pickup aerocap design question
 

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.

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

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.

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.

http://kamperbob.com/images/aero/Concept4-Closed.png

http://kamperbob.com/images/aero/Concept4-Open.png

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).

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.

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.

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. :)

If you have stake holes in yer bed...you could use them. My aero cap build...with steps and piccies! ;)

Mine is made from wood and coroplast as well... :D

You might want to consider foam sandwiched with coroplast... ;)

Pretty neat idea. I was thinking of something similar, but I do not have the time to start something of that scope.

I did mine in steps after much planning and brainstorming. You build it like a house...first the foundation, second the frame, third the walls, and last the roof. :D

If you were closer...I'd even give you a hand. Maybe when I git my next version going with the tapered sides....I can git you going on yers with some step by step instructions/piccies. :D

Concept 2006, Reality 2007
 

All you guys who are designing and building your own aerocaps, you have my greatest respect.

While I was building the Aerolid, I always remembered what Thomas Edison said about Invention. He said, " Invention is 10% inspiration, 90% perspiration." Frankly, he was right.

Bondo

Maybe some ideas for inspiration... I never got a chance to long term test mine, but here is kinda how it was done....

http://ecomodder.com/forum/showthrea...html#post25264

Brett, your work set a high standard, and inspired me to contribute. :)

I have been thinking about your design. Perhaps an interesting ABA test would be lid on versus off. What I'm thinking is the wedge sides would limit vortices feeding the bed bubble. Those who need to haul something a little taller might not incur too much aero penalty (compared to fully open box) by simply removing the lid. It could be a selling point if test data supports the hypothesis. :)

Intersting idea, never considered that.
 

That would be an intersting aspect of the Aerolid to study. I do need to do a complete ABA testing of the Aerolid and this spring will be a good time to do it. Warm temperatures and calm winds are ideal for this type of test.

The idea of running the Aerolid without the roof on it, which I have done very little of, is indeed something to look into.

Thanks KamperBob and you guys are my inspiration too,

Bondo

cap 90% complete
 

the hatches are double hinged and completely removable with latches and simple bolts/wing nuts as pivots. side windows are pending a cheap place to buy Plexiglas. The first hatch is for loading simple items (groceries, bags, etc). it comes out in seconds if larger items need to be loaded.

I had to sacrifice some form for functionality. I haul road bikes and mountain bikes weekly so i had to abandon rounded sides for the extra storage space. It ends up at roughly 10 degree slop leading edge to trailing with a slight camber.

no economy results yet. I expect all initial results to vary largely to weather (Minnesota). I'm still hauling around a couple hundred lbs in sandbags for traction so summer will be the first real mpg estimates i want to consider to added/lost efficiency.

Pictures

Looking good...how water tight is it?

about equal to the cheap Chinese child labored water resistant watches that claim "up to 15 feet." It definitely has its flaws. I'm planning on sealing up some seams when i'm satisfied with its mechanical reliability and functionality. I want everything to be accessible to changes and modifications while it goes through its growing pains.

Menards had some acrylic windows at a very affordable rate that I'm planning on installing this week. Positioning it is going to be difficult to maximize visibility with minimal windows.

I have found through my own testing to git a similar product to "test/play" with so I don't have to commit to a design until I'm fully satisfied with the results. You might want to try some of the floral clear wrap as a test material fer yer windows. ;)

Since I'm in the process of working on my next version...I've pulled the hatch (weighed about 30-40lbs) off and will be running without it fer awhile while I gather some numbers on this setup. :D

http://i677.photobucket.com/albums/v...s/IMG_8126.jpg

http://i677.photobucket.com/albums/v...s/IMG_8127.jpg

Ducktail...
http://i677.photobucket.com/albums/v...s/IMG_8132.jpg

curious to see what your mileage does with that setup. Keep us posted.

I've estimated my MPG has dropped about .5MPG without the hatch. I just revamped my aero cap fer my up coming road trip...so running around longer without the hatch isn't possible at this point.

It was nice to see more of the road without the hatch though...fer a short time. ;)

Could you fill in that picture? How does that compare with open bed. Also, could you put a stake in the ground with absolute numbers? That way we can see the relative impact over your baseline. TIA!

I only have my Ultra-Gauge numbers that I'm using at the moment. I haven't refueled yet but will at the end of the week. But that will mean about 60-80 miles of the hatch being back on and the new aero cap design in place. I'm seeing about 18.4MPG on the Ultra-Gauge when I should be seeing close to 19MPG with the hatch on. ;)

Open bed numbers were approximately around 17MPG... :(

BZP, first of all thanks for anchoring your numbers. Big help! :)

Second, I have to believe you have more upside to your numbers. Especially if your T100 doesn't have V8 and/or 4WD options. (ref point)

bam- I have to add that I drove around for about 150 miles with out the top panels of my cap and saw a similar drop. There are many variables that would play into my limited results, however they were negative gains in relationship to my baseline of "no cap."

My baseline average mpg lately with the sub zero temps was 21mpg (same station same pump calculations). open box with sides of the cap only resulted in 19mpg (same work route, same time of day/traffic driving style).

2 tanks of the fully assembled cap were 24.35 and 27.2mpg (again same pump same station). temperature fluctuations lately are largely in part of the difference in economy. the first tank averaged sub zero ambient temps with snow covered road and slower traffic. the second tank saw 20 degree weather, calm winds and clean roads.

Had to refuel today...numbers are (aero cap hatch off, ducktail in place) 18.44MPG.

1998 Toyota T-100 extended cab SR5 Gas Mileage (BZP T-100 (2011)) - EcoModder.com

I know it's not definitive...but there is a slight drop fer my setup. I did have to carry some stuff in the back during one trip...so not sure if that skewed my numbers too much or not. I did notice that after I put my new aero cap and hatch together...my Ultra-Gauge did go up by .1MPG on the trip MPG before I refueled today...and that was over the 70 miles after I put the new aero cap and hatch together. :D

Just took the hatch off of my current aero cap and I'll refill later today so it will have a fresh start. I should be able to run this way fer a few tanks before I git ready fer a road trip in June. :D

This will also give me time to clean up the edges on the aero cap, too. ;)

So right now...the bed looks kinda like a Honda Ridgeline. :eek::rolleyes:

Currently running without the ducktail. :(

BZP, if you want any help presenting your data, maybe I could help with a small web page with pix, curves, tables, etc. (If so email me direct.)

I half expect better numbers with tapered sides but open top than open sides too. One big eddy bubble with softer end radii may take less energy to stir than two halves (comparable total volume). The difference may be small though.

Rock on!

2nd set of numbers are in...pretty close to the first one without the hatch.

18.39mpg... :D

Gitting ready to put the hatch back on... ;)