improving a "battleship"

[ajac]

Well with my truck there was no rear quarter panels, just like another cab chassis/ flatbed out there. So I was thinking that if i was getting 17 mpg (20 on highway) without any panels, i could hop it up with some fabbed panels.

all this was exposed along with diesel tank
http://s274.photobucket.com/albums/jj270/ajac302/?action=view&current=P1100032.jpg

first off a little prototyping in foam
http://s274.photobucket.com/albums/jj270/ajac302/?action=view&current=P1100042.jpg

then some 1/4 luan just have to blend it to the body tomorrow and paint
http://s274.photobucket.com/albums/jj270/ajac302/?action=view&current=P1100047.jpg

and here is the full view
http://s274.photobucket.com/albums/jj270/ajac302/?action=view&current=P1100045.jpg
* oh I also popped off the headache rack, must have removed at least 3 sq ft of frontal area

[Red]

Looks good. Wondering what would happen if you extented the panels to hub height or to the bottom of the rim then wrap it back to the frame?

Welcome to GS :)

[kamesama980]

or just see where it sits at max capacity and leave about an inch of clearance at that point.

[ajac]

HAHA , max capacity that is a good one. max capacity is about 3-4 ton on payload. but that wont be functional for me. because in upstate ny hills and pot holes would ripoff anything lower than 8 inches from the ground.

well besides that today before work i painted the panels flat black, all sides. it is looking pretty good i'll get a pic tomorrow, as well as fill the tank. and then we'll see if mileage improves.

*on the plus side cabin noise is down drastically, now it is a dull roar

[Big Dave]

Good start, ajac.

[kamesama980]

ah, good ol' new york...you could make it out of assembly line rubber like I've seen some guys make front air dams for trucks out of. that's stuffs pretty durable. only make the mottom foot or 2 out of it for cost reasons. I'm just throwing out ideas here

[ajac]

yeah is doesn't help that the ski resort i work at doesn't plow or at least have a paved lot. then all the back roads is is great fun when they aren't plowed i feel like i am going to burst something, everyday.

her she is
http://s274.photobucket.com/albums/jj270/ajac302/?action=view&current=P1130023small.jpg

how about this? keeping it all flat like a hinging tonneu cover set at a slant, that way it can still be used as a truck.
http://s274.photobucket.com/albums/jj270/ajac302/?action=view&current=fastbacksmall.jpg

The question is will there be any gain to having a 16" over 96" (8') slope versus a 16" high flat box. Which would be similar in height to a normal bed witha tonneu cover

[mattW]

If you are making a fastback cover then you want to have it running from flush with the roof at an angle of 11 degrees to the back of the truck so you would drop about 18.5 inches (if my trig is right) across the 96 of the bed. If you look at the following diagram under the "A" column it shows the drag and lift produced my a fastback shape at different angles with the lowest drag at 11 degrees:
http://www.ibsinger.com/excerpt/gaer/gaer.01/bentley.gaer.ch6.5.2002.nov.06.sd.gif
So there is a definite advantage to have sloping rather than horizontal because as long as the air stays attached (and it will at 11 degrees) you would be reducing the area of your wake which is what slows you down the most. It might also be good to have the last bit of the fastback curve back to the horizontal so that you aren't producing any lift. You could use plexyglass for a rear window in the cover if you are worried about viability.

[ajac]

I have been thinking about that. But since it is actually my moms spare truck that I am driving since my truck's motor blew due to lack of coolant by the origional owner. So I am only doing minor things. That she agrees to. The full tear drop wouldn't work with that truck, i would love to do it. The truck is used as a truck, it moves industrial machines from shop to shop.

Plus a sheet of polycarbonate 4' x 8' would run me $100. While I am trying to do what I can with little to no cost. I have $10 in spray paint only so far.

[Big Dave]

Question for mattW: Where did you get those curves? I’d need to peruse them more closely. Obviously one want to produce minimum lift (positive or negative) if drag minimization is desired as we do. I thought my 17 degree setup was good but if 11 degrees is better, maybe I better reconsider. This makes sense. These exotic solar-electric cars are of nearly flat slope. 11 degrees might well describe them.

[DarbyWalters]

That is spectacular in it's own way...even bothering with an old truck like that deserves a "bow"...

[mattW]

It from the book Race Car Aerodynamics by Joseph Katz with an excerpt on this website (http://www.ibsinger.com/gallery.htm?code=GAER&galleryId=768). Here is an excerpt if you don't want to read the whole thing:
As the slant angle is increased from zero, a positive lift will develop, which increases up to ? = 30°. At slant angles larger than 10° the rearward projection of this negative pressure causes quite a large increase in drag, as shown in this Figure. The most interesting feature of this data is that above a critical angle (close to ? = 30°) the vortex structure breaks down and the drag and lift contribution of the slanted surface is much smaller. This fact has an effect on hatchback automobile design, where rear window inclination angle should be more than 35° or less than, say, 25°. Also, note that in this case, the basic body (with ? = 0°) has negative lift due to ground effect, similar to the case with the ellipsoid, shown in Fig. 2.22.

