Closing the trailer gap - Ideas & Theory
 

I have spent a bit of time reading old threads as I have a trailer project coming up and the gap seems to be one of the major drag issues amongst all the threads I even read Aeroheads trailer thread which was like one of the old "Norse Saga's" or maybe "Gone with the Wind":D.
Anyway, some of the things that stuck out for me were the numbers quoted for the gap, >48" and the two vehicles can be treated seperately and at <24" benefits wil begin to be realised 0 gap of course begins to approach perfection. The issue is how to deal with the articulation problem both vertical and horizontal, now it appears we do not need solid or perfectly smooth panels, this thread has a little discussion on how the appropriate membrane may actually be superior:
http://ecomodder.com/forum/showthrea...rms-23084.html

The bottom is probably the easiest as there is some verticle flex, but overall dimensionally the vertical change is minimal, so effectively it is primarily the horizontal plane which requires greater movement. An idea I had for the bottom was simply use two plywood panels joined by a piano hinge at the hitch point, this would alow the vertical plane flex, then the car side panel is fixed to the vehicle under the towbar. The other panel to be hung under the trailer with a mechanism that allows a little forward/back movement, but a lot of side to side movement when turning, this could be something like small double rollers and aluminium track, two sets of rollers back to back at 90 degrees the long side to side track fixed under trailer facing down and the front back track fixed to gapping panel. The panel would move with vehicle movement and just hang under drawbar(tongue).

As we move up to the sides and particularly the top, both dimensiones move the same amount potentially so we can go from 48"gap to nearly 0.
Aerohead had a solution that served his purpose, but IMO, possibly would have showed up deficiencies in extended use, so I suppose it would be good to quantify how much work is required to recover 80-90% of the gap drag loss and maintain real world utility.

The images I saw posted of drafting numbers between vehicles suggests simply filling the draw bar with a half dome that went from hitch point to either side and up to top of trailer would probably be half the job putting in a vertical splitter of deformable soft foam centred infront of this close to the vehicle would further reduce gap drag and crosswind effects, so maybe up to 60%, what else could be done to get to the 80-90%. Some of the things I read indicated just filling the void in any way possible improved the situation, I even thought of a canvas bag of appropriate dimensions filled with inflatable balls or ping pong balls, they would act as a fluid to some degree and shift around in the bag as it was distorted from vehicle movements but the mayhem that would ensue if the bag popped at full speed on a busy major highway ruled the idea out.

What about those elastic cargo nets pulled 60% tight to give enough flex in turns but still hold tight in a straight line, if the grid size was small enough, the angle of aproach to airflow it would almost appear to be a solid surface. Thinking about it, they only need to be tensioned lengthwise and this would help them maintain form, additional hoops of poly pipe or fibreglass rods could be used to support shape progressively from vehicle to trailer, these would be attached to the elastic and move with it in corners or over bumps. The elastic force may well add a few lb's to the tongue hitch weight, but probably minor. As a further developement it could be skinned in durable shade cloth (90% blockout) or light weight canvas which would help with the durability of the elastic as it rapidly deteriorates in UV light.

If one were really daring (or crazy) what about rather than the cloth use coroplast or a similar thing and attach scales to the cargo net, would take forever but if scaled (ha ha) to right size, say about two hands per scale long and they were cut to teardrop shape, that would be something to see slithering down the highway, why bother with a trailer skin, just scale the entire thing from the back of the car to the tail. Actually it wouldn't be that hard to do, stamp out the scales on mass, maybe plastics or garment manufacturer would have machinery, use large staple gun like for packaging with penetrating jaws to fold staples, stretch out on flat frame to right dimensions, anyway enough of those "pipe" dreams, must be having flasbacks from my youth.

Any ideas to add,
most of all how much of that gap do we need to kill to get 80-90% of the benefits?

How about this? ;)

If you had a camper top/aero cap that extended past the pivot point on the hitch and the trailer was the same height or below the extended edge, you could reasonably use some shaped doors attached to the sides of the tow vehicle that should allow articulation of the trailer. Doors could be pulled tight with some bungee cords.

