Repainting a car for Aerodynamics?
 

I really need to repaint my car.... 30yo paint... crappy 1st attempt at repainting/bodywork/restoration on half the car... so I'm wondering about any effect on aerodynamics

Thoughts:

1. Non-Smooth/Matte Surface
2. Glossy Surface (I'm thinking this may be the least aerodynamic)
3. Textured Surface (i.e. GOLFBALL :D )

I think the general knowledge on this subject is that air doesn't really glide along the surface of the car as much as slightly above it, so how smooth it is to the touch really doesn't make any difference.

Paint won't make any difference, except possibly from its added weight ;). Other than the very front of the vehicle (where flow may be laminar), the air in the boundary layer immediately over the paint surface is already turbulent.

So golf ball dimples will have no effect.

And that's also why washing/waxing makes no difference. (Unless you're washing off great hunks of mud, or birds & small animals.)

LOL... priceless sig material right there

I didn't know... figured it would be worth asking. Guess in the case of a Golfball its "spinning" so at any one moment the surface of the ball somewhere is coming in contact with air movement

Never hurts to ask. (Unless someone yells at you to use the search feature :))

With the golf ball, the dimples aren't really about spin/no-spin. They help reduce the size of the wake behind the ball by tripping laminar flow into a turbulent boundary layer, which permits the point where flow completely separates from the ball to happen further back.

(Since a car already has a turbulent boundary layer, that's why adding dimples won't help anything.)

USE THE SEARCH FEATURE!!!!!!!!

Or, you could just listen to MetroMPG... he's a smart cookie!

*Sorry, had to do it.*

I've heard of people painting their cars with anti-radar paint, but maybe this should go in the speeding thread ;)

And a pretty picture!

http://2.bp.blogspot.com/_urSQl6wUA5...lf_ball_03.jpg

From: Fore! Here Comes the Ultimate Golf Ball | LiveScience

There's some video at that site too, if you're interested.

actually near the end of WW2 american warplanes where left bare metal, in part to save weight. (but also because in this stage in the way planes rolled out of factories faster than the axis could shoot them out of the sky...)

anyway, if you have an aluminum car like an audi A2 or a delorean you could save a few more grams by not painting them.

if you paint the car choose a relaxing color, this will cause you to drive slower and save gas!

You could also wait a bit longer (what's another year?) for PhotoVoltaic paint, which will help ease the costs of the alternator, and get you better FE.

They also didn't paint the fighter planes because the allies had air superiority at the time, so ground camo was unnecessary.

I'd love to have a bare alloy body with black paint over the areas that could reflect into your eyes. Would be hell for a lazy person like me to keep up, though.

I don't know about the relaxing color thing—pink's supposed to be relaxing, right? If I drove a pink car I'd drive faster so no one would see me in it. I think it's such an ugly color.

the Delorian was Stainless Steel :P

Actually... I planned to paint it a "light" color as my car doesn't have AC and it gets upwards of 100-105 here in the summer LOL

hahaha... that would be AWESOME!

Blue, my friend.

I was planning on the "$50 Paint Job" (google it ;))

it comes in a "Sand" Color... a nice light beige color :)

There was an EPA joke about this, right around April Fool's day... the word was that the EPA was going to ban new cars from being painted black in California... they said that the extra heat created by the black paint caused car owners to run the A/C more frequently, which obviously burned more fuel.

It was a hoax. The study that the EPA actually did, showed that the temps inside the car are not affected by exterior color. If I can find the source, I'll link it.

BTW - the $50 paint job is not economical in any way, and doesn't work out to actually costing less than $100 for many people. Rustoleum isn't car paint, either. And don't use latex, please. LOL

Oh - if you want better interior temps, get some tint. There are many tints which reduce UV light, but don't ruin the transmittance of light through the window (you can still see, and the cop can still see in, to a legal extent.)

not if your car has NO insulation... I can burn the **** out of my hand by touching the inside of the roof of my car in the summer ;)

and, yes, I know it can total upwards of $100 (or more)... but sure beats $600-$1000 for a shop job

Ok, wait until the hottest day you can, and get a friend with a lighter car. Put a thermometer in his car, and yours, in the same place, out of the sun.

Park them both in a parking lot in the sun, right next to each other.

Come back in like 4 hours. If there is more than a 2% difference in the temps, and you can repeat that effect over 3 or more tests, I'll call the EPA a bunch of morons.

*I've actually done this, back when I used to call them a bunch of morons.. now I call them scientifically correct morons.*

The fact that you can burn your hand by touching hot metal does not make it true that your interior is any hotter.

Oh, and removing the entire interior of my Civic made no difference in the temperature each day at 3 PM after being parked in the sun from dawn.

your telling me radiant heat doesn't make the interior hotter... well **** I think you just undid physics :thumbup:

Do the test. Then think about what you're saying.

In order for heat to radiate effectively, there has to be a differential between the area that is heated, and the area that is being heated. Hence the term radiation.

