Spoiler (Jeep XJ)

[Taylor95]

I read the other thread on here on spoilers, but I don't want to steal it. I'm seeking the opinion of all of you experienced ecomodders on here. Do you think the spoiler pictured will provide any meaningful reduction in drag? Or does it create more drag? This spoiler is $200 online, and that is worth it if it reduces drag. The European XJs have spoilers like that one, so I'm thinking it helps reduce drag. The Jeep on the left has no spoiler for reference.

[Taylor95]

Here's a different angle

[oil pan 4]

Removing cross pieces of the roof racks would probably make a bigger difference.

I say once you get a baseline mock up one out of cardboard and duct tape and try it out.
$200 is a lot to spend on an aero mod on a passenger vehicle that may or may not work.

[kach22i]

As shown, you will never recoup the $200.

This type of rear spoiler acts by providing a clean edge for the air to release from, plus a little box cavity action of a single plane cantilever (some air will get sucked down/in).

As pointed out on the first response the roof racks "as shown" are an issue. They are an issue for a rear spoiler than relies on attached air-flow. The disruptive turbulence the racks cause will negate any benefits of a rear roof spoiler.

The spoiler is a nice looking, but on this vehicle of little value.

Originally Posted by oil pan 4

Removing cross pieces of the roof racks would probably make a bigger difference.

I agree.

One could remove the roof rack altogether as well.

Then the rear roof spoiler would have a chance to work, but I remain doubtful that it would ever pay for it's self.

It is a nice piece of bling though.

[Taylor95]

Yes, the looks are one reason I would get it. It is unique; none of the American XJs came with those.

I have already removed the roof racks and that has made a measurable change in my mpg. Well, I removed the sidebars anyway.

I'll have to try that with the cardboard, although I'm not sure how I will measure the difference, or how I will attach it. Any suggestions?

Some spoilers claim to reduce drag on a vehicle by like 17%! I'm thinking that would be possible on a vehicle like an SUV.

[kach22i]

Originally Posted by Taylor95

I'll have to try that with the cardboard, although I'm not sure how I will measure the difference, or how I will attach it. Any suggestions?

There might be an old thread or two on how to do a roll down the hill test (roll test), look it up.

It may even be a sticky.

If you know your vehicle really well, a good old "feel of the pants" feeling may give some feedback, but is very subjective.

[Taylor95]

In the DIY section, I presume.

I'll have to take a look there.

[jakobnev]

I'd consider fabricating a box cavity for the whole rear instead.

[kach22i]

I just remembered the guy writing the book on ecomods had a section of why roll down test do not work.

I don't remember what he thought was better, but it's in the later half of his thread.

EDIT: It is a video I have not yet watched, see link below post #78.

Writer seeking examples of tested DIY aero mods for book
Does coast-down testing work?
https://ecomodder.com/forum/showthre...k-35843-8.html

[kach22i]

Originally Posted by jakobnev

I'd consider fabricating a box cavity for the whole rear instead.

I almost bought a Jeep Cherokee 9-years ago but an ice storm hit so I had to wait and ended up with my S-10 Pickup truck.

I sometimes wonder what my path of modifications would have been.

I'm pretty sure it would have a box cavity of some kind, maybe in the form of adjustable flapper vanes - like tail rudders canted inward located along the trailing edges of the boxy body.

With the ground clearance of the Cherokee I may have even gotten into making a belly pan in lieu of my multiple chin spoiler projects.