? about Golf ball dimples
 

I have several large areas I want to cover with a material that will have golf ball dimples on it, (I will be making them, figured out how to do that). My ? is; is SCALE important, for the effect. I'd like to make them 1 1/2" in diameter, and 1/8" deep.Its a lot more labor to make them say, 1/2" in diameter.(Imagine doing a 4'x8' sheet, using a drill to make each dimple. A LOT more work!) So, will I still get the effect of reducing the boundary layer, and therefore the drag on the air flowing over the surface? Anybody Know??Dutch

Perhaps someone more highly degreed than me can answer, but I do know that even on golf balls the dimple size & pattern vary from manufacturer to manufacturer. They all have their own different (but similar) beliefs on the dimple patterns, so I doubt that anyone here will have such a conclusive answer.

-BC

I also have something to add. Would dimples behave the same way? We're talking about totally different shapes. A sphere and a flat plane. Certainly this should have some effect as to their behavior, and maybe influence optimal size, proximity, and arrangement?

-Jay

Ok, no expert here, but looking for Wikipedia (ha! again even less expert there!) anyway:
Most golf balls on sale today have about 250 – 450 dimples.
Golf equipment maker Callaway has introduced a ball with hexagonal dimples to increase the dimpled area on a golf ball, as hexagons tesselate unlike circles.
Super-distance balls have deeper dimples and are heavier than allowed by regulation, which allows them first to maintain momentum and second to maintain a thicker "envelope" of still air around them which reduces turbulence and wind resistance. Marketers of these balls generally advertise a 12-yard gain on most distance shots.


Those are the points of interest as I see it. So clearly amount of dimples can vary, they also talk about some balls with 400 larger diameter holes and 600 pinhead holes, so clearly you can mix size too.
The thought of hexagonal is interesting, I wonder how much it helps/affects.
The 'super distance' ball notes that deeper dimples cause a tighter envelope of air. Not sure if that's true but definately I would want to look in to an anaylsis on depth and how much it helps.

Where are you planning to place these on a car? Since it's about the boundary layer I would think it's main purpose would be to keep air connected even when the shape of the object is dropping off too fast to normally maintain attachment.

That is, if you already had a perfect teardrop, dimpling would not help, and neither would it help a kammback, because it's too severe, but dimpling the back of a VW bug should have some pretty amazing results, becuase it's not smooth enough as a rain drop to maintain flow and will lead to turbulence, but close enough that dimples should keep flow attached further.

I know a lot have talked about it, and I read that Smokey U did it, but I'm doing it! I'm making a duct, 34" wide and 16" high, from the tranny to behind the rear axle. At the front, it gets about 5' wide, also at the back. Complete belly pan, even under the engine bay. Object is to carry the positive pressure air at the front of the truck to the negative pressure air at the back.Going to use this material to line the duct, as well as on the bottom surface of the belly pan.May also use it on the roof, from the top of the windshield to the back, and maybe on the last 3' of the sides.The truck is a Divco milk truck; google Divco club of America to see what it looks like, or imagine a slightly smaller version of the early UPS trucks.Also going to put a visor on it, and cut holes between the bottom of the visor, and the top of the windshield, and duct that to the back as well.All the ducted air will join up at the back, and 'come out' of a large hole above the back bumper, 16"x34", 2 holes about 16' square on each side of that, and 2-3" dia. holes running up the back, at the corner, and across the top.(Of the back).So, thats the plan.If I drive down a dusty dirt road, and have very little dust behind me, I'll know it worked! Jim

I originally wanted to post a joke about "speed holes", but Google Images wouldn't give me the picture I need for it. :) Then I read that you're using it in a belly pan which reminded me of something I've got...

My 2008 Volkswagen has golf ball dimples on some of the factory belly pan parts.

https://lh4.ggpht.com/ronanian/SBm-Ya...0/IMG_0658.jpg

Is this part from a Golf? :rolleyes:

LOL, now that you mention is, "Rabbit" is just US-marketspeak for "Golf". :p

Can't tell from the pic, What is the diameter, and depth, approximately, in inches, please!!That looks like what I'm talking about!!Jim

Vaguely speaking from memory, the diameter was about 1 inch and depth was almost nothing, at most a quarter of an inch. I just reviewed my photos of it and of the car's undercarriage, but they're not very good for determining size. I'll leave myself a note to try to get measurements when I get home from work.