[ajac]

Thanks for the Bow. Well as of right now barely into the first tank. Even though it is quitter and the wake is less turbulent (much less snow being kicked up). This project will come to a close. About an hour ago I bought a beater car. 1988 Buick Century 3.8L Auto has only 40K original miles. The guy I bought it from (my father) just bought it and did the run through, parts replacement. Best of all $1100, for 28mpg highway. It should be quite a change from the 15/17 I am used to.

Depending on how long it takes to process and insure the truck may see another tank or two. And when the truck needs some "exercise"

[itjstagame]

If you are making a fastback cover then you want to have it running from flush with the roof at an angle of 11 degrees to the back of the truck so you would drop about 18.5 inches (if my trig is right) across the 96 of the bed. If you look at the following diagram under the "A" column it shows the drag and lift produced my a fastback shape at different angles with the lowest drag at 11 degrees:
http://www.ibsinger.com/excerpt/gaer/gaer.01/bentley.gaer.ch6.5.2002.nov.06.sd.gif
So there is a definite advantage to have sloping rather than horizontal because as long as the air stays attached (and it will at 11 degrees) you would be reducing the area of your wake which is what slows you down the most. It might also be good to have the last bit of the fastback curve back to the horizontal so that you aren't producing any lift. You could use plexyglass for a rear window in the cover if you are worried about viability.


Hey, so I have a question for you. I am planning on making a fastback for my pickup come spring, after reading this I took some measurements and found out if I make a fastback from the top of my cab to the back of my truck (top of the tailgate while up), it creates an 11.36 degree slope, so that sounds perfect.

But my original thought was to open the tailgate up and create a fastback from the top of the cab to the very end of the tailgate while down. As you can imagine this will increase the slope, it calculates out to exactly 18 degrees. Will flow stay attached at that angle?

Basically is it better to have an almost optimal slope (11.3 degrees) and THEN a 20" flat/drop off where the tailgate is while up, or would it be better to eliminate the flat surface of the tailgate by making an even bigger fastback but having it only be 18 degrees?

[fabrio]

Hi boys.

could you help this photo?

http://img301.imageshack.us/img301/552/rearendgk4.jpg (http://imageshack.us)

;)

[mattW]

I would say that it would be better to have the tailgate up and the smaller angle as you would have less turbulence at that angle and smoother flow. A boat tail with the end chopped square is called a kammback (http://en.wikipedia.org/wiki/Kammback), here is a quote from wikipedia:
Kamm showed that a better drag-reducing tail end design for a car is one that tapers and is then cut off abruptly.

The point at which this must happen, in order for the design to be a true Kammback, is controversial. A popular definition is that the cut-off should occur where the cross sectional area is approximately 50% of the car's maximum cross-section. Thus a minivan is not a Kammback.

Prior to Kamm's thesis, a teardrop shape that tapered smoothly to a point was considered optimal. Kamm showed that an abbreviated teardrop actually worked better; the air still flowed as if the entire teardrop were still there, but without the surface drag of the long point.

To minimise lift you would want the cover to start at horizontal at the roof of the truck, curve gently to 11 degrees and then back to horizontal at the tailgate so that the air flowing off the back doesn't produce force. To minimise total drag it will probably be better to let it continue at 10-11 degrees at the tailgate (but still make it gently at the roof). That way you would still have more room in the bed for carrying stuff as well so its win-win.

[itjstagame]

Hmm, I don't know about the Kamm's theory. It seems widely accepted by the general public though, but I always felt that was more for looks.

If what you're saying is true than basjoos would do better with his two piece tail.

I guess I can always try both but it's really hard to compare improvements with my horrible MPG. Anyway it's an experiment for the future.

[mattW]

I am sceptical that the friction forces would be greater than the added turbulence that must occur at the cut off but I am pretty sure that the 10 degree angle will work better than the 18 degree one because there will be less turbulence where the sides and top meet. It would make a good simple experiment with some A-B-A testing, empty bed vs kammback vs higher angle boat-tail, all you would need is a large sheet of ply and something to secure it with and a loop of highway...

[ajac]

I was under the impression that a Kammback was a compromise. Where the Kammback cut-off is dictated by the utility of it. While a complete teardrop is the most aerodynamic it is also somewhat unpractical. Especially on a truck or any tall vehicle. Because a teardroped truck would have a very long tail, 14 foot. Assuming two 11 degree slopes with a 6' tall cab. This long tail would swing out in turns. I think that the Kammback was developed to get similar CD decreases( similar but not equal) while being much shorter -> more practical to drive with one equipped.

I hope that makes sense.

[ajac]

I know , I know one tanks means nothing. But for the past couple weeks during my lunch breaks from college. I have been running scrap metal loads. This whole tank has been 1/2 metal loads and 1/2 3 mile city trips (my dd needed repairs). The scrap runs were anywhere from 1000 - 3500 lbs of metal. Then empty on the way back.

This tank averaged 22mpg. Which is better than the previous best. 20 mpg I got last fall 100% highway.

[red91sit]

That's some impressive mileage from such a large vehicle!!