The trailer would need something that tucked up under the extended cap...probably something inflatable so you could easily take it down to unhitch the trailer. ;)

Defining the shape/size of the trailer contemplated would be a help to this thread even where a specific project is confined to another thread of its' own).

A 10' tall travel trailer that is also 8'6" wide is quite a different beast to mate to a much lower/narrower tow vehicle than a slick little teardrop trailer both shorter and narrower than the tow vehicle.

Trailers -- open, enclosed, etc -- can be most any size & shape.

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That's kind of what Aerohead did, but top and sides were articulated and had rollers on the trailer, the inflatable thing is good, but always seems to be unreliable unless many $$$ expended. I did also think of a series of inflatable hoops, like air matresses, but continuous over the shape, half inflated only so as one side compressed the air would expand the other side.

The biggest issue I came across when thinking about this is the fact that even though we call them bricks (cars) there isn't a staright panel or right angle anywhere on them they are all compound curves, so trying to hinge anything requires straight lines.

When I think back on all the threads I read Aeroheads was the only build that actually made any attempt at this, all others were mostly to build out the trailer with a radius or V shape or to put a spoiler on the back to deflect air over a bigger trailer.

I was looking for all ideas, in my case the trailer will be just slightly smaller than the vehicle in profile, but am also interested in ideas to tackle bigger trailers. There is likely to be a crossover of techniques and solutions that may be applied to either case with a bit of tweaking.

ideas
 

They'll be dozens of solutions.Just like automatic coffee makers.
The thing I kept attempting to be as smart as,was all the possible deflections and their orientations such that I'd never find any of the structures binding or colliding.
I experienced almost daily failures when in early testing.
If you can get your trailer onto wheel dollies,where you can observe full range of motion side to side,while anticipating pothole and pressure ridge airborne antics.
It's like a monkey ______g a football.

I lose 3 to 4 MPG when towing my little trailer empty.
About 1MPG is form the added weight and the rest is added drag.

Mine's not to bad only 1-2mpg down, though it is only a 6'x4', the only time it climbs is when it is over weight and/or the load is very high and obstructs the airflow above.

What do you mean about "pressure ridge airborne antics"?
Is that to do with varying vortexes and pressures causing oscilations and unwanted flap?

Some sort of flexable/deformable gap filler would be ideal as long as it didn't flap around in the wind.

I have started thinking about ideas for the "ever elussive" inflatable gap filler/boattail idea, just read through some old discussions on the topic:
http://ecomodder.com/forum/showthrea...ack-22480.html

Seems the biggest hurdles are:
Inflation & deployment system.
Maintaining appropriate shape.
Minimising oscilations.
Fabric durability.
Simple attachment & removal for utility.

Would shade cloth be a suitable external skin "aero wise" as it is a fairly course weave?
From what I have read all indications are it should be fine as I have seen many comments suggesting that in reality we are actually moving turbulant air all the time so finer details of skin surface are irrelevant, the basic form is what is important.

*I'm thinking if one were to make the outer skin out of this type of material, it is cheap, strong & durable and is often used on the front of trailers as a stone deflector to minimise paint damage.
*Then use a lighter weight material internally to create a grid pattern lengthwise so individual inflation tubes could be inserted, visually looking into it like a case of wine, 12 individual bottles each in its own compartment.
*The inflation tubes could just be a suitable length and cross section of plastic tube sealed at one end and inflation tube attached to the other end.
*All tubes connected to one inflation manifold, where you can have one way valve, inflate/deflate etc.
*Use a duct from the front stagnation point, highest pressure on vehicle is there I imagine, supplementary blower if required. I could probably duct 2-3" flexitube without increasing frontal area on my car.
*Mount on vehicle side with a solid frame of appropriate design and angle to begin boattail shape.
*Trailer rotates around shape deforming as required, maybe a slip sheet of thick plastic attached flat on trailer face to protect trailer finish.
* Have one larger cell (or multiple) in the centre with a large rubber balloon attached to the manifold, this would remain deflated in normal operation, but when the trailer was turned hard the pressure from the compressed cells would inflate it temporarily until vehicle was in straight line again.
*It is a relativly low pressure set up < 3 psi most likely, aim to use commonly available materials to keep cost, but moreso inevitable maintenance down, one way valves can easily be made with ping pong ball, rubber O-ring and PVC pipe fittings, the pump/fan would be a negative, best avoided if possible.
*HD zippers, like on tents or larger luggage bags, could be incorporated for easy attachment and removal etc.

