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. ;)

As an additional intermediary, you can add some vertical partial baffles as a vortex trap, to reduce cross winds. Similar to semi trailer application, though probably needs to be scaled to fit better.
https://encrypted-tbn1.gstatic.com/i...XmQ7arq9GFm9ig

Nice project!

nice work.
 

Very clean. I like it :)

Numbers on the way to Utah
 

Update from the road with nose of trailer dropped, no skirt yet:

Today's Trip distance: 466 miles (Boulder, CO to Park City, UT)
Average Speed: 58 mph (with speeds jumping to 65-70 mph occasionally)
Terrain: Good bit of climbing and descending.
AC On
Average MPG: 35.2 mpg

We're pretty happy with this result, I'd love to see 40 mpg after the skirt/gap filler. If we have enough energy tonight after this long drive, we'll be stitching up a skirt at the campsite tonight. We'll then test it tomorrow on our 750 mile trip to San Francisco.

Update soon.

Cheers,
Tommy

from a friend who was out camping in UT, if you have alcohol at the campsite skirts will come to you. wait.. wrong skirts... nevermind. :o

watching how this turns out since I am hoping to do a trailer similar to what you have behind my Jetta TDi. I was thinking of rounding or angling the front corners more tho.

:thumbup:

The front edges should definitely be radiused if you're not going to go with some skirt, we just ran out of time for the skirt before we left.

Update: Still no skirt.

Today's Trip distance: 200 miles (Flagstaff, AZ to Thoreau, NM)
Average Speed: 55 mph
Terrain: Rolling
AC On
Net Elevation Change: -400 ft.
Average Elevation: ~6700 ft
Average MPG: 43.0 mpg!


Thrilled with the numbers today, we finally exceeded our goal of 40 mpg for this project.

We caught up with our package in Flagstaff that contains our Spandex, sewing machine, LaminX and velcro for making our skirt/gap filler.

If the skirt doesn't work out, we'll do some sort of pool noodle setup to radius the edges, I like that idea Metro!

If things go well stitching up our spandex skirt at the campsite tonight, we may leave our campsite with some type of gap filler tomorrow!

Cheers

Successful Skirt
 

Last night was an interesting night.

We rolled into camp last night around 6, probably the most reasonable time we've hung up the keys for the day yet on our trip. We settled in at Bluewater Lake State Park campground, outside of Albuquerque, NM, a good sized lake held at bay by an even larger dam.

The place was a real party with RV's surrounding the lake, smoke rising off of charcoal grills, and some guitars pulling off some cool riffs. The cold water was calling as we were all parched from the dry desert air.

We took a dip, then started on the skirt. We powered our sewing machine with the 44 volt lithium battery on our Outrider 422 Alpha. Plenty of power to run a sewing machine for days. The 44V is inverted into 110V AC, then fed into the sewing machine.

The Pain of the project was the elasticity of the fabric, making it difficult to mock up and measure, combined with the extremely sticky adhesive on the industrial velcro, which did a great job of gumming up the needle and repeatedly breaking the upper thread. It was good practice of threading the machine for me, something that I probably did almost 50 times last night.

Our strategy with the fabric was to stretch it as tight as humanly possible in every direction, to minimize wrinkling, and maximize tension to reduce flapping in the wind. We'll just remove the skirt on exit ramps, and reapply it on entrance ramps, so as to not overstress the stitches on tight turns. This setup is primarily for our long tours anyways, and will have minimal benefit around town.

It was a three man job, Dan and Jesse stretching the fabric while I stitched in the velcro.

We used LaminX as a base coat on the car, and then attached the hooked velcro to the car on top of the LaminX, so the Velcro's adhesive doesn't damage the car's paint.

This system seems to work very well, as we can put a ton of tension on the fabric, and it stays firmly attached.

I triple stitched everything, and threw in the towel at 4AM with Jesse, leaving the finishing trimming and hemming the lower edges for the morning.

We got it all mounted up, and pulled tight, and there are really no wrinkles to speak of in the fabric. Flapping in the wind is minimal, much less than I expected, mostly because of the very high tension we put on the fabric.

