AeroJeep (remote cousin of aerocivic)
 

I have been contemplating my truck's aerodynamic drag for, well, ever since I have had it. I've also been contemplating improving my Cd. My truck's cruising shortcomings became much more obvious after driving a better eco-vehicle than my own.

This summer, I drove my friend's chevy prizm. It was shocking just how big of a difference there was between it and my jeep. It was lightweight, had a smoother engine, and, the best part, was aerodynamic and had low frictional losses (well, it felt like that anyway), oh, and don't forget; it was a stick shift.

Anyway, I was able to coast much farther than in my truck on the same hills at the same highway speeds. I'm sure some of that comes from its smaller tires and that it is FWD, but it also seems much more aerodynamic than my truck.

Getting 30 MPG highway in a cube is not too shabby. Especially considering that it has more than 10 inches of ground clearance. How much farther do you think I could push my FE (highway, of course) with aeromods? Another 30%, I'd imagine (and hope for).

I would like to build a more aerodynamic bed cover than the one I have now. As it is, I may as well be driving an SUV with my profile. I would like to make a full boat tail, that comes to a point, behind the rear bumper. Like this:

(the line on the front is not something I am too concerned with ATM)
http://i37.tinypic.com/25qbors.jpg
(dimensions of my truck are here, and are probably accurate within a few inches. They are just to give a general idea.)


It comes off of the back of the truck in a gradual sweep, is tangent with the tailgate, then ends 3 (or 4, or 5) feet beyond the back of the truck.

For the construction, I am thinking 4 mil plastic stretched over a 1/2" PVC frame. 4 mil plastic because I have a roll that shrinks when heated, and PVC because it is A) cheap, B) lightweight, and C) easy to machine/cut/shape.


This is a project that will be on the back burner for a while, with my schedule filling up again, so I am collecting ideas until I start building, whence this will become my build thread.

I have a few questions. What are your opinions of the profile in general? Should I delete the lower part of the "wing" profile? How would I attach the plastic sheet to the PVC? Other thoughts, etc?

1) You'll definitely need some sort of rigid material to supplement the PVC and the plastic. Coroplast would be a good idea when making an underbody cover, and good for creating that wing profile you want.
2) Attaching the plastic to PVC pipes would be pretty easy. Duct tape/packing tape the plastic to the frame, use zip ties and punched holes to attach the panels to the pipes and tension the plastic.
3) An alternator delete, cooling fan delete (possibly replaced with a squirrel fan) and suspension lowering would have more impact on the MPG than the just altering the Cd.
4) The front area could be aeromodded like an old NASCAR "superbird", that is the grille block/frontal area is extended and tapered/streamlined.

I should also mention that I am not concerned with seeing behind me; the rear window on this truck is opaque plastic. Speaking of seeing behind me, I should relocate the mirrors inside (they are pretty bad, just by their looks)

Deleting the alternator is feasible (I had a relay on the field control for a while, but noticed no big gains). As a matter of fact, as soon as I swap out the tie rod ends (which should tighten up a bit of the steering slop) I plan to remove the belt to the P/S pump (convenient that there are two belts on the engine, and one of them goes only to the P/S).

I also have an electric fan. When I took off the mechanical one (that had a bad clutch) I could actually feel the difference in the ol' butt-o-meter.

I keep waiting for these stupid 30 inch all terrain tires to wear out, but filling them to their 55 PSI with a 50 PSI sidewall (or 55, not sure) and with their 1900 lb. rating, on a 3500 lb truck... Yeah. They ain't going anywhere soon. I have thought about selling them (They have enough tread) and buying some stock size tires. What are the differences between passenger and LT tires? I wonder what the lowest RR tires would be. Probably something with a very high load rating (for hard rubber. I don't really care about performance traction: it is a truck!).

If I wanted to lower the ride height, I could get some tires below stock size, but I doubt that would help MPG because my engine would be spinning faster at highway speed. I suppose I could make my rear end taller, but I don't really want to rebuild my rear axle until I have to.

Besides, I don't really want to have a "lowrider" or slammed truck :rolleyes: (not to say you were suggesting something that drastic). I think a skirt kit would help (but look kind of silly on a Jeep. Oh well, it's all in the name of efficiency, and I can still load down and haul stuff)

An air dam is another thing I could consider seriously. I would probably weld the supports (although nothing beefy) right onto the frame/bumper. (hmm, that wouldn't be too hard, welding supports onto the bumper and putting skin over it). A grille block is definitely another thing I am open to. My grille is very open to air AND has "bucket" headlights.

You could always go get a Prizm

Lol, that is a logical conclusion. I am not looking to be as good as a prizm, just help my truck out a little. And keep my truck. For the time being, at least

Having 30s (most measure around 29.5" or so) on an XJ isn't going to make much of a difference over the stock 28s or 29s in terms of mpg. Do a little aero modding around the wheel wells, and on the highway, the taller effective gearing may even help. In city traffic, however, the extra rotating weight will hurt.

I just looked up the stock size, and converted to inches, and you are right. Stock is 28". I wonder how my speedometer is 30% off... (reads 80% ish of actual speed)

I do a lot of city driving, so getting new tires would probably help by weight reduction alone... if I get lightweight tires.

On a side note, the comanche is an MJ, cherokee is an XJ. But they are basically the same truck.

Today I decided that I don't need to wait to take off the P/S pump. I removed that belt, and now have a truck that steers like a bear. Cool, less load on the engine. Looking forward to what this tank looks like without it

I will make an airdam/grille block whenever I get the time. I have cut some pipe to extend the bumper about a foot, then take a piece of 4 mil plastic from over the grille to the bottom of the airdam frame.

I wouldn't be suprised if I had to cut holes into the sheet to let air flow through to the radiator. I think I will cut two holes out the same size/shape as the holes in the grille. The sheet will also cover the "bucket" headlights and wrap around the truck a little.

I'll tighten the whole thing up with whatever heat gun/source I can jimmy rig up

Whoops. I forgot yours is a Comanche. But either way, an MJ is pretty much an XJ made into a pickup.

Regarding no power steering, it may be a bit too much, depending on the steering box ratio. In my ZJ, even at 30, the steering is STIFF without power assist. At a stop, the only way to turn it more than a little is while standing next to the Jeep with the door open, ripping at the wheel hulk-style. You could, however, probably get a slower-ratio manual box.