Which Trailer shape is better??
 

Hi All,
I've scrounged up some power tools and will start building my truck Duck Tail and a small "Teardrop" camping trailer. I can easily build either trailer type and both strike the right cord as to style to my eye, so my question is which shape would be better aerodynamically? The car in the scene is a Toyota Matrix which is the real tow car.

The trailer is built on a Harbor Freight 4'x8' trailer kit. Basic dimensions are set at 4' wide, 4' high and 8'-10' long(could go a little longer if that helps).
Thanks for any input.
Ed :turtle:

Oh and much thanks to someone who mentioned Google SketchUP. What a fantastic free tool. I worked AutoCad a long time ago and this free tool would have cost a few grand back then. Computers and programers are simply amazing:thumbup:

I'm no aero expert, but I would guess that the trailer with the wedge shaped front end would be better due to the fact that its front end will be closer to the airflow coming off the vehicle in front. It also has more storage space.
I notice you have too steep of an angle on the rooflines of both designs.

Rather than have a pointed front to the design, you will most likely see more benefit if you flip your design backwards and have it boattail at the rear.

Edit : sorry . Didn't open the thumbnail ( waiting at dentist )
I would go with the rounded ( I thought it was a wedge ) from end for sure.

(The one farthest from the car )

Would you consider something that was basically shaped like a boat tail extension to the car?

Lay this shape over the side of your blueprint. Scale it so the ground on the drawing is at the ground position for your trailer, and the top of the car in the drawing is at the top of your trailer (4'?).

Choose how long you want to make your trailer and cut off the rear end of the drawing at that point. The longer you make it, the better the aero. If possible apply the second drawing to the plan view so the thing tapers in both directions. If you're really adventurous, generous radii on the edges will also help.

*Make sure you cover the bottom of the trailer so it is smooth. If you can, build the body of the trailer out to cover the wheels.

http://forums.pelicanparts.com/uploa...1314378865.jpg

http://ecomodder.com/forum/member-ba...g-template.png

The answer is (c), none of the above.

The closer you can keep the front of the trailer to the back of the tow vehicle, the better. If the back of the tow vehicle is large and flat (pickup and cap), (a) would be best, but put radii on all the leading edges. If the tow vehicle is a small sedan or some such, (b) might be better if the flat trailer front largely matches up with the rear of the trunk. Many, many trailers, especially the teardrops, have sharp junctions from sides to front/top/rear. It is expedient for construction but hurts aero. Then, teardropping more sharp than template leads to the rear being in turbulence... probably just as well off making a squareback then and enjoying the extra interior space.

Also, think in 3-D; boattail it in plan view as well.

Frank, won't the trailer be too far back for any real benefit if he doesn't have skirts to fill in the huge gap that will inevitably be there?

Here's my design in Alias. It combines space efficiency with aero efficiency and complicated surfaces with ease of construction.

http://i43.tinypic.com/1zg72ph.jpg

http://i40.tinypic.com/mh38k2.jpg

I forget, is it 36"+ where the gap really starts to hurt aero?

It's a tough situation, making it as close as possible without risking hitting the trailer when backing up, going over bumps and dips, etc. Perhaps the best solution is some sort of flexy fairing that can close or at least partially close the gap without damaging anything... I seen some attempts at that on tractor/trailers but not at the consumer level.

I'd call Sven's proposal (c)- it looks quite good... now put radii on the leading and upper edges.

Depends what Ed wants to make- an aero pull-behind that also hauls cargo, or a trailer that is also aerodynamic. Yours, Frank, is the former, and mine is the latter. Two different ways to approach the same problem.

I don't know how big the gap would need to be to really hurt the mileage but it'll probably be a meter or so from the get-go.

Thank you. I didn't put radii on partly because I'm pressed for time and partly because building a consistent radius in real life would probably be a struggle with metal HVAC ducting, if you could even pull it off. I'd go sans the radii and see how it does. With that shape it should give some pretty awesome mileage right out of the box :thumbup:

Hah? :confused:

It's a statement about sacrifices. You can have great aero or great cargo capacity... the more of one, the less of the other.