Wow that's pretty cool, I'd be really curious to see placement on the vehicle and try to figure out what the VW guys were thinking.

Of course I'd also really like to see how ducting works. I'm not an engineer, so I can't 'solve' how good it'll work. It certainly 'seems' like it'd save the energy of the car 'crushing' the air and building pressure in front and underneathe and also using energy to 'suck' air back in to the void behind.

Actually, that answers my question; they don't have to be the diameter they use on golf balls;they CAN big larger diameter. Thanks, All!! Jim

My vague memory was accurate, they are about an inch in diameter and less than a quarter inch deep. That's amazing, usually my memory fails at everything.

The dimpled areas are flat (well, except for the dimples) surfaces on the undercarriage, mostly running along the sides, between the front and rear wheels, from the edge of the car to about 16 inches in.

I don't know why they used those dimples, I can't say for sure that they were for aerodynamic drag reduction (then again, what else could they be for?).

Yeah, they say memories ,...either the first or last thing to go, but I can't remember which!,....You sayed it, What else COULD they be for? The ducting should work pretty well. Seems straightforward.The one thing I'm curious about; obviously at low to medium speed, the air in the front will flow to the back. As you increase speed, the "amount" of drag increases at a greater rate than the speed increase. I'm wondering if, at a certain point, the negative pressure will begin to move 'up' (actually forward) the duct.While I'm not counting on it by any means, I'll be on the lookout for a pulling or surging at say 60-65 m.p.h.Could happen, I don't know.;-)

Just thinking out loud here, but it could be for structural reinforcement on those panels... A dimpled piece of plastic is less prone to flexion than a thin, flat sheet of plastic. Any aerodynamic benefit may be purely accidental.

-Jay

I thought that, but how much less flex would dimples cause? Wouldn't it be more effective to use ribs? They are decently thick plastic and don't span very far (about 16" wide).

Yes, I agree ribs would work better, but right now we're trying to guess what a German engineer was thinking when he designed it. If the panels aren't that large then making them more aerodymanic would have very little, if any effect on the vehicle's economy as a whole.

-Jay

Well, that begs the question...

One could make a couple other cases.

Big dimples could be used to reduce the amount or material in the covering while maintaining a web of structure for the piece. Might only be an accountant, a money engineer, that is doing any work on that piece beyond its initial value.

I can understand that... I ran into a similar problem a few years ago when remodeling a bathroom that was in a townhouse. I tried to put a smaller vanity in the bathroom to make it look like a larger room, but I found out that the builder didn't put tile under the vanity, and of course 20 years later you'll never find tile to match. I thought "Damn cheap builder..." It maybe would have cost the builder an extra $10 or $15 to tile the whole floor, but then I got to thinking... Saving 8 tiles in each bathroom * 3 bathrooms = 24 tiles per unit saved. 24 tiles * 150 townhouses in the neighborhood = 3,600 tiles the builder didn't have to buy. In each unit very little was saved, but overall across the entire production there was a signifigant savings.

-Jay

Besides the cumulative cost savings on materials, the labor savings add up too. Not only is labor saved, but then everything is done just a little bit sooner, each trade can get in and out that much sooner, and payment comes sooner. Time is money.

You'd have to factor in the costs of making the molds, with dimples as opposed to flat or with ribs. As no one else has weighed in to my original question, I'm gonna take this as an answer, that the dimples on a larger surface can be bigger, and still function, and move forward.If I can figure out how to post pictures, I'll try to do so, when I get the ductwork complete. Might be a little while, as this is part of making the new floor, which the body will mount on. Some structural stuff, precise measuring, etc. involved.Should be challenging and fun!! Thanks again for the answer!! Jim

It might not be about the size of the surface, but the speed or size of the object, or the airflow expected between the vehicle and the ground, or any of a million other variables...

