34 vs 38- any actual data?

I hear alot that to get tangible aerodynamic benefits from a wheel the rim must be at least 38 mm deep. This assumption has led some people to argue that an "aero" rim 34 mm deep would be all but worthless. Is this really true? Does anyone have any actual data to back this claim up? From what I've read, the 38 mm figure comes from the Zipp website- not too reliable in my eyes since theyre clearly trying to sell their own wheels. Could you point me in the right direction to find out more about this?

I saw a report on a wind tunnel test a couple of years ago that supported the 38mm claim, but it isn't going to do you much good, because I can't remember where and I think it might have been the same one they refer to on the website.
As a practical matter, though, how much difference could there be between 34 and 38? It may exist, and if it does it probably can be measured, but that extra 4mm isn't going to make you get on the bike and say, "Wow, feel THAT!"

I think the basic aerodyanic principle is that the length (cord) of the airfoil needs to be twice (or a little higher?) the width, or something like that. With a 23 mm tire, the rim of 38 mm plus the tire of 23 mm (or somethat less buried in the rim) would be 61 mm, or about twice.

Those numbers might be a little off, but I think that's the principle. Nonetheless, I have not heard that there is a critical number under which there is zero benefit and over which there is 100% benefit. I think testing shows that longer is always better (with a disc being optimal), but for other reasons, too; longer (deeper) rims mean less exposed and fewer spokes, which are a large source of wheel drag.

Doug

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Originally Posted by OokieCookie

I hear alot that to get tangible aerodynamic benefits from a wheel the rim must be at least 38 mm deep. This assumption has led some people to argue that an "aero" rim 34 mm deep would be all but worthless. Is this really true? Does anyone have any actual data to back this claim up? From what I've read, the 38 mm figure comes from the Zipp website- not too reliable in my eyes since theyre clearly trying to sell their own wheels. Could you point me in the right direction to find out more about this?

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You might find something here, but I'm not sure how detailed it is:

http://www.analyticcycling.com/

The problem with these sorts of "rules" is that they don't reflect reality. Reality is that 38 is better than 34, but 38 is not "good" while 34 is "zero." If you plotted drag vs. rim depth, you would see a reduction, and maybe even a change in slope of the reduction, but you would not see discontinuities. These kinds of "rules" suggest that there ARE discontinuities, but physical systems don't behave that way in most cases.

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Originally Posted by Kerry Irons

These kinds of "rules" suggest that there ARE discontinuities, but physical systems don't behave that way in most cases.

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I would guess that a factor such as cross winds would play a part in variability?

Well, I found the following in an article for Leonard Zinn on insidetri.com:

"In general, any increase in rim depth and any reduction in spoke count and/or thickness will be faster than standard wheels with 32 round spokes. Wind tunnels show that you get small aerodynamic improvements from rim depths in the 30-45mm range, but you have to get to 60mm before the aerodynamic improvements are great."

So as far as Zinn is concerned the 38 figure doesn't mean squat. Interesting.

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Originally Posted by OokieCookie

Well, I found the following in an article for Leonard Zinn on insidetri.com:

"In general, any increase in rim depth and any reduction in spoke count and/or thickness will be faster than standard wheels with 32 round spokes. Wind tunnels show that you get small aerodynamic improvements from rim depths in the 30-45mm range, but you have to get to 60mm before the aerodynamic improvements are great."

So as far as Zinn is concerned the 38 figure doesn't mean squat. Interesting.

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I do not remember where I got this info, but I have heard also that a v-section rims (e.g. Velocity Aerohead, Ritchey WCS/Pro, Campy Shamal) are better than box section (Open Pro, Campy Neutron, etc). the v section reduces the turbulence created over a box section or something.

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Originally Posted by weiwentg

I do not remember where I got this info, but I have heard also that a v-section rims (e.g. Velocity Aerohead, Ritchey WCS/Pro, Campy Shamal) are better than box section (Open Pro, Campy Neutron, etc). the v section reduces the turbulence created over a box section or something.

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Like most of the advertising in cycling, it is misleading. It isn't just the chord to thickness ratio, but the profile. The profile of every rim I've seen less than the Zip/Reynold's 58mm carbon rim looks nasty. I'd like a rim that with an inflated tubular on it that closely matches the NACA 2412 airfoil profile.