Aerodynamic trailer design
 

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

http://i.imgur.com/NBHi39F.jpg

http://i.imgur.com/SWNzfee.jpg


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.

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
https://www.etrailer.com/Trailer-Sus...SR1200S02.html

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

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

Have you considered a single wheel trailer similar to my trailer project?
http://ecomodder.com/forum/showthrea...ler-26997.html

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. ;)
SWIVELWHEEL-ECO

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!

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.

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.

http://i.imgur.com/HVeOVZi.jpg

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

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.

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:

http://yves.fungiart.com/images/bluetrailer/blue2.jpg

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

What am I doing wrong here?

http://i.imgur.com/sSuZfH7.png

Ecky,

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.

http://ecomodder.com/forum/372995-post533.html
http://ecomodder.com/forum/373016-post534.html

Ecky,

I use PowerPoint 2010 to do my overlays, put both pictures on one slide and drag the AST-II over the picture of the car. The AST-II will go semitransparent and you can see them overlaid quite nicely. Use the built in scale and rotate tools to make them match up.
Your car is already fairly boat-tailed in the back, making a trailer that continues the boat tail to reduce drag will result in a fairly small trailer.

How is the quality of harbor freight's trailers? A lot of reviewers have given them good ratings but mention that they watch for the hubs getting hot. If I can make something for not too much more that has better suspension, is less likely to rust away quickly, and doesn't have crap bearings, etc. I would prefer that, but I'm giving consideration to spending $180-230 on one of those as a base and building on it. Working on some drawings based on one of those right now.

I once made a trailer axle from a piece of 2" EMT (thin wall electrical conduit) by welding in a pair of front wheel spindles from the junkyard. I mounted the axle solid to the trailer frame. No springs, no suspension.

The trailer towed well on the highway without bouncing. I stress tested it with an estimated 1500 pounds of sand and concrete blocks. No problem. Another stress test filled it with firewood and pulled through a field watching the firewood bounce out. No problem.

I sold after several years. I ran into a friend of the new owner about 20 years later, and he said the trailer was still in use.

Harbor Freight
 

*My trailer from the 1980s is essentially what Harbor Freight sells today.
*They go on sale regularly so you might get a price break there.
*I made a plywood flatbed,then created a top.
*The top is 1/2-" EMT,bent with an EMT bender and gas welded into a space frame,covered with 1/8" Lauan plywood/fiberglass,and cast fiberglass radii,laid up inside split PVC pipe 'molds.'
*The wheels were 13" from a CRX,reversed.(I had to grind the wheel centers to fit over the hubs.
*All-Season radials from Discount tire were okay for my light loads (1/4-ton max)
*As pulled,with no belly or gap-fillers I lost 2-mpg @ 55-mph (50-mpg) Denton-to-Los Angeles round trip.
*If you're spooked by the Chinese wheel bearings,you could pop in Timken or another brand.Just set enough 'slop' in the preload and use nothing but wheel bearing grease.
http://i1271.photobucket.com/albums/...ntitled-10.jpg
http://i1271.photobucket.com/albums/...titled-8-1.jpg
This is a study I did for your car.It is excessively long, enough for camping.As with BamZipPow's 1-wheeler,the idea was to get the streamlining good enough that you'd 'gain' mpg while pulling it.
Another:
http://i1271.photobucket.com/albums/...titled-6-1.jpg
This profile would be okay for plan taper
http://i1271.photobucket.com/albums/...ad2/Bottom.jpg

Much appreciated!

Here's a sketch I've made of HF's trailer with 12" wheels, as it had more complete data than the 8" version. There's still some missing data so I had to extrapolate some of the measurements.

http://i.imgur.com/HMHLS1Z.jpg


From the front of the platform, I'd have about 33" to the bumper, which would almost allow 90° turns.

In the interests of keeping tongue weight down I would like to keep most of the weight approximately over the axle, which may necessitate moving it forward given the taper, or perhaps extending the trailer back another ~24 inches, giving a total usable area of around 40" x 72". The front looks like it should be roughly 15" above the platform which doesn't leave a lot of room for taper, but if I need to haul more I could always leave the cover off. If I extended the box forward another 12 inches it would allow for a depth of approximately 19-21" but I'd be eating into my turning radius.

What do you guys think?

~~

One thing I'm confused about:

Heavy Duty Utility Trailer - 870 Lb. Capacity
40-1/2" x 48" Small Utility Trailer - 1090 Lb. Cargo Capacity

^According to the overview, both of those trailers have the same height despite one having 8" wheels and the other having 12" wheels, but I'm skeptical. Would the trailer with the smaller wheels really have the same deck height?

They would have the same deck height at the hitch but not at the wheels. ;)

This is IMHO only, as you will probably have your trailer on the road before I have mine.

