Teardrop trailer aerodynamic advice
 

Hi all. I am new to this forum but have spent several nights reading old posts. I am building a teardrop trailer and am in the design / materials purchase mode. I will note several of the choices I have made so far and materials I have acquired, and then pose some design questions.

* I have read here that the traditional teardrop design is not really that aerodynamic, so I am trying to design in aerodynamic features that are not associated with a traditional teardrop as seen in the TNTTT forum.

* I will pull this trailer with a 2004 Jetta TDI Sportwagen.

* I have acquired sheets of 5052 aluminum sheet with a clear anodized gloss finish that is 12 feet long that I will use for the skin. I hope to attach this to the frame using 3M double side ultra tape to minimize rivets.

* I have acquired a Harbor Freight trailer with 1720# capacity that I will stretch to be 4x10 foot frame. I plan to skin the underside of the trailer with a sheet of continuous aluminum hiding the axle within the skin and place the axle on top the springs to lower the profile and reduce area under the trailer.

* I plan a Kamm back design using your AST template for the top surface, running out to the 50% to 60% bulkhead.

* The front of the trailer will lift up, hinged at the rear to increase headroom and interior space, while minimizing height while traveling. (Picture a car hood or hatchback.)

Now for the questions...

1. ? For the front of the trailer profile, I am thinking of several options.

a. try to recreate the shape of the rear of the Jetta, making the front of the trailer concave to try to match the convex profile of the rear door of the Jetta and matching the shape of the Jetta as seen from the rear. (although I wouldn't be able to run the trailer THAT close to the rear of the Jetta.

b. make the front traditional with a rounded front like the leading edge of a wing and the sides of the trailer flat (very easy to construct with flat panels.)

c. use the superellipse or square half body plans documented by freebeard... (the square would be much easier to construct.)

d. try to reproduce schlorwagen shape and raise the top half of the trailer with pop up trailer jacks.

2. It there that great of a benefit to round / slope the trailer sides from bottom to top or are straight sides acceptable?

3. How great a benefit are rounded edges at the corners.

I thank you in advance for some discussion and apologize for my newbie questions :-)

Once I am further in the design process and make some decisions I will post some drawings and pictures...

Of course a fully rounded nose and aerodynamic shape would cut Cd. And making it five feed wide with wooden extensions to HF trailer makes it more comfy for two but also sticks out wider and increases Cd. I'd still make it five wide.

I guess the low profile and tucking into the cars slipstream is the real drag reducing design of the teardrop. The aero side profile is perhaps more style than substance apart from that. some people even put luggage racks on top of them adding lots of braking effect;-0 There are several builder forums you might want to Google for advice and ideas.

It sounds like you're onto a good start.

Yes, I am going to re-post this to the TNTTT (Teardrop Trailer) Forum. Thank you for the advice... BB

I don't know if you saw this thread: http://ecomodder.com/forum/showthrea...tml#post383403

I mention it reluctantly, because I haven't updated it in 2 weeks; but a lot of what I had to say is there. There's a lot I haven't touched on, though:

• curving corrugated materials
• kitchen cabinets on 4-bar links
• telescoping canopies and pop tops
• fiberglass-rod-and-fabric teardrop fenders

As to you questions; I'd go with a blunt nose making it generic instead of tying it irrevocably to one tow vehicle. The superellipse, in case it isn't apparent, is achievable with the 'Roswell' design.

Your Harbor Freight trailer frame is twice as heavy as a purpose-built frame and you may not have thought through the implications of burying a live axle within the floor.
You need to get 5 posts in before you can post pictures.

My Harbor Fright supposedly weighs 272. Off and on I think about putting a removeable streamlined bolt-on shell on it, for camping or hauling stuff or flatbed use.

I did see the thread, but just scanned it. I am trying to digest some of the 12 billion posts here :-)

I have done several "thinking drawings" one of which superimposed the template over the top of my Jetta. The ground to the top of the frame with 1" of flooring material with the lowered trailer measures 18". The ground to the top of the template just in back of the Jetta measures 54". This gives me 36" of trailer height (not much) so I did plan on some sort of pop up. With the taper of the template I get 18" in the back of the trailer. If I telescoped the whole thing I get 72" in the front and 36" in the back.

I have thought of attaching 3/4 plywood to the bottom of the trailer frame members and have a "sunken space" which gets me back 4.5".

I have also thought of fairing at least the top of the Jetta / Trailer unit to maintain flow as I have seen in other parts of the Forum.

I plan on doing a little welding and reducing the weight by eliminating some of the frame cross members in order to reduce weight.

The axle would not be attached to the bottom of the trailer if I enclosed it. It would float in the open space. Are there other implications of that technique that you had in mind?