Here are the shots with the skirt mounted:
http://img.photobucket.com/albums/v2...n/CIMG2933.jpg
http://img.photobucket.com/albums/v2...n/CIMG2935.jpg
http://img.photobucket.com/albums/v2...n/CIMG2938.jpg


I think it actually looks pretty neat. We're about 100 miles in for the day so far, and the numbers look very promising. I don't want to post them yet, as we were decending a bit. I'll post up the numbers at the end of our driving today, we should have a 300+ mile day to pull data, and we'll be using cruise control almost all day, so they should be pretty reliable numbers.

It looks like the skirt is going to make a significant difference.

Cheers,
Tommy

Since the fabric is stretched super tight...do you have any problems when you make turns? ;)

I think it's most likely that the velcro would detatch itself around a tight turn, but it's hard to say.

Pretty much all of our driving is I-40, for thousands of miles, so we're just detatching the skirt on the exit ramps, and reattaching it on the on ramps.

I'd rather avoid testing it so I don't have to pull out the sewing machine again, I want at least a week to go by before I have to run it again. :)

Here are the numbers from the driving so far today:

Today's Trip distance: 335 miles (Near Albuquerque, NM to near Amarillo, TX)
Average Speed: 55 mph (cruise typically set at 58)
Terrain: Rolling
AC On
Net Elevation Change: -2000 ft.
Average Elevation: ~4500 ft
Average MPG: 45.7 mpg (with skirt attached)

Definitely an improvement, we were seeing a solid 50 mpg for a good while today, then we pulled into a Albuquerque and did some idling and tooling around town. Had to have the AC running when parked because it's blazing hot.

Pretty incredible numbers considering we have three people in the car, 200 pounds of gear, and then about 1200 lbs gross trailer weight. All in all, I'd estimate our total payload at about 2000 lbs, we'll try to get on some scales. I'm super happy with the result!

We're planning on putting in 800+ miles tomorrow. I'll post an update tomorrow!

Cheers,
Tommy

How about your fuel economy east of Albuquerque?

Tommy, absolutely amazing. This is the kind of thing that advances the science. Keep it up!

Amazing results. I'll keep an eye peeled for you guys to come through Knoxville. I probably won't be able to break away to say hi though, I have volunteers to run.

east of Albuquerque
 

When Pop Sci did their San Francisco-to-New York economy run in a Geo Metro LSi they got their single best tank between Cheyenne,Wyoming and the town of Goehmer,about 500-miles east.
It will interesting to see just where the 'sweetest' tank turns up.

Any updates on your trailer? I am in the process of putting together a similar setup for my 2004 Jetta TDI Wagon for an RV / Teardrop trailer based on a Harbor Freight 1720# load frame. Have you done any permanent work on the skirt? What is your MPG without the trailer? I saw a big racing hubcap, then it was gone... any experience with that?

PS I didnt see any dimension... how much space under the lid? Height interior, total height with trailer... front height rear height... width... rib materials? Any information would be useful.

4862forestgrove -- You visited the O.P.'s Profile page, did you notice this?

Need to throw in a caution about trailer tires. Next to none are rated for above 65-mph (unless in the fine print accoriding to several conditions met). An upgrade to a decent tire/wheel size and best quality brand is highly recommended for any trailer of any sort. I would not want to put Mile One on an HF trailer with original, as-purchased tires.

Weigh the trailer, with as close to travel condition load as possible. Weigh across axle, and weigh each separately on a public scale. One wants to see -- in worst loading -- 15% "reserve" in loading. IOW, a tire rated at 1,840-lbs ought to have on it no more than 1,564-lbs.

All posts by tire engineer CapriRacer are to this end. An HF trailer, for example, has the most minimal tires possible . . and an expectation that it is hauling a riding mower twice a year for service. Across town, not the kind of miles some of you are contemplating.

Wow nice rig, low n long! And a bike hauler, sheesh! I gotta say nice metal work, and very creative gap filler... you're on to something there

Would be nice to know more about this one. Was reminded of it earlier that a diesel TDI can pull an Airstream up to about 20' at over 30-mpg. Similar speeds to above. I don't know what the upper end is for the car, but this rig is just getting started.