I was thinking something similar a few days ago, with regard to textured clothing supposedly being more aerodynamic than smooth lycra. I presume the same would be true for textured material vs slick helmet and bike surfaces, so (durability aside) why not cover everything in fabric?

It's a moot point for someone as slow as me, but an interesting thought nonetheless.

A matte finish isn't a texture thing, its a sheen thing. However i don't think a textured paint job would make any real difference. the dimples that are popular on other parts are engineered and very specifically sized to reduce drag. an orange peel paint job would just be bad painting and would probably hurt you aerodynamically more then it would help.

However, if you are looking to shave a half second off your IM time then it might be worth looking in to further.

You may be right, but what does this mean?



I thought that matte vs glossy was indeed a texture thing, in that glossy finishes have zero or many very very very tiny scratches. While matte finishes had lots more and much bigger scratches (still very small to the human eye, but hugely bigger than those on a glossy finish).
Or, is this not correct?

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Not sure how (or if) it relates to aerodynamics, but years ago surfboard makers started sanding the gloss coat on some boards so that they had a matte (or semi-matte) finish to them, vs the previously-favoured high gloss. Supposedly it made them faster/more slippery by trapping a layer of water (boundary layer?) against the board that passing water would then "slip" over more readily.

Again, not sure if the same idea would apply to air as it was supposed to with water.

I think this, or a similar question, was asked of Chris @ Spec in the new Venge thread. My recollection was that they didn't note a difference in testing.

Matte implies diffuse reflection, like most interior walls, whereas glossy surfaces create specular reflection, like mirrors. Pitting and scratches can create these differences, but the scale tends to be much smaller than that which affects aerodynamics. Internal refraction of translucent materials can also create a diffusion or matte look by letting light pass through the surface, while having many internal imperfections. Many "white" objects are actually composed of clear fibers that refract light to create a diffuse "white". This is why paper and t-shirts look clear when wetted.

Roughness that can change the flow is larger than the laminar sublayer (search for Hydraulically Smooth). For bicycling Reynolds numbers, that roughness would have to exceed something like 0.02" or so, which would be much larger than most paint surface features (common lingo is morphology).

Great info, thanks.

I'll do some more research on this as well.

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Textured surfaces initiate turbulence earlier than it would occur with a smooth surface. The trade-off is that the increased skin friction is offset by allowing the flow to stay attached longer, reducing the wake size. The effectiveness depends on how well the designers design the texture, surface, for the intended speeds and fluids.

For surfboards, I find it hard to believe that texture would help. The water is already impinging the high-pressure side of the board, not a "suction surface." Here it only serves to add skin friction. The marine industry spends lots of money on anti-fouling - organic growth that increases skin friction on hulls; if anything, research is being done on superhydrophobic coatings, such as the food-safe research product from MIT. That may help surfers more than wax.

I recall some tests done back at the TA&M tunnel looking to see if rough paint (they added fine grain sand) improved aerodynamics; it did not.

The tests were done with round tubed steel frames, and if they did not see a benefit then it's highly doubtful that aero section tubes (greater surface area, and inherently lower pressure drag) would improve with rough paint.

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There are several reasons why they could have observed those results:

• Texture too small for regime
• Drag improvements too small for tunnel sensitivity
• Paint thickness increased frontal area simultaneously
• Freestream turbulence intensity artificially high (poor maintenance, screens, fan pitch, etc.)
• Maybe other specifics that I'm not privy to

I'm not really advocating textured paint for bicycles. My post was more of an explanation of when initiating turbulence would be beneficial, and how I doubt it works well for surfboards.


That said, I don't think the argument of airfoils not benefiting due to higher surface area and lower "pressure drag" is sound. Everything in aerodynamics is scale and regime dependent. Consider an airfoil which has just stalled: it will have a large wake and associated drag. Adding vortex generators upstream of the separation will likely reattach the flow and reduce drag significantly.


Under normal cruise conditions (low angles of attack) a designer would not want to depend on vortex generators to attach flow, but it may be necessary for take-off and landing.


I think that a skilled aerodynamicist should be able to reduce drag on a round-cylinder tubed bicycle, though by how much I wouldn't guess at the moment. It would never be the ideal solution unless weight was a primary concern.

We can certainly agree that the skilled aerodynamicist wouldn't opt for a homogeneous coating (ex. rougher paint). I bring this up in the context of the OP's question about matte vs. gloss finishes (ignoring for the moment that the imperfections in the matte finish are not aerodynamically significant).

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I've wondered about this as well (and may have even started a post on this topic a few months back???) but I don't think its about matte vs glossy. I think you'd need an actual texture like what Pearl Izumi has on the Octane sleeves or perhaps grooves similar to the face of a golf club (to act as a trip wire at yaw).

With that said, I really doubt there's much left on the table in terms of aero gains for the frame itself (over the Speed Concept/P5/etc). IMO, from here on out it will be mostly about integration, comfort, and serviceability (making the bike easier to wrench on). One thing on my wish list is clearance for bigger rear tires. I doubt there would be much of an aero penalty (maybe even a benefit?) but you could pick up some noticeable comfort (just imagine 28s or even 30s...on a purpose built disc of course).

Some years ago the original ridley noah and Dean had a special textured paint that was supposed to initiate turbulence and slow boundary layer separation. I'm not sure if it still has this.