That's today's idea, see what else comes along.

Mine is a 4'x8' and it could very well go down to 1-2mpg if I took the wood sides off. But I built them because I use them.
It does a lot, I should put up a post. Little trailers like that could eliminate most of the need for a "pickup", they are very cheap buy, own and maintain.

Totally agreed, they're light easy to tow and can do most things regular folk require, mine's maybe just a bit short, fitting 4'x8' sheets in is a bit of a challenge, but I have managed it otherwise it services my needs well, sometimes an extra foot of length would be nice but most times not required.

Keeping the width in the vehicles wake is the main goal, that's half the battle.

antics
 

A couple years ago,returning from a Bonneville trip with the trailer,she went airborne a number of times near Vail,Colorado on I-70 when I hit pressure ridges caused by winter freezing.
In a curve,the trailer would come completely off the ground and swing away due to centrifugal force.Bizzare!
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*As to inflatables,If you're really serious,I recommend you look into the aerospace fabrics used with outdoor inflatable advertising.They'll be something like a Hypalon-coated fabric which can endure arctic temps while remaining supple.
You'll also need to invest in,lease,rent,or borrow a commercial sewing machine as used for automotive,marine,and aircraft upholstery.You need the double walking-foot and ability to plow through heavy material without snapping the needle off.
With these fabrics I don't think you'll need internal bracing structures.
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The envelope can be secured with VELCRO.
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I've used a 12-VDC ventilation fan from a VW Westfalia Campmobile for inflation.Just providing inflation tension,it doesn't use enough Watts to affect mpg appreciably.They'll run day and night no problem.
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I recommend that you build a mockup and begin testing.As reality intrudes and things get ripped apart it will evolve into a lasting design.
There are so many dynamics involved with a articulating trailer that you've just got to jump in and see how it goes.
I've had many failures on my path,but as my project sits right now,I could take her around the world without event.

This is a variable wheelbase trike. But it could easily be a 4-wheel vehicle with a hinged, *retractable* single wheel trailer.
http://i.imgur.com/cCrh5.jpg

Double wall inflatable, like a bouncy castle.

Else:

• A net with coroplast fish-scales
• Chain mail
• Telescoping vanes like an armidillo

Looks a bit like a lobster on wheels, if front wheel fairing was painted up like claws to complete the picture.
But nature does often provide the solutions,
Still like the inflatable idea for bulking the fill and possibly with a complimentary technology using foam or coroplast for added rigidity.

As long as they were aerodynamic lobster claws, I guess.

I'm going to chase that idea a little more. I made this late last night for another thread.
http://i.imgur.com/TRVD3B4.png


suppose those 8 triangles that are lighter than the rest folded out. The hinge line would be at the sides of the pyramidal cap and it would have a trailer wheel and tire at the 6-way vertex. The trailer tire would normally fly just above the ground, but with an overload and bad departure angle it would touch down like aircraft landing gear.

For those of you that just went :eek: that would be a four-wheeler, not a trike.

I'm playing with some of the same issues with my camper. I get about 50MPG clean in my 2003 Civic Hybrid and 30 MPG towing my teardrop camper at 60MPH in 4th gear.

I was thinking about some sort of rubber sheet wrapped around the front of camper and the back of the car. You could use cargo ratchet straps or even bungee cords if more elasticity was needed for turns etc.

Alternatively, I wondered about a v-shaped tongue box that would double as storage space ? I also thought about a plywood fairing that I could install on the back when I'm towing and would strap down to the trunk lid like a bicycle rack or even attach to the bike rack that is already there?