We know from physics that everything will follow the path of least resistance, which in this case, is the area with the largest temperature differential.

The heat is actually radiating OFF the roof, into the open air. The headliner is there so you can't touch the hot roof, it's there for aesthetics, and for sound insulation. It's not there for temperature insulation. (Cardboard is crappy insulation anyway.)

hunk o' hunk o' burnin' mud
 

Sorry! "great hunks of mud" somehow made me think of Elvis.Yeah,I know.I need help!

obviously you know about as much about a VW as I do about Civics (other than that a lawnmower has more Torque :P ). I dare you to look under the headliner and tell me there isn't insulation under there, I dare ya :thumbup: I can show you the remains of the headliner insulation that is (was) permanently glued at the factory to the inside of the metal roof

Radiation from the sun heats the vehicle, and the amount of energy absorbed depends directly on the albedo of the surface.

The rate of energy transfer to the surrounding air depends on the temperature difference. Radiation from the car depends on other stuff.

insulation
 

How I see it,is that the ultraviolet radiation,which is streaming in from the sun,is converted to infrared as it is intercepted by the metal roof.For the albedo of the roof,heat will build until it reaches some equilibrium temp.Inside the car,you'd be exposed to infrared radiation eminating from the sheetmetal above,and since the roof and headliner has a composite heat transfer coefficient ( the inverse of it's R-value) the air inside the car would also build do to conduction across the temp differential(delta-T)from the roof to cabin air temp,until some equilibrium is reached ( about 140-degrees F around here in the summer).The headliner(mine has some polypropylene foam in it) acts as a buffer,providing some shielding from the infrared,and also slowing heat transfer,to give the AC a little break.So I'll think of the headliner in terms of heat gain or heat loss for either the heater or AC to deal with.

I'm sure the headliner does have a buffering effect, but equalization will still happen, insulation or not. Just because a house is completely insulated and sealed does not mean it will never reach outside temperature.

Heat from the sun is entering the car primarily through the transmission of light energy passing through the window into heat. The metal actually should act like a giant heat sink, helping to relieve the car of that heat which passes through the window.

Regardless of any of this, paint color does not (has not, and never will) affect interior temperature of a vehicle.

It will affect your perception though, as we all well know that you get hotter wearing a black shirt in the summer.

Would be interested in seeing how this was done, as well as the results... Was temp taken only twice, once at the start and once at the end? Or was it taken "continuously" over the 4 hours (like every 5 or 10 mins)?

Curious if the rate of temp increase is equal between the two colors... does make sense that after some amount of time they'd equalize.

Of course, perception is sometimes good enough too.

When I took the temps, we were at work, so it was after an 8 hour shift, right in the hottest part of the day. (right around 3 pm)

All we were testing was that lighter colors don't mean the car will stay cooler.

It would have been nice to use an IR camera to test both cars, and track temp changes over the course of the day, because that would also show that the glass is the largest area for heat transfer into the vehicle. Alas, we're construction workers... the only thing we use IR for is detecting leaks LOL.

yes... and obviously... if the insulation or what ever prevents (delays) the absorption of thermal energy... would be like needing to turn on the house AC because it got to hot inside in May instead of mid to late June.

Trust me, if you have ever driven 4 hours in a 120 degree car (105-110 with the windows all the way down, and you know what happens to your gas mileage with teh windows all the way down)... if it wasn't 120 degrees the whole damn time... 90, then slowly climbs to 100, then maybe 110... then finally gets to 120... its a HUGE ****ING DIFFERENCE when you don't have AC. I'm not going to disagree that a huge amount of energy that heats the interior does come through the glass, THAT IS NOT WHAT I'M TALKING ABOUT

there... we all happy now? :thumbup:

When did you get into your car that it wasn't already hotter than the outside temp? Fact is, it doesn't take very long for your car to heat up when it's parked in the sun. If you want to keep it from heating up on the inside, get it out of the sun. Simple as that. Or tint your windows.

Paint color will not change how hot your interior gets, period.

And no, it doesn't make a discernable difference from 110 to 120 degrees. You're sweating your proverbial balls off as soon as you get into the car anyway. I don't use A/C in any of my cars except the van, and I use it in the van sparingly. I also can't drive w/ the window down, since the wind going in my ear bothers me more than sweating profusely does.

Maybe it's just me? (And a group of scientists who have already proven that paint doesn't affect interior temps....)

gawd.. are you a Troll? If so, I'll remember to plain ignore your arguments in the future. otherwise, I'm done because you don't seem to understand or want to understand

Not trolling at all, but you still seem to be avoiding the original argument, that you said you wanted to paint your car a lighter color so the interior would stay cooler. As many times as you veer from that subject, I just brought it back up.

What exactly don't I understand? That you don't want to listen to fact, because you have a preconception of what "must" be happening with your vehicle? If that's what you're referring to, then you're right, I don't understand.