I suggest that it's the next step you need to look at, The trailer box 50% outside The Template in both the top and plan view. You could put a little template on the trailer box. :)

http://yves.fungiart.com/images/bluetrailer/blue2.jpg

Personally, I like this one. You could take a size-graduated set of inner tubes, string they on a bungee between the trailer and car and then inflate them. Match the body color with Plastidip S.

Or make it like a bouncy castle.

It would like like a caterpillar or the larval stage of a flying car. :thumbup:

I think you have two main options. One is to make a small trailer that continues to follow the template off of the car which may actually help MPG, but has limited inside space and usability. The second, as Freebeard mentioned, is to make a larger trailer which will have more space and usability, but may actually slightly hurt MPG.

It is all about your intended use. If you are planning on dragging this thing around all the time, I would suggest option 1. If this is just an occasional use trailer for specific hauling needs, I would suggest option two.

For an option 1 trailer, I think you will need to use some sort of gap fillers to get to a net positive effect. Freebeard has some ideas above, and you can also look at Aerohead’s pictures and BamZipPow’s trailer. I would also be wary of using a harbor freight trailer axle for a 100% follow behind the car everywhere trailer, depending on how much you drive it may not be designed for that much use. I think the swivel wheel trailer as BamZipPow is using is the best idea for this sort of trailer, as it simplifies the gap fillers and would maximize interior space and since you wouldn’t have to worry about turning radius, but you would need to be cautions of your new tail sticking out the back.

For an option two trailer, I would build something like the white trailer Aerohead posted. Make it 48”+ between the wheelwells to maximize utility. Even if you lost 2MPG pulling it, you would still be lightyears ahead of anything else that could haul that much stock. Only use it when you need it.

Lastly, don’t forget safety. Your car probably would be able to have a class 1 or 2 hitch at the most, using a 1.25” receiver. Make sure your trailer will not exceed the recommended capacity of the car or hitch. If the weight of the trailer is going to be significant compared to the weight of the car, I wouldn’t attempt this without trailer brakes. I know that you know all of this, just keep it in mind.

axle width/deck height (leveling)/wheel diameter
 

I wanted to mention that the smaller trailer has a C-Channel axle,with stub-axle hubs captured by clamps.
Narrowing the axle would be straight forward if you chose to do that.
The frame members would have to be re-drilled for the new width,depending upon how much you shrunk the dimensions.
It would allow some flexibility with respect to the axle location.
--------------------------------------------------------------------------
I suppose that the receiver tow bar could be customized for a proper vertical ball position to keep the deck level.
--------------------------------------------------------------------------
The 12" wheels should have a rolling resistance advantage over the 8".If you can get the 14"s on there,it would really slow down the wheel bearings.

Thanks for the tips. My intended uses for this trailer will be mixed, though at a later date I may choose to go with two different ones for different uses, or detachable tops to go on the platform. Aero box for trips, open top or rack for plywood/lumber.

Next summer I'm moving a load of stuff from Florida to Vermont, and a 40x48" trailer should be just about right for what I need. If a Harbor Freight trailer won't survive a 24 hour drive, it's of no use to me. After that, I'm probably going to do a tour of the country before heading to Asia for a year or two. I may or may not use the trailer for this. When I get back, the trailer will be likely be used mostly for bringing extra cargo along on trips, as well as hauling things around town that will not fit in the hatch.

Subscribed!

Sexy:

http://i1271.photobucket.com/albums/...titled-6-1.jpg

Been out of town for a few days, but I'm back to drawing. Not sure yet where I want to fall in terms of compromises between absolute width (e.g. 48" plywood sheets), internal area, aero impact and turning radius, but having the box on top be replaceable means I can build several tops for different uses.

The Insight has about 28,150 cubic inches of cargo space in the hatch + hidden cargo area under the floor, or 16.3 cubic feet.

If I were to pick up a stock HF trailer with a platform of 40x48" I would have a maximum angle between trailer and car of ~82.5° with the trailer sitting 30" behind the ball. Internal area at 15" uniform height is 28,800CI, or 25,920CI if I taper it to 12" at the rear.

Widening the trailer to 48" would not affect turning much - max angle would still be ~80°. Almost all of the trailer would still be out of the windstream, with only the top corners poking out, though Cd would be increased compared to a narrower trailer. I'd have roughly 34,100CI of internal volume assuming a uniform 15" height, and 30,700CI with a taper, both accounting for recessing the wheel wells to fit the wider box. Alternatively, I could look for another trailer that already comes 48" wide, so I wouldn't have recessed wheel wells, which would give an extra ~400CI of volume.

Extending the trailer platform forward 6" and leaving it at 40" wide reduces angle before the trailer contacts the car to approx. 75°, increases the height at the front of the box to ~17" while not hurting aero and total volume ends up at ~32,880CI/30,000CI.

