enclosed trailer aero kammback
 

Hey all
been reading about the "pot of gold" that's lurking at the back of the trailer.
Been getting about 13mpg on average towing my racing trailer. I'm shooting for 16 for now.

Read on wiki that a reduction of area about 50 Percentgets close to the efficiency of a full boat tail. So i've started with 96 wide by about 85 high. Tail should then be about 68 x 60 I believe.

Using a 15 degree slope the length of the addition ends up around 44 on the angled part. Does that seem reasonable?

Building a frame from 3/4 conduit and bending and welding as needed. I plan to skin with some plastic sheet I can get at work. Heavier than coroplast but very available and very free. Probably just rivet it on.

I've attached a couple pics of back of trailer and start of frame. Thinking the pieces that connect the two will have several small bends to get closer to teardrop shape.

Anyone have any feedback on this. Would like to build once and be done.

The whole deal will be hinged on topside to get it out of the way to unload the racecar.

It depends greatly on the execution, your pictures did not post. Just make another post then it will let you, or do as I have and create a Tinypic"dot"com account and just refer to your pictures in here, it is real easy to do. Let me know how I can help ya.

Don't feel bed creating another couple of one word posts to get your count up...your project is VERY interesting to all here and we would all love to help!

I seem to have 2 issues.1 is the file size I think. I'll try again if I can resize or try the other account like you said.

Picture size is funny here too, it needs to be less than 650 pixels wide to not be resized and < 800 wide to not look really bad when expanded. Keep that in mind. I really think having a Tinypic account is the best way to go due to the flexibility, the ecomodder blog is real finicky.

It seems you are correct. I'll have to do the tinypic deal.

tail
 

with an 'ideal' boat tail nearly all the drag reduction has occurred by 80% of the length.Chopping it off at 50% would leave 26% of the drag on the table.
If you wanted to guarantee your job,you might want to search here at Ecomodder for 'Aerodynamic Streamlining Template Part-C'.this was intended for these kinds of projects.
Curves are your friend,angles will get you part way.
Edge curvature is also very important to a boat-tails performance.
I'll wait for your photos.

Sorry for the delay on the pics. They aren't great but do show the trailer and the start of my framework.

Aerohead - I didn't know that 50% was a less than ideal number. Got it from widipedia. Perhaps not a great source, but when I searched for Kamm back that's what came up first, so . . . .

I kind of thought it seemed "too easy" to get there. Was planning on making it deeper, but then at 50% cross section and 15 deg. it was only 44" deep on the hypotenuse.

Anyway,
The piece was going to start out with straight corners since the corners on the trailer are straight anyway. Does this make sense or not? Could have easily used the bender to make them 6" radius curves.

The rear of the tail was going to just use the curves that the bender produces (seen in photo). I guess that's probably not enough curvature to make the most of it.

Thanks for the heads up on the template. that's what I needed yesterday but I didn't know what to look for. I'll look at that before I go any further.

Let's see if these pics will show up from photobucket.

First the trailer

http://i1164.photobucket.com/albums/...0528-00804.jpg

Then the rear section of frame.

http://i1164.photobucket.com/albums/...0528-00803.jpg

three different angles involved.

the top is REALLY safe at 20 degrees. 20 degrees is ABOUT 1 inch of drop in 3 inches of run.

The sides are "pretty safe" at 20 degrees, and REALLY safe at 15 degrees. Again, 1 inch of drop in 3 inches of run is pretty safe for the side.

the bottom is best left at 10 degrees. one in 5.

so. assume 96 wide. each side is 48 inches "wide". one in 3 would mean the back would be 144 inches long, which is pretty silly.

if you can make it 60 inches long you will get the vast majority of the benefit, as your top and bottom also taper.

When you are saying "safe" are you assuming I'm making the sides a curved surface as best I can, or is that also ok even if the sides are just flat? I'm going to try to make the sides curved as best I can, but I don't have a way of rolling the tubing, I can only make multiple small bends in the tubing to approximate a large radius bend.

60" long seems like a fairly logical length from a feasibility standpoint. Any more than that is going to get unmanageable. Trailer is already 26' long without tongue.

I wonder why none of the new trailers out there have such features. Do ALL people pulling trailers seriously not care about fuel mileage? At $4/gallon for diesel I have to care. Would think they would have fuel saving options and show valid proof of such an addition. Seems like people would be interested if it was built in a way that looked "factory".

Is there a commercially available "drag reduction semi-cone" for the rear of trailers?