I decided to design a trailer with decent aero and decent capacity. :)

I described the same thing you sketched. It's a convergence, like it or not.

K. I didn't worry about "[keeping] the front of the trailer to the back of the tow vehicle". But they accomplish the same goal.

That's a nice design you built Sven7. I have a hard time seeing how the interior accommodations would be installed. Bed and Galley. Can I get that model in to Sketchup to play with?

And yes radii at edges are better BUT, ease of construction is also a requirement. A radius in one direction is easy, two is very hard, given time, materials and tools at hand. That's why I was curious which direction of leading edge might be better, horizontal vs vertical.

I like the idea of bringing the trailing edge up to aero profile and chopping the end off. I'm limited to about 10'-12' total length.

Does anyone know if that aero shape is in the Google Sketch up library? And if so the name.

This camper is to be towed 90+% of the time behind a small car like our Toyota Matrix hence the limited size and weight.
Thanks for the input.
Ed :turtle:

I think a or c or some blend thereof for the front, and c for the back would work best behind the Matrix.

Sorry I've lost track of a,b,c. Here is a new model incorporating the raised rear.
Do these look better?
Thanks
Ed :turtle:

Still way too steep of an angle at the rear. Can you insert canvas images into Sketchup to build over? If so, do that. If not, overlay the aero template from last page until it matches. Print them out if need be. I posted that image because it's pretty close to the "perfect" low drag vehicle shape. Any steeper an angle will detach the airflow, resulting in eddies and vortexes.

I don't know how to build in Sketchup or convert from Alias, if it's even possible. The best way to do it would be building over an image. Also, if you're making it 4x4x10, define those dimensions in the model! My model is built almost exactly to those dimensions and you can see how much longer it is than either A or B. (That is, unless 4' is measured from the axle up)

If you only make a curve in one direction I'd do A because it will have a smaller frontal area in crosswinds.

Edit: As far as accommodations go, you should be able to fit a bed in there with a little space to walk around, especially if you cantilever the body over the wheels for an extra 10 inches or so.

Sven7-Thanks I figured out getting that aero image in to sketchup. And WOW I didn't think the trailing edge would be that high. With the rear that high I have to really rethink the layout.

As for accommodations I should have been clearer. Bed for two, min 48" wide 6ft long.
Thanks
Ed :turtle:

My two bits says make the front fill in as much of the space/volume between the tailer and the truck as posible. the tail end of the trailer looks great.

^^ I agree with yaris. You could gain a few "free" feet too.

The template is not placed correctly. The "ground" in the drawing should line up with the "ground" in real life. Correct that and it will look great!

Good luck on the build. :)

Tractor-Trailer gap is ideal at 24" or less. Above 30" is frowned upon.

Looks like fun, Ed. What "accommodations" will this trailer feature. That might help what-to-put-where and point a particular direction.

Have you a floorplan in mind? Or, interior wall-plan? I've seen some good give-and-take on AIR around these questions (exterior shape is given, but interior plans can go a very long way to using space).

slowmover-Not much to a floor plan in a 4'x11' area:) But yeah, the bed is forward the rear is a "chuck wagon" type area. I think I'll make the rear hinge up to an awning position to give my wife coverage when at the camp stove.

