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Why are Zipps dimpled?

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Why are Zipps dimpled?

Old 07-09-10, 07:14 AM
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Rocket-Sauce 
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Why are Zipps dimpled?

Why are Zipp rims and hubs dimpled? I get the golf ball reasoning, but if that is so, then why are no other aero products dimpled? Airplanes, cars, cars etc all face greater aerodynamic loads than a bicycle wheel, and yet none are dimpled.

But then the Mythbusters worked on this a while back:
http://www.autoblog.com/2009/10/22/m...-dimpling-mpg/
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Old 07-09-10, 07:29 AM
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Because it would be a hell of a lot more expensive/time consuming than it already is to dimple every car/airplane/etc that is made. Plus, a dimpled car would look ugly as balls, as shown by the Mythbusters.
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Old 07-09-10, 07:36 AM
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Marketing. The rider is the biggest aerodynamic impediment. I LOL when I see some totally "aero'ed out" dude with a loose, fluttering jersey. Of course, if you are a PRO, everything little thing counts, but for the average Joe? Naaaah.
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Old 07-09-10, 08:21 AM
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Originally Posted by robbie_vlad View Post
Because it would be a hell of a lot more expensive/time consuming than it already is to dimple every car/airplane/etc that is made. Plus, a dimpled car would look ugly as balls, as shown by the Mythbusters.
OK, but then why not things where money is no concern, like fighter planes? And why is Zipp the only company dimpling their cycling gear?
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Old 07-09-10, 08:30 AM
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Actually, money is a concern even for fighter planes.

And if dimples made commercial planes use less fuel, they would pay for themselves very quickly.

I think the real answer has to do with wind velocity and profile, like the dimples only work in some transitional Reynolds number area. But I don't really know much about aero stuff.
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Old 07-09-10, 08:34 AM
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Originally Posted by Rocket-Sauce View Post
OK, but then why not things where money is no concern, like fighter planes? And why is Zipp the only company dimpling their cycling gear?
I have to facts to support this, but: money is always a concern, even when it's US defense budget money. Dimpling adds huge amounts of surface area, so maybe all that extra material, when it's some exotic composite with super low tolerances for error actually is a limiting factor. Maybe it's not money and LM has done tests and found dimpled material expands differently at mach 1 than flat material. Maybe it has a worse radar signature. Really not sure, but I'm wiling to bet if the golf industry has figured it out, the airforce has at least looked into it. As far as other cycling products, they either don't believe it's worth the R&D costs (wrongly or rightly, I don't know), they just haven't thought about it, or they think putting they efforts into something else (lighter material, better bearings, etc) is a better return on investment. There's no cycling product design newsletter that all of a sudden gives every company the lowdown on each development; sometimes it just takes time for everyone to catch up.
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Old 07-09-10, 08:35 AM
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Originally Posted by Rocket-Sauce View Post
OK, but then why not things where money is no concern, like fighter planes? And why is Zipp the only company dimpling their cycling gear?
For fighter planes, they've tried the same concept but it wasn't worth the trouble. Fighter planes are powered and, sometimes, they need those surfaces to exert big drag for maneuverability. (They also tried it with pressurized air outlets on the skin. It worked better than dimples, but bugs kept plugging up the holes, again, making the whole plan worthless.)

Golf balls are traveling much faster than bicycles and the forward-spinning side is traveling very fast, thus making vortex generation possible and productive. The rearward-spinning side (lower velocity, but still faster than a bike) sees much less benefit, the interaction thus producing lift. A perfectly smooth golf ball doesn't travel nearly as far -- and neither does a golf ball without spin. (That's why a "knuckle-ball" doesn't fly as far.)

Zipp isn't the ONLY company dimpling their cycling gear. A frame manufacturer or two has dimpled the surface of their frames. But, ultimately, the dimples don't make a measurable performance difference at cycling speeds on such an insignificant part of the total surface area of the bike. At cycling speeds, they are better revenue generators than vortex generators.

Last edited by FlashBazbo; 07-09-10 at 08:42 AM.
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Old 07-09-10, 08:41 AM
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I think I read an article where airliners will be dimpled some day.

Fighter jets on the other hand, have to deal with upwards of nine G's, and have to be repairable in the battlefield. Fuel economy and max speed aren't the biggest issues facing a fighter jet.

Dimples are also bad for stealth...
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Old 07-09-10, 08:50 AM
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And, to incorporate another favorite BF debate: Hairly legs really should offer lower drag than shaven legs. The hair disturbs the boundary layer of air -- same concept as a dimpled surface.
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Old 07-09-10, 08:50 AM
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You can google "airliners dimpled"... here's a good thread: http://www.airliners.net/aviation-fo...ad.main/51641/
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Old 07-09-10, 09:02 AM
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they can't dimple commercial planes like golf balls, if they do, then 'frequent flier miles' would have to be changed to 'frequent flier yards'
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Old 07-09-10, 09:20 AM
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If dimpling cars was a good idea, then hail-damaged cars would be worth more, right?
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Old 07-09-10, 09:40 AM
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Does no one remember Ullrich's dimpled TT helmet?
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Old 07-09-10, 09:44 AM
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Originally Posted by Rocket-Sauce View Post
Why are Zipp rims and hubs dimpled? I get the golf ball reasoning, but if that is so, then why are no other aero products dimpled? Airplanes, cars, cars etc all face greater aerodynamic loads than a bicycle wheel, and yet none are dimpled.

