They make wind guards for light bars:
Perhaps something like that could be adapted
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Speaking as someone who has had sheets of plywood rip in half while driving down the highway because they were sitting on my roof rack without properly being secured in the frontā¦ you wonāt really be able to tell exactly what you need to do for your vehicle without some kind of smoke chamber or extreme modeling software. The slope and shape of your car was designed to minimize drag as air goes up and over your hood and windshield, and you are adding things to it which was not part of the design.
One way to do it is to use flexible solar panels and just adhere them directly to the surface of your vehicle. Of course they are lower efficiency and more expensive, so this might not be a practical option.
If you are going to mount them to an existing roof rack, I would suggest hanging them under the rack rails instead of on top. This allows you to continue to use the rack rails for cargo. The nice thing about mounting panels to your roof rack rails is that there was at least some design consideration for airflow with cargo on your rails.
As long as your panels are flat and mounted to the rails, you shouldnāt have any serious issues. Problems arise when the material is flexible and air is given a chance to get under it. With just a little bit of deviation, more air gets under it, it lifts moreā¦ the force exponentially increases, rapidly leading to catastrophic failure. Since your panels will be rigid and they will essentially be flat facing the oncoming air, you shouldnāt have any serious problems.
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There are aero blocking kits for panels.
Or you could use it as a fast-n-furious style mega spoiler.
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Thatās exactly what I want to avoid.
I saw someone strap a flexible panel flat to the roof, but yes, I was leaning towards a rigid panel for the better efficiency. One thing I wonder is whether a greater number of smaller panels would be more secure than one large panel, and if so, whether it would be better to get panels that run down the length of the vehicle or that span the breadth of it. ||| or ā¤ (or a bit of both).
And Iāll look more into these aero blocking wind bars. I guess the principle is to break the wind before it enters the slit between the panels and the roof?
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Strap? Iāve done it before but I used construction adhesive and drilled holes in the metal hood panel for the wiring. Worked great.
I donāt think so. Not unless you were mounting them all to a very rigid frame. Larger panels should be rigid enough on their own if they have their own aluminum frames. Smaller panels mean more flex points. You may want to carefully reinforce either way though.
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I might have glued some brackets for solar panels on a van some years back. With travels from WA to UT to SoCal and everywhere in between for years. It had phenomenal shear strength. Your way is waaaaay less likely to fly off 
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Sadly, itās missing the āeverything is going to sh*tā stress. But itās a good graphic either way. 
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Neighbour used bungee cords. I donāt know that they do any highway driving.
What I was thinking with the smaller panels is the gap between them letting the air flow around it more easily instead of building up pressure as it passes between the roof and a big panel before it has anywhere to escape to. Thereās no existing rack, so Iāll be bolting them directly to the frame, probably elevated enough I can clean under it with a pressure hose. So each one would be bolted to the roof separately rather than being part of a flexible sheet.
If that held, I feel more confident I can get mine to.
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If youāre bolting into the vehicle, why leave any space under?
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Panels need airgap. Heat kills output.
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Fair ā¦ but while driving the airflow over the top will be more than sufficient for cooling.
When it sits under the hot sun for hours you can cook eggs on 'em. Output falls off sharply with increased temps.
I assume that use is only without engine running, else the alternator would be better by far.
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Yepā¦ makes sense.
Yes, while the vehicle is parked for most of the day, the solar panels will help charge a separate power station that I use for running various devices, along with plugging the station into the grid when needed. That way I donāt overtax the vehicleās own battery.
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