Project: Aero Trailer for TDi
 

I wanted to take a few minutes to share what I've been working on with Outrider, the company I work with.

We have a 8000 mile summer tour, and decided to make an aerodynamic trailer to carry our ultralight adventure vehicles and compliment the TDi Sportwagen that's pulling it.

This project has a few goals:
1. Maximum possible aerodynamics
2. Easy Access to UAVs
3. Weatherproof

We decided to use a utility trailer as the base, and add an aluminum shell that tapers the bodyshape of the Sportwagen that's pulling it.

We used aluminum tubing as the skeleton, and then covered it with thin sheetmetal.

Here's what we started with, a 5x11 utility trailer with Ipe decking:
http://img.photobucket.com/albums/v2...n/CIMG2714.jpg

The ribs in progress:
http://img.photobucket.com/albums/v2...n/CIMG2743.jpg

Finished ribs:
http://img.photobucket.com/albums/v2...n/CIMG2788.jpg

Aluminum sheet added:
http://img.photobucket.com/albums/v2...n/CIMG2826.jpg

Shell partially opened:
http://img.photobucket.com/albums/v2...n/CIMG2822.jpg

Shell fully opened:
http://img.photobucket.com/albums/v2...n/CIMG2807.jpg

View from the rear:
http://img.photobucket.com/albums/v2...n/CIMG2831.jpg

The vehicles we're carrying:
http://img.photobucket.com/albums/v2...phaOffroad.jpg

Our maiden journey with the new aero trailer was from Asheville, NC to Vail, CO on a nonstop trip to show at the GoPro Mountain Games.

For reference, the car is loaded with three 180 lb men, and about 200 lbs of gear. The trailer has a gross weight of about 1200 lbs with two Outriders, camping gear, and tools. We started out cruising at about 65 mph average, and we were averaging around 38 MPG. Then we realized we were running late, and we upped the average speed to 75 mph, dropping our mileage to 33 MPG average for the 1500 mile trip.

The trailer worked great. We have a few improvements in mind, however, to further improve aerodynamics.

After arriving to Vail, we've changed from a 2" rise hitch, to a 1" drop hitch to better align the nose of the trailer with the rear end of the car. We haven't yet collected numbers from this setup, as we've been in the mountains and it's difficult to quantify, but it's clear visually that this will improve the aerodynamics further.

We're now in Boulder, CO. When we arrive in Park City, UT, we plan on adding a spandex skirt that stretches from the vehicle to the trailer, filling the gap between the two, and keeping aeroflow attached over the whole shape. I imagine we'll see good returns from this modification.

Our overall goals are to see over 40 MPG on the return trip back across the country to NC. I imagine we'll hit that number, with the addition of the skirt, the better aligned trailer, and reduced speeds on the return trip.

I'll soon add photos of the trailer with the shell fully opened, a shot that shows the improved alignment after the 1" drop hitch, and some detailed shots when we add the skirt.

We had one of our coworkers at Outrider ship us our sewing machine, spandex, laminX, and industrial velcro. We plan on sewing up the skirt in the woods of Utah, powering the sewing machine off of an inverter that runs off the batteries on one of our Outrider vehicles. Should be a trip. :p

Cheers!
Tommy

Very very cool
I keep saying I want to build a similar setup for my TDI.

In this sense, a spandex skirt is very logical, but I keep shuddering because I cannot prevent clothing imagery.

I probably should have kept that to myself! :D

Great work!

Nice project! The spandex bridge between the car & trailer should help a lot, since it appears (am I wrong?) the leading edges of the trailer have no radii.

Fun looking trikes too! (I have trike envy.)

Added to the front page.

Nice project. Really looking forward to following. Subscribed!

PS- since not unmanned aerial vehicle what does UAV mean in this case?

How tight will the spandex be?

It makes the TDi have the capacity of a truck, with the efficiency of a TDi. We're loving it.

:D That is indeed fascinating imagery! :D

Thanks! There is essentially no radii on the leading edge, so hopefully the spandex does work out. Thanks for the complements on the trikes, they are a ball.

Thanks Bob. UAV in our case stands for Ultralight Adventure Vehicle.

We're going to make it as tight as it can possibly be, without it ripping itself off the car around corners. We'll lay LaminX on the car first to protect the paint, then 3" industrial velcro around the whole rear edge of the car, over the LaminX. I expect it to have pretty strong holding power, so provided the spandex has enough stretch, we should be able to get it pretty tight.

Before and After the Hitch Drop
 

Here's a shot of the alignment of the trailer with the 4" rise hitch (the setup that returned 33 MPG at 70+ mph on our 1500 mile trip):

http://img.photobucket.com/albums/v2...n/CIMG2883.jpg

And here's a shot of the trailer alignment with the 1.25" drop hitch (much improved, but yet to be quantified):

http://img.photobucket.com/albums/v2...MG2875crop.jpg

This means we've dropped the nose of the trailer 5.25", significantly reducing the frontal area of the trailer. I'm excited to see what numbers it returns on some flat roads where we can get good numbers, we might get a flat stretch on our way to Park City, UT tomorrow where we can collect some good data.

Cheers

Another option (until you get that gap-filler / bridge) is to affix a half-round shape (border) around the leading edges of the trailer. (EG. something like pool noodles, or PVC pipe cut in half lengthwise.) You'd gain a flow-friendly transition from the forward face of the trailer to the sides, and the shallow "pool" that would create on the face won't harm its effectiveness.

I think you will find that the Spandex will stretch significantly when it inflates/deflates according to the air pressures being exerted on the fabric. My thinking comes from my 1985 Toyota Celica convertible top inflating at the top edge of my roof line when traveling at speed (anything over 40mph). The material my top is made from is black Cambrio cloth and is pretty thick along with support spars to keep it in place. My top will rise in the front about 3" when traveling at speed. Something to think about before you put a lot of effort in yer gap cover.

You could try some testing of the fabric by taping it down with some duct tape before you do all the fab work and do a few test runs. ;)