Okay,

The Template as it is called. From what I can tell Aerohead you are a big part of this drawing I couldn't read all the pages on that thread tonight.

http://i1164.photobucket.com/albums/...1334620232.gif

I say thanks for making such info available.

So how do I apply it to the back of my trailer? I guess I can transpose it to a CAD program and apply that way - and go back 60" and see where it comes out.

Also - I am planning to start the tail right below the top row of lights there on the trailer so that I don't have to make recesses in the tail itself for the lights. That puts me 2" below the top before the tail starts - this seemed insignificant to me at first, but now I"m wondering if that's really the case. Thoughts?

1) To use the template properly, you start with the end of the trailer at the 0 degree point. Scale the template to match the height of the trailer.
2) you will have a pronounced separation with that 2" step. It will re-attach before the end of your tail, provided you keep the angle shallow.

I'm a minority of one.

I think you take your flat trailer, and put flat angles on the back of the trailer and get a much better result.

wiki/source/numbers
 

Wikipedia as a data source is way out ahead of nothing,but we've been able to go straight to the horses mouth through original publications.
The 50% 'thing' originates with a 'practical' length issue,when considering traffic and driveways,and parking.
Also,Koenig-Fachsenfeld's model (of which Kamm is given credit) would suffer a ground-strike exactly at the 50% point due to the 10-degree departure clearance angle.So he just lobbed off the tail there,and added an inflated boat tail to get it out to 80% when out of town.
If you look at the 'Template',you see that there's essentially very little curvature at all where the tail begins.This is essential for high performance.If you just jerk the body into an arbitrary angle, boundary layer separation will be triggered right there and you're basically screwed.
NASA did a relatively primitive boat tail behind a Ford Econoline but respected the subtle beginning curvature.There are images of it online and here at EcoModder somewhere.
If I were doing your trailer,I'd use the height as the layout parameter,since the trailer is wider than it is tall.the With the imaginary end-point 1.78X of the heigh behind the trailer.
Your 50% frontal area point would be at around 51% of that dimension. Kinda 'long.' This is why Don Burr started his curvature 'early' on his trailer so it wasn't all 'behind' the trailer.
The final length is something you'll have to determine.I would recommend a short but proper chopped-off tail section over an 'aggressive' but 'dirty' longer tail.
I use an EMT bender to make small incremental bends of large radius.
Harbor Freight has a nice entry-level tubing bender at reasonable price.If you have any awning companies in your area,they can do perfect curves and might take on your work at a reasonable fee.
If the lengths scare you off,let's talk about DR Miller's idea.

flat angles
 

The lengths for hotrod's trailer tail ARE 'something.'
I have only one alternative which was part of Texas Tech's Project Tailwind,involving General Motor's 'Optimum' boat tail.
It has more simple angles but with definite edge radii all around.I'd have to dig it out.
So there may be a minority of 'two.'

Here is what you want to try and do. You'll want to radius the corners of the tail extension as much as possible to help you in a cross wind. My angle may be a bit too steep, you certainly would not want to go steeper, going at less of an angle would probably get nearly the same result.

Notice how the shape starts out "Blended" into your trailer? Then it slowly curves to an angle, this is what Aerohead meant, I believe, when he said you don't want flat sides creating an abrupt transition.

I have about a 13° overall change and the back of the shape is at about 21° before it cuts off there. FWIW

Here is a link to a bigger slightly more detailed version of the same drawing as below.
Bigger Pic Of Trailer Kamm Dwg

http://i45.tinypic.com/dxokjl.jpg

but there is only one top, and two sides.

And the advantages on the bottom are TOUGH to not screw up.

Again, I'm not very sure on this, and am TOTALLY willing to discuss it.

I think you do 1:3 on the top and sides, and radius about a foot at the transition from the tail to the trailer.

Be CAREFUL during that foot - don't do anything to create separation, but don't worry about the 4 corners around your tail- if you screw up the corners, you create a vortex which helps fix any separation.

Thanks for the illustration. That looks like the ticket - it's about what I was thinking I guess. I understand the importance of the tail curve being tangent to the vertical of the rear of the trailer. Makes sense. Gradual transition so as not to upset the flow so to speak.

So, the 2" drop at the top due to the lights- it sounds like this is not a huge deal breaker?

Just looking at the illustration - I wonder what kind of drag reduction that will be. Appears to be such a small cross section change but if that's enough to make a significant difference then I'll roll with it.

I guess at the end of the day longer is better, but how much would the difference be if I were to jump from 60" deep on the tail to a 90" or something.

That trailer sits pretty low actually in the rear and very rarely scrapes, so the length would only be a pain getting fuel, etc. I think. Mostly highway miles, so maybe not a huge disadvantage, other than more weight and more fab time?

If I get most of the benefit from the pictured above, and only an additional few % by extending, I would then say it's not worth making it really long.

So that brings me to this question: is there a rule of thumb for amount of effective drag reduction along the length direction? As in, at the first line on the template you get 50% benefit, next line is 70%, next line is 80%, next is 85, next is 89, so on, so on?? A sort of curve that nears 100% reduction as you get closer to the 1.78x dimension that Aerohead referred to as the end point?

DR - so this means a radiused area to go from the flat top and sides to get a nice transition to the flat sides of the tail section - and then not worrying about actual curved sides on the tail itself correct?

Fab would obviously be easier on this concept. Does it end up too abrupt and still create separation?

You've got the gist of my idea.

Danged great questions.

I think it would work fine. I'm working on building one between all my other projects.

I'm a bit surprised Aero agrees it should work - let him noodle on the idea a bit, and lets see what he comes back with.

top/sides/bottom
 

*Looking through my materials I've noticed that others have apparently used the 'width' as the datum for streamlining when the vehicle was 'tall',and the height as the datum when wider than tall.
*When I did my VW Bus I used 20-degrees for top and sides incorporating some beginning curvature provided by VW's original design.There was no 'diffuser',as in 1980,there was no talk of such things yet.
*NASA did the Econoline slowly wrapping the rear corners into a progressive slope,ultimately acheiving 20-degrees.
*GM's 'OPTIMUM' boat tail which appeared on the cover of Texas Tech's Engineering magazine appears to brake directly into a constant 19-20 degree slope at top,maybe same at sides (no true-length images provided in article),10-degrees at bottom,with outer edges softened with radii.
GM's tail extends back 65% of trailer height and ends at about 40% of frontal projected area of the trailer box.
No figures are given,as this may have been a proprietary project (they attempted to camouflage the tail markings) and may not have been for public consumption.
*This tail would be a lot easier to construct.
*I couldn't vouch for any numbers,but GM's got some really sharp aerodynamicists and this may have been an 'optimized' tail when looking at the entire life-cycle-cost-analysis,versus an 'ideal' tail.I'd be compelled to give it my blessing!

length vs Cd
 

You're rig will basically respond to the tail as any other vehicle,save for the exception of losses between the tow vehicle and trailer face.
In a perfect world,where there was no gap in your rig,all your wheels were flush and had covers,and everything was bellypanned,you'd essentially have a 'bus' or 'motorhome.'
At the following lengths of tail,you'd have the potential for:
50% = Cd 0.177
60% = Cd 0.153
70% = Cd 0.14
80% = Cd 0.133
90% = Cd 0.13
100% = Cd 0.13
So you can see that the last 20% is statistically insignificant.This region is 'phantom' tail,where the turbulence itself is behaving as a solid structure,with the outer flow ricochetting off of it.
--------------------------------------------------------------------------
The whole thing about the tail is that it is converting dynamic pressure back into static pressure.With 80% of it,you end up with the forward stagnation pressure very close to that of the rear base pressure,with only the unavoidable skin-friction losses in between.Which according to Hucho and others,THE fundamental premise of all road vehicle streamlining.
PS,if you'll go to the 'full-boat-tail-trailer' thread,at permalink 245 or so,there are some pictorial drag tables for different boat-tailed vehicles.Look around,you'll find 'em.

aero,
Thank you for your comments and thoughts.

Just to bring it back for perspective for me, do you have any estimates for
0
10
20
30
40 percent?????

Sorry, just to confirm here.
The % all relate to "full" length tail, which is the 1.78X relationship right? And the X is the height of the trailer in this case? If trailer is 85" high or so, than a full tail is 151.3", a fairly "healthy" length. So the 50% is then 75.5 or so long. Skinning a tail that long will be more difficult with the material I have, but I will work around it.

And speaking of other aero treatments, I was thinking of side skirts that go forward to a point behind the hitch, and also perhaps some fender
skirts on the trailer as well. Hoping to avoid a pan on the trailer that way. In the grand scheme of things are the skirts worth doing on a trailer?

Check out this post Doc Miller.

Aero Drag Reduction Potential Thread

Summary is this.
zero------ Cd 0.488
10 %----- Cd 0.375
20 %----- Cd 0.299
30 %----- Cd 0.240
40 %----- Cd 0.194
50 %----- Cd 0.162
60 %----- Cd 0.135
70 %----- Cd 0.125
80 %----- Cd 0.118
90 %----- Cd 0.118
100 %---- Cd 0.115

Here is a chart I made for myself to help quantify the difference.

I use it by looking at where I start, ie. at 30% length (3 in the left column), and I figure out how much I'm going to add on to the shape to bring it closer to the template, so if I go to 60% (6 in the left column) a change of 30%, I get a 44% improvement on the Cd (44% in the far right column).

I hope it's not too complicated. The tool helps me figure it out better.

http://i50.tinypic.com/35b6vcw.jpg

Chaz, Thank You.

I assumed the corners are rounded at the back. And I assumed 7 feet by 8 feet, for 56 square feet.
Then I went to the calculator, and typed in 15,000 pounds as vehicle weight just as a place to start.

So, I WAGged .40 for rounded corners and Cd.

I got 10.8 mpg at 60 mph, which passes a sanity test but maybe a bit high based on personal experience.

Then I typed in .177 as the 50 percent mark.

I got 16.7 mpg at 60 mph. HOLY CRAP.

So. Assume a 1000 mile trip, and assume 4 dollars per gallon.
(1000/10.8)*4 = 370 dollars without.

(1000/16.7 ) *4 = 240 dollars with.

Wow. HUGE difference!!!!

So if you look at my chart, and you went 30% longer or 45 inches, you'd gain a 51% decrease in your Cd. Not too shabby Scoobie Doo. The biggest gains happen as soon as you start adding anything.

Of course all these numbers assume you're dealing with an ideal type shape to begin with, the variables are daunting to try and accommodate, but still, even if you only got half of 50%, you're still looking great. Well worth the effort because the gains are very significant.

Chas, There is almost NOTHING more ideal then a LOOOOOONG straight trailer.

I believe we will see the benefits listed, perhaps a bit more.

As I interpret Aero's comments, he is saying the same thing. We might be off 5 or 10 percent, but the savings are HUGE.

Keep in mind that the "aero drag" part of the fuel economy equation here is still only a one part of the overall picture. Sheer engine size, large frontal areas, unsavory aerodynamic interactions between the truck and trailer, and even skin drag on such a large vehicle comes into play more. None of these items are being affected by the addition of the tail cone, so a 50% improvement at the rear, is still only 50% of maybe 20% of the equation (WAGing myself at this point) The improvements at the rear are only going to get you 10-15% better mileage tops if done properly. Driving habits are still the biggest area for improvement.

I highly doubt you'll go from 11MPG to 16MPG by slapping a tailpiece on your rig. If your definition of HUGE is 11MPG to 13MPG, and good driving habits end up getting you to 14MPG, then I guess we're talking the same language.

Donbur's experience shows that a curved trailer is better than a straight one - even at the expense of some added frontal area.
If they didn't add that extra area, the results would be better still.

Curving the sides as well as the top, would further increase the gains.

Length is limited by legal and practical issues, so loooooooooooong really can't be that loooooooooooong.

%
 

The 'Template' is based on a vehicle and its mirror-image,as if the air was seeing your vehicle and another identical to it,upside-down and joined at the tire bottoms.
Paul Jaray came up with this ground-proximity, aerodynamic relationship and Ludwig Prandtl and Edmund Rumpler developed it into a wind tunnel test protocol,which is still used today within all Computational Fluid Dynamic modelling.
So your 85" Height is actually doubled to 170",and the 'virtual' ideal tail would crash into the ground 302.6" behind the trailer.You'd cut a vertical slice away for your normal ground clearance,but it would still be really long!!!!!!!!!!!!!
(this is why I've been pushing the idea of a boat-tailed trailer who's sole purpose is to provide enough length for significant streamlining)
If you did a tail which was only 20% of the full tail you'd be looking at 60.5",which is quite a tail.
GM's tail on the 18-wheeler was 0.65-H,which would be 55.25" for your rig.
--------------------------------------------------------------------------
In Hucho's 2nd Edition book,Fig.4.14,he shows a 'angular' boat tail with 25-degree top,10-degree sides,10-degree diffuser bottom,and chamfered top edges,of 17.5% length which demonstrated a 14.4% drag reduction behind a bus-like body;which is very much like the GM 'Optimum' boat tail described above.
How's that for muddying the waters?:p

straight
 

Until we get 'curved' trailers we'll have to work with what we've got.
It might be that trailers like Haulmark and Featherlite are better candidates for mods,as they have the radius'd top edges and don't blow over in a cross-wind like slab-sided rigs.
Also,their wheels are integrated into the body better,with less interference drag as a result.
We have local beer distributors with really fine, low-profile,rounded corner trailers,no higher than the truck cab.And if you brake down,there's no risk that you'll die of thirst!