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dimples

Old 01-20-05, 10:33 PM
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extomesm
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dimples

dimples on a golf ball make it more aerodynamic, dimples on zipp wheels are supposed to be aerodynamic. would dimples on the frame amount to a more aerodynamic bicycle?
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Old 01-20-05, 10:58 PM
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Properly implemented, I would think so. I'd be interested to hear someone with some authority on the subject address the question.
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Old 01-20-05, 11:03 PM
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Why wait, where's my ball peen hammer?
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Old 01-20-05, 11:05 PM
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Dimpled jerseys & helmets would be more effective... more surface area than the bike itself.
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Old 01-21-05, 04:46 AM
  #5  
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no authority. But seems to me the goal is the same. Spinning down the road. Spinning golf ball.'
'
Geez..
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Old 01-21-05, 04:59 AM
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I always thought the dimples on a golf ball were to give it more "bite" in the air so you could put spin on it and curve the trajectory. Seems to me that would make something less aerodynamic. Then again, I could be talking out of my rear.
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Old 01-21-05, 08:33 AM
  #7  
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As the air flows over the surface, friction causes the boundary layer of the air to lose energy. The dimples stir up the air to bring air that was further away from the surface into contact with the skin. This then gives the boundary layer sufficient energy to stick to the surface when it flows around the back of the ball. They put vortex generators on some aircraft in front of the control surfaces to get the same effect. There has been discussion of sewing seems into jerseys and swimsuits to cut drag in this way - they may have made it illegal.
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Old 01-21-05, 08:45 AM
  #8  
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Originally Posted by tarman_x
I always thought the dimples on a golf ball were to give it more "bite" in the air so you could put spin on it and curve the trajectory. Seems to me that would make something less aerodynamic. Then again, I could be talking out of my rear.
Actually The dimples on a golf ball are to generate lift and keep the ball in the air. Dimple design is varried between different ball types due to differing initial spin rates. The cover (and mantle/core) softness is what creates the amount of spin on a golf ball. To me with the frame being non rotational simply dimpling it would cause air turbulance which would result in drag which would be bad. Maybe dimpling in specific areas (trailing edges?) can help the turbulance of the air coming off the tubes to help aerodynamics. Got to admit I am not a fluid dynamics engineer or anything so I could be wrong. I do know that they dimple the under side of some cars to aide with niose retuction caused by air turbulance going under the car so maybe there is something to it.
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Old 01-21-05, 09:02 AM
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Nah, dimples will probably do nothing. For example no car manufacturer ever considered it and they can benefit from less drag. Even if dimples helped on a bike, that bike better be moving really fast (much faster than your average road speed) to make a real difference.

Atomic makes their skis with such a surface on the top skin. It is nothing more than marketing hype, trust me.
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Old 01-21-05, 09:17 AM
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When cyclists start launching off the start line at 160 mph, we'll see dimples. Instead of "On your left!" you'll hear "Fore left!"
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Old 01-21-05, 03:21 PM
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Dimples on a golf ball work to create lift and allow the ball to fly farther before hitting the ground because the ball is spinning (if hit properly) with backspin. That is, the ball is rotating on an axis parallel to the ground and perpendicular to the direction of flight, rotating in the opposite direction from that which it would be going if it were rolling on the ground.
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Old 01-21-05, 03:35 PM
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I was allow under the impress that you do not need dimples to produce lift on a golf ball, and the example that I was given was a baseball/tennis ball which does not have dimples. I was told there is enough friction to one side of the ball to create an area of lower air pressure which is in the direction of the curve in a curve ball, and the high air pressure side would than "push" towards the low pressure side, "pushing" the ball
I was always told that dimples allow for turbulate flow, where there is less contact with the surface, thus less friction that a ball without dimples would have laminar flow (flows like smooth water, always in cotact), but I guess to know you would have to look at the Reynolds number (not the metal company, the fluid dynaimics number)
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Old 01-21-05, 03:48 PM
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yes a smooth golf ball can generate larger pressure differences on its surface with same spin rates as a dimpled golf ball

let's talk ping-pong...

and yeah, i think the Reynold's number for your body/bike at 30 makes body and frame dippling moot
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Old 01-21-05, 03:56 PM
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Originally Posted by extomesm
dimples on a golf ball make it more aerodynamic, dimples on zipp wheels are supposed to be aerodynamic. would dimples on the frame amount to a more aerodynamic bicycle?
What dimples on a golf ball is trip the boundary layer to turbulent flow which can better resist flow separation on the aft side of the ball. The dimples increase friction drag but decrease pressure drag, with the net result (at least on a golf ball) being and overall drag decrease. If there's no flow separation, as would be the case with a streamlined object, then dimples hurt rather than help. Bike tubes operate at a much lower Reynolds number that golf balls, so dimples might help. I doubt that dimples help on Zipp wheels.
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Old 01-21-05, 11:48 PM
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Originally Posted by Chorus_Girl
Bike tubes operate at a much lower Reynolds number that golf balls, so dimples might help. I doubt that dimples help on Zipp wheels.
Hmm...hadn't thought of that. That'd be sort of nifty -- seeing smoke flow patterns on a rotating wheel. I'd aim for helmets, though...wonder what the critical Reynolds number of my bike hat is, and if a ball-peen hammer would improve it.

PS: Where'd all the Mech/Aero engineers crawl out of the woodwork from? Feel free to ignore the silliness of an EE here...
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Old 01-22-05, 12:12 AM
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these dimples occur natrally in mtbs, but they're called dents. Gimme a week w/ you bike and i'll do it for you
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Old 01-22-05, 12:58 AM
  #17  
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the simplest way to make a bike and its rider more aerodynamic is to apply a faired surface to direct air around the entire structure. then you can worry about dimples.
as for dimpled wheels, i think the dimples are meant to decrease rotating resistance, which would then, theoretically, increase its forward potential.

if i remember correctly, trek did supposedly apply the dimple idea to a TT frame a few years back, but i haven't heard of it since.

but again, i spite of what i know, i'm probably wrong.
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Old 01-22-05, 08:22 AM
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The "lift" theory is wrong, the dimples reduce drag by maintaining boundary attachment longer, thus increasing the pressure at the rear of the object, hence the drag.There is a classic demonstration of this in most science museums: 2 balls (one dimpled, one smooth) each suspended from a wire, in a wind tunnel.

On a bike, the portion of drag attribuable to the frame is marginal to that attribuable to the rider. You can get a lot more from a position standpoint (playing the compromise of aero efficiency vs athletic efficiency, keeping within the bounds of UCI regulations) than from gimmicks on the frame. Graeme Obree gave a pretty good demonstration of this in 1992. It would be interesting to see the drag measurement difference between a simple road bike and a full-blown TT bike with aero tubes, seat tube blending into the rear wheel, disc wheels, etc. - all at the same rider position. The wheels probably have the biggest impact. Note that Obree's tuck position was measured at 15% drag improvement over traditional track position; so you can't expect more than perhaps 1% from anything you do to the frame.
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Old 01-22-05, 08:25 AM
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got the info secondhand, the manager of my shop spoke to the zipp reps at Interbike who said, basically, that the dimples are just a marketing ploy. Just so Zipp can tell customers that the 909 disk is dimpled and more aero, like a golf ball. just what i heard though, believe what you want.
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Old 01-22-05, 08:47 AM
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Helmets have already been done.. this is from 2002:


(from cyclingnews.com)

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