Aero rim advantage?

[herbert.nrp]

I am interested to know if anybody is aware of data regarding the difference in aerodynamic drag between spoked wheels with different rim sections. For example, what would be the difference between the drag coefficients for otherwise identical wheels if one had a rim section of, say, 24mm and the other had a rim section of, say 34mm? Of course, the wheels are not likely to be identical, because the wheel with the thicker rim may also have fewer spokes.

I have seen data comparing the drag coefficients of spoked wheels and aero disc or three-spoked wheels (for example, at Analytic Cycling, www.analyticcycling.com), but never comparing spoked wheels with different rim sections.

Thank you for your responses.

 

[shokhead1]

I wouldnt worry about it unless you ride at 20 mph or more most the time.

 

[marlon1]

20mph isn't that fast is it...!!!

Here is the difference in Power, so you can compare it with a hometrainer with power measurement



Power drag= 0.5*Cw*A*p*v^3

Cw= constant number mostly bewtween 0.1 and 1. It tells how the thing is shaped. A flat Rim has Cw=1 and a aero rim 50mm: Cw=0.4

A=frontal aera in meters. This stays the same. You can take:
213/3.1215 cm* 2.3cm

p=thickness of the air. Take 1.2 minus 0.1 each 1km altitude

v= you speed in km/h


You fill it in for both and you see what the power difference is.


For 42km/h it is about 10watt.


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Now I don't calculate the extra force needed to rotate the deeper rim. But that is is not much.

 

[dhk]

Originally posted by marlon1
20mph isn't that fast is it...!!!

Here is the difference in Power, so you can compare it with a hometrainer with power measurement



Power drag= 0.5*Cw*A*p*v^3

Cw= constant number mostly bewtween 0.1 and 1. It tells how the thing is shaped. A flat Rim has Cw=1 and a aero rim 50mm: Cw=0.4

A=frontal aera in meters. This stays the same. You can take:
213/3.1215 cm* 2.3cm

p=thickness of the air. Take 1.2 minus 0.1 each 1km altitude

v= you speed in km/h

Agree with your equation, but I think your estimate overstates the benefits a bit for two reasons. First, the drag coefficient of 0.4 vs 1.0, and the assumption that the frontal area stays the same, only works for the section of the rim that's most forward, ie, when the aero rim is lined up into the wind. For the sections that are at the top and bottom, believe A would increase, and the difference in Cd wouldn't be as great.

Second, the highest airspeed and drag would be on the section of the rim at the top of the wheel, since the rotational speed would add to the forward speed of the bike. Seems to me the aero section doesn't have a clear advantage over a box rim at this point, since it's "on edge".

I've heard a good aero wheel is worth up to 30 seconds in a one hour TT at 30 mph....something less than a 1% power savings.






You fill it in for both and you see what the power difference is.


For 42km/h it is about 10watt.


----------------


Now I don't calculate the extra force needed to rotate the deeper rim. But that is is not much.

 

[murky]

Originally posted by herbert.nrp
I am interested to know if anybody is aware of data regarding the difference in aerodynamic drag between spoked wheels with different rim sections. For example, what would be the difference between the drag coefficients for otherwise identical wheels if one had a rim section of, say, 24mm and the other had a rim section of, say 34mm? Of course, the wheels are not likely to be identical, because the wheel with the thicker rim may also have fewer spokes.

I have seen data comparing the drag coefficients of spoked wheels and aero disc or three-spoked wheels (for example, at Analytic Cycling, www.analyticcycling.com), but never comparing spoked wheels with different rim sections.

Thank you for your responses.

Check out the ZIPP website.

http://www.zipp.com/TechInfo/wheel_rim_discussions/index.html

They have discovered/published some interesting things about rim depth.

Besides, it's a good read.

 

[marlon1]

ok...

speed of the rim = 2.13* normal speed.

the frontal area stays the same. Because seen from the front the deep rim is behind the tubular.

Now I can say the I hav less wind resistance with a deeper rim, becuase my spokes are shorter, and the mean speed of the spokes thru air is less (the outside of the wheels is rotating the fastest)

And I can say a deeper rim has some more wind resistance because the of the extra surface.

etc.
etc.


so only looking at shapes of the rim + tubular.


I think the difference is more. That 30sec. is for a Disk.



If you think it will make you faster (and it is... how much is an other question), you will always cycle faster!