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Old 09-16-2009, 06:00 PM   #1 (permalink)
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Do You Know? Help on drag question

I joined EcoModder to find the answer to two questions. I have sent several querys / PMs to individuals but have not gotten an answer. Below is one:

Hi, I did what you said. I have read years worth of threads on EcoModder and other forums. And I have spent weeks of time researching on the Internet using various search terms. My question remains unanswered.

On a Pickup truck with a aeroshel bed cover. On the Trailing edge of the aeroshell to tailgate junction. Do you know if any spoiler lip / ridge / shelf or whatever it might be called can help the after-body drag behind the tailgate?

Most cars today have a little lip either on the roof dropoff or trunk dropoff. Even the Kammbacks. I cannot find out if this is just styling or is it functional in reducing afterbody drag? Since so many have it I think functional because the Mfg are doing lots more to improve gas mileage. Even the Prius (see the little spoiler lip under the window). Do You know the answer?

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Old 09-16-2009, 06:08 PM   #2 (permalink)
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I answered your question here

http://ecomodder.com/forum/showthrea...tml#post125572

and you blew that off.

The answers to trailing edge questions are here. If you don't find them satisfactory, get an aerodynamics book, as also mentioned here.

Seriously that little lip isn't worth all this obsessing.
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Old 09-16-2009, 06:20 PM   #3 (permalink)
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I'm with Frank Lee on this. Those little lips cannot be doing much. Now one guy (I forget his name) put a big honkin' World of Outlaws whale-tail on a VW Beetle and got good results but it was far bigger than those little ornaments.

Of course, you could prove us wrong. Get some cardboard and duct duct and make your own little lip and tell us how much it improved MPG.
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Old 09-16-2009, 07:22 PM   #4 (permalink)
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So sorry Frank Lee. I have not 'blown off' anybody. I just found your answer too brief and uninformative.

I have a library of over 20 books on aerodynamics and aircraft design from when I was an active aircraft mechanic, EAA and experimental aircraft builder. I have a further dozen books on marine hull design and flow dynamics along with analysis programs like Delftship, Michlet Water Resistance Calculator, ArchimedesMB Flowdesign. None of them give specific answers to my questions. I have books on racing suspension, cooling and general body aerodynamics that have not answered my question.

Plus: It was suggested (by you and several other people on multiple threads) that instead of driving a truck/ other un-aerodynamic / high HP vehicle to just buy an economy car. My answer to that is:
>>>>>>>
Why Do I Drive A Gas Guzzling Truck

All the details here to prevent those unusable helpful suggestions -
Driving a pickup truck is a necessity, not an option for some of us. I travel to a job site, unload and then live locally for a while [2 weeks to a couple months]. While were ever I am, I am driving solo with an empty bed. The load I carry occasionally prevents me from using a flat hard tonneau or a shell. I tried renting a trailer but sometimes I have to leave one job site to travel to the next on hours notice making rental impossible (told at 5PM to go pack and be at new job site 2/3/4/5 hundred miles away tomorrow or day after but really just barely make-able; now find a rental place open). I even tried a car/trailer combo for a year but hotels and motels parking policies make it difficult to just impossible. Or it gets broken into and vandalized, three times in a year. Or, finally, the tire I had removed and locked inside did not make a difference, I got back from the job one day and it was gone. Back to the truck. So now you understand why I drive a pickup truck and can not just “buy a beater for the commute” like I see suggested so many times on the threads.
<<<<<<<<<

I know that just smoothing out the airflow is not always the best thing. I know that sometimes the after body drag / turbulence can be so high as to negate or even worsen mileage. IE: the tailgate being up is better on trucks is counter-intuitive and takes wind tunnel or CFD to 'see' it. And various streamlined cars have had to have drag inducing spoilers added to restore performance and handling lost to poor air flow AFTER the car [ex; the TT].

Quote:
Seriously that little lip isn't worth all this obsessing.
Recently here on the forum there was even a video link here about how Ford trailing edge tweaks made such a difference in mileage. You may have seen it but did you SEE the point? Little lips sometimes DO make a difference.