+1, I remember this, it was around 2008.

"To improve the air flow in areas of high resistance, our engineers developed and patented F-Surface aerodynamic paint which re-establishes the laminar flow. F-Surface reduces the drag on the frame by up to 4,1%, as measured in our wind tunnel tests."

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Yes, this was my line of thinking. Nonetheless, I think the gains to be made on paint on bike frames is miniscule compared to clothing and helmets. I look at the big, bluff faces on aero helmets/visors and can�t help but think that there�s scope for improvement there.

I presume dimpled helmets like the Louis Garneau Vortice simply don�t test any faster than alternatives, otherwise everyone would be doing it?

The old Cervelo P3sl had a "shot peened" finish that was very rough to the touch. Lots of folks put in really, really fast times on this bike. Fastest 10 mile TT I ever did was on a P3sl and I've owned a lot of "superbikes" since. But I'm also getting old. Regardless, that P3sl was a fast little bike!

You are not seeing dimples on anything these days.

Did you ever reach a conclusion? I have been looking up and reading over F1 threads, but maybe you can save me some time.

Seems like matte vs gloss paint really is no different.

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From my reading here and on other places on the interwebs, I think the bottom lines are:

- It is not clear if gloss paint is faster or slower than matte paint at cycling speeds. {Remember that in some cases a smooth surface ends up being overall faster than a rough/dimpled surface, in other cases it is the other way around. This is because, while a rough surface always creates more local drag than a smooth surface (via skin or surface drag), in some cases, this additional skin drag can affect and lower 'form drag' (or induced drag) by a lot more than the minor increase in skin friction, this all relates to the boundary layer of air, much like in the case of dimpled balls or dimpled rims being sometimes faster than their smooth equivalents.}

- That said, any difference in drag between gloss paint and matte paint (at cycling speeds), if one exists, appears to be so low as to be well below the threshold of measurement of most wind tunnels.

( But gloss paint is sure is easier to keep clean ! )

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and airplanes are generally glossy, except for certain spots. In a case like this, where fuel costs are huge, crazy money is spent on paint jobs and waxing. I doubt it would matter on a bike, but IMO a glossy bike just feels faster.

I've probably spent 150 hours of tunnel time looking at wax, coatings, surface finish, etc.. both during and after our dimple patent and would add this..

Codygo is spot on, all of this is regime dependent, so what works on a racecar or commercial aircraft is pretty different from what we are looking at here... of course, aircraft, race cars and boats all use turbulators in some form or another (dimples are one form) to reduce separation. I've never done hydrodynamics but know some who do and there are some key differences due to the surface tension effects of the water..so the roughness on a surfboard may be more about surface tension than or some other boundary condition effect than about pure hydrodynamic drag.

I've seen very small order gains/losses at cycling speeds between surface coatings, wax, polishing, liquid coatings, etc... none of them have proven robust enough or repeatable enough to matter. The key to the dimples was that they proved over and over to improve reattachment of flow to the rim after it had become detached. So if you run product in the wind tunnel from 0-30 degrees, you get one curve that generally shows flow detaching at 12ish degrees sometimes more or less..but as you yaw from 30-0 (something most aren't doing) the reattachment generally occurs at 4-6 degrees.. back in the early 2000's we realized that roughness on the rim could improve this and the original 808 had a detachment angle of 13 and a reattachment of 12 which was a pretty big deal at the time. \

So, like most of this aero stuff, the answer really lies more in the conditions and specifics of the situation, but things like surface roughness are likely to have effects in very limited regimes.. which may or may not be significant for all athletes. Similar to this, I'd point out the vortex generators (a turbulator) on the top surfaces of aircraft wings.. they are really only adding drag for 99% of the flying time of the aircraft as they aren't necessary or beneficial for steady-level flight, but they can be critical to flow attachment or reattachment during high pitch conditions occurring during takeoff and landing, and for this reason, you will continue to see them for years to come.

Josh

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Very interesting!

Don’t know much about surfboards, but yachts have always had their hull sanded below the waterline to improve speed. Yachts with a sanded bottom are always faster than ones that are polished. For the racing sailor job #1 when getting a nice shiny new boat is to bring out the 800 grade wet and dry paper to give it a good sanding below the waterline.

I cannot tell you why this works I will leave that to the ST scientists on the forum. But there must be some parallels into aerodynamics.

While water tunnels can substitute for some wind tunnel studies, one has to be careful of effects like cavitation.

Some calculted and well placed hull roughness could cause local air bubbles which have a net reduction in drag, but I do known that ships endure organic fouling cleaning and treatments to reduce drag from roughness that naturally accumulates on them. This biofouling is calculated to cost a large sum in increased fuel usage.

A surface which is a “streamline” and with no friction is theoretically optimal. Any other solution removes energy from the flow in an attempt to reduce an aerodynamic sin elsewhere. The amount of energy one gives up by creating permanent “flow mixing devices” may not always pay for themselves because the sins they intend to fix might not exist, or may have caused less drag than the flow mixing devices introduce.

So, if something benefits from adding roughness, there likely exists a better smooth surface that does not require roughness to outperform it aerodynamically, provided it is feasible according to additional design constraints. There are a lot of designs out in the wild that appear to have evolved as a “local optimum” of what was likely a bad initial design. That said, I am not saying all non-smooth solutions are suboptimal :)