I would love to be able to cruise in 5th gear, although at 2,000 RPM the engine still is probably not turning enough to power that load in 5th, will have to see.

https://sphotos-a.xx.fbcdn.net/hphot...18889243_n.jpg

I've been thinking about this as well. What about "stretching" a durable rubberized fabric (((so its fairly "loaded" but has the stretch capacity to expand for turning?

My other thought is a fiberglass or wooden air dam on the car itself...could make it just strap down temporarily like a bike rack and trim as needed to allow the trailer to jackknife as needed, would be simpler for trunk access as well

elastic structures
 

I'm suspicious of an elastic membranes ability to 'hold its shape' under aerodynamic loading.I think you'll see oscillations and flutter at road speeds unless you could provide extreme tension.
I would tend towards an inflated envelope.Even ballistic missiles and rockets with extremely thin skin section can withstand aeroelastic effects if the airframe is provided with a modest inflation pressure.

Keeping the width in the vehicles wake is the main goal, that's half the battle.

I think, if you ask around, that most of us find that it is height (of the trailer) that degrades FE faster than width.

While the total amount -- and shape -- of the frontal area of the trailer seems fixed, it is that an "aero" cargo trailer (trying to use conventional shapes) benefits more by having been increased in length (even though it extracts a penalty) over a heigtht increase (where total cubic capacity is unchanged).

Again, the shape of the trailer is important. Defining the job[s] defines the trailer. From there the ways to guide the air between tow vehicle and trailer become more limited. A trailer one must be able to stand up within is a far different cry than one which has a lid or other hard cover for cargo.

A trailer that is in service to such an extent that the two are rarely uncoupled for engine-on hours needs to be different than one that is used even twice per week. Or, month. The number of miles covered against solo miles is a percentage. And a small percentage is only worth so much work, IMO.

Second to that is the expected weight of the trailer. Manufacturer guidelines don't tell the story very well as hitch rigging has more to it than what is realized. On-road only, or some unpaved road miles is another. Etc.

Taking a stab at the thing with a common template (trailer frame dimensions, clearances, etc), is where I'd go next. And to start with a common-enough vehicle (for it need not be a truck as we know around here).

So, is the vehicle one which runs most miles solo, or under a load? Most of the aero decisions are right there.

Whatever its merits, I've always felt Phil's solution was way too complex to work IRL.

It's got to be done the easy way to work - KISS.

* Optimised for straight line duty and slight turns, i.e. highway style driving.
--- Don't bother about streamlining in slow hard turns

* Gap filling plates hinged on the tow vehicle
--- outboard movement to be restrained by bungee cords
--- inboard movement physically restricted by solid stops, yes, this will leave a gap on the outboard side in a hard turn - that's not an issue
--- downward movement of the top plate to be physically stopped by the sideplates , upward restrained by bungees
--- bungees might be fixed to the physical stops for the gap-filling plates, but not too far aft on the plates in order not to bend these.

* front side and top of the trailer to be rounded
--- helps even without the gap-filling plates
--- rear end of the gap-filling plates to rest on the rounded front, using teflon wheels on the plates
--- small, locally applied teflon plates on the trailer where the above wheels go as the plates slide over the front of the trailer

Not sure if there is value in copying it, but the Seattle area buses look like this:
http://a.abcnews.com/images/Business..._111201_wg.jpg
It's basically a permenantly attached trailer on the back of a bus, so there are three total axles and it articulates in the middle. The front and rear are connected with an accordian-like material.

I've thought about building a triangular shaped plywood box on the tongue of my travel trailer. My thought is that is would improve aerodynamics while towing, but also provide some storage space for trailer related items that I could get out of the cabinets in the trailer (it is somewhat light on storage space). There may not be a lot of gain economy-wise, but the dual-purpose nature would make up for that.

