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Old 01-13-2015, 12:55 AM   #1 (permalink)
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full belly pan vs side skirts and chin spoiler?

Which one would work better? I like the full belly pan idea, but it would be easier for me to put a bigger chin spoiler on the front of the truck and removable side skirts than full belly pan.

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Old 01-13-2015, 07:48 AM   #2 (permalink)
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If you look at most low drag concept cars, you will see that they almost all use under body smoothing, indicating that ideally a full belly pan is "best." However as you have probably gathered they are also a lot of work on a typical truck, and you have lots of pitfalls you need to avoid for success, such as exhaust heat removal, airflow for transmission and differential cooling, etcetera.

The chin spoiler is comparatively easier, however be forewarned that bigger is not always better. A good starting place is as low as existing components, i.e. if the lowest point on the underside of the truck (excluding the rear differential housing) is 12" from the ground, a good place to start with your chin spoiler is 12" off the ground.

Many on here have used rubber or other flexible materials for air dams and side skirts so that they can stand up to truck use.
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Old 01-13-2015, 06:06 PM   #3 (permalink)
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skirts vs pan

Quote:
Originally Posted by nemesis View Post
Which one would work better? I like the full belly pan idea, but it would be easier for me to put a bigger chin spoiler on the front of the truck and removable side skirts than full belly pan.
There is very little data for a comparison,as the convention is to panel the underside.
*The 1982 Trans Am Firebird realized a 4.4% drag reduction with skirts.
*Same car with skirts and airdam,7%.
*A full belly pan with integral diffuser is on the order of 12%.
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Old 01-13-2015, 06:31 PM   #4 (permalink)
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Quote:
Originally Posted by aerohead View Post
There is very little data for a comparison,as the convention is to panel the underside.
*The 1982 Trans Am Firebird realized a 4.4% drag reduction with skirts.
*Same car with skirts and airdam,7%.
*A full belly pan with integral diffuser is on the order of 12%.
would the full belly pan with integral diffuser be 12% on it's own or would it be 5% on top of 7%?
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Old 01-13-2015, 07:17 PM   #5 (permalink)
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would

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Originally Posted by tvbd56 View Post
would the full belly pan with integral diffuser be 12% on it's own or would it be 5% on top of 7%?
I don't have everything with me to give you a decent answer.
*Hucho is showing Potthoff's research on diffusers.
Pothoff's test vehicle started at around Cd 0.256,and then with the 2.5-degree diffuser dropped to 0.231 or thereabouts,for around a delta- 0.025 improvement.
That's a 7.9% improvement on top of whatever the bellypan gave him (which isn't given).
*Hucho gives Carr's research on pans but I don't have it.(Members?)
I think that Carr gave 0.07 for a complete pan,but I need to verify if the tailpiece was just level,or a diffuser.(And 0.07 needs to be given a context as to the test vehicle analyzed).
*I'll get that,but maybe in the meantime another member will catch this and chime in.Sorry for the delay.
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I got to my materials and here are some additional numbers
*Both Carr and Buchheim et al came up with a delta-Cd 0.045 for a full bellypan.
*Buchheim's test vehicle was a 1982 Audi 100 III,Cd 0.30.
*With the full pan the Audi's Cd falls from 0.30,to 0.255 (which agrees with the table for the diffuser.
*With the diffuser added,the Cd falls to 0.23 (which is in close agreement with the table.
*So in this particular instance,the Audi's drag is reduced 23.3% with a 2.5-degree diffuser and complete paneling.
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*The Ford Probe III had Cd 0.22 with active valance and full belly pan.
*The production Sierra/Merkur had Cd 0.34 without these features.
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*R.G.S.White indicated a delta-0.038 drop for a smooth belly pan on a higher drag (Cd 0.58 ) car (6.57% drag reduction ).
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*CAR and DRIVER reported that NASA got a 15% drag reduction on their Ford Econoline van with a carefully executed belly pan in the mid-70s.
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*University of Wichita showed a 6.3% drag reduction in early research.
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*University of Michigan showed a 5.8% reduction likewise.
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*In 1933,Fachsenfeld showed a drag reduction,from Cd 0.40,to Cd 0.27 with a scale model Omnibus intercity bus.
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*In 1963,Walter Korff showed up to a delta-Cd 0.12 difference with a full pan on a car with exposed frame/axles,springs.
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*Ford Motor Co. showed delta-Cd 0.04 for a full pan on a 3/8-scale sedan car model in the University of Maryland wind tunnel.
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*Chrysler belly-panned their 1934 Chrysler DeSoto Airflow aerodynamic test mule,which helped it achieve Cd 0.244,but did not present the numbers in such a way as to isolate the pans individual performance.
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*On my 1970 VW Transporter,I estimated a 1.038 mpg improvement with a full pan ( 8.726% drag reduction).
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I looked,but couldn't find any additional info just for skirting.It's out there somewhere,but probably only as internal memos in corporate labs.
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Last edited by aerohead; 01-14-2015 at 06:54 PM.. Reason: add info
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Old 01-13-2015, 11:14 PM   #6 (permalink)
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great info so far. Keep them coming.
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Old 01-14-2015, 01:03 AM   #7 (permalink)
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Interesting question, i was wondering about side skirts too and came across a paper from two students at Chalmers University... had trouble copying the link so heres the title

CFD Analysis of Aerodynamic Trailer Devices for Drag Reduction of ...
Masters Thesis by Hakansson and Lenngren

Very interesting paper as they look at various aero treatments for semi trailers, and has some great pictures!

The aha moment for me was the testing at 0 degrees/straight headwind and 5 degree side wind, as i drive in massive sidewinds here on the sask praires.

The good stuff is in the second half of the paper, enjoy!
EDIT: page 31 has results of side skirts and smooth underbody.
this paper focuses on semi trailer aero, but IMO aero is all about minimizing the wake, regardless of the size of the vehicle

Last edited by rumdog; 01-14-2015 at 08:22 PM.. Reason: more info
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Old 01-14-2015, 06:57 PM   #8 (permalink)
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added info

I went back and added some additional data at #5 (permalink) just to keep everything together.

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