Other than that, I'm sure this discussion has outlived it's usefulness, so I guess we agree.

NachtRitter -

That's what I was thinking. Each car would reach the same equilibrium temp, but the black car would get there first.

I think the best way to reduce the heat gain is to stop the light from getting inside in the first place. There are some window covers that are installed externally, but those are problematic (weather and thievery).

MetroMPG -

Regarding your golf ball picture. Would a racing disk gain benefit from having "dimples"? The racing disk is obviously not a sphere, but it does rotate. (Maybe what we really need are sphere-shaped tires with dimple-tread)

CarloSW2

Ya know those stupid foil windshield things? They actually help. Of course, if you have the option, you could always just park inside, in or shade.

Tinting the windows helps tremendously. There are also anti-UV coatings that are virtually clear, and can be applied to windows, but I don't know how effective they are or how long they last. (I just heard about them on the Green channel the other day, haven't looked into it fully yet.)

Oh - there are also those window coverings which turn white when power is applied... you could use the power of the sun (solar panel to power them) to block out the power of the sun!

SPD Smartglass. I just wonder what it costs. I'd love to use it, but doubt it will be financially practical.

OK, here's what I was looking for...

Christ, I think what I understand you to be saying is that over time (you tested with 8hrs, EPA says 4 hrs, and it may be as little as 1 hour), the interior temp of a car is going to be identical regardless of the outside color when sitting in direct sun. Makes perfect sense, especially since the car isn't very well insulated to begin with.

I think what Edward is trying to say is that the rate of heating within the car would be higher with darker colors vs a lighter color... so if he is parked in the shade (and the car is cool inside) and then drives out in the direct sun, the interior will get hotter faster if it is dark colored vs a lighter colored car. Eventually though (in 1 hour, 4 hours, whatever), it will still reach the same temp. Likewise, in order to keep the interior cooler, the AC (if used) would need to work harder to counteract the rate of heating in a darker colored car.

One study done by Florida Power & Light on houses indicates that a lighter colored roof helps reduce electric bills... (see Residential | FPL | Keep the Heat Out and the Savings In) not directly applicable to the discussion at hand, I know... but a similar idea. Obviously not the only thing that'll help... I agree that window tinting and those foil "sun shades" would help a lot as well.

NachtRitter - My primary concern in saying that the paint really doesn't matter is that the glass transmits more heat energy into the car than the metal ever would. Glass transmits light into the cabin of the vehicle, and since there is a large surface area of clear glass to act as an intensifier for the light, the vehicle's interior surface will heat much faster as a result.

It's the interior surfaces absorbing the heat from light energy, not heat radiance from metal panels.

If the color of your car makes any difference at all, it would be an extremely negligible difference, at best. Possibly a few degrees over a 2 or 3 hour period, hardly something worth painting your car over.

Think about how efficient heat transfer can be:

Glass transmits light, into the surface area of the vehicle's interior,
The surface of the interior upholstery absorbs the light energy as heat.

Light hits metal painted panels, which absorb that energy as heat.
While the temperature below those panels is different from the temperature of the panel, heat radiates slowly into the vehicle.
As the temperature increases, the differential decreases, and heat transfer slows. Eventually, the interior temperature is equal to the temp of the metal surfaces, and the surfaces begin dissipating heat into the open air.

Obviously, black absorbs more heat than white, but I don't think the light absorption occurs any faster. (I'm not sure, correct me if I'm wrong, and show a source.)

In any event, the windows are the primary source of heat being absorbed into the car, indirectly via the larger surface area of the upholstery inside the car.

That was the whole argument in a nutshell, by the way.

AB experience - twice
 

Southcross,

paint it light - it makes a difference
did this on a an 1988 S10 - dark brown to a light tan pewter - it helped
(neither were pretty)

just changed from forest green to silver on a chrysler mini van - it helped

not arguing it was hugh difference
- obviously it does not keep people from buy black cars


there are lots of good thoughts but really bad thermo data in this thread
not going to challenge it, except to say - check this out as a data point

http://www.sae.org/altrefrigerant/presentations/sun.pdf

Christ,

I'm with you, in general. Like most things, there is really is no one "right" or "complete" answer.

For the OP, who was planning to repaint anyway, or for anyone that has the opportunity to choose the color of their vehicle (e.g. new vehicle buyer), evidence provided in this thread seems to indicate that a lighter color can help reduce the rate of heat build-up within the car. True, it may be negligible in some cases, especially if the windows are large and the interior is a dark color.

Certainly would not advise anyone running out and rattle-canning their new car white if they currently have a dark exterior color.

I would expect a light colored interior will help a little as well. And tinted windows would help even more (or instead of).

In the end, though, a car sitting out in direct sun for several hours will be friggin hot no matter what. :cool: Best bet, as you said, is to park in shade or indoors, if that's an option. Maybe something like those 'bikini' car covers that just go over the passenger area with UV protection...