Aerodynamic of polished/waxed surface on a car?

Question

Aerodynamic of polished/waxed surface on a car?

kingnero (Mechanical)

(OP)

12 Apr 20 11:48

Not trying to get this thread into a debate, just curious:
Has there ever been done testing about the efficiency (however you measure it or whatever the variables are) of extremely dirty and extremely smooth exteriors vs. "standard" surface?

I believe that an extremely dirty car is much more ineffecient vs. a standard car surface than when a fine polished, waxed surface treatment can gain (I'm not native english, so forgive the explanation: it boils down to that the "delta" of a dirty car is much bigger than the delta a polished car can give.

Now, don't ask me to define " a standard car surface". Let's assume this is how an average household upkeeps their daily driver car, at the car's age of 3 year old. Or whatever.

I know it's a long shot, but has anybody ever asked (or even better, investigated) this question before?

IRstuff (Aerospace) · Answer 1 · 2020-04-12 15:26:43

Mostly an urban myth. https://mythresults.com/dirty-vs-clean-car they showed that at ONE specific speed, 65 mph, with a SERIOUSLY dirty car, they got about 10% worse mileage.

At lower speeds, and stop&go, and with a less dirty car, it's probably negligible.

I believe this has been a myth foisted on the rest of us by neat freaks, just like the myth that a cluttered desk equates to a clutter mind.

TTFN (ta ta for now)
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kingnero (Mechanical) · Answer 2 · 2020-04-12 15:54:34

10% is huge, making me doubt their research...

And no, I'm certainly no neat freak, I am having a discussion with one right now, I'm telling him I'm thinking he has less to gain by detailing his car, than can be "lost" by not washing. I guess it's a bit of a weird premise, but hey, I've seen worse discussions here

.

BrianPetersen (Mechanical) · Answer 3 · 2020-04-12 16:29:38

The boundary layer is still there next to any stationary surface. The thickness of the boundary layer considerably exceeds the thickness of dirt.

Compositepro (Chemical) · Answer 4 · 2020-04-12 17:29:29

A certain amount of surface roughness is well known to improve aerodynamics. Take, for example, the dimples on a golf ball or the sandpaper roughness of shark skin. The micro turbulence induced in the boundary layer prevents the formation of large scale turbulence and flow separation from the surface, which absorbs energy, thus increasing drag.

kingnero (Mechanical) · Answer 5 · 2020-04-12 18:05:15

@ BrianPeterson: would you care to guess (or share) the thickness of that still boundary layer? I am not sure it is "considerably" thicker than the dirt on top...

@ Compositepro: I deduce from your reply that a dirty car is more efficient than a polished car? I'm not sure that the dirt on a car represents the carefully chosen dimple pattern on a golf ball...

IRstuff (Aerospace) · Answer 6 · 2020-04-12 19:04:07

If you're really curious, the equation is here https://en.wikipedia.org/wiki/Boundary_layer_thick...

TTFN (ta ta for now)
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GregLocock (Automotive) · Answer 7 · 2020-04-12 19:37:11

On model aircraft we use sandpaper to improve the stall performance of the wings. The boundary layer thickness of a model aircraft wing is much thinner than that of a car at 65 mph, and hopefully your car isn't as dirty as a piece of sandpaper. On full size gliders some modern wing sections have a drop in performance if they accumulate too many squashed insect on the leading edge.

I don't know if this has been measured, or if it is just confirmation bias.

Cheers

Greg Locock

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EdStainless (Materials) · Answer 8 · 2020-04-13 12:58:59

look up the old myth busters episode on that.
If you take a car and put thin packing tape over all of the seams and joints, skirt the wheel openings, pan the bottom side and such and you get significant gains. But it isn't very practical.
You have to balance aerodynamic drag against other losses. For nearly all cases with cars weight is more important because of acceleration.

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P.E. Metallurgy

cibachrome (Automotive) · Answer 9 · 2020-04-13 16:08:35

It shows up in coast down tests used to set chassis dyno load settings for EPA fuel economy tests. Front of the hood at the top is a major initiator. Sometimes it actually hurts the ratings. First noticed during testing of 2 IDENTICAL trucks. One driven to the Ann Arbor lab and one trucked there (40 years ago or more). I wrote the initial coast down test software and was pressed to explain the 'problem' to management. This was done at the time on a GE Time sharing system terminal in Fortran, based on an SAE paper by Korst & White. They made an error in the derivation, I checked it out as a fresh college graduate, and the rest is history !

cibachrome (Automotive) · Answer 10 · 2020-04-13 16:12:19

Hey, I even got to write an Official GM Engineering report on it. Thus began a series of 'Round Robin' tests by the Big 3: Everybody gets to test the same vehicle.

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Aerodynamic of polished/waxed surface on a car?