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We have an extended Chevy express with a 24" high-top, on which we intend to install roof-mounted PV.

I'm really concerned with how to properly install the panels. We've selected six 100 W Renogy panels and a 400 Ah LiFePO4 battery pack, but it might be too much for our roof. I'm worried that if we are going 80 mph up I-5 and hit a 40 mph cross-breeze we could tear the roof off the van, rip the panels off the roof, destabilizing and roll the vehicle, etc. How can I avoid or minimize these risks?

I was initially thinking of laying three panels long-ways on each side of the van, with a ~20" section down the middle I can walk down and hang out or service the panels, and where the roof vent will go. Each section then would only be 22" wide (140" long) with space on both sides for air to dissipate to either side.

My plan is to build my own rack with bent heavy conduit and bolts. The rack should be pretty strong. But could unwieldy pressures blow out the panels? I don't know what thresholds they should withstand. The product shows 2400 Pa high winds and 5400 snow, but what does that correlate to in driving down the road?

You'll notice that with the current layout, the panels must go over each side by 4.5 inches. To me, this means they can be more affected by wind. If the roof was just flat instead of the bubble top, mounting might be much easier.

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    $\begingroup$ Hi Liandri, welcome to Engineering SE. This question seems to be to be on the edge of what sorts of problems we address. I've edited your problem statement to bring it more in line with our asking guidelines but you can do more to improve this question and attract better answers - for starters, a diagram is always preferred to imprecise descriptions like "long-ways on each side." And any specs on equipment, materials and fasteners helps - there are bolts and then there are bolts. $\endgroup$ – Air Jul 6 '16 at 22:31
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    $\begingroup$ Got it. I can make a more thorough post when I get home from work! $\endgroup$ – Liandri Jul 6 '16 at 23:21
  • $\begingroup$ Have you calculated how much extra fuel you're going to burn, with the drag of the panels? I can't see how this makes sense from an energy perspective: why not just charge the batteries from the van's engine in the same way that it charges its own battery? $\endgroup$ – 410 gone Jul 7 '16 at 12:22
  • $\begingroup$ Good thoughts, but most of the driving is done around town, and the van is usually sitting most of the day. We'd never drive far enough during the week to charge the batteries unless we can pull like 200A from the alternator during a 10min trip. The best chance is obviously on a roadtrip, but then camping out for 3-5 days has the same problem. I think effect on mpg could be negligible if the frame is built "correctly". Adding the 1000lb 24" top had zero effect on mpg (dutifully logged via my OBD-II reader). I think the 5.8L V8 will burn gas regardless. $\endgroup$ – Liandri Jul 7 '16 at 16:00
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I work for a company that, builds custom truck bodies and upfits vans, though we do not work with GM/Chevy vehicles.

Your best source of information will be in the Body Builder Guide that GM provides to upfitters and others that use their vehicles as a base for their own custom vehicles.

You'll also want to review GM's Best Practices (found at the same link).

In general, you'll want to tie in whatever structure you create to the structure of the van itself, following the Body Builder's Guide and the Best Practices. Deviate from those at your peril/liability.

I would not use metal conduit as a structural material. It's not intended for that kind of use.

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I would suggest building a support structure on the interior under the roof to help root the panels as well as distribute any force from driving more evenly. to help prevent air pockets and high upward pressure i would angle the panels with the front part being on the downward slope. this will increase drag but you should not have to worry about the panels or roof ripping off. also consider cutting off the bubble top and welding a flat piece of metal.

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  • $\begingroup$ I imagined a slope may help as well... to what degree, 1" across the 150", or something larger like a 5" slope? $\endgroup$ – Liandri Jul 7 '16 at 18:15
  • $\begingroup$ the larger the difference the less upward pressure you will have but then you will have more force on the top of the panels but that can be accommodated with proper panel support. i would suggest a difference of between 3"-5". also if you are worried about lateral air you could build an endplate for the panels to keep air flow uniform under them. $\endgroup$ – connor67 Jul 7 '16 at 18:38
  • $\begingroup$ You should be aware that such a slope will create downforce at the front edge of the panel, it can also create a significant amount of lift further back on the panel. $\endgroup$ – DLS3141 Jul 13 '16 at 19:06
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There are a few different issues here.

Firstly is the integrity of the frame itself. As well as the loads from the weight of the panels and aerodynamic drag it will be subject to fairly significant vibration and weather exposure over it's life so corrosion and fatigue are potential concerns.

The next thing is establishing decent mounting points to the vehicle. Ideally you want to be bolting into structural members in the body of the vehicle rather than just single skin panels, even if this involves fabricating brackets to link the rack mounting points to something solid. Also the top section of extra high roof vans are often fibreglass or moulded plastic so you may well need to add an internal frame to extend the load down to structurally rigid parts of the vehicle. If you are lucky you may find unused seat-belt mounting points (if the same body is used for a people-carrier) which can be used. Otherwise you may have to drill into a pillar and back it with a plate and captive nut.

The final consideration is aerodynamic forces. Here the main concern is to avoid generating lift by getting airflow underneath the panels. A simple spoiler to direct airflow over the top of the rack will help a lot here but ideally you would want to have the whole thing sealed all the way around and perhaps have it slightly higher at the back. If you can create a smooth blend between the panels and the roof of the van you should avoid any serious issues.

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