Aero cap VS cross winds

[NewMPGfrontier]

I've been thinking about making an Aero cap for my Nissan Frontier 4X4. I haven't been able to find a lower kit for the rear-end of the 4x4 version all of the kits specify that they are for the 4x2 version only. If I can not lower the rear I might as well cap it. The majority of my driving is north south driving and living in the midwest with lots of farm fields I have a lot of cross winds
Does an aero cap help hurt with cross wind drag?
I have read a lot of discussion about boat tails and cross winds. It seams that the same might apply to aero caps except that most aero caps have a slightly different shape.
Could I be better off with a tonneau cover giver the issue with crosswinds?
A tonneau would be easier to make than trying to get the curves and angles correct for an areo cap.
The materials I have on hand are sheet metal and cattle panel I could buy plywood but the most savings comes from using what I already have. I could easily make a 2/3 cover too as the diagrams I have seen show that has some improvements as well.

[adam728]

Some Google-fu makes it look like the Frontier was still spring-under on the rear, 2wd and 4wd.

If that's true (your leaf springs mount under the axle), then lowing an inch or two couldn't get any simpler. Right width block, the thickness equals drop, and new u-bolts to mount it up. Degree shims if needed for driveline alignment to avoid vibration.

[NewMPGfrontier]

A previous knuckle head must have raised it. I ran out to my garage to take a second look and the leaf springs are over the axle. Grabbed my Chilton's manual looked up the rear suspension and all the pictures show what you described, spring-under suspension. Looks like I have some work to do. Lower it back down and see what my mileage does. I'm at 23.4mpg now with Torque HQ running I hit 24mpg on a clear day but its dropped the last few days due to strong winds from storms that have been moving through.
Thanks

[NewMPGfrontier]

Opps, after a bit more google-ing on the rear axle I find out that the 4x2 trucks are axle over spring and the 4x4's are spring over axle. If I want to move the leaf springs below the axles I have to move the perch to the bottom of the axles from the top. That requires cutting and welding on the axles. Not so sure about my welding skills on such an important part.

[freebeard]

You'll want it level or down in the nose, a negative angle of attack. If you lower the back and not the front it will wander all over, and topping a rise into a headwind may offer a big surprise. Leave it up and add spats to the wheels.

As for the aerocap, I don't have one, but I recall others saying better directional stability and more resistance to buffeting from oncoming trucks. Read what you can find on aerohead's T-100. Even if the crosswind's speed equals your forward progress, that's an apparent angle of 45° and the aerocap's taper will be 22 1/2°. 30mph crosswind at 60mph would be 30°. etc.

Maybe a tail fin?

[aardvarcus]

If you want to lower the rear, you could look into drop shackles or you could have a custom set of leaf springs made from a company like Alcan, who can build a leaf pack to your specs. Realize that changing the angle of attack from front to back has some implications as to where the aerodynamic streamlining template would fall with regards to your bed. If you haven’t already, get a good side view photo of your truck, and so a template overlay.

A “typical” boat tail on a car sticks out the back, well aft of the rear axle. Any side wind loading on a boat tailed car creates a lever arm type effect attempting to rotate the car around the rear axle.

In contrast, an aeroshell on a truck bed will be mainly centered above the rear axle, so there wouldn’t be a significant lever arm, so you could think of the wind as just a normal side load. Also don’t forget that your truck is heavier than a typical car.

So will there be any impact? Yes.
Will the impact be significant? Only way to know for sure is to build it.

[Big Dave]

Couple of thoughts on your initial post.

Yeah, a tonneau would be easier to make with the materials on hand but (in my experience) an aerocap is more effective.

For my truck a hard, flat tonneau netted a 1.5 MPG improvement over an open bed.

For the same truck, my old crude aerocap (see avatar) showed a 3.0 MPG improvement over the open bed.

Name your poison.

Like you, I live in the Midwest where the wind can mortally howl out of any direction and farm fields offer no shelter. With either the tonneau or the aerocap, I never had any problem with crosswinds (the 7,500 lb truck overwhelmed the problem). I haven't heard anything from any of the site's aerocap builders about crosswind effects.

Whichever you use, I'd advise bolting the cover to the bed. Last spring a 50 MPH crosswind got under my tonneau and broke TWO C-clamps and I had to chase my tonneau 50 yards out into a muddy corn field.

[NewMPGfrontier]

Thanks for the input. The front end of the truck is already much lower than the rear (why it was easy for me to initially think that someone had raised the rear.) With a couple hundred pounds the rear will level out. I think I will continue on the cattle panel route for making an aero cap. I've gotten good at bending the panel making tomato cages. If I am understanding all the info I have found and read above I want a 12 degree slope (relative to the ground) going back and a 22.5 degree angle from the back of the cab to the tail gate to mitigate crosswind effects. I'd like to do as little extra bending and shaping as possible so it looks like I will have three triangles. Perhaps I should draw on some of the pictures of the truck to get a good idea how it will eventually work, and do some trig to get the shapes correct.

[freebeard]

What is cattle panel? If fit will spring into simple curves, clamp the bottom edges and pull curved edges together, joining as you go. You'll wind up with this:

[NewMPGfrontier]

Cattle panel is a heavy duty 4ga welded wire fence. The sheets I am buying are 50in high by 16ft long with 8inX8in at the top and 8inX6in towards the bottom. I bend 4'8" sections into circles for tomato cages. The panels cost me about $20each and the cages are better than anything sold at the garden center. I get 3 cages and an end piece from each panel, I'm working with end pieces to keep cost down.
The drawings look interesting, I had thought about cutting the bed down to the wheel wells and constructing a fin / boat-tail type of structure, but I still want storage space and am not quite ready to commit to 0 resale value.