Modifying a full sized van top for aerodynamics?
 

I'm planning to buy a late 90s Ford 7.3l diesel to convert into a RV and had some questions. I'm going to cut the roof off and box in a heightened roof (roughly 24" taller) out of 1/2 square steel and I will skin that in 20-22g cold rolled. This will allow me to make a really custom top with welded seams and no chance of leaking.

Since I'm already making a new top for it, what would be the most aerodynamic design for that? I would like to extend the front a little or ideally the front and back but I don't know if sticking the front out over the windshield will impact the MPG a lot or not. I have no experience with this short of what intuitively makes sense so any pointing in the right direction would be wonderful!

The van profile looks like this:

I don't have enough posts to include a profile picture for the van but you can google that.

Do you follow stuff at the Expedition Portal? Lots of ideas there. (Gotta be a member to view pictures in the forums.)

Like the "Jackur" van. Folding top gives lots of space parked, but minimal additional frontal area on the road:

http://www.youtube.com/watch?v=pCPPcvvTKlA

If you haven't done so already, check out the Sticky:Aerodynamic Streamline Template near the top of the list of Aerodynamics threads.

Then perhaps incorporate a partial boattail into the extension at the rear.

I have a '92 Ford E-150 conversion van with Mark III kit, a fiberglass roof extension which sticks up another 2' or so. Has 5.8 liter engine, TV, captains chairs, fold-a-bed in the back, etc. Wanna buy it?

Otherwise, I'd start with a Home Depot garden edging air dam, or wider if possible, and Coroplast undertray for the underside of the van, otherwise an aerodynamic abortion and prime example of what NOT to do when designing a vehicle. In other words, a target-rich environment for an Ecomodder.

Ever considered, instead of expanding vertically (with consequent increase in frontal area), doing the boat-tail thing on your van? You could make a boat tail which would also shield a bike rack or whatever else you need to haul on vacation, which would essentially pay for itself in fuel savings. Remove when not needed. Doesn't make the van top heavy. Better in crosswinds. Much cheaper to do than going vertically. Does not ruin resale value as cutting your roof off will, since the mod can be easily taken off to return to stock configuration.

My Ford aluminum wheels are prime candidates for pizza pan covers, and the Dumbo ears outside mirrors are bigger than the law should allow.

Got that diesel running on cooking oil yet? Home heating oil? There's really no need to waste money at the pump, y'know.

RV top
 

A local tile floor contractor has an Econoline conversion with 2-ft roof extension.Even with his wet saw and all his gear,he gets blown all around on the highway.
*If you could do some sort of pop-up top as already recommended,it would hurt you the least at the pump.
*If you must do a rigid top,then ideally,you'd want a bulbous nose as you'll see with some trailers ranchers use hauling livestock.This is best when driving into quartering headwinds.
*A 6-inch radius all the way down the top edges will reduce roll moments in a crosswind and gusts.
*And if you can teardrop the roofline towards the rear as Don Bur has done with their semi-trailer rigs,this will help the air to think that your RV is actually a bit smaller.
*As mentioned,the Aerodynamic Streamlining Template is created to help with this part of the design.The new roof would want to look like that of the 2014 Corvette Stingray but with a more complex windshield.

Um, I was wondering, if you haven't yet bought the van, have you considered simply buying a trailer for use as a camper, and modding that?

We have some superb trailer guys and designs on this blog, so there is plenty of good information. A trailer need only be used occasionally, so the tow vehicle need not be burdened with that extra load (or extra fuel burned) during normal use. A well designed trailer may actually improve your highway mileage, as noted by our best trailer guys. You could start with a cheapo Northern Tool trailer with a 4X8 bed, expanded to cover the tongue area for extra floor space, and enclose it like a mini horse trailer. Some of these trailers fold and can be stood on end to save space in the garage. Light, they can be towed by a small car, so you don't need a big, heavy, thirsty van like mine to pull it. This is what I wish I'd known and done before I bought my big heavy thirsty Ford E-150 conversion van.