Extending the trailer platform forward 12" reduces the turning angle to around 55°, increases the height at the front to 19" and increases total volume to ~37440CI/34560CI.

Attaching a 20" hollow semicircular nose to the front of the trailer (as it looks like aerohead did in his CRX trailer) would give a maximum angle of ~60°, allow a maximum height at the front of the trailer to be 22", and give an internal volume of approx. 40,800CI/37,920CI. The tip of the cargo box would be 10" behind the ball.

Attaching a 20" hollow semicircular nose to the trailer and setting it about 6 inches back from the front of the platform would allow for a maximum turning angle of ~80°, a height of 20" at the front and a total volume of approximately 36,700CI / 34,180CI. The tip of the cargo box would be 16" behind the ball.


It looks like a rounded nose is the way to go for maximizing internal volume while maintaining turning angle, though it does increase the difficulty of building the trailer unless I can find something prefabricated that I can adapt.

http://i.imgur.com/PHXzqUP.jpg

http://i.imgur.com/6jWkWTN.jpg

Please excuse the slightly sloppy drawings. ^^

Any thoughts on the ability of the HF 12" trailer's bearings/axle to survive many thousands of miles with 3-500lbs on them?

bearings
 

I went from Texas out to the West Coast and back,with over 600-lbs,and just the 8" trailer wheels.I think you'd be okay.
---------------------------------------------------------------------------
Thinking about the nose.For something 'pre-made',A wheelbarrow body,whether steel or plastic,has some fine shape when inverted.This 'nose' could be cut in half to form the sides of a wider trailer nose,then just fill in the void.
It would give you side radii and top radii,plus a slope-back 'Stromform' nose,considered one of the best.
http://ecx.images-amazon.com/images/...S._SL1200_.jpg

Leaving it upright might be even better. The rounded top should clear the bumper and give some extra volume with a given turning radius.

http://i.imgur.com/4Bj0dmM.jpg

http://i.imgur.com/LzZ5zAf.jpg

http://www.katewerblegallery.com/fil...temper-091.jpg

I checked back through the thread and I haven't posted this here, so here:

http://ecomodder.com/forum/member-fr...-trdrp-frm.jpg

This is what I would do before a HF folder: It steals the construction details from a 1952 Silver Streak Clipper and places it over the Sacred Template. Silver Streak used the simplest possible construction; with a central tube from the hitch, C-channel crossmembers and a carriage bolt pinning the end of the crossmember to the deck. Size the parts and compare the weight. The center tube in the pic looks fat because it was intended to be a fresh water tank.

Instead of a wheelbarrow, I'd look for a 2nd hand car-topper.

Not sure what kind of vehicles would have a topper that's 40-48" wide.

Found this on craigslist:

https://jacksonville.craigslist.org/tls/4791231640.html

57x38x20", 20 cubic feet of internal volume (34560 CI)

http://images.craigslist.org/00u0u_3...YF_600x450.jpg

Taper isn't right, but I may find the right box.

Another:

http://cdn3.static-tgdp.com/ui/produ...ed_450x300.jpg

67 x 35 x 16, 15 cubic feet.

EDIT: Does a premade cargo box exist that doesn't have a terrible aerodynamic shape? They're not even close to the profile.

http://i.imgur.com/UgqaIJJ.png

http://i.imgur.com/SHMN0x4.png

http://i.imgur.com/BOv7BaJ.png

http://i.imgur.com/coeTqEv.png

http://i.imgur.com/YK0CLgV.png


Thinking building from scratch, or maybe using the wheel barrow front and building the rear from scratch would return the best results.

The first example is like the one I have. Cut in half and plan-tapered it would be a good fit for a VW Beetle, which is 42" across above the rear axle line. I chose not to cut mine up.

The second one could be quartered and spread at the front, horizontally and vertically. It would be a big step ahead of the square-framed box you started with. I'd put the lower section below the deck.

I notice you haven't put the wheelbarrow to The Template. That would be a first. :)

First, I wouldn't be scared of the HF trailers. I have one. The key is to remove the shipping/storage grease from the bearings and hubs and put real wheel bearing grease in them. I think people that have problems never did that. Also, quality control is spotty on any Chinese stuff; I found weld spatter on the axle where the grease seal rides. Fine sandpaper knocked that off in a jiffy.

Second, you might want to just get one and determine how close you want to shorten the hitch and everything by experimentation. I know for a fact that short lengths between hitch and trailer axle can easily "out-steer" the car, by that I mean when backing up the trailer can easily turn sharper than your ability to steer resulting in a jack-knife and/or many many fore-n-aft corrective actions. I also know we want to suck that trailer in as close as possible for aero; therein lies the conundrum. I considered all this and stuck with the stock hitch length.

