Building an Aerodynamic trailer for VW Jetta

[bentring]

It's been a long time since I've done anything but lurk here but I'm finally getting tired of dealing with the elements and am getting serious about enclosing my scaffold trailer for work. This would be so much more simple if I still had a hatchback, but I really enjoy the Jetta. I'm looking for help with the basic shape of the enclosure. Everything will be built using sheet metal of some fashion and probably skinned later for appearances. I can build and test with easier to work with materials, since nothing is permanent yet except the base of the trailer.

I would prefer to not have to make major modifications to the rear of the car and rather adapt the front of the trailer to help with the air leaving it by way of something similar to a tongue toolbox eventually, but for now I'm more focused on the shape of the box itself. I assume the ideal thing would be to make something similar to a teardrop trailer using the template, but I'm open to ideas. I do have the option of shrinking the width of the rear sidewall of the trailer as well as the height but not at the base, and could only lean them in a few inches but I'm undecided if the extra effort would be worth the trouble when building the rear wall/doors.

I've attached a pic with the car and trailer with the sidewalls that will be cut to shape. I'm willing to lower the top of the sidewalls a bit, but I sometimes carry multiple scaffolds that stack about as high as the trunk so it couldn't be by a lot.

[oil pan 4]

Do you have to have a trailer that big?
I picked up a cheap trailer that pulls behind a motorcycle for $125. It's so small it should easily be able to hide in any vehicles aero wake.
I will be towing it behind my leaf any day now.
The leaf is very sensitive to any additional drag so I will know right away.

[aardvarcus]

I assume by scaffold you mean Mason scaffold with five-by-five frames or similar. I would suggest streamlining the trailer independently from the car if you are not willing to make any modifications to the car. Hard to tell in the picture, I'm imagining you're laying the frames down flat.The length of the tongue being what it is, I'm not imagining close coupling of the vehicle and trailer is really an option. I am also guessing that trailer is wider than the car by a decent amount. I'm not sure how many highway miles you put on this combination to make aero mods really worth it, But I would consider a lid hinged off of the tongueWith a frame that coveredThe entire trailer including fenders in a teardrop style shape. If you're just looking to keep the bulk water off the scaffold I don't think you need to go fully enclosed, a simple lid would go a long way.Skin it with cheap 29ga sheet metal from a roofing company, Buy it without the rolled profile. If you need something much better than that, you might be better off trying to find an appropriately sized already enclosed trailer.

[bentring]

I will get some better pictures of the car and trailer posted, but the scaffolds are interior baker style that when loaded on the trailer make a box about six and a half ft long, three feet wide, and about eight inches tall. I've strongly considered an extremely small swivel wheel system that would only hold scaffolds, but I often have to carry a minor amount of material, or other equipment that won't easily fit in the back seat. The trunk is always filled with tools, and some of them will live in the trailer, as well as a ladder, push brooms etc when it's completed.

I've been using this car/trailer combo exclusively for almost 4 years, and with normal driving does pretty well. I've never had the opportunity to compare full tanks with and without the trailer, but I'm guessing it robs around ten percent when loaded. A real world comparison that comes to mind is on a vacation trip with the wife, I was able to (no trailer, car was empty, and by the cars info) drive at an average of 70 MPH for an hour and it read 40.1 MPG, and on an hour and a quarter drive to a job (three adults, trunk crammed with tools, trailer with three scaffolds) drive at an average speed of 66 MPH at 37 MPG. I've tried recreating the loaded scenario and have come within a couple of MPGs, but I suspect the loading of the scaffolds just happened to be perfect for drag that day, or just a good tailwind lol.

[bentring]

Hopefully a couple of other angles will be easier to make out what I'm working with. I've considered things from just leaving it flat with minimal siderails with a tarp to something like a utility truck bed by enclosing the tires and making the whole thing follow the suggested template with a removable lid. The only other regular use this trailer has is taking off the garbage, because I don't have a truck.

The trailer measures 7ftx4ft at the base, and from level ground the top of trunk measures 42 inches while the top of the trailer sides are at 52 inches. The mocked lean at the rear measures 48 inches at the base with a slope to 36 inches at the clamps. I only have about 28 inches from the inside of the base to the top of the sides if I left it all square, but I'm willing to lose a few due to sloping.

The base is stainless steel from a job, and it's installed on the bottom lip of the frame for streamlining underneath, which was a fun afternoon... The large sidewalls are the aluminum sheet base when I bought the trailer used, and the permanent sidewalls are 10 inch galvanized heavy gauge metal studs. I'm planning to use the same studs for fabricating a rear frame for access, as well as the wall/roof support and other stainless as the top and front.

I'm planning on a tailgate type access panel or maybe french doors for the rear with a minimum size of 36 inches wide by 24 inches tall. I've considered a clam shell lid or installing the access panel on the sloping roof but those would probably increase the complexity of actually loading and unloading for regular use.

I'm assuming a proper curve rather than an angle would be best for the front of the trailer where the air leaves the car and meets the trailer. I'm willing to lose a bit of space between the car and trailer, but I can currently back the trailer up considerably farther than 90 degrees, and it's quite useful in tight locations.

[aardvarcus]

Honestly only losing 10% and being at 37mpg with a loaded trailer is extremely good already. I would personally refrain from adding height to the front just to allow slope (unless you need that wedge of space) because you are trying to minimize both cd and area, adding area just to slope may not be a net benefit. I think the gap is too great to assume close coupling. I assume your extra tools travel beside or on top of the scaffold, accessed from the back. I would consider making your box a squared shape, as low as reasonable to fit your supplies. Then you could put an add on to the front that adds radius to the trailers leading edge. And maybe a trailer tail type add on to the rear door, which could flip up to open out if your way. You could do an add on to add radius to the front of the fenders, and run the back of the fenders sloping in to the rear of the trailer. But given how good your mpg with the existing trailer are, I wouldn't fret about it too much.

[oil pan 4]

It's so long there's not a whole lot you can do beyond making really slick wheel covers and rounding corners.

[jakobnev]

I would prefer to not have to make major modifications to the rear of the car...

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

[bentring]

Thanks for the replies. I agree with just rounding the corners and keeping it simple, but it looks like I won't be doing a bit of guesswork and hope for the best. I was explaining what and why I was doing to the youngest and he's been enjoying brainstorming and problem solving methods for adjustable temporary shapes. He's looking forward to tuft testing.

[freebeard]

It's good that you have an accomplice.

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



With your sidewalls, you could reduce it down to a ¼-circular arc. Hoop and canvas or metal.

I'm planning on a tailgate type access panel or maybe french doors for the rear with a minimum size of 36 inches wide by 24 inches tall.

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