pickup aerocap design question - Fuel Economy, Hypermiling, EcoModding News and Forum

laack · 02-05-2011, 11:27 AM

I've gradually been acquiring materials all winter and its time to get my a$$ in gear.

design of overall shape: I'm trying to maximize space in the bed while still resulting in an improvement in fuel economy. overall slope is planned to be between 12 and 15 degrees leading edge to trailing edge with a 4-6inch over hang on upper trailing edge and sides of the cap on the trailing edge to act as a kamm.

question:

Would you guess there would be any appreciable gain rounding the edges that slope towards the center so that it resembles more of a cylinder to reduce wake vorticies or just a 90 degree corner along the sides resulting in a slightly higher amount of storage.

To further clarify the over all shape in question, if you were standing at the rear looking at a 2 dimensional image of the trailing surface: shape 1 with rounded top would resemble a fraction of a circle vs a rectangle.

IsaacCarlson · 02-05-2011, 11:37 AM

I built a mini prototype of mine and it has corners that mate up to the cab and get round as they go back.

http://ecomodder.com/forum/showthrea...ver-13560.html

KamperBob · 02-06-2011, 09:50 AM

Rounded is better for aero. Sloped top means surface area on each side decreases front to rear while pressure on top also decreases. Side pressure needs to relieve itself with flow over the top. Unless, the sides are tapered as well. No rollover flow requires balanced pressure. In theory that is possible but tricky. In practice cross winds introduce asymmetry. So a generous radius is your friend aero wise.

Cargo wise, sharper corners only help if what you carry needs those precious few inches due to combined length, width and height. Bondo provides a clever solution. With his lid in the up position soft sides make it behave like a conventional topper with full rear height and width. Best of both worlds.

Ref: this post.

laack · 02-09-2011, 12:56 PM

thanks for the replys. post #85 in the referenced link is the closest drawing to what I had in mind with some slight variations to accomidate larger items that I haul around.

at what point does additional weight to framing the cap offset the fuel savings. my drive to work is 80/20 highway/city. recreational drives hauling items (mountian bikes and road bikes) are almost all highway.

structure materials- coroplast for the skin. skeleton - ?? wood= higher weight, pvc= flimsy, other plumbing piping= unknown strength vs weight ratio.

what materials have been tested on here that produced positive outcomes. I want to consider safety of other drivers vs maximizing fuel efficiency/testing.

KamperBob · 02-09-2011, 01:24 PM

Quote:

Originally Posted by laack

thanks for the replys. post #85 in the referenced link is the closest drawing to what I had in mind with some slight variations to accomidate larger items that I haul around.

at what point does additional weight to framing the cap offset the fuel savings. my drive to work is 80/20 highway/city. recreational drives hauling items (mountian bikes and road bikes) are almost all highway.

structure materials- coroplast for the skin. skeleton - ?? wood= higher weight, pvc= flimsy, other plumbing piping= unknown strength vs weight ratio.

what materials have been tested on here that produced positive outcomes. I want to consider safety of other drivers vs maximizing fuel efficiency/testing.

I expect that cap would weigh under 100 pounds. The foam skin and paint are almost negligible. The wood framing is probably less than 10. Hardware may be another 10 (C clamps, metal hinges & latches, gas struts, plexi windows & rubber seals). Fiberglass could add 50 (depending on #plys).

3-Wheeler · 02-09-2011, 02:39 PM

Just my opinion, but a nicely designed topper with foam and fiberglass could be built at under 40 pounds. This would utilize similar construction techniques as the Tail.

Hardware to hold it on the truck should only add 5 more pounds.

If on the other hand, you're expecting heavy usage, then as KamperBob said above, you may want to beef things up. I guess it really depends on your particular usage/application.

Jim.

sid · 02-09-2011, 09:49 PM

For a point of reference on weight:

I just built this topper for my new truck last year.

http://www.stonemarmot.com/images/reardriver700x406.jpg

It has a wood frame, shown here:

http://www.stonemarmot.com/images/framelside700x503.jpg

The outside skin is 4 mm thick marine plywood, covered on the outside with 5 oz. fiberglass and on the inside with 5 oz. kevlar. The inside skin is also 4 mm thick marine plywood, covered with 5 oz. kevlar on the inside. The space between the skins was filled with urethane foam.

The whole topper without the windows and doors weighed 78 lb. Each side window (glass with aluminum frame) weighed 10 lb. and the doors (aluminum with glass windows) weighed 30.5 lb. total. Add in mounting hardware, wiring, curtains and rods, and hard points for the quick disconnect carrying racks, the completed, mounted topper added 138 lb. to the vehicle, minus 36 lb. for the tailgate, which is no longer needed.

This may be heavier duty than you need. I frequently carry musical instrument gear inside it, so I need something reasonably secure. That is also one reason it is still box-y and not aerodynamic like many discuss here (another reason is I frequently camp in the back of my truck). I also wanted to be able to mount carrying racks on top to carry kayaks, canoes, plywood, etc., which I do a lot (another reason it is box-y, so I can mount level racks). Since considerable weight may be mounted on top, this topper is strong enough to stand on, which I have done already.

Again, this is just a real world point of reference for weight discussions.

laack · 02-10-2011, 11:56 AM

building materials were mostly picked up last night. Im going to be using mostly wood as its structure, and planning on coroplast as its skin (not purchased yet). I picked up a few door hinges and struts for its pivot point for opening and holding open, and a mix of pvc fittings and pipe. I doubt i'll use the pvc since I expect it will get brittle and break in the extreme minnesota winters. Mounting wise, I dont want to perminantly fix anything to the vehicle, so I'm just going to pinch the frame to the bed rail with clamps and red lock tight on the screw jack to keep them in place and reduce risk of loosening. I figure this will hold it down in combination with the the posts going into the 4 tie down holes in the corners.

Pictures hopefully this weekend. I'm installing some kitchen cabinets over the next few days so this will take a back seat.

KamperBob · 02-10-2011, 01:47 PM

Quote:

Originally Posted by laack

building materials were mostly picked up last night. Im going to be using mostly wood as its structure, and planning on coroplast as its skin (not purchased yet). I picked up a few door hinges and struts for its pivot point for opening and holding open, and a mix of pvc fittings and pipe. I doubt i'll use the pvc since I expect it will get brittle and break in the extreme minnesota winters. Mounting wise, I dont want to perminantly fix anything to the vehicle, so I'm just going to pinch the frame to the bed rail with clamps and red lock tight on the screw jack to keep them in place and reduce risk of loosening. I figure this will hold it down in combination with the the posts going into the 4 tie down holes in the corners.

Pictures hopefully this weekend. I'm installing some kitchen cabinets over the next few days so this will take a back seat.

That's great! I look forward to following your progress, whenever your schedule permits. May the force be with you!

Mine is also a back burner project. So far I have only made a cab template and sil plate for the side walls, mainly to get accurate dimensions for computer design. Hopefully I can nail the compound miter angles the first time that way.