Third, backing up with a small trailer can be quite a chore due to visibility as in, not being able to see it until it starts jack-knifing. My HF behind Moon Unit is almost invisible out the windows and mirrors unless it's turning.

Thanks Frank. I'll probably stick with the stock length too, given that info.

Got my hitch installed:


http://i.imgur.com/UxIVOdw.jpg

http://i.imgur.com/1yrhLs1.jpg

Will probably purchase a trailer and begin construction after the new year.

but then, closing the gap with the towing vehicle gets harder - and that's where a lot of mpg's are lost pulling a trailer.

Updated designs:

http://i.imgur.com/TwCIUra.jpg

http://i.imgur.com/3Lr1zpN.jpg

http://i.imgur.com/RlkoHvg.jpg

http://i.imgur.com/CEG29Xt.jpg


Design two has less volume and less turning radius, but would be easier to construct and will likely have more aero benefit. It also has a wonderful symmetry to it: 6' x 4', 12" in the rear and 18" in the front, 24" from the ball, 30 cubic feet, which effectively triples what I can bring with me. I would likely split the top with hinges on either side, so I can open it and carry large items such as appliances.

The more I think about it, the more I like your CRX trailer and find my design converging on it, aerohead. It's an elegant solution.

This is what I'm up to now:

http://i.imgur.com/XPXlH3d.jpg


I decided it would be possible to keep the rounded nose so long as I didn't slope it at the top, which would require that I cut two fitting ovals. Turning angle is down to 65° if I don't set the nose back, but I'm thinking of constructing the trailer with a plywood frame, bottom, and lid, and siding that you'd use in an RV shower which would give and return to shape if it hits the bumper; plus it's waterproof. 7' length keeps the slope shallow enough that I should hopefully have reattachment of air before the tail end, and lets me cut it from two 4x8 sheets of plywood. I don't think I'll be able to easily round the top edges like you did.

What I haven't decided on yet is what to do about the fenders and underside. As I understand, extending the fenders down to cover the front of the tires should provide aero benefit, but that might be a bit ambitious for my first iteration.

EDIT: I miscalculated the volume of my design #2 in post #33, it should have been 22.5 cubic feet. Volume of this design is ~32 cubic feet.

EDIT2: Should I lower the front an inch or two, do you think? The leading edge of the cylindrical nose is within the template, but moving back, it's not.

Here are some inexpensive torsion half axles from Northern tool. These are probably the same cheap axles from amazon.
Reliable Rubber Torsion Axle Suspension Units | Rubber Torsion Axle Suspension Units| Northern Tool + Equipment
I have towed torsion axles before, and the ride is far better than the typical spring setup.
If you fab your own and put a telescoping tongue on it for storage, you can make a couple of intermediate length holes for testing purposes.

I will admit I didn't follow the streamlining template revision discussion closely. Should should I update the tool with a revised image?

should I
 

I like the idea of efficient use of materials,and I think your nose will basically slip into the wake area as you've drawn it.There'll be turbulence due to the gap but I think you'll have reattachment everywhere except the belly.
http://i1271.photobucket.com/albums/...titled-7-1.jpg
With enough length,even a sharp-edged box will regain attached flow.
http://i1271.photobucket.com/albums/...ad2/12-061.jpg
The front of the trailer will be embedded within the Insight's wake.Here's a shot of a 1st-gen Golf/Rabbit wake.
http://i1271.photobucket.com/albums/...ad2/Scans4.jpg
The gap-drag is an unknown quantity.Here you can see the drag of a Clark-Y airfoil as a function of gap location,etc..
http://i1271.photobucket.com/albums/...Untitled-1.jpg
As to the fenders and bottom,I've gone ahead and enclosed mine,with full sides,complete belly pan,and I'm about half-way with complete wheel fairings.
You might just do simple skirts down to where a belly pan with diffuser might be,and some simple fairings before and after the wheels.
Or just skip it,get a baseline for whatever you do,then later,if you add the bottom cleanup,you can compare performance to see what difference the bottom made.
On low-drag cars,the wheel drag alone can constitute half of the overall drag,so the 'cleaner' you get,these naughty bits become more and more important.

This is the template aardvarcus linked, and the one I've been using:

https://lh6.googleusercontent.com/cg...AcY=w1342-h513

Trailer frame assembled. Here's a very rough (not to scale) sketch:

http://i.imgur.com/pTQwxFj.jpg


Down the road I'm hoping it'll look a bit more like this:

http://i.imgur.com/9N95Nry.jpg


When I asked my father his opinion on what color I should paint it, he said that because black is the fastest color, it's probably also the most aerodynamic. Thinking of just extending the car's color scheme, though I might add a stripe somewhere.

EDIT: Top view, without the addition of fenders

http://i.imgur.com/OLxYONr.jpg

Would there be a way to have it switchable in the tool or 2 versions of the tool (one with each version). It would be interesting for comparison.