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Ecky 12-08-2014 12:21 AM

Aerodynamic trailer design
I'm starting some sketches of my aero trailer design, and wanted some feedback.

Ideally the trailer should be approximately 7ft x 3.6ft at the front and rear, 2ft high in the front tapering to 1ft in the back. The width was selected so it could sit 1.5ft behind the car and stay roughly within the car's wind shadow. I can probably widen it to 3.75ft safely.

The trailer would sit 1.5' back from the bumper, which would allow for an approximate 45 turn. Is this enough? Going wider would slightly reduce that, while pushing the trailer further back would require that I slightly narrow it to stay within the wind shadow.

I plan to use 14" wheels, same as on the Insight, but was wondering about the rolling resistance properties of trailer tires vs something like a Potenza RE92 @ max sidewall. I don't plan to ever put more than maybe 3-400lbs in it.

How difficult would it be to source an axle of the width I'd like to use? It's possible I could repurpose a trailer that's already 3ft wide, and have recessed wheel wells, but it would take up a lot of useful floor space.

Any thoughts on the shape/design? My goal is for it to be relatively easy and cheap to build, add no frontal area and reduce Cd.

Hersbird 12-08-2014 12:45 AM

Instead of a straight axle you could use a torsion axle which allows any width you want and easier to make lower, and easier to make a smooth underbelly.
Something like these

Ecky 12-08-2014 12:53 AM

Those seem rather expensive, but it's nice that the "suspension" is built in.

BamZipPow 12-08-2014 02:21 AM

How much are you looking to spend on this trailer project? ;)

Have you considered a single wheel trailer similar to my trailer project?

There are a few companies that have single/swivel wheel trailers available. ;)

Swivelwheel ECO is about $900 and would be a great platform to work with. They can handle up to 500lbs. They use a torsion suspension setup. ;)

Toy-Tote has dual wheels in a swivel wheel setup which I would love to git. They have the highest weight hauling in the swivel wheel setup. Keep in mind that these trailers are not anywhere close to being cheap. I'm not sure on what sort of suspension they are using on their setup. ;)
Toy Tote - Toy Tote

Smart-Trailers should be based in Lakeland, Florida which might be close to you.
Smart Trailers — Guaranteed Never To Jackknife!

Ecky 12-08-2014 02:33 AM

I'd like to have something usable for under $300, and I can work on the details after that. $300 axles makes that difficult, but Amazon seems to have a pair for $150, though there are no reviews on them. I can probably build it mostly out of scraps aside from the lights and axles. Good wheels can come later, as can hinges, struts, and a nice paint job. I'll do the welding myself.

Ecky 12-08-2014 08:24 AM

To keep costs down, I'm thinking I'll start with a bolted tube frame design and fold some thin aluminum sheeting around it, and I'll weld it together at a later date. Lay down some plywood on the bottom, bolt the axles to the two beams going across in the middle. Something like 1 inch tubular? Might have to beef it up on the bottom, but given my low weight requirements it might be alright. 400lbs with most of it on the middle should be safe if I use some beefy bolts.

Have some supported sheet aluminum on top with a hinge, probably two sheets so it opens like a box, and can be left open if needed.

Later on, I'll get fancy with the aero-fenders and make them into usable space.

aardvarcus 12-08-2014 08:29 AM

2 Attachment(s)
Your sketch and write-up shows the trailer very close to the car, with the caveat that the maximum angle between the car and trailer is 45 degrees. I would want more maneuverability than that, the small wheelbase of your car is an advantage to trailer maneuverability, but you will limit yourself if impose this limitation on it.

Think about being in a full parking lot and having to pull out of a parking space with cars on either side of you and across the aisle in front of you, where you have to make a quick turn to the side to get into the parking lot isle. You have to keep going straight so that you don’t drag the trailer into the cars beside you, and then cut the wheel hard at the last minute to make the turn. You might be able to swing it with a small car with a small trailer, but in a truck with a trailer you would have a hard time getting out of there with a 45 degree limit.

An important part of trailer design is deciding the theoretical load it is designed to haul. Your trailer inside width is less than 48”, so it doesn’t look like you are trying to haul 4x8 sheet goods. I would want to design the trailer with the wheels inside the body work, but that would limit your width in between the wheels at the axle. If you were just hauling smaller boxes or items, that wouldn’t be that big of a deal though.

Also, your streamlining templates don’t look quite right to me. Attached are top and side views of AST-II overlays on your existing drawings.

Edit: I think the side overlay originally posted didn't align the ground planes correctly, I think it aligned the ground of the car with the under body of the template. Notice on the new drawing posted the template perfectly follows your car. Also on the top view drawing originally posted, it was following the greenhouse of the car, but based on the height of the trailer the new drawing fit the template to the bodywork on the sides of the car. Notice how perfectly your car fits the template from this view as well. If you are trying to reduce CD, I would try to follow the template as much as possible.

Ecky 12-08-2014 08:49 AM

Much appreciated. I used the tool here on this site to overlay the template, and scaled it so the bottom of the wheels and rooflines were on the same levels. What did you use? Regardless, it looks like I messed up somehow.

Given that, I'll probably rework some of my numbers then, and likely the trailer will end up a bit less tall, probably closer to 1.5' in the front if I bring it back 6-12 inches to give a better turning radius.

EDIT: It's tempting to upsize it to fit 4x8's, but originally I was considering going as small as this:

The tail lights are only 4.7ft apart edge to edge, and the further the trailer sits back, the narrower it needs to be.

Ecky 12-08-2014 09:01 AM

What am I doing wrong here?

aardvarcus 12-08-2014 11:26 AM


I rescind my former statement, you are using the tool correctly. However the tool that you are using is using an older version of the Aerodynamic Streamlining Template. There is a newer version which is slightly more aggressive, the AST-II.

If you notice, your car (which is extremely low drag) doesn't fit the AST-I, but perfectly fits the AST-II.

I will find a link to the online version and post it here.

Edit:As promised, here are the links to the AST-II, and Aerohead's discussion about the differences between the two.

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