Spoiler (Jeep XJ)
 

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.

Here's a different angle

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.

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.

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.

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.

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.

In the DIY section, I presume.

I'll have to take a look there.

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

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

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.

There's a lot of low hanging fruit when it comes to the Cherokee. One guy was able to get his to 25 mpg highway on a modest parts budget!

https://www.jeepforum.com/forum/f11/...5-ats-1551782/

It appears your hatch wraps to the point of aerodynamic separation. If your device has a rolled leading edge that you can pull over the edge of the hatch, then all you need is a detail that crimps it on.

https://www.vehicleclips.co.uk/media...fcx-7004_1.jpg
http://www.vehicleclips.co.uk/fasten...ess-panel.html

You could box the three sides of the hatch and put a clear plastic fin on the taillight lens to fill down to the bumper. Voila! Boxed cavity.

That's an idea! And yes, the hatch opens at just the right spot to be able to do that

It has been a long time but I am more serious about buying this spoiler now. I will definitely modify it so it angles downward some.

I am thinking about adding small fins on the outside edges of the spoiler to guide the air. In addition to this, I may add a lip to the spoiler to help decrease lift. Thoughts on this?

You probably don't need to try to reduce lift - squareback shapes with a roofline basically parallel to the ground don't develop a lot of lift.

As has already been suggested, I'd experiment with different configurations first.

This took a lot of tweaking but ended up working pretty well (as far as I could tell; I didn't know about throttle stop testing then):

https://i.postimg.cc/WbtZJ1rz/Figure-6-34.jpg

spoiler
 

In addition to a any small drag benefit, for regions of the country which experience truly four-seasons weather patterns, that overhang might mean less snow and ice to clear off the rear glass.

Was hoping the addition solved the boil up after the car. I have the same problem in my 13 golf..... ten minutes on a snowy or wet road and you can't see out the rear window for all the road grime from the rear. Vw's fix is to turn n on the wiper back there.

Keep us posted. I'm curious about your plan.

The company that makes those spoilers wanted $75 for shipping on top of $225 for the spoiler, so I decided I did not want to pay that. I am currently thinking about how I can make my own spoiler. I have some plexiglass laying around, so that may be an option if I can figure out how to attach it.