I suspect that your conclusion is right anyway.

Similar dimples can be seen on the bottom of some wake and knee boards. They are typically at the ends of the boards, presumeably to provide less drag when turned laterally.

Although it was hinted at earlier, I think it worth pointing out that dimples over the leading surfaces of an object are ineffective compared to a totally smooth surface. Where they impose their greatest benefit is at the rear surfaces, where laminar flow would tend to otherwise seperate from the surface. I.e., golf balls only have dimples all the way around because there is no way to guarantee which face will be fore/aft, but it assures that there will always be dimples at the critical zone of seperation. The extra dimples actually add to the drag, but it remains a net reduction overall.

For that reason, I think vortex generators or trip strips (turbulator etc.) are just as effective on a vehicle since we can generally deduce where they need to be on the body work. VW for examply already has that figured out with the turbo beetle extending spoiler. For design reasons however, it may not be in the optimal spot or as effective as it could be, but I doubt dimples would offer any improvement at least in the area where the spoiler exists.

I was looking at the Festiva today and the back hatch isn't that dramatic, I thought it was almost vertical, but it looks like maybe a 60 deg slope. Granted this is more than on a round golf ball, but it still might help to have some dimples at the transition and halfway down the hatch. If only I could find some clear, stick, dimpled tape. I'm assuming larger dimples would be better, maybe 1/2" diameter, rather than tiny pinhole size on golf balls.

Any thoughts?

And yes, I was saying earlier that dimples will actually hurt compared to smoother or bubblier front sections. The point is that they create turbulence, but this turbulence helps keep the flow attached and seperate further down the shape of the object. Good turbulence. But only in the rear where it otherwise would be seperated and more turbulent, not in the front where it'd have been smooth anyway.

https://www.dimpletape.com/ https://www.lessgasmoremiles.com/

I've never used them, but do they still ship?

Cool.

A search of dimple tape on google got me an old Gassavers thread on this exact subject: https://www.gassavers.org/showthread.php?t=1508

Also this place has the same tape for much cheaper: https://www.wingsandwheels.com/page29.htm The first link would be 10feet for $49, this is 10 meters for $38. They say the zigzag, turbulator is better though.

I wanted to post some great links I found.

This one provides a great explaination of why the dimples work and how and also talks about vortex generators to improve flow over an already streamlined body: https://www.aerospaceweb.org/question...cs/q0215.shtml

This next one is an article talking about applying dimples to car and planes and lots of other things, stating that not a lot of testing has been done but improvements can be seen. The one guy owns Zipp which produces some AWESOME stuff for bicycle racing, check this out: https://www.zipp.com/wheels/detail.php?ID=33
And here's the article: https://www.racecar-engineering.com/a...-surfaces.html

That guy has a world patent on dimples on disk/rotors. I don't really understand the point on the disk/surface itself, but around the ellipsoidal wheel/tire makes tons of sense. They also have a patent on more ellipsoidal wheels with lips that transistion the flow better. They claim the wheel will actually pull you forward from lift!

Now, I really want to try this on the festiva, in lots of places too. I don't really like the dimple tape because there's very little information and it looks like one tiny line of dimples in the center, doesn't sound too helpful.

I saw other gassavers using bumps instead of dimples, do you think this would have the same effect? We need someone like trebuchet to chime in. A thought that just occurred to me, which I might actually try. Is velcro! Mostly because I have a bunch of super strong, 4" wide stuff laying around. They say a rough surface helps as much as dimples, I'm not sure whether I'd want to use the 'fuzz', which would make me like a tennis ball, or the 'hook' side, which even though they're tall and straight I think there's enough that it'd provide a rough surface. They're both kind of tall though...

Oh and both of those sources mentioned the same thing that wikipedia did, that hexagon dimples seem to show even more improvement and that lots of testing needs to be done to actually verify correct diameter, depth and proximity, etc. Any idea for creating hexagonal dimples? Maybe an 8mm hex driver and a hammer? :-)

Not mentioned thus far is the fact golf balls spin in the air as they travel. The dimples delay the onset of drag and thus provide some measure of stability, which is a vital aspect of the golf ball.