But with this extra volume dictated by aero considerations I'm thinking about a "garage" area for a bike(actually a "Catrike Trail"). I'm riding a Trident ST2 Trike from Maine to Washington state this spring/summer. My wife will SAG for me. We have tent camped always in the past but 90+ days of tenting is a little extreme for us. So this little trailer is just one step up from tent, but without all the fiddling that a standard Pop-up requires.
Ed:turtle:

Here is the newest top view. I can't upload too much on a single post.
Ed:turtle:

Ed, I would make any nice aero shape you want so long as there are a minimum 2' tall straight sides on it, and then make it a two piece slider or pop-up. I have several ideas regarding hard sided pop-ups. No fiddling, no tent section, just slide it up or down in a hurry.
Stand up = momma happy :D

Ed, those HF kit trailers are 4' wide, right? If you build the deck wider with wheel wells and skirts then you could increase the beam to match the tow vehicle (somewhere between 5 and 6 feet). Without any elbow room on the sides of the mattress you'd probably have to crawl over it. If offset to one side there might be enough isle to get past it. Think about where you want the door (assuming only one) how big it can/must be. That will constrain body geometry. Then I'd use the template to maximize interior space with least aero penalty.

More food for thought:

1. Airstream Basecamp
http://www.roamingtimes.com/a/consum...m-basecamp.jpg
http://images.gizmag.com/hero/4279_150705113152.jpg

2. Frankenhauler: web page forum thread

3. Tiny toy hauler (Scamp hack): blog post

skyking- I would love to see those popup designs you have, don't know if I have the build time but it would be very interesting. Can you email them or post. Dry Bed that doesn't rock == momma happy:D We lived on sailboats for 5 years and my wifes first question is always "how are you going to keep the rain out". Also what's that plane in your avatar? Love the old war birds:thumbup:

Kamperbob,-yes HF base is 4'x8', I've been thinking of widening to tire width but it's not giving much more utility. The design has a his/her door on each side. But craw over isn't that bad:D, had to do it on our last two boats.
That Airstream looks sharp but I was glad to be sitting down when I saw some used prices OMG:eek: Also weight becomes an issue. I have an uncle towing a Scamp 13' with a Matrix, IMHO he's nuts. I want to keep empty trailer weight under 600lbs, loaded for highway 1000lbs.

plan taper
 

I know that you'd lose interior space,but allowing body plan taper would give upwards of an additional 38% drag reduction.Both Walter E.Lay and Wunibald Kamm confirmed this during wind tunnel research.
The gap is hurting you also.If the gap must remain,then a bulbous nose will give the lowest drag,especially in crosswinds.
If you can look at Hucho's books,you'll find hard numbers attached to different trailer architectures at varying degrees of yaw.

Nice idea but he needs to be able to get in it.

The hatch extension is probably a good idea, modifiable to be useful even on a trailer that is taller than the car.

I bought a Toyota RV-2 promotional card today and thought it might apply to this project. Have you considered having the roof "open" for more interior space when parked?

http://retrothing.typepad.com/photos.../toyotarv2.jpg

A two part boat hatch on the front edge where it it tallest:

Project: Ericson 29 Teak Cabin Hatch Boards Refinish | I Sail Away

Ed,for a pop up version of any of the trailers these guys have drawn up, I need to know what materials you had in mind. Work with what is comfortable for you. If sheetmetal, we could make the top section out of EMT and skin and insulate it for example. As small as that trailer is, the roof is in range of "crouch into it, lift up roof by standing up" with no mechanisms. I can design a simple toggle that you pre-set, once the roof is high enough the toggle falls into place at each corner and you let it back down.
To reverse the process, you re-position the weight on the toggle. Lift up and it unlatches for you, let roof down.
Hang the door on the roof section, opening out. After lift, install a 2' door section on the bottom to fill that hole, done.
It opens up those aero designs like jacobnev's or sven7's (very nice) and lets you build low and live high. Both of those designs have that 2' section of straight side for the slip.
If building for cold weather, you need at least a 2" wall insulation. making it pop up will reduce interior width 4.5", 2" each side plus .25" for clearance. Small price to pay for stand up comfort.
Use sliders made from UHMW plastic strips at the corners, installed with countersunk rivets. This stuff is very very slick and tough, and will prevent binding.
My material of choice is a XPS foam core covered with very thin veneer inside and out, layer of glass/epoxy, and finish with linear poly. That may not be in your comfort zone but it is what the really light and fast boat builders would do.