The body would be wide enough to enclose the outside of the tires / fenders at that point.

by blunt do you mean like a rounded wing leading edge as in a standard teardrop trailer or squared off in the front with a flat panel like a cargo trailer?

Kitchen cabinets on 4 bar links??

BB

Attached picture of the Jetta with the template laid over the top. Height at first dotted line from ground is about 54-58". Distance between "bulkheads" is about 5 feet.

BB

Just ran onto this trailer thread for a TDI wagon and am loving it!!

http://ecomodder.com/forum/showthrea...tdi-26132.html

BB

Subscribed. Looking forward to visuals to watch where this is headed.

To start with, here is a picture of your wagon with an aerotemplate in a debatable placement due to the fact that it is a wagon.....no real hard & fast rule for where it should go. I'm putting it here because it seems if I move it forward, the back of the roof pokes into it and I have no idea how you're supposed to recover after that.

Anyway, this is close to optimal if you were going to build a boat tail on the wagon, which you're not, so in essence, this is a fairly meaningless image. I just wanted to show what the ideal thing to do in the back would be.

http://i39.tinypic.com/5luzgh.jpg

Since there is no practical way to build a trailer that would be livable in the space within the template and also because it would not be close coupled thereby having a fair gap, you really need to think about the front shape of what you're towing a lot to minimize its parachute effect on your aerodynamics. And of course, the rear of the trailer will have a good impact as well.

If I were to do something like this, I would look into making a hard pop-up version of something that was very aero and low while towing, but could be expanded easily and quickly when it was time to set-up camp.

So here's what I got for you, this was fun.

http://i42.tinypic.com/1y0bhg.jpg

Much Bigger Version Here

I envision a solid box (gold dash) under a aero shell (Dk Blue) when moving. To set up camp, lift the front, pull out gold solid part and reattach, (Red) Then lift the back to attach to gold dash, (Green).

Lift, slide, lift, camp. I figure you got about 5 feet of head room and 10 feet of length all under solid sides. A door could be engineered between the front of the wheel and back of gold solid giving you about 6 feet in back for the bed/dinette and 4 feet forward.

Have not given this any thought in plan (top) view, I just worked out the basic shell and hinge point dimensions (Dk Blue Arc Line) here.

It's a start of something I suppose.

You really need to consider break over and under angles here or you'll constantly be dragging the back or front of your trailer with any kind of ramp angle you encounter. Aerodynamically, I don't think there is any significant advantage to "Low as you can go" on a trailer behind a vehicle. It could be argued that too low might hurt aero drag. I'd be very careful on the ground clearance of the trailer, or build in some extreme skid plates while engineering the trailer to not fold in half when suspended by both ends.

ChazInMT demonstrates canonical use of the template.

Maybe save that for v2.0. That's a can of worms.

So you see the problem.

Where is this open space? Is it between the top of the deck and the bottom of the aluminum skin? I'd be looking at independent rubber torsion or elastomeric snubbers.

Half circular or V-shaped. I'd look at the wake from your Jetta, and fill it, e.g. wide and low. A bluff forebody will let it reattach the turbulent flow off the tow vehicle.

Hold that thought. I'm trying to spend a few days out of town before August is gone.

Created the following attachment today...

1. I decided to do out and measure actual values on my Jetta today rather than relying on computer pictures and estimated curves. I measured the peak of the roof line to be back 48 inches from the back of the Jetta. Maximum height at that point was 56.75 inches. Drop to the back was 1 inch. Height at back was 55.75 inches.

2. I used these values along with the xy coordinates of the AST II template to scale a spreadsheet graph and then fit an image of the Sportwagen under the curve.

3. I generated a scale factor to create the rest of the xy coordinates.

4. The top of the trailer frame is 17". The frame is 3.5" channel, so the underside is 13.5". The A frame tongue is 42" from the front of the frame to the hitch.

5. Using these constraints, I generated a trailer body at 8', 9' and 10' lengths of the frame, fitting close to the back of the vehicle

6. The top will be a simple lift of back first... block, then front... block. I'm thinking a simple pull out block (shelf) once the trailer body is lifted.

Thoughts so far?

BB

thoughts
 

*The 'full-boat-tail trailer for T-100' thread may have some pictorial drag tables which will help you think about your project.Also the half-body charts at 'AST-C additional drag tables' thread.
*The body drag of your combination will be factored from the new base area of the trailer's transom area,plus a penalty for the car-trailer inter-spacial gap interference(look at the Clark-Y wing with various gap combinations),and then the wheel/tire drag superimposed back onto the body (call that portion Cd 0.05 ).
*If the trailer nose is unshielded,make it as complex as you can stomach (Airstream's design is very good).This will help in crosswind.

