16 foot single wheel unusual TD build

[MtnDon]

I had the same initial thought. But on thinking it over if the two yoke ends are solidly secured / welded to the tow vehicle and the the trailer there will be no rotational motion around the longitudinal axis like there is when used in a driveshaft. The u-joint will then provide lateral movement (turning) as well as up / down movement. I'm uncertain if there will be any binding when a turn involves a dip for example. Would a double cardan joint be better, worse or unnecessary???


That will be some long turning circle though with the rear wheel way back there; might be a learning curve when corners on narrow roads.

[WeirdDogGuy]

If my single foamie had a "regular" ball and socket trailer hitch, it would do just that. Since the foamy is not perfectly balanced, it would just flop around the hitch like a fish out of water. A ball and socket joint like a trailer hitch has too many degrees of freedom. It pivots side to side (like when you are making a turn) and it pivots up and down (like when the TV enters a steep driveway) and it also rotates around the long axis of the trailer. It can't rotate 360 degrees of course, because the receiver socket will hit the tongue base where the ball screws in-but it can rotate partially. A universal joint does not posses this rotational degree of freedom. It is constrained in that plane. So a universal will pivot side to side and up and down but it can't rotate by design. Look at my pix with the universal joint. In your mind's eye, pivot it side to side and up and down. Now try wringing it like a wet dish rag. You can't. When you twist the right yoke, the left yoke twists right along with it. So the TV and trailer rotate equally along the long axis of the rig. They mimic each others angulation. If I'm driving down the road and I'm in a stiff cross wind from the passenger side, my TV will list to port and so will my trailer. If the wind is from the drivers side, the TV will list to starboard again, at the same angle as the trailer. A universal joint cannot allow a differential rotation between the two vehicles. Can you see that?

[WeirdDogGuy]

I think the plan is to limit the u-joint to pitch and yaw, but to weld the shaft at either end to eliminate roll.

U-Joint looks small to me, like from a PTO of steering shaft. What is the shaft diameter? We can do a torque analysis and see if it has any chance of survival.

That's right. No roll.

It is small, just like the rest of my build, just like my TV. There is no more shaft. I welded the shoulder of the yoke to the mounting block, which will be welded into the center tube or "spine". If it fails, it will fail in sheer at the crossmember of the universal. The diameter of the steel cross member as it enters the yoke bearing is 0.473 inches. Or maybe that's what you meant by "shaft diameter"?

[KCStudly]

The original shaft diameter would be a good indication of what the anticipated or original design load was.

So lets do some back of the napkin math here. Lacking the shaft diameter, assuming that the u-joint neck cross section is the weak link, and that it is at least mild steel (not cast iron), we can look at its ability to resist shear.

Pie are square, right? (.473/2)^2 x Pi x 24kips = 4217 lbs, where 24kips is 2/3 the tensile strength of steel. I'm using single shear based on the open geometry of the u-joints yoke.

Now don't get too excited, we still have to look at the lever arms.

Let's assume that the neck of the joint is 3/4 inch from the center. That gives us a moment of 3163 lbs-in. Still seems pretty stout compared to the projected weight of the trailer. But not so fast.

So most people miss their projected weight, some by a little, some by a lot. Let's be realistic and assume that it ends up at 1000 lbs loaded, and let's further assume that just half of that rests on the tire (...so the other half is on the u-joint, but let's not worry about that right now). So that's 500 lbs on the tire... static, not dynamic.

Now let's take that 500 lbs and see how many inches it can be offset. 3163 lbs-in / 500 lbs = 6.3 inches. At yield.

Add a minimum 1.5 safety factor (because just barely not breaking isn't okay) and we get about 4.25 inch allowable offset... static.

So that's suggesting that we might have a problem when we consider the combined stress on the u-joint from dead weight and torque, not to mention that dynamic loading can typically be as high as 10x static (I wouldn't consider anything less than 4x for dynamic).

Oh, and don't forget about torque induced by side winds.

Conclusion: I'm not a licensed engineer (so please refute any errors), but it looks sketchy to me.