I've also put some thought into lowering the trailer, as it sits quite high. I think this was done to keep the fenders from intruding so much into the trailer, but the overall height are much higher than most of the other trailers I see at the campgrounds.

plates
 

The most challenging hurdle I ran into was when in a low speed turn,with trailer cocked,and on the sloping crown of the roadway,and the tow vehicle was ascending a laterally-level up-slope.
With only aluminum skins on the outside of steel-truss skeleton plate frames,the side plates would snag underneath the top plate and destroy it as it was displaced span-wise.
By moving the side plates onto the trailer by use of the lolly-shafted C-frame,the side plates cannot bind against the trailer during any type of orientation,nor the integrator on the back of the tow rig.
The upper frame has skids and rollers which are in constant contact with the trailer.As soon as the trailer displaces in any manner,the upper plate can raise to clear.An upward displacement of the trailers tail causes no interference.
I've yet to finish the lower plate(s).Those will have to wait 'til after the A2 Wind Tunnel.
As far as bungees or springs,I found that the side plates would be required to swing a full 90-degrees to clear the trailer face at 'full-lock',and no bungee or spring could maintain tension while 'straight',without being stretched beyond it's elastic limit and permanently deform when at full 'turn.'
I ended up with something akin to cable/pulley/double-hung window sash weights, riding vertically inside no-snag elevator shafts.These can handle the entire range of motion without damage.
It is complicated but has survived over 5,000 test miles without a hitch.
I recommend that the nose of the trailer be just as close to the tow ball as possible to allow the side plates to be as short and as light as possible.
When you strike a pothole or pressure ridge in the roadway it causes 'jerk,' which is an instantaneous acceleration,and can easily destroy structures who's inertia/momentum loads can be overcome during these 'instant' transient loads.:o

accordian
 

I like the accordian type integration very much.I do not know how they do when torsion is introduced.
The Monorail at Disney Land in Anaheim,California has used it since the 1950s.But it runs on a track and experiences no torsional loading.
Bus systems typically stay on the streets without dealing with driveway slopes and the like.
Same for passenger trains.
I remain shy of it,as I cannot fathom how they would perform with radical,multi-orientations thrown at them when in passenger car/trailer applications.

The most challenging hurdle I ran into was when in a low speed turn,with trailer cocked,and on the sloping crown of the roadway,and the tow vehicle was ascending a laterally-level up-slope.

A problem for any of us with low ground clearance trailers. My TT (travel trailer; a longer one than in my current sig pic) has a very long rear overhang and not much clearance. So, fuel stops and the rest are best planned in advance (reference materials and Google Map confirmation).

One must also (with Texas law and other states) pulll over at a "safe place" to allow traffic around one. So, one is always scanning the road shoulder on two-lane highways to recognize those places. There is a pretty good burden in being a draft horse out on a race course. It cannot be ignored. (In the same vein: mirrors on the TV must allow visual convergence on an object 200' to ones rear. This is not optional. What is past (behind one) is (actually) "future".


I recommend that the nose of the trailer be just as close to the tow ball as possible to allow the side plates to be as short and as light as possible.

For purposes of tow vehicle stability (steering gradient index, and braking) keeping yaw to a minimum, the tow vehicle hitch receiver and the trailer coupler are ideally as short as possible. Shorter than what most off-the-shelf stuff measures to (givens are that one must be able to clear a 90-degree turn; in backing one WILL hit the trailer with the tow vehicle otherwise). There is nearly always room to shorten by a few inches.

Custom hitch builders still take this into account.

IMO, this is where the money is as combined vehicle dynamics are vital. Reducing driver inputs to maintain lane-centeredness is the first goal. Aero is secondary . . icing on the cake that is a well-chosen pair of vehicles, well-hitched, and on a well-planned trip. Aero won't mean much without those first done.

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You're in a better position to know, but I thought the significant ratio was tow-vehicle rear axle centerline to hitch vs hitch to trailer axle centerline. I'm not picturing how to shorten the hitch/coupler.

1] Hitch receiver design. Inches count

2] Length of drawbar. Inches count

For any given tow vehicle.

Shortest rear overhang -- from tow vehicle axle center to hitch ball -- is best, as you note, but separate from the above.

The "best" trailer for a given use (shape + weight, first; second are road performance issues such as COG, ground clearance, suspension and brake type) drives the design for best tow vehicle. "Given use" drives the length of the trailer (among other dimensions).