That said I have yet to see any such dimples / hexagons or anything similar appearing on 747's etc which suggests to me a smoother panel may be a better option in this instance.

The panel from the Rabbit / Golf may be so shaped to return to it's initial shape after impact if it is in the area likely to be damaged from road debris.

Cheers , Pete.

If your car isn't spinning in the air, you haven't been driving fast enough. :D

Actually when I started looking for dimple tape I found a lot of the DIY light-weight hangglider type planes using it and zigzag tape to try to get better results out of their smaller, tarp like wings. It seems it's first invention and use was for planes.

Here's another strange article using them https://www.dimpletape.com/article.htm

From what I understand, airplanes are shaped so the airstream remains attached from tip to tail. For shapes where airflow detaches, dimples/zigzag tape help the air stay attached a little longer.

Trip the boundary layer
 

The big airliners don't have them because the passengers pay for the fuel and fuel has been cheap for a long time... cheaper than the extra engineering that boundary layer trips would cost added to the purchase of a big plane.

Ribbing, zig-zag strips, and dimples work on most trailing surfaces, even with designs that have laminar flow designs.

Anything that can trip the boundary layer and get it to stay attached to the form will help with the aerodynamic (or hydrodynamic) performance.

I've seen the effects of this myself in my streamlined recumbent racing vehicle development.

Best regards,

Jeff Bales
Tucson, AZ

I think I've read that flow only attaches at a max of 30 degrees (from horizontal). I don't think there is any way you will get it to stay attached at 60

I agree. I did some tuft testing on my car and found that airflow became turbulent at around 1/2 way down the back of the rear hatch on my car. Here's a nice side view of my car, https://www.gassavers.org/garage_imag...0pum2sbw4z.jpg
The angle of the hatch is around 20 degrees at that point. Something else to keep in mind is that the boundary layer grown thicker farther back on the vehicle, so I might have been seeing the tufts fluttering from that affect, too.

I want to try adding a trip layer to the rear window on my car, but it would be a b i t c h! to clean, and I'm sure it would collect dirt and look really nasty in no time.

What is a trip layer?

Golf ball dimples don't reduce drag. The dimples increase drag.

So why do dimples make golf balls travel farther?

It's because the flying balls have backspin imparted by the angled club head. The backspin creates a low pressure area above the ball and a high pressure area under the ball, so that it stays in the air longer. Longer time in the air = longer distance. Dimples accentuate this "Bernouli" effect.

Dimples will increase automobile drag.

I was wondering about this. I could have sworn I heard that the dimples help a golf ball because it spins, but then I couldn't think of what would make a golf ball spin... So it's the angled club.

Semantically, the issue of dimples vs. drag really isn't so black and white. I think that in terms of automotive application it pretty much is, however it cannot be ignored that when utilized in the right way, they can reduce the wake of an object, and consequently, the drag. Utilized incorrectly, they will increase drag.

It's also worth noting that dimples work for objects with varied orientation dynamics to the direction of airflow. In other words, on a golf ball, it doesn't matter which side is facing the airflow, the dimple pattern is always oriented in a way to create the desired effect. Cars, boats, and planes in contrast are always more or less oriented the same way into the flow, so there is no need to use the more dynamically effective dimple design, and instead the more directional linear boundary trip could provide greater benefit in the right place.

So instead of dimples, what about a strip of thick tape just before a rear window drops off? That's the idea with the zigzag tape right?

Perhaps I don't have the correct term?
I meant that I wanted to add something to trip the boundary layer and help airflow remain more closely attached to the rear of the car, something like zig-zag tape or?

I was thinking of using clear silicone caulking. It can be applied almost anywhere, in various shapes and patterns, the thickness can vary, it's not too noticeable, and it's removable. The drawback is that, as I mentioned, if applied to a window it would collect dirt and be tough to clean for good visibility.