So someone is bound to ask, "what's the big deal? The hitch pin on my big a%% trailer hitch is only 5/8 diameter." True dat, but it is captured in double shear, sees no torque and has 75/ct more cross sectional area.

[WeirdDogGuy]

The original shaft diameter would be a good indication of what the anticipated or original design load was.

So lets do some back of the napkin math here. Lacking the shaft diameter, assuming that the u-joint neck cross section is the weak link, and that it is at least mild steel (not cast iron), we can look at its ability to resist shear.

Pie are square, right? (.473/2)^2 x Pi x 24kips = 4217 lbs, where 24kips is 2/3 the tensile strength of steel. I'm using single shear based on the open geometry of the u-joints yoke.

Now don't get too excited, we still have to look at the lever arms.

Let's assume that the neck of the joint is 3/4 inch from the center. That gives us a moment of 3163 lbs-in. Still seems pretty stout compared to the projected weight of the trailer. But not so fast.

So most people miss their projected weight, some by a little, some by a lot. Let's be realistic and assume that it ends up at 1000 lbs loaded, and let's further assume that just half of that rests on the tire (...so the other half is on the u-joint, but let's not worry about that right now). So that's 500 lbs on the tire... static, not dynamic.

Now let's take that 500 lbs and see how many inches it can be offset. 3163 lbs-in / 500 lbs = 6.3 inches. At yield.

Add a minimum 1.5 safety factor (because just barely not breaking isn't okay) and we get about 4.25 inch allowable offset... static.

So that's suggesting that we might have a problem when we consider the combined stress on the u-joint from dead weight and torque, not to mention that dynamic loading can typically be as high as 10x static (I wouldn't consider anything less than 4x for dynamic).

Oh, and don't forget about torque induced by side winds.

Conclusion: I'm not a licensed engineer (so please refute any errors), but it looks sketchy to me.

So someone is bound to ask, "what's the big deal? The hitch pin on my big a%% trailer hitch is only 5/8 diameter." True dat, but it is captured in double shear, sees no torque and has 75/ct more cross sectional area.

Hey thx for the number crunch. It's interesting to look at different loading scenarios. I have 2 comments. 1) This is a single wheel/axle foamie and will never be anywhere near 1000 pounds all up. No way. It will be much closer to 500 than it will be to a 1000. My kitchenette will be under my hatch in the TV. A grand will never happen. 2) If you really want something to worry about, look at the loading that will cause the walls of my 2-1/2, 14 ga steel tubing to deform and collapse due to torsional loading. I haven't crunched numbers but I suspect the tube will collapse before the u joint fails. I would expect the failure to occur where the tube exits the front of the foamie body. All the stress that the foamy body is carrying gets dumped into the center tube and discontinuities like that usually act as stress concentrators. Thx for your comments.

[eggsalad]

Just don't get me wrong - I'm not in any way trying to put you down. I love interesting new designs, and I want to encourage you!

Now that I've got my brain wrapped around what you're trying to do with the U-joint, I'll encourage you to look at some of the off-road builds. The folks who build hardcore off-road trailers do something very similar to what you're trying to accomplish with the U-joint, but they do it with two separate "hinges", spaced a couple inches apart - one for the y-axis, and another for the z-axis.

[WeirdDogGuy]

Oh, I'm not put off at all. Criticism of my design is helpful and gives me new things to think about. Regarding the 2 hinges that the off roaders use; I think that's a pretty good explanation of a u joint except that the hinges aren't a couple of inches apart-they occur at the same place (the cross member of the u joint). Thx.Andy

[GuitarPhotog]

I think the OP should consult Roly Nelson here on the forum. He started a single wheel teardrop but had issues and abandoned the plan. I don't know if the issues were legal or structural.

Make sure your state will approve a one-wheel trailer for use on the roads.

