16 foot single wheel unusual TD build

[WeirdDogGuy]

Welcome to my build. Here are my design criteria:

1) My TD must fit my mountain bike
2) My TD must fit my windsurfer
3) My TD must fit my kayak
4) My TD must fit me
5) My TD must be light enough to be pulled by my tow vehicle, a 1600cc Subaru.

So I ended up with an unusual TD; a 16 foot foamie with a single wheel. Here it is:

[eggsalad]

I'm very interested in single-wheel suspensions and hitches. I'll be keeping an eye on this thread.

[WeirdDogGuy]

The first decision that I had to make was what type of wheel to use. I had 3 options in my scrap pile. 1) the rear triangle of a suspended mountain bike 2) the rear triangle of a motorcycle, also suspended 3) the rear axle of a 1983 VW Rabbit. I need to build light but I was concerned about building too light. Afterall, I'll be towing what amounts to a large sail behind my car. I don't want to be pushed around by large passing trucks on the interstate so I chose the heaviest option, the VW axle. A VW Rabbit is front wheel drive and the rear axle is just a tag-along. This is what it looks like:

[WeirdDogGuy]

Of course, