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Old 03-07-2011, 06:19 AM   #21 (permalink)
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Originally Posted by Sularus View Post
From what I have gathered on this site though is you will not realize enough potential from a boattail/kammback until you have smoothed out most of the car starting at the front.
You won't get the full potential for improvement of a boattail until you sort out everything in front.
But you do gain a serious improvement even if the front of the vehicle is a brick.


Streamlining a brick : see what the air resistance does when you add fairings to a brick .

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Old 03-07-2011, 12:51 PM   #22 (permalink)
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Then why are we talking about boattails. Working from the front to the back is both cheaper before you get to the boat tail, but easier as well. This is about hood-windshield transition.

This is what was done to aerocivic. As far as the angle goes I am not sure, but I would think an angle meeting up with the top edge of the windshield where the roof begins would help and it somewhat bisects the difference of the hood-windshield transition.
Aerocivic - aerodynamic mods for maximum fuel economy - aerocivic.com boat tail storage space

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Old 03-07-2011, 03:20 PM   #23 (permalink)
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Quote:
Originally Posted by Sularus View Post
This is what was done to aerocivic. As far as the angle goes I am not sure, but I would think an angle meeting up with the top edge of the windshield where the roof begins would help and it somewhat bisects the difference of the hood-windshield transition.
Aerocivic - aerodynamic mods for maximum fuel economy - aerocivic.com
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Those are wiper deflectors, rather than an attempt to reduce the apparent windshield angle (which is already smaller than on trucks).
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Old 03-07-2011, 06:33 PM   #24 (permalink)
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Quote:
Originally Posted by Joenavy85 View Post
how's this look??
It looks awefully steep on the top. You want a smooth transition into about a 15 degree angle. I did a 4 foot kamm back on my car and realized a 6% improvement in MPG (12% drag reduction). Also if you have roof racks, you will want them off when the boat tail is on. Removal of the roof racks can be a 3% (or there abouts) improvement in MPG, and the boat tail will have much smoother flow coming into it.
I also cleaned up the front end of the car with a grill block (over 50% blocked), and a belly pan that goes to the firewall. This was also good for about 6% MPG improvement.
If you remove the roof racks, and have attached flow on your roof (easy to test with tufts of yarn and some tape, window down and mirror in hand at 35 mph with NO TRAFFIC) you will be good to go on the boat tail, as long as you follow the above advise.
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Old 03-07-2011, 07:30 PM   #25 (permalink)
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angle was due to size of cargo box and how tall i can make it without obstructing my window too much. i would have the box go to about 5-6 inches above the spare tire, which would only block about the bottom 1/4-1/3 of the rear window.
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Old 03-07-2011, 11:09 PM   #26 (permalink)
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Making sure the flow attaches nicely to the front of the hood above the grill will help keep the airflow energized. Probably already good.

Venting the hood to allow used cooling air up through it (first third of the hood, probably, would require verification) would effectively increase the hood angle without being too garish (see Mitsubishi Evo line). Probably make hot air get sucked into the ventilation system, though.

Gains should be available by increasing the radius of the A pillars to reduce the vortices that form where they meet the roof (that little patch of swirling water on the side window when you drive in the rain). I've been trying to figure out a nice way to do this on my own project.

Mirrors, of course.

A little rounded hump right behind the windshield might help. It looks like the roofline slopes, so a small hump could smooth the transition, reduce pressure drop, improve pressure gradient on the roof, and leave you with more energy to make the curve onto the Kammback, without increasing the profile. I've seen something like it on the front of a bus, IIRC.

Alternatively, cab-over semis sometimes use turning vanes on sharp corners to help simulate a bigger radius. Like those station wagon rear-window wind deflectors, but on the front. Never seen it done here, but again, I've seen it on trucks, and probably wouldn't look out of place on a jeep.

If it's like most rigs, especially 4WD, the undercarriage is probably where you'll find the biggest easy gains, though. Air dam is quick and easy, belly pan allows for better diffuser performance and better ground clearance. Steep approach angle on the jeep should make the air dam less likely to scrape than on other rigs.

'course, it's all just book learnin' for me. Haven't gotten to test it yet.
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Old 03-08-2011, 07:34 AM   #27 (permalink)
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There is a thread here but i can't link it due to only having 3 posts yet. I will quote it instead - it's in my bookmarks.

To quote....
1. Narrowing the body at the front and rear,
2. Side window angle (22 and 30),
3. Length of front bumper,
4. Angle of grille (0, 12.5, 25),
5. Angle of hood/bonnet (5, 7, 9),
6. Angle of windshield (25, 30, 35),
7. Angle of rear window (30, 35, 40, 45, 50),
8. Inward angling of rear pillar,
9. Angle of trunk/boot (4, 8, 12),
10. Geometry of rear spoiler.


The effect of each of these elements on the drag coefficient was tested, then the propotional change in Cx (Cd) was calculated. Here is an ordering of those effects:

1. 46.5% - Angle of hood/bonnet (element #5)
2. 17.0% - Angle of grille (element #4)
3. 11.7% - Narrowing the body at the front and rear (element #1)
4. 11.5% - Angle of rear window (element #7)
5. 3.9% - Inward angling of rear pillar (element #8)
6. 1.9% - Relationship between rear window angle and spoiler
7. 1.7% - Relationship between windshield and rear window angles


As the above list shows, the hood/bonnet angle has the largest effect on the drag coefficient Cx of a sedan shaped vehicle. Increasing it from 5 to 9 reduced Cd from 0.52 to 0.47.
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Old 03-08-2011, 03:09 PM   #28 (permalink)
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Quote:
Originally Posted by Joenavy85 View Post
angle was due to size of cargo box and how tall i can make it without obstructing my window too much. i would have the box go to about 5-6 inches above the spare tire, which would only block about the bottom 1/4-1/3 of the rear window.
What I was suggesting (a 15 or less degree slope) would allow you MORE visibility and LESS drag. You will find that pressure drag would indeed be less if done like your drawing, but total drag would be more. Total drag usually bottoms out with a slope around 15 degrees, at least for a "fastback" design as opposed to a full out boat tail.
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Old 03-08-2011, 06:15 PM   #29 (permalink)
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hood-windshield

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Originally Posted by Joenavy85 View Post
yeah i've looked through his stuff quite a bit (hence the statement of "doesn't look like a giant cowl induction hood scoop", like on his pickup (not knocking him, i have no doubt it works, just saying i need something simpler)) but not sure of what i could really do on the backside of my Jeep
Sorry! Late catching up.
My impression on the Liberty, is that the Germans did a good job with the fore-body,the flow should be attached all the way to the rear, and as Frank has inferred,it's the huge turbulent wake you should be targeting.
If I had one,to begin with,I'd install a 2" receiver-hitch,buy a $59 cargo platform from Harbor Freight Tools and build a slide-on boat-tail,extending the body at least 24," with plan and elevational taper,with diffuser section.
If you do the first 24" real well,you can add on in the future for even lower drag."Plug-and-play."
If you get the tail "wrong," it will prevent maximum performance.
The reason for the blister fairing on the T-100 is to help reduce the pressure spike from the abrupt windshield eruption only properly cured with a $3,000,compound-curvature, GTP laminated safety glass replacement and all the attendant body work to integrate it.
Hucho informs us that once these hood/windshield areas are 'optimized' and we have attached flow,no further streamlining will reduce drag.And there are studies in his book which relate the validity of his comment.
I'm going for around Cd 0.10 with the truck and kinda going overboard.
PS,the mod your thinking about did nothing for my CRX at Bonneville.The boat tail did almost everything!

Last edited by aerohead; 03-08-2011 at 06:18 PM.. Reason: add PS
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Old 03-08-2011, 06:27 PM   #30 (permalink)
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W.E.Lay 1933

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Originally Posted by Frank Lee View Post
Really?

Can you find that?
Frank,wherever I said that is gonna come from a comment Hucho said,regarding Walter E.Lays research in 1933.
Lay did a bunch of scale model car wind-tunnel studies,got about five of 'em down to Cd 0.12 with full boat tails.
He found,as did Fachsenfeld and Kamm,that if the fore-body had crappy flow,the boat tail was a moot point,as flow would be un-attached coming to it,rendering it useless.
Hucho goes on to say that virtually all production cars circa 1986 and later will be clean enough to ignore fore-body streamlining and to basically concentrate on the back end.

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