<Chas>

[mezmo]

Hi WeirdDogguy,

Good to see further info on your approach. I had some other comments,
but they were concerning your design done as a single-swivel-wheel set-up as that
is what it seemed like you were doing. Since that isn't what you are doing they
don't pertain - Except - I would consider locating the single-wheel to more towards
a mid-body location, or even better yet, use both of the VW wheels/suspension you
have, and tuck them in the body taper - alligator leg style - at about the "60" position
for wheel center, using the "60/40 axle location" rule of thumb. That should make it
much easier to manoever when towing. Your design is so aero that what little extra
weight that using two wheels would entail really wouldn't have much of an effect.

Meanwhile, I think your design is very singular. I had thought you were planning
on carrying all the equipment [kayak,winfsurfer,mountain bike] on the inside
at once, hence the length. So now, carrying them individually will obviously
make your trip packing tasks much easier. The design will also be quite "aero" with
the front cross-section fitting within your tow-vehicle's cross-section, and then
the nice long shallow taper to the end 'point'.

I was wondering on your build method. The 'composite torque-tube' explains alot
but I wonder if the casein glue will be robust enough ? I admit to the average ignorance
concerning glue qualities, but most that I've run across seem rather brittle, so I'm
sure you must be checking all that out. I'd guess the most common or default skinning
technique would be fiberglass cloth and epoxy.

The way your design example looks, it reminds me of the early aircraft of the 1920s with their
tubular truss and fabric covered fuselages. I'll just throw out some ideas related to that for any
benefit they may give you. I'd do a triangular triangulated base frame, with a full perimeter
bumper around it [just a tube] - like South African Off Road Caravans]


and have receivers integrated , 3per side, one on each corner, and one ahead of each wheel.
This would be useful to be able to attach various set-up/camping items to it, like leveling jacks,
tables, awnings etc. , and provide a place to tie off things to.
See: [This is one of The Best ideas I've run across on the forum here.]
TD Manufacture Suggestion: 4 Receiver Tubes? Here's Why...
http://tnttt.com/viewtopic.php?f=2&t=30925

Maybe use a galvanized emt truss for body sides and roof truss members, and fit foam in the
truss spaces and smooth with greatstuff, fair out, cover with "glue-n-canvas" - OR - use bead and cove foam strips applied over the trusses [Like they are doing over hull molds on some
self-build boat web sites.], then skin. I'd also make the door somewhat wider/larger or do a
door within a door for easier loading of the bulky items. [A smaller [but larger] entry door
within an overall larger cargo door.]

Your universal joint based hitch gives me pause as It seems to rely on a very small amount
of metal to handle all the towing stresses - or are you using an extra large U-joint? I'm sure
you've more experience in that area or you wouldn't be taking that approach, ['doing so to
maybe 'see if the idea will work'?] but what extra safety factor are you bulding in? At face
value you would probably have an easier time of it all by using two wheels on the trailer,
positioned more traditionally, and some version of a multi-axis hitch like the Lock-N-Roll. [ https://locknroll.com/ ]

Vastly new designs always draw comments. Mine are meant to be constructive and hopefully
beneficial to you as you persue the build.

We'll have fun watching.

Cheers,
Norm/mezmo

[rowerwet]

I know static the joint will work for the load you have, dynamic loading is a whole bigger box of worms. I just wonder about the torque on the tongue/hitch/bumper and what will twist/fail. single wheel trailers work when kept short and low for motorcycles, high sides/long length, impose much higher twisting forces.
I love the concept, but think this is an exercise for serious engineering, after all your experiment is putting every other driver on the road at risk if it fails. You don't want to be explaining that you thought it was a great idea to a judge/lawyer/insurance co. Road safety is a serious issue with permanent consequences.

[rowerwet]

I don't want to be too harsh, but I have a friend who was trapped in his car for a few hours, and ended up with a broken hand and stitches (put him out of work for weeks) along with other cuts and bruises, while his daughter was still suffering bouts of nausea dizyness and nightmares, and his wife had seatbelt burns, all because somebody going the other way didn't have their trailer secured correctly.

[WeirdDogGuy]

.... Your design is so aero that what little extra
weight that using two wheels would entail really wouldn't have much of an effect.

This is developing into the most common criticism; that one wheel is wrong. If 2 wheels are better than 1, then wouldn't 10 wheels be better than 2 ?????

