Building an Aerodynamic trailer for VW Jetta

[bentring]

Originally Posted by jakobnev

Maybe a transparent piece about this shape that sits on top of the trunk lid and reaches about half way up the rear window

I've thought about maybe a lip on the trunk edge but I would be worried about making things worse overall if I interrupt the air flow on the rear glass. I think it's attached pretty well going by how quickly rain is removed when driving.

Originally Posted by freebeard

It's good that you have an accomplice.

Here's my solution for a trailer but not with a rectangular plan...

Split tailgate? That would lend itself to a boat tail or box cavity.

The accomplice is useful for keeping me from shortcuts as well as giving an extra hand. I've seen your trailer pics multiple times, and it's been influential in my thought process, as well as many other projects here such as the T100 from BamZipPow. I plan on having at least a drip edge box cavity in the rear, and we're fairly sure we can taper the side walls (other than the 10 inch base) to help create somewhat of an overall shrinking profile.

Once my son got interested this turned into a full fledged experiment, lol. He's decided to work with me after the upcoming school year is over and he's gaining skills that will help with that, so explaining and learning is as time intensive as working. Good times all around though. On to a couple of progress pics.

We picked up a few sheets of floor underlayment to make templates with. Made a couple of smaller temporary sidewalls with one and we let some water and the sun do work forming a general curve we can start with. We've decided to make three or four quite different curves and positions to test and we will use the one that works the best and tweak it until we are satisfied then we'll move to doing it again with metal.

[aerohead]

If the amount of use and distance traveled justifies the cost, a contemporary, best-case commercial product for ideas would be Peter Brock's 'Aerovault' trailer, @ bre2.net/aerovault.info.
Peter designed the Corvette Sting Ray and Shelby Daytona Coupe.
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Also, U-HAUL trailers utilize aerodynamics researched by NASA in the 1970s. Their fenders suck, but Brock's can be an example of a solution for that.
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AeroStealth did a couple of You-Tube videos on trailer aerodynamics based on the NASA data.You might search for John Gilkison Aerodynamics videos.
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Ideally, the trailer and Jetta would be the same frontal area where they meet, with no gap ( articulated buses and railcars ), and then boat-tail from there back. And then reality intrudes!
You're the only one who knows what you're willing to put up with.
Make sure the pain justifies the cure.

[freebeard]

aerohead isn't big on hyperlinks.

https://www.bre2.net/aerovault.info/

https://www.youtube.com/channel/UCWl...PkSNS9oWkMoMXg

[jakobnev]

I've thought about maybe a lip on the trunk edge but I would be worried about making things worse overall if I interrupt the air flow on the rear glass. I think it's attached pretty well going by how quickly rain is removed when driving.

But you don't want the flow to go down with the back of the car and end up in front of the trailer, you want the flow to stay up and go past the top of the trailer.

[JulianEdgar]

Originally Posted by freebeard

aerohead isn't big on hyperlinks.

https://www.bre2.net/aerovault.info/

That's a lovely looking trailer.

[freebeard]

"Your book is unique! It was high time that someone covered vehicle aerodynamics through the practical eyes of someone like you." - Dr Wolf-Heinrich Hucho, the founder of modern vehicle aerodynamics

[bentring]

Planning to tuft test the basic teardrop shapes in the next few days, and still haven't totally decided if we'll use a tailgate or clamshell/hatchback style system.

Originally Posted by jakobnev

But you don't want the flow to go down with the back of the car and end up in front of the trailer, you want the flow to stay up and go past the top of the trailer.

Yes, I understand that, and I've considered aftermarket rear window spoilers before but didn't find any that were proven to be beneficial at the time. The car is fairly slippery stock and I didn't want to introduce things discussed in this thread. I'm happy enough with the car being stock for now and am focusing on the general aerodynamics of the trailer since the tow vehicle could (and likely will) change in the future but I'll always use the trailer at times.

[bentring]

The weather has kept me in the garage and my help has abandoned the project so progress has been slow... It's been interesting but determination, ingenuity, and clamps are a solo worker's best friend. I've gotten the aluminum sidewalls cut to shape and attached to the side rails and have the first sheet of stainless started on the front.

[JulianEdgar]

You do not want that sharp front edge! With it, it is quite likely that flow will be separated down the side of the trailer, increasing drag.

The sharp edge needs to have a radius. I don't think the radius needs to be all that large (50mm?) but without it, drag is very likely to be much higher than it would otherwise be.

If you want to avoid my guesswork (always a good thing) tuft test the front and sides of the trailer as it is now, before doing any further work.

Addition:

I've never really thought much about trailers, but if you're happy with fabrication, you could make a temporary coupling with a spring within it, that extends with the aero and rolling resistance of the trailer. (Obviously add a safety device so that if the spring fails, the coupling doesn't come apart.) Add a pointer and scale and attach a video camera or similar. Then on a flat road at a constant speed, you could directly measure changes in aerodynamic drag in real conditions.

Further addition

You may well have your own ideas about how to form such a radius but I'd probably use thin wall steel or aluminium tube and roll it to the curve, or uPVC plastic pipe and heat/bend it to shape. In either case, I'd then rivet or glue the panels to the tube, so forming the radius. (Some fibreglass cloth and resin could then cover any 'steps' that developed.)

Or, if you have a bead roller (my new toy, thanks to prompting from Freebeard), you could roll the radius'd curve in aluminum or steel using a urethane lower roller and curved upper roller.

[freebeard]

Thanks. I thought I coincidentally mentioned it as it was about to arrive.



I suggest roll the edge to 45° and then add a 90° flange. It leaves a seam in the middle of a 90° edge. (The sample was Polymetal so it burst the outside of the bend)