affordable Aero mods for LARGE SUV

[aerohead]

I did a back-of-the-envelope calculation on the Tahoe, and relying strictly on aero, I don't see how you could have any probability of seeing above 36.6-mpg.
That's at Cd 0.12.
Without adding any length, and considering all the usual suspects, you might get to Cd 0.265.
Below that, you'd need elongation. Enough to get your wake down to about 14.5 square-feet, from about 32.0.
The 'transition zone', building into the ultimate slope angle would require 64.5-inches by itself, putting you at 266-inches, total length, from 202".
The Tahoe's squareback is your enemy. There's just no facility for meaningful drag reduction without boat-tailing, top, sides, and diffuser ( once you get one ).
You could probably pull it off with $800.00. Lots of time! Man-hours.
And then, dealing with over five feet of tail.

[Isaac Zachary]

Originally Posted by aerohead

I did a back-of-the-envelope calculation on the Tahoe, and relying strictly on aero, I don't see how you could have any probability of seeing above 36.6-mpg.
That's at Cd 0.12.
Without adding any length, and considering all the usual suspects, you might get to Cd 0.265.
Below that, you'd need elongation. Enough to get your wake down to about 14.5 square-feet, from about 32.0.
The 'transition zone', building into the ultimate slope angle would require 64.5-inches by itself, putting you at 266-inches, total length, from 202".
The Tahoe's squareback is your enemy. There's just no facility for meaningful drag reduction without boat-tailing, top, sides, and diffuser ( once you get one ).
You could probably pull it off with $800.00. Lots of time! Man-hours.
And then, dealing with over five feet of tail.

This is what I would have suggested, guessed too.

This is also why a lot of SUV manufacturers are starting to promote "coupé" SUV's, like the Tesla Model Y.

If they start making a coupé Tahoe, that would get you closer than adding a huge boat tail. You could still add a boat tail to a coupé SUV, only it could be made much smaller.

Square rear ends are terrible for aerodynamics. Try to shape the Tahoe into an Aptera as best as possible for best high speed fuel mileage.

[aerohead]

Right!
The Audi E-tron is illustrative.
This SUV can be purchased with:
Cd 0.28 raked-back
Cd 0.26 Sportback
Cd 0.22 fastback GT
depending, essentially on which roofline one chooses.

[Isaac Zachary]

What about the BMW X4? It may be smaller than a Tahoe, but it's still considered an SUV and is in the Coupe flavor. Just a quick glance, it costs about the same as the Tahoe but gets better fuel mileage. I'm not sure what the safety ratings are though.

[aerohead]

There's one in Denton that I see at lunch, driving around the University of North Texas.
It's 'hideous' to me, and better looking in the photographs.
I don't have a 'blueprint' of it, so I can't evaluate it's roofline.
This may have been the car which got the guy who signed off on styling fired. BMW spent $ 260-million to 'fix' the design.
Personally, I prefer the fastback coupe design because of the aero. They're turbulent-quiet and laminar-loud. Raging calmness. But I understand that many buyers have expectations about 3-row seating and gobs of interior room.

[Phase]

Originally Posted by aerohead

Right!
The Audi E-tron is illustrative.
This SUV can be purchased with:
Cd 0.28 raked-back
Cd 0.26 Sportback
Cd 0.22 fastback GT
depending, essentially on which roofline one chooses.

is there a reason rooflines seem to be more important when tapering, verus tapering the sides? isnt closing the wake the same thing, whether horizontally or vertically?

[Isaac Zachary]

Originally Posted by Phase

is there a reason rooflines seem to be more important when tapering, verus tapering the sides? isnt closing the wake the same thing, whether horizontally or vertically?

I would agree. Didn't the EV1 have tapered sides too?

But as with anything, "does it look cool?" is the most important argument.

I really, really, really hope that Aptera takes off. Right now, the future seems to be an SUV invasion apocalypse.

[Phase]

Originally Posted by Isaac Zachary

I would agree. Didn't the EV1 have tapered sides too?

But as with anything, "does it look cool?" is the most important argument.

I really, really, really hope that Aptera takes off. Right now, the future seems to be an SUV invasion apocalypse.

well apparently narrowing the roofline has a bigger impact than the sides. doing both is obviously optimal

[freebeard]

Out of curiousity I went looking for the Cd of thr BMW Isetta.

According to https://www.automobile-catalog.com/c...html#gsc.tab=0 its 0.60.

Too much of a good thing?

[aerohead]

Originally Posted by Phase

is there a reason rooflines seem to be more important when tapering, verus tapering the sides? isnt closing the wake the same thing, whether horizontally or vertically?

It's conditional.
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On a notchback, it cannot be predicted with the probability of fastbacks and squarebacks.
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On mass-produced fastbacks, the roofline is doing the lion's share of pressure recovery, while the sides cannot begin any meaningful section reduction until the rear wheels are passed.
Then, the same care as the roofline would have to be respected, to ensure that no super-deceleration and adverse pressure gradient were produced, otherwise, this 'boat-tailing' would provide no benefit.
Beyond the rear wheels, the added plan-view boat-tailing adds a synergetic benefit to pressure recovery.
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For an 'actual' squareback, where the top, sides, and bottom end at the same place on the body, the sides and diffuser shape would be of equal importance as the roofline. No surface would be allowed to be so 'steep', as to produce the adverse pressure gradient super-deceleration Hucho, and all others tell us to avoid like the plague.
Boat-tailing top and sides along a simple straight angle provides a 60% drag reduction benefit compared to just doing a straight, angled roofline. And if you don't radius the edges you lose 13% of the drag reduction potential.
You lose also if the diffuser isn't elongated ( let's wait until Jeff Howell conducts this research for these numbers, and let's wait until Jeff Howell conducts tests for tumblehome impact, and ditto for 'cambered' boat-tail surfaces all around ).
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For notchbacks, one would initially modify the aft-body into a 'fastback', then, afterwards, you would proceed as with a fastback.
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After another 3-generations of boat-tailing investigations, Jeff Howell et al. will have caught up to the aerodynamic state-of-the-art for boat-tails, as of 1986.
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The trick is to reconstruct the 'entire' longtail vehicle from it's blueprint ( photographs cannot provide the dimensional information we need ), top, sides, and bottom, all the way to it's farthest extremity; then, when elongating 'ANY' surface of the body, you stay along this imaginary contour suggested by the longtail blueprint you've created.
Hucho has used 'contour comparison' for this type of exercise.
It's why it's so important that everyone know how to draft and be able to perform scale measurements, or CAD, AUTOCAD, etc..
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Once you 'know' where 'the rest' of the vehicle would be, streamlining becomes 'modular'. 'Plug-and-play.
Just pick a 'length' you think you can live with, then simply elongate the body out to that length, along the imaginary contour you drew.
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Drag reduction will be a function of the reduction of wake area, compared to the OEM wake area.
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