The design will also be quite "aero" with
the front cross-section fitting within your tow-vehicle's cross-section, and then
the nice long shallow taper to the end 'point'.

Well spotted!!! That is exactly how I sized the coach. I wanted it as big as I could get it yet still fit into the wake of my TV so it would draft behind it

I was wondering on your build method. The 'composite torque-tube' explains alot
but I wonder if the casein glue will be robust enough ? I admit to the average ignorance
concerning glue qualities, but most that I've run across seem rather brittle, so I'm
sure you must be checking all that out. I'd guess the most common or default skinning
technique would be fiberglass cloth and epoxy.

That casein is pretty tough stuff. In a composite build, the glue does provide strength but perhaps more importantly, it holds and orients the fibers of the fabric in place. The fibers do most of the work and the loads get spread out over the millions of individual fibers, that's why it's so strong. I've never vacuum bagged but I'm thinking about bagging some birch ply or veneer onto my foam panels. I kinda have my heart set on birch for the inside. I'll probably need stations on the inside to support the walls as they go up and the roof goes on. Once the structure is glassed, it will be plenty strong and I can remove the stations.

The way your design example looks, it reminds me of the early aircraft of the 1920s

I'm glad you picked up on this. I'm going for a retro look.

Your universal joint based hitch gives me pause as It seems to rely on a very small amount
of metal to handle all the towing stresses

It's all relative. I don't consider a .473 diameter steel hinge pin a "very small amount of metal" but your mileage may vary.....

At face
value you would probably have an easier time of it all by using two wheels on the trailer,
positioned more traditionally, and some version of a multi-axis hitch like the Lock-N-Rol

Again with the two wheels!!!!!

[KCStudly]

I'm not opposed to the single wheel idea at all, so long as the loads are understood and accounted for there is no reason that it could not work. That said, I personally am not convinced that due diligence has been given; at least not based on your words so far. On one hand it sounds like you have a good understanding of what you are getting into, but on the other you indicate that you want to use stuff at hand.

The two wheel vs. one wheel "argument" is the exact thing that you explained about the fibers. Two wheels spread the loads out in exactly the same way that the multiple fibers do, spreading a larger load out over a greater area. By transferring roll from side to side on a two wheel setup, the chassis is being loaded transversely in bending, not torque; the side to side reactions transfer weight from one side to the other with center of gravity helping to "right the ship"; there is no torque load concentration at the front wall, and no torque load on the hitch or TV.

Think about the cornering characteristics of your TV and what the uni-wheel trailer will do to that. Where will the CG be vertically?

[WeirdDogGuy]

On one hand it sounds like you have a good understanding of what you are getting into, but on the other you indicate that you want to use stuff at hand.

Both true.

The two wheel vs. one wheel "argument" is the exact thing that you explained about the fibers. Two wheels spread the loads out in exactly the same way that the multiple fibers do, spreading a larger load out over a greater area. By transferring roll from side to side on a two wheel setup, the chassis is being loaded transversely in bending, not torque; the side to side reactions transfer weight from one side to the other with center of gravity helping to "right the ship"; there is no torque load concentration at the front wall, and no torque load on the hitch or TV.

OK. But I don't want 2 wheels. I want 10 wheels because more is better. So should I build with 10 wheels? If not, why not? Please be specific.

Think about the cornering characteristics of your TV and what the uni-wheel trailer will do to that. Where will the CG be vertically?

Depends on the distribution of mass. Foam is high and steel is low to keep the CG reasonable. I could have easily used a motorcycle wheel for this build but one of the perks of the vw axle and bigger wheel is helping to lower the CG.

Thx for your comments. Andy

[MtnDon]

I want 10 wheels because more is better. So should I build with 10 wheels? If not, why not? Please be specific.

Not.
Tire scrub on turns, because of all the different radii of the tires/wheels.

With a single wheel as described there are or will be forces at work that are different from those that have to be dealt with using a conventional axle and two wheel arrangement. I believe that is the basis of the expressed concerns. It is somewhat uncharted ground. When we run into unconventional processes in designing and constructing houses we refer to an engineer.