But then the Mythbusters worked on this a while back:
http://www.autoblog.com/2009/10/22/m...-dimpling-mpg/
Could be because of wheels are a rotating mass? Aerospace engineers might know. The other rotating mass would be the helicopter propellers.
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Old 07-09-10, 09:53 AM
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Seeing as the skin on aircraft can be a stressed member of the airframe, I don't think you would want to dimple it. Think of a dimpled coke can. Not very good at bearing stress.
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Old 07-09-10, 09:56 AM
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Originally Posted by calamarichris View Post
Does no one remember Ullrich's dimpled TT helmet?
Some companies still make dimpled helmets...

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Old 07-09-10, 09:57 AM
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Originally Posted by robbie_vlad View Post
Because it would be a hell of a lot more expensive/time consuming than it already is to dimple every car/airplane/etc that is made.
Not really. Sheet metal is formed in several hundred ton hydraulic presses. Adding dimples to the dies would have a negligible cost increase.

Originally Posted by robbie_vlad View Post
Plus, a dimpled car would look ugly as balls, as shown by the Mythbusters.
Definitely.
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Old 07-09-10, 10:02 AM
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Originally Posted by timster View Post
I understand aerodynamics is frequently counter-intuitive, but that looks like the most un-aero object yet devised by man.
OTOH, it also looks like it has cool Halloween potential if you wore it backwards and with the beak tilted slighted downward.
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Old 07-09-10, 10:15 AM
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Here is a little cost/fuel economy aviation related story to think about........

My brother-in-law is an exec. for a big aircraft firm, he once told me about these new fasteners for the upholstery/trim in the large jets that weighed less than the current ones. These took big bucks to develop.

I asked what the weight savings were. He said it was roughly 54 pounds..........not per seat etc., but for the entire aircraft. Yes, the huge aircraft would weigh only 54 pounds less! That just illustrates the importance of efficiency in the aircraft business.

If the average Joe knew just how expensive it is to run these beasts, they wouldn't bring so much darn luggage and quit *****ing about ticket prices.

Back to the dimples.........
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Old 07-09-10, 10:19 AM
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Dimpled jerseys and bib shorts FTW!
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Old 07-09-10, 10:20 AM
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The dimples have something to do with breaking up the boundary layer of air that clings to the surface passing through it. you wouldn't want to do that (except under certain conditions) to an airplanes wing, you loose lift and stall the wing.
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Old 07-09-10, 10:27 AM
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Originally Posted by ls01 View Post
The dimples have something to do with breaking up the boundary layer of air that clings to the surface passing through it. you wouldn't want to do that (except under certain conditions) to an airplanes wing, you loose lift and stall the wing.
Maybe not to the wings, but to the fuselage?
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Old 07-09-10, 10:45 AM
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I googled dimple tape and stumbled upon this aviation discussion and post 16 seems to answer the question of why and where to use dimples. There are some other posts where pilots tried dimple tape on their props and wings with varying results. http://www.oshkosh365.org/ok365_Disc...5&topicid=2346 So I suspect that like the golf ball the bicycle wheel is a less than ideal airfoil and as such the rim benefits from dimpling.


I can't even spell enigener but from what I have read, dimple tape and vortex generators do the same thing, They increase the turbulent boundary layer which can fill in areas of a less than ideal airfoil. G.A. AIRFOILS ,Harry Riblett. In his book he shows the benefit - need of VGs on less than ideal airfoils and how to improve airfoils. Assuming you can change your airfoil.
The golf ball is less then an ideal airfoil and needs the dimples. A well designed airfoil for a specific purpose should not need them.
Any disturbance to the boundary layer increases drag, but if this disturbance is approximately 20% or greater from the leading edge the amount of drag is significantly reduced. THEORY OF WING SECTIONS, ****tt and Doenhoff.
Maybe if your wing - prop airfoil is somewhere between a golf ball and a well designed section they should be an advantage.
As for wing tips, drooped or upright they should not be needed if the wing platform was well design for a specific purpose. I got this from a seminar by "Raymer" at Oshkosh this year. He also has a book on design.
So if you had a good airfoil for the purpose no "improvements" should be needed. Not much help huh.
As for Marvel Mystery Oil on the farm we found trans fluid did about the same thing.
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Old 07-09-10, 11:03 AM
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Originally Posted by timster View Post
Maybe not to the wings, but to the fuselage?

But then to what effect. the fuse is already fairly aero, would the extra cost of manufacturing, engineering, etc ever be recouped by the slight improvement in efficiency? And some fuselages are a lifting surface to, while not completely they do contribute.
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Old 07-09-10, 11:04 AM
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Further to my other post...I have a Zipp Sub9 dimpled disc. However, HED has a very nice web app that compares the aero efficiency of wheels as a function of yaw angle. http://www.hedcycling.com/aerodynamics_technology/ If you click on the sub9 button, you will see that the wheel performs best when the wind is at an angle. Hence, the surface of the disc is not a great airfoil at all wind directions. I suspect Zipp played around in the wind tunnel to come up with the best design and dimples improved aero performance over a greater range of yaw angles.
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