be that wheel inertial forces are nearly insignificant, when the advertisements say that inertia is so important? Quite simply, inertial forces are a function of acceleration. In bike racing this peak acceleration is about .1 to .2 g’s and is generally only seen when beginning from an initial velocity of 0 (see criterium race data in Appendix D ). Furthermore, the 0.3kg/0.66lb difference in wheels, even if this mass is out at the rim, is so small compared to your body mass that the differences in wheel inertia will be unperceivable. Any difference in acceleration due to bicycle wheels that is claimed by your riding buddies is primarily due to cognitive dissonance, or the placebo effect (they paid a lot of money for the wheels so there must be some perceivable gain).

jkv357 11-12-2011 10:39 PM

Here's some more very technical info about bicycle wheel aerodynamics from:

47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition
5 - 8 January 2009, Orlando, Florida


Titled: An Aerodynamic Study of Bicycle Wheel Performance using CFD

By: Matthew N. Godo1
JMSI dba Intelligent Light, RutherFord, NJ, 07070
David Corson2
ACUSIM Software, Inc., Clifton Park, NY 12065
and
Steve M. Legensky3
JMSI dba Intelligent Light, RutherFord, NJ, 07070


http://www.acusim.com/papers/AIAA-2009-322-649.pdf


Jay


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