Edit 1: I would build the trailer out over the wheels, as close to the Tow vehicle width as possible. Add a center divider over the hitch tongue to keep the crosswinds from crossing over the gap. aerohead has these ideas around here somewhere and explains them well.
The XPS/veneer/epoxy/glass sandwich weighs in at 0.6 lbs per foot at 2" thickness. take off 0.08 lbs at 1.5" thickness.
5'x11' plus 2' side is about 120 feet. it would weigh in at 70~80 lbs.
Use the bluecor .25 foam and build it up, making radii as you lay it up. each layer bonded into a radius gets stronger and stronger, think motorcycle helmet.
http://building.dow.com/na/en/produc...on/bluecor.htm

Ed, I agree those Airstream Basecamps are outrageously priced. Even if I won the lottery, rather than buy one I'd rather build a factory and make my own. LOL

Yes, weight is important. I have many camper designs swirling around my head. I've sketched up some to work out geometry and trade-off puzzles but currently lack the facilities to build anything. But I have varied skills and experiences working with a range of materials. My current favorite approach for projects of this magnitude is fiberglass coated foam board. Some years ago a gentleman used this approach to make a full slide-in camper that he said weighed only 600 pounds! Sadly, I lost the link... (pout)

A splitter plate on the centerline would do the trick.
A single plate wouldn't be an issue when turning or backing up.
It can go as far forward as the hitch, but then it should angle back a bit to clear the tow vehicle.

Teardrop trailers all have too much / too fast a drop on the rear .
They often end up as nearly semi-circular, the kind of shape you really need to avoid on the back end.

Longer helps - it gives you more volume, and the teardrop can be longer / less steep / more gradual, so it more closely matches the streamline template.

upper edges
 

I know that it would be a pain to fabricate,but to have upper edge radii could shave as much as 0.043 off the Cd.It would really reduce the roll moment as well,which knocked Sheppard's beautiful new trailer over.

gap & Cd
 

In Hucho's 2nd Edition,p.180,Fig. 4.98,he shows a Morris Minor 1000 notchback,and Traveller squareback pulling a small caravan trailer.
For the notchback,the drag curve shows a maximum @ 36" gap.
For the squareback,the drag maximum is at about 39.3".
For SAE Paper 870714,with matched-height,parallel-face tug/trailer, a 21.6" gap produces only a 2.3% drag increase.
For stock cars in a 2-car draft,the trailing car,going from bumpers touching,to about a 36" gap would raise the Cd from 0.185,to 0.193,about 4.3%.
For two identical Mercedes-Benz buses,going from touching bumpers to around a 36" gap,would raise the Cd of the trailing bus from 0.167,to 0.21,about a 25% drag increase.
Since the buses don't have 'bumpers' per se,they might best reflect the significance of gap drag.
In Abbott and Von Doenhoff's Theory of Wing Sections,they depict a Clark-Y airfoil with varying degrees of gap(slots) along different positions of the cord.The Cd of the wing can vary from a minimum of 0.0152,to 0.0208,based on surface area skin friction drag.
NASA reported 19% drag for their C.O.E.semi-trailer with 31.2" gap.

aerohead- Yes the model Sven7 made is beautiful, needs to be a little farther forward in the trailer I think in order to get the departure angle high enough for our driveway. My problem is my skill set and tools are limited to plywood and lumber and basic shapes. I'm having a devil of a time trying to fit that form in to tortured plywood. WHILE being frugal, i.e. using dimensional lumber without a lot of waste. That's kind of the beauty of the original TearDrop design, 3 pieces of 1/2" 4'x8' for floor and walls.

But I'm still trying to come up with a build design that I can actually build:D

skyking- thanks, looking into those material.

THANKS EVERYONE:thumbup: No matter what I build it will be better because of this forum.
Ed :turtle:


{edit} By pulling out the sides to enclose the wheels and give the slick aero plan shape and lowering cd, would the increase of an estimated 8sqft of frontal area of the trailer and the needed mirror extensions on the car offset any decrease in cd?