Repainting a car for Aerodynamics?

[Christ]

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.)

[aerohead]

Originally Posted by MetroMPG

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.)

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

[Southcross]

Originally Posted by Christ

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.)

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 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

[winkosmosis]

Originally Posted by Christ

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.)

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.

[aerohead]

Originally Posted by Christ

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.)

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.

[Christ]

Originally Posted by aerohead

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.

[NachtRitter]

Originally Posted by Christ

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.

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.

[Christ]

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.

[Southcross]

Originally Posted by Christ

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.

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?

[Christ]

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....)