And Big Dave, I like where you are going with your truck. I am following a similar path. However, my first duct tape, lath n plastic tarp tuft testing showed there is HUGE turbulence on the side corners at the tail lights and on the tailgate. The tufts did not just lie in different directions, they move back and forth and swirl around. That means there is a LOT of drag and because the basic shape I formed has the right angles (less than 12 degrees over the top and complying with the template and tapering on the sides at 7 degrees)
I do not think there is any significant change to be made to the cover shape. That leaves trailing edge spoilers and vortex generators.

On another thread a reference to vortex generators on the trailing edge generated disparaging remarks that what good would they do because they worked to keep the slipstream attached to the following surface and if there was no surface there would be nothing to work on. And that is just not true. Search for yourself, lots of truck testing has positively shown that vortex generators on the trailing edge break up the large drag inducing vacuum vortex into multiple smaller ones that cause less drag. Drag, meaning mostly after-body drag is a major reason for lower mileage!!!

So there is a problem that I want to fix as best as possible. I am looking for that little tweak that I am sure is there. I might have to build a sub-kammback on the tailgate itself but that is why I am asking if anyone else knows about this. If I can find a "Ford Fix" it would be easier.

PS: I have installed an electric fan, blocked the grill and have consumed 3 sheets of coroplast sheathing the underside, including a difuser from the axle to the bumper. I am still looking for more information on correctly shaped tire dams.
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Old 09-17-2009, 02:03 PM   #5 (permalink)
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windrider919, just curious what truck you're driving and your before and after modification mileage. Seems you've done most of the big gain mods.

Sounds like you're asking a question for which nobody here knows the complete answer. This forum is for sharing new ideas and testing new ideas and is by no means complete. Since few trucks have the small spoiler you are asking about, do a test and report your findings. This may be a beneficial improvement to the teardrop bed cap - or not.
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Old 09-17-2009, 02:21 PM   #6 (permalink)
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The purpose of those mini-lips, apart from styling, is to create a single large vortex pattern in the wake. Only testing can determine the success of any particular aerodynamic detail subject to other influences. A lip probably works best if it gets unobstructed airflow both from ahead and beneath.
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Old 09-17-2009, 06:01 PM   #7 (permalink)
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lip

The big boys encourage case-specific scenarios over broad generalities and shy away from making a claim they know will fail in certain applications.------ After Texas Tech published their pickup aero research findings with SAE,Sport Truck Magazine picked up on their data for a piece on MPG.--------- Industry gurus were interviewed and they felt it inappropriate to make any sweeping claims,say in the debate over tailgate up vs tailgate down.--------- The F-150 demonstrated lower drag with gate up,but we're splitting hairs here,as the difference was 1% in drag.--------GM published data in the 1970s which indicated positive gains with tonneau.I've never seen data from Chrysler or Daimler.--------------- Presently,GM incorporates a token lip spoiler on Silverado,etc..Aerodynamically,the thing is a wash.It's only purpose is the hope that the owner will keep the tailgate "UP",for lower drag,as the target market for those trucks statistically will be following the NASCAR Truck Series.---------------- You'll notice that all those race trucks have full tonneau covers.It's the only thing which allows the tailgate spoiler to function for downforce.--------------- Certainly,anything which increases the fineness ratio or prevents separation is going to reduce drag,but for significant reduction the vehicle is going to need something rather dramatic.No cheap,easy fixes.
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Old 09-17-2009, 09:20 PM   #8 (permalink)
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What kind of truck do you drive, Windrider? Engine? Transmission? Gearing?

If you can't make a tonneau or sloped bed cover work...well that's trucking, but it basically shoots aero improvements in the head as the rear of any vehicle is the aerodynamic big enchilada.


Can you hack numerically lower gearing? I found that to be very effective.
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Old 09-20-2009, 12:40 AM   #9 (permalink)
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These lip spoilers greatly depend on the vehicle they are attached to.

However they all try to serve a similar purpose in promoting clean separation. At the rear, you want the smallest radius at the trailing edge. For many reasons, the vehicle body often does not have this ideal radius, so a spoiler may be beneficial.

Will this make a huge difference? No. But if you're like me and take it one count of drag at a time, things like this will add up.
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Old 09-20-2009, 01:26 AM   #10 (permalink)
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Remember the old Taurus that looked like a fish? Ford put a little lip on the trunk kid to counter lift and reduce drag.


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