Did my initial structure/mpg test on my light weight aero cap made of EMT conduit and 4mil plastic sheeting. No significant flapping and cursory +3 MPG on scan guage.
materials
7 lengths 1/2" emt conduit
1 -1/2 coupler (1/2")
2 - 1/2 conduit 90* elbows
5- 1/2 emt hold down brackets
desired hardeware (1/4" nut and bolts)
bag of 3/4" hose clamps
8- 1/2" L brackets
4mil plastic sheeting (paint dep), 6mil would be better but only came in 100ft lengths
roll packing tape
2 rolls transparent duct tape
6- 1/2" wrap around foam insulation sleeves (comes in 3ft lengths?)
Bend B pillar arch..(tip-bend one side, measure to center cab, mark center cab length on arch, trace half arch on floor, flip emt, align mark, bend to match trace)
attach 90 elbows between base of b-pillar arch and bed rail conduit.
bend bed rail lengths over tailgate, apx 2" over hang tailgate. tail gate lengths will overlap, cut both somewhere in overlap, allow 1/4" gap for coupler, install coupler.
check for fit.
install arch crossbrace with L brackets and hose clamps
check fit, adjust tailgate coupler if needed
install down bars with hose clamps and L brackets, note aero angle. Permanent mount upper bracket, temp lower/tailgate and do not cut to length yet.
side braces are under tension, make long enough to bow down bars up(note will lean in until down bar brace is in)
side braces-flatten emt ends in vice or hammer bend over to appropriate angle to fit hose clamp on down bar
cut down bar "tails" to length and perm install
install down bar cross brace with brackets and clamps, make length to desired horizontal bow/curve.
make sure all hose clamps are positioned so sharp edges are inward(away from plastic)
Check fit
install 1/2 emt hold down brackets(drill/bolt)-my design has passenger side semi-perm installed and act as hinge(need to be inline for hinge to work well)
Driver side quick disconnect- ran bolts up from bottom, bolted to bed with threads sticking up so emt clamps fit over them.
I wanted to install the quick pin release for all driver side mounts but the angle of the sheeting did not allow for my design. For the quick release I made an F channel by welding strap to angle iron.
I installed the plastic over the top and sides and temp taped with clear packing tape, stretching the best I could, then used the transparent duct tape along the entire edge apx 6" over the emt seemed to work well, all taping done inside the cap.
I then covered the arch face, taping having to be done on the outside.
I put a layer of duct tap on the edges that touch the bed rail to protect from abrasion.
I was considering how to prevent abrasion from the down bars..I had some left over pipe insulation and squeezed it around the down bars and down bar cross brace, which had the added benefit of tightening the plastic more.
I also put the insulation on the arch cross base to pad and space the cap from the cab.
I tack welded some of the emt connections for extra strength( don't breath fumes)
When putting on the duct tape I used two widths.
There is noticeable increase in mileage, and actually noise reduction.
I drove with it 2200 miles on a trip. the only wear is on the tailgate edge overhang, water got under the tape through openings where the down bars meet. I drove through 1700 miles of rain and snow during this trip. Another layer of tape and close off the gaps I think will help.
The overhang was slightly bigger than planned but an unplanned benefit is I can open the tailgate with out lifting the cap, and I think there is a little air pressure that gets into the cap and keeps it from flapping.
The kamback is not perfect but it was easier to design this way, and some is better than none.
I wanted to fold my mirrors in but during the day the cap is not clear enough..at night you can see the headlights.
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