The more choices one has in appropriate tow vehicles, the "better" is the trailer design, IOW. A Euro turbodiesel SUV or sedan may return all the best numbers . . but how affordable, in main? Avoiding the need for a pickup truck ought to drive things -- so to speak -- for a non-dedicated combination (where solo miles predominate).

If HP demand can be settled to the comfort of the owner, then the lash-up can be optimized for a given tow vehicle.

It's circular, but the trailer drives all other decisions. Trailer weight is secondary to trailer shape, on the one hand, but climate & terrain (mainly altitude) are the tow vehicle limiting factors. Paved, or unpaved. Etc.

Subsidized roads and subsidized fuel are the largest picture. Without them this is an exercise in futility.

More closely, we might say that the scaled weight of the payload, and it's shape, is what is at stake.

How quickly or slowly, and at what cost it arrives at it's destination is what needs sorting as secondary concerns (that folks often assume as being primary).

Shipping that same load by rail or truck versus transporting it ourselves reveals that we find convenience to be of a higher value than true lowest cost. Where do we draw a line? What are the upper and lower limits of payload shapes and weights. Etc.

Hitch rigging (lash up) comes down to specifics of the vehicles chosen. Aero is simply unknown until that point.

A pickup with the bed removed pulling gooseneck-hitched trailer may well be the best candidate for an aero trailer "package". But how useful is that tow vehicle to a non-commercial user in solo miles?

Etc.

.

The 'fifth-wheel' hitch needs to be at approximately the axle line, right? I know with farm tractors if the drawbar is above the axle line bad things happen.

Suppose the hitch is drawn up under the body, which terminates in a convex spherical sector that is centered on the hitch ball. Then a trailer with a long tongue and a concave front bulkhead could make torsional and angular excursions without impacting the tow vehicle.

The PULLRITE hitch essentially replicates the 5'er / gooseneck advantage. Is this what you were thinking? (As to "safety" -- minimizing / eliminating trailer sway -- it is an excellent choice).

.

Since I'd never seen it before, no. But it does point up a problem with what I *was* thinking. Which was the tow vehicle (or a profile extension) shaped as a quarter sphere with a half cone carved out of the bottom center for a single tongue to fit into. (How do you latch the hitch onto the ball?)

The Class III and IV hitches the Pullrite is intended for use an A-frame on the trailer, which pretty much kills the idea for me.

I tend to think of the lighter end of the scale.http://i.imgur.com/bnY0ZwJ.png

But it seems to scale up: http://www.youtube.com/watch?v=dgJ3lSbOqqw

nose/ball geometry
 

The 'aerodynamic' nose of the trailer could differ from the 'cargo area' nose,maintaining tongue length and tongue weight (as with the inflated trailer nose from England).

I must have missed it. There is a current aftermarket "inflated trailer nose" ( similar to the NOSECONE hard piece)?

I've seen other references to an older one (ca. 1982) bought up you, aerohead, but not any offerings of recent.

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recent
 

Nope,so far it only exists as an old half-tone photograph in a magazine.
An idea I tried to sell Gilkison on for his APEX RV trailer,was to get a local awning company to create a welded-aluminum space-frame which mimic'd Airstream's nose,and just skin it with the UV-resistant cotton duck,or whatever it is that they use for the outdoor awnings.
It wouldn't be as 'pretty' as a hard-shell nose,but the air would love it.It would weigh essentially nothing!
It could detach and be used for a nice sun shade as well.

Yes, I have saved your reference. And searched it more, but without success. And thought about it for both FF and RR on my TT. Thanks for clarification.

Anybody for re-upping it so I can see if it's something I missed?

Imagine this with the rear wheel and tailpipes replaced with a trailer tongue and hitch:http://ecomodder.com/forum/member-fr...nd-inflate.jpg

trailers

Something like this . . but in actuality an inflated fabric stretched over a framwork on the trailer frontal area, (not just a gravel protector as the lower panel). My thoughts were to do both. Nose-entry, and "belly pan" (versus nose-entry and big honkin' mudguards hanging from truck rear)

The reference was for an inflatable trailer nose, from England IIRC, ca. 1982 in Pop Sci mag. Naturally, I can't find my own link, but it is referenced in a thread on this site.