Repainting a car for Aerodynamics?

[theunchosen]

Originally Posted by evolutionmovement

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

Its not terribly expensive. Its used in ALL the windows in a data center I toured. As they give the tour its all whited out like frosted and as they talk about pieces and parts they flip the switches and you're standing in a glass hallway and can see the whole complex.

It is a little heavier than regular window pane glass, but its the same thickness as wind-screen glass. I doubt it has the same fracture quailities though.

To regards to the OP.

Glass is EVIL.

Glass has the horrible ability to allow light to pass straight through the first time and then after it hits something and turns infrared(virtually all light does this after striking a solid object) the glass will not allow it through. Don't believe me hang a plate of glass in front of an IR camera and all its going to see is whats behind it. IR perfectly reflects off glass. So that 12-18 watts per square foot of electromagnetic-radiant energy once in does not get out unless it can convect through a surface or leak out.

Even the difference between solid black paint and white paint won't really matter. The paint has the ability to radiate heat away from the surface and also the IR spectrum bounces off the car paint(black absorbs alot of it, but glass blocks ALL of it from escaping so all of it gets converted to heat energy).

The interior of the car will climb in temperature MUCH faster than the paint. So if the time in question is short enough for the car not to reach its current max temperature your best bet is to rip the insulation out to allow the car to convect heat through the black paint. But you don't want to rip insulation out otherwise when you have the heat on in the winter heats going to convect out very quickly.

[evolutionmovement]

That's the partition kind. There's a version for outdoor use, like aircraft and cars, that goes from black to translucent and can be used with plastics as well as glass. With my preoccupation for aircraft canopies, I think it would be a perfect match for a polycarbonate top. I think I read somewhere that its use is supposed to substantially increases the UV resistance of the plastic as well. Ferrari has used it, likely with glass, but that's also Ferrari and I don't have that kind of budget. It might not be bad in pre-made sections, but I wonder what applying it to a pre-existing piece would be or, at least, custom made pieces (I could build a canopy with 3 flat pieces of plastic instead of buying a 1-piece bubble canopy).

[Piwoslaw]

I remember reading about solar heating of a swimming pool, and it said it doesn't matter whether the bottom of the pool is painted black, white, blue or any other color, the water will be just as warm.

[theunchosen]

Originally Posted by Piwoslaw

I remember reading about solar heating of a swimming pool, and it said it doesn't matter whether the bottom of the pool is painted black, white, blue or any other color, the water will be just as warm.

Thats the same effect that the glass has more or less. The light that gets into the water will never escape because the OHO bond absorbs almost all the IR energy tha radiant light emits after contact.

The only difference is the specific heat of water is so much higher so it takes a long time.

[davidgrey50]

A semi-relevant & possibly true factoid for what it's worth: VW vans of the 70's - a vast number of which were sold in California - all had white roofs to help keep them cool. If I recall, this was according to Road & Track magazine.

[theunchosen]

The only situation in which I can conceivably think of that paint plays a role. . .is if the windows are open and the car is not moving at all.

In that case black paint could absorb more heat than the open-to-air vehicle, but as Christ said most of the heat is going to dissipate into the air even if their is as much as a nominal grass-rustling breeze, because thats easier to do than to go into the hot interior.

So unless you leave your car windows all down when parked/stuck in traffic for extended periods it doesn't matter.

Although White paint will last longer usually because it absorbs less energy than other colors. Blue and green colored cars fade among the fastest because thats the most prevalent part of the spectrum(both from our sun and that breaks through our atmosphere).

[winkosmosis]

Originally Posted by theunchosen

The only situation in which I can conceivably think of that paint plays a role. . .is if the windows are open and the car is not moving at all.

In that case black paint could absorb more heat than the open-to-air vehicle, but as Christ said most of the heat is going to dissipate into the air even if their is as much as a nominal grass-rustling breeze, because thats easier to do than to go into the hot interior.

So unless you leave your car windows all down when parked/stuck in traffic for extended periods it doesn't matter.

Although White paint will last longer usually because it absorbs less energy than other colors. Blue and green colored cars fade among the fastest because thats the most prevalent part of the spectrum(both from our sun and that breaks through our atmosphere).

Having the windows open would lessen the effect of dark paint, because the interior would be much closer to outside temperature without the glass to block flow and insulate.

[theunchosen]

Actually. . .If its hot the interior of your car is going to be hot(if its sunny its going to be hot). IF your car has no insulation and the metal above you is the metal on the roof and if you are coming back within the next few minutes it matters. any more than 6-8 minutes and it doesn't matter.