Adding curved form to roof - good or bad??

[Tesla]

I have a large 4x4 (Nissan Patrol wagon), and was looking at some aerodynamic improvements, have read a lot of the stuff here and today blocked the grill area to start some testing on the frontal area. But I am getting the impression that the biggest improvements on a flat back vehicle like mine are on the rear drag, using some boat tail design.
To get any significant effect with just the boat tail, means having too much overhang at the rear.
So I was wondering if I ran a panel along the roof from the windscreen, then raising it 6 -8" just above driver area and tapering back down to the rear, with maybe 1ft of overhang. This would then start to resemble the ideal shape.
Would any airflow benefits be negated by the increased frontal profile?

[drmiller100]

that will make your car "bigger" which will hurt your mileage.

[Sven7]

Yep, it will increase your frontal area significantly and slow you down.

[ChazInMT]

Contrary to others here, I'd think that the improvement in shape would overcome the increase in frontal area. You could make a test shape out of cardboard, then do a coast down test or find a hill somewhere and coast down it to see how it changes your speeds with and without the tester on top.

[Frank Lee]

Seems like those humpback semi trailers to me, which I think to be gimmicks.

But I suppose if you have a flow detachment issue just aft of the top of the windshield and you can correct it with a SMALL ENOUGH hump such that loads of frontal area aren't being added, it's possible it could help. Still, if the flow on the stock roof reattaches well enough towards the rear (no racks and stuff up there) maybe the whole exercise is moot.

[wyatt]

My copy of Hucho shows very clearly what you are talking about, but they use a standard sedan, not a "wagon" type vehicle. What we are shooting for is CdA reduction, not just Area reduction. If the re-shaping reduces the Cd enough, the CdA will reduce enough to offset or overcome the increased Area. In Hucho's book, it shows how increasing the height of a standard sedan's roof impacts the overall drag force (CdA) encountered. It's Section 4.4.4 Roof, and Figure 4.41. In his example, l_r is used for the length of the roof, and a_r is used for the increased height of the roof. He calls a_r/l_r the "roof camber". He finds the lowest Cd at roof camber = 0.065 (ish), but the lowest CdA is 0.0175 (ish) or less. Remember, this was for a sedan, not a wagon, so YMMV.
Do you have attached flow on the roof of your vehicle now? If so, it is less likely that messing with the roof will help, and more likely that adding some boat tailing onto the rear hatch will help. If your vehicle has poor attachment/no attachment on the roof, you must correct that before a boat tail will do anything positive.
If you try "raising the roof", document what you do, and what the results are. I would start out doing a tuft test on the roof of the car. Video tape it with your phone and see what's happening.
Good attachment?
If your roof is 9' long (108"), I would try a 2" rise, peaking at the center. This will give your tail end a bit of a downward angle that should help your boat tail be a bit more effective.
Poor attachment?
Keep adding height and rounding the FRONT edge of the car over until you get good attached flow. Following a gentle arc, take the rounding you started at the front all the way to the back of the car. The Ideal Template would be a good place to start .

[kach22i]

Don't raise your roof as others have commented.

1. Chin spoiler (see link in my signature)

2. If no chin spoiler, then full belly pan

3. A trailing edge wing spoiler like on the new ones should help (see image below)

January 31, 2010


EDIT:
http://s184.photobucket.com/albums/x...%20and%20Ends/


Asking if you could lower it, and asking if anyone thinks the hood raise is a good idea.

[Frank Lee]

Keep in mind that any extra height up there will probably be detrimental in x-winds, even if by chance it improves the 0 yaw condition.

[aerohead]

Originally Posted by Tesla

I have a large 4x4 (Nissan Patrol wagon), and was looking at some aerodynamic improvements, have read a lot of the stuff here and today blocked the grill area to start some testing on the frontal area. But I am getting the impression that the biggest improvements on a flat back vehicle like mine are on the rear drag, using some boat tail design.
To get any significant effect with just the boat tail, means having too much overhang at the rear.
So I was wondering if I ran a panel along the roof from the windscreen, then raising it 6 -8" just above driver area and tapering back down to the rear, with maybe 1ft of overhang. This would then start to resemble the ideal shape.
Would any airflow benefits be negated by the increased frontal profile?

Dr.Hucho,who did actual streamlining for Volkswagen,and who has gone on to publish a number of textbooks on road vehicle aerodynamics reported that curving the roof would provide a drag reduction,so long as frontal area was not increased in the process.
Don Burr reports that their cambered-roof tractor-trailer rig gets a drag reduction in spite of increased frontal area.
Is your 4X4 based on the Armada SUV?

[wyatt]

Yes, a picture of your vehicle would help. and Aerohead's comment on rounding helping so long as you don't increase frontal area is only applicable if you have good attached flow. Cross winds should not be a problem, since you shouldn't be adding much area, probably much less than a pizza delivery guy's sign.