Rear Wheel arch ducting to rear to minimise base drag??
 

I've been looking at what to do with the rear end, being a solid axle 4x4 makes it hard to do the rear undertray/diffuser because of axle movement & towbar position & it is also hard to close off wheel arches to underside. I have read that the wheel arches develop pressure at the top and need to be ducted, and even more when I do the covers. Looking at all the mechanical mayhem in the area, there are large cavities going to rear of vehicle so I removed rear step today which has opened two slots about 1.5" high and about 3' long one between body and chassis crossmember and a lower one between chassis and tow bar. Before that the air from underneath was only exiting below towbar, the higher exit now is about 8" higher.
Will this help fill the vacuum at the back and reduce drag?
I went for a drive and it feels better, but you know how placebo works.
I am looking at designing a new rear bar & step, so want to begin the basis of a Kamm back style with it. The current bumper sticks out about 3" either side so if I taper the new one in from the wheel arch to skimm the body corner, it comes in at about 15 degrees and taking that line back about 6" should take about 8" out of the width at that point.
If I duct into the lower portion of the rear end, what effect will there be on the rear drag?

Rear end of what?

Suggest you enter your vehicle details in the garage, so they appear below your username when you post.

That way we automatically have the context/details of questions & comments.

Done, entered into garage.
Basically it is a big SUV type, wagon body, sitting on a steel chassis, running on solid axles at both ends. The axles articulate quite a bit when off road, so getting a continuous undertray set up is difficult. I've been looking and researching a lot on the wheel covers & skirts, but quickly realised anything done on the rear wheels needs to tie in closely with the rear work on the vehicle.

The 1st generation Honda Insight, Cd 0.25 has factory rear fender skirts, and no special ducting from within the wheel wells.

Of course, every vehicle is different. I do recall reading that Mercedes ducted the rear wheel well area through to the edge of the rear tail lights, claiming the pressurized air promoted cleaner flow separation & drag reduction. (If you search, you'll find a thread about it here.)

But the improvement is going to be so minor, and so difficult to test, I'd be concerned with the lower hanging fruit first.

Good point. Glad you're aware of that!

A 15 degrees change is probably too abrupt a transition to maintain attached flow. If you've seen the streamlining/teardrop template you can visualize how much maximum transition is possible before heading off toward airflow separation (and wasted body work time/effort past that point).

I don't believe it's possible to even guesstimate about this one.

http://wwwm.coventry.ac.uk/researchn...estigation.pdf

You might find this interesting.

Salient line from the PDF:

I would still be wary of assuming that you can automatically apply the results from a single vehicle to another, particularly different vehicle types (a sedan with fastback roof -- the one in the test the PDF describes -- to a SUV).

Thanks for the replies, was hoping for yes or no answer, rather than maybe or maybe not.
Cd - I did read that UK study, and also found some others related, but not really much detail and they were not done on a moving ground or tested in real world, so not really conclusive, but still a possibility.
Metro MPG - I will look up the Merc discussion, but about the 15 degrees being too sharp, as I understood it an overall between 12-22 degrees is suitable depending on other factors, 22 being doubtfull if other factors are not supportive. Where I say 15 degrees, I'm not talking a sharp edge 15 degrees, it would likely be about 6-8 inches transitioning from 0 - 15 degrees, so a curve leading into the straight, is this ok or have I missed something fundamental in everything I have read here.
I have put the spare back on the rear, std factory position, just looking at ways of filling & disturbing that vortex, FE is still trending down, last fill 9.87l/100km, recent average since mods began, down at 10.5l/100, average prior to mods 11.4l/100, so all still moving in the right direction.

FYI, the Merc ducting to the tail light was very similar (though more subtle) to the ducting modification in the PDF link, above.

As for tapering the bodywork, glad you're thinking curves rather than just angles.

15 degrees in 6-8 inches seems like it may be too much, too soon. FYI, attached flow at 22 degrees is only achievable as the ultimate angle/tangent in the curve for a given shape. If you get to 22 degrees (or any angle) "too soon", separation occurs.

Have you seen this thread? http://ecomodder.com/forum/showthrea...rt-c-9287.html

PS - congrats on the fuel economy improvement so far!

Thanks,
Yes I have read the template thread, and have a read of most threads on a daily basis, but the deeper I go, the more questions arise, seems there is more than one way to skin this cat.
I think my FE will crack the 10l/100 mark, but no declaring that till I get a few more tanks through.

MetroMPG
Found that Merc thread with the tail light ducting below,
http://ecomodder.com/forum/showthrea...trol-6075.html
The tiny slits they have would not be capable of any serious ducting, I think thaey were there primarily to give clear seperation at the body edge, like vortex generators, so rather than putting a sharp edge on the lights like they are doing moreso now, they chose to eject some sharp straeams of air, which simulate a sharp edge and give clear seperation of air from the body.
That's my interpretation.
My direction would require quite a bit more volume, to help drop pressure at rear and and hence reduce drag, more like the uk study linked in one of the posts above..