Thanks for the input... could you take a quick look at the attached drawing of the treatment of the front radius and tell me which your intuition favors... I believe there are equally easy to build. Thanks in advance. BB

which
 

I would recommend the convex nose.If you never do gap-fillers you'll still be protected in crosswinds.
If you eventually do gap-fillers,you'll have a proper ball-and-socket articulation with the air shingling down the filler panel 'socket' and onto the trailer 'ball.'
Anything you can do to close off the gap will help cut fuel.But remember that the trailer will have range of motion in pitch,roll,and yaw,and sometimes all three simultaneously.It's easy to get things bound up.

Great I'll think about that... any other MUST READ posts you can suggest would be great... Thanks again for your input. BB

Must Read
 

You might do a Search EcoModder for 'Range Extending Trailers' and go to #10 permalink to see some projects and thought experiments.:)

Don't make the front of the trailer into a scoop that scoops air between the towing vehicle and the trailer.
Use a convex shape.


Don't use any circular shapes at the rear.
They'll destabilise the trailer.
Cut it off sharply at the rear, if possible with the sides / top / bottom plates extending a bit (2-4 inches) beyond the rear wall - i.e. forming a shallow box of thin plates around the tail end.


If you get around to covering the gap, the covers can rest on / slide over the convex front of the trailer.

Thanks Euromodder... looks like that's the advice I'm getting, but, however.... The convex shape was intended to make gap filling much easier to do, not make a scoop or allow a gap between the tow vehicle and the trailer. The intention would be to immediately make a fairing to join the car with the trailer like the "range extending trailers" I have seen on this site. Given that I was intending to create fairings before taking it out on the road, and only tow with the Jetta and not other tow vehicles, and the further intention would be long distance cross country trips, would aerohead and your advice still be the same?? BB

Also, have you guys seen any of the theory and testing associated with KF (kline fogleman) airfoils? They are stepped foils and seem to show that the steps associated with a fairing from the vehicle to the fairing and then from the fairing to the towed trailer might not penalize the drag calculations very much and might make the fairing easier to design and construct... KF Airfoils :: explained by Dave Powers | RCPowers.com

BB

This is a fairing idea for the curved to the front concave design... added hollow "box" at the rear squared off. I'll try a couple of different views this weekend. Top and rear. BB

What purpose does the hollow box serve?

I was told squaring the back and adding a hollow box at the rear would reduce drag...
BB

Seems to me that hollow box thing is for when it isn't practical to put a real boattail (or built-in boattail form) on.

If you have garage and parkability concerns and make it folding, then yeah. Then it may as well go further to a point.

Still in the planning stage right? Aluminum sheet and a Harbor freight trailer?

Re: the nose: Definitely convex. That concave curve you like should be turned around and attached to the back of you Jetta. Customize your tow vehicle to the trailer, but leave it generic so you can move to another tow vehicle.

...When it survives those nasty crosswinds. Be sure to watch the UK Top Gear caravan racing videos before you head out on the road. :thumbup:

In the back: Full -tail? Flat plate? Box Cavity? Eomodder starts to reveal it's schismism. :) I say go out one nite with a piece of fiberglass pre-preg [activated] and drape it over the back bumper of your neighbor's Prius. Remove it carefully with a spatula before dawn and narrow it up after it hardens.

The frame: Sell it on? You could weigh some different trailers, old boat trailers and such. Then you'll know how much penalty there is for that folding feature. A commercial axle and hitch with some recycled steel stock? It ain't rocket science.

I have one of these and I will be going to him for the parts to get it on the road.
http://images.thesamba.com/vw/classi...ix/3865649.jpg
All you need is a round stub axle. Scavenge the swing arm or roll you own and just buy the elastomeric suspension module you can see just inboard of the trailing arm.

As for the aluminum skin, roll the edges and get some aluminum H-channel. Curve that to the required shape and rivet through one skin, the web of the H and the other skin. Easy peasy.

Edit: If you built your own trailing arms you would want to maintain that angular geometry but you could make them straight so they skim the road right behind the tire; and then you could build a full teardrop light-plane style pantaloon off the back of it. I'm ditching the stock fender on my Westy as of now.

The 1720# HF has no folding feature :-) I plan some welding and a lot of modifications to reduce weight. I'm being talked into the convex shape, your comment about putting the curve on the back of the Jetta is tempting, since I will be fairing the back anyway. I may just do a square front and then try different shells attached to the front of the trailer for experiments. The square would be easiest to build and then I could make the shell I like the best permanent. The shell would cover the batteries, propane, and storage in the front of the trailer.

I'm only into the trailer $270 and can mod if for weight much easier than finding another one.

Thanks for the other points I am considering all input carefully.

PS - I have a Prius too, that belongs to my wife, so I don't need no stinkin' neighbor, I'll just sneak out in the middle of the night to my garage when she's asleep :-) :-)

Epiphany! Thinking about using shaped foam rubber like couch cushions coated with liquid plastic for a smoother surface as a flexible gap filler with velcro attachments to trailer... This should allow trailer turns without binding just letting the foam rubber squish and should maintain my radius and be easy to create. I'm sure someone must have thought of this, but, I haven't been able to find it anywhere. BB

http://ecomodder.com/forum/member-fr...ni-desktop.jpg
There's a Vanagon/traailer thread on the samba.com. This post is interesting for the aluminum frame, single trailing arms and fender shaped like a bicyclist helmet.

Lots of Harbor Freight-based trailers in that thread

Clean, sharp air detachment.

There's nothing there to attach to even when it swirls back 180°.

Lights, plate and fittings can go inside the "box" - out of reach of the airflow.

Have you done the math on how much space you need to allow for cornering, meaning the distance from the front corners of your trailer to the back of your car? Looks to me like the minimum practical is 1/2 W of the trailer so at 5 feet, you'll need 30", which in a tight turn will shrink to 6" on one side and stretch to 54" on the other. The shrink is easy to engineer, the stretch is where it gets interesting. But I suppose you really don't need to fill the gap in a turn since at highway speeds the gap differences from side to side will be small, so all it really needs to do is crush. Still, a daunting challenge, which is why I think we see so few gaps filled.

I'm working on a rear profile, floor plan / top view, and other details this weekend. At some point I'll put it on Sketchup or Blender.

Your train of thought and then reasoning at the end of the post followed mine... I only need it to crush and won't worry about even large gaps when turning, since almost all turns at highway speed are very gentle. The gaps will occur in town when aerodynamics is not needed. The application doesn't seem too daunting with my proposed method :-) :

1. Needs to crush / deform very well, 2. can not be abrasive or solid enough to dent things, but, 3. needs to have a "slick" rigid surface, 4. needs to be easy to shape (cut to form), and scalabe 5. Must be easy to attach and will not detach when needed but easily removable. I think my method meets all criteria and seems rather simple to apply.

1. Foam crushes very small, especially low density, 2. rubberized surface is "soft" and should be non-abrasive, 3. Rubberized surface is "slick" and "rigid" (like a wrestling mat), 4. can shape foam with hot wire adn make it big or small (like pillows, mattresses or couches). 5. Velcro attachments should be easy to install and remove.

Major problems might be... uh...storage bulk, (I also thought of air, because of deflation but that causes other problems, such as difficulty of manufacture, and not as "crushy" without popping, might have to attach to air reservoir or pump).

Can you think of other possible problems or engineering criteria?

Can of worms.

Some things you might try:

• Air mattress type thing on each side with a connecting air tube so when one side compresses it inflates on the other side.
• A tongue like thing that extends and retracts on the top front of the trailer. The tip would have a roller ball that bears on the tow vehicle roof and can roll sideways in turns and front to back as the trailer rises and falls on the hitch.

To understand how a box cavity works, look at the advantage offered by a half-tonneau on a pickup truck. It is a cavity that points up right behind the rear wall of the cab.

hot-linked from pinterest
http://2.bp.blogspot.com/-GXx1Yx7Wih...Harley+Rig.jpg
You have to get past that siamesed front fork, and a few other things ( ;) ), but I noticed how the flare on the tow vehicle, in a turn, would lay against that beveled corner. It looks like with a combination of turn and pitch or roll they would trade paint, though.

I wonder if a steering front axle, like a farm implement trailer, would reduce the vehicle-trailer angle for any given turn. I need to stop looking at the picture.

It's like gawking at an accident.

No need to understand how it works - just take advantage of the fact that it does work beyond its looks ... :)

As a simple add-on, the box cavity is not nearly as aerodynamically pretty as a boat tail, it doesn't nearly conform to the ideal teardrop shape - yet it does help quite a bit.

Much as we don't understand how electrons work, yet we converse on Internet?

There're straight, curved and inset boxes. Which is best? Would a half-box on the back of a fat fender work?

a horrific accedent

best?
 

I suspect that a curved wall would win simply because it could give the gentle pressure gradient the boundary layer needs and would not generate a vortex of which we'd have to pay for all the circulation.
If NASA could have cheated on the boat tail for the Space Shuttle I believe they would have.Since they went with a more 'ideal' form,perhaps its form substantially out-performed the other design candidates.
That's not to say that it would 'win' with truckers.
I think that any form of cavity would help on a 'fatty.'