Cargo trailer drag estimating

[Fourbtgait]

Hello everyone.
New to the forum though I have been lurking for months learning from different posts across a wide range of subjects.
Now I realize a pre-built cargo trailer is not the most efficient shape, better if I built my own, which I may still do.
But....
Tow vehicle is a 2014 Toyota 4runner, on highway at 65 mph about 22 mpg.
In town average 19 mpg.
Both figured by pencil.
I need a small camp trailer, though a teardrop is ummm cute, not the most efficient. Thoughts are a 5' x 10' cargo trailer. Tow vehicle is 6' wide, 6' tall. Thoughts are a 5' wide, flat front (I would install a front nose cone) would stay inside the vehicle drag better than a 6'. The wheels/fenders would stick out 4" each side, about 24" tall. If I kept the trailer not over 6' tall it would "fit" better into the slipstream. But the inside height would only be about 4' 6".
Question is, is there a formula I have missed in life to look at flat front drag area? Trailer above would only "show" the wheels/fenders beyond the tow vehicle. Yes, I realize the slip stream begins to come back together behind the tow vehicle.
A better size or usable space trailer would be 6' x 10' trailer, 7' 6" tall. But then extends past the vehicle about 12+ sq. ft. exposed vs. 1 sq. ft. of the smaller trailer.

Always before I had diesel pickups, large travel trailers or gooseneck horse trailers, never worried. Now, I do not want to go from 20 mpg down to 10/12 mpg with large frontal areas. I would be comfortable dropping from 22 mpg to say 18 mpg driving 55-60 mph.

Thoughts?

[ennored]

A couple thoughts:

Unless you can get the gap between the truck and trailer very small, the trailer needs to be have the very best aero it can, regardless of what is in front of it.

Of course you could connect them:
Spandex connection

Other wise, a few threads come to mind:
Eco-RV thoughts

Aero car hauler

[Fourbtgait]

And getting that gap close will not work. Read all those stories, methods.
I have read the other two threads also.
And about radiousing the front edges, top edhes, tapering the rear top/sides (15-17*?)
Hence my question of any formula for frontal area penalty.

[Sven7]

If you bought that small trailer, added a healthy nose cone, covered and boat tailed the wheels and built a bit of a kammback/boat tail, it probably wouldn't be too bad. I wouldn't put a ton of time into it if you have the time and skills to build a properly streamlined trailer later on.

Otherwise you'll be looking at elaborate gap filling like aerohead and BamZipPow have done- it doesn't look easy!

[Fourbtgait]

I agree that the smaller trailer with slight modifications would be more aereodynamic.
But smaller is not always practical for some people in a situation.
Hence my question of a methodology/formula for establishing the loss of or increase of aereodynamics for a given frontal area.

[Sven7]

That's true; you're not going to be happy with a trailer that doesn't fit your needs, or one that wastes fuel. I'd say get one that you can use, then aero-mod the crap out of it! The amount of work you want to put in is up to you. Trailer aero is not well documented, so you'll have to do some designing.

[aerohead]

Originally Posted by Fourbtgait

I agree that the smaller trailer with slight modifications would be more aereodynamic.
But smaller is not always practical for some people in a situation.
Hence my question of a methodology/formula for establishing the loss of or increase of aereodynamics for a given frontal area.

Hucho offered a drag table from D.M.Waters' research,presented in Proceedings of the 1st Symposium on Road Vehicle Aerodynamics,London,1969,Paper 4.
*Waters shows the drag of a flat-faced trailer,tow vehicle combination with Cd 0.82.
*By softening all the leading edges of the trailer face,the composite drag coefficient drops to a minimum when the radius used is equal to 0.39% of the square-root of the frontal area,achieving Cd 0.546.
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If you had a topper on the Tacoma that possessed a height and width equal to the height and width of the radii origins,the trailer would basically train behind the Toyota,and the radii would get the flow over the difference.
This would make for nice drafting.

[freebeard]

Hence my question of a methodology/formula for establishing the loss of or increase of aereodynamics for a given frontal area.

The formula is Cd x A = CdA. Establishing Cd is a black art.

Maybe what you are looking for is A[sub]tow vehicle[/sub] / A[sub]towed vehicle[/sub] x [fudge factor]. I don't know about that, as long as it's greater than 1 you should be good.

For inspiration you could compare the classic Airstream to the Airstream Basecamp. It is a very utilitarian vehicle reminiscent of the Rumpler Tropfenwagen.

[Fourbtgait]

Thank you aerohead and freebeard.
Yes, I figured my question was on the edge of the black arts.
If I were to buold, it would be along the lines of the basecamp.
Ease of a cargo trailer is a factor, though curious of the factor of frontal wall 6' wide 1'6" tall sticking up above the tow vehicle.
I have given thought if I went with total 6' height, to cut/weld a lift top to increase inside height.
I do know from several conversations with people that the Chalet/Aliner trailers typically only drop mileage 2 mpg. Due to folded height way below the top of most tow vehicles though just under 7' in width. Even used ate pricey.

[freebeard]

Curiously, the Basecamp is similar to Wally Byam's first trailer:



Once you go expandable, the sky is the limit.


https://www.pinterest.com/pin/279715826829704846/