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Old 10-13-2011, 10:36 AM   #1 (permalink)
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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?

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Old 10-13-2011, 07:58 PM   #2 (permalink)
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put smooth covers on the rims and that will help.
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Old 10-14-2011, 01:21 PM   #3 (permalink)
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Quote:
Originally Posted by bikin' Ed View Post
How does the backward teardrop shape fair in this moving air?
Better than the square section of ordinary rims.
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Old 10-14-2011, 04:01 PM   #4 (permalink)
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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?
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Old 10-14-2011, 06:41 PM   #5 (permalink)
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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.
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Old 10-14-2011, 09:12 PM   #6 (permalink)
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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.
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Old 10-14-2011, 09:59 PM   #7 (permalink)
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Old 10-14-2011, 10:47 PM   #8 (permalink)
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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.
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Old 10-15-2011, 12:28 AM   #9 (permalink)
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"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.




"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
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Old 10-15-2011, 06:51 AM   #10 (permalink)
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Quote:
Originally Posted by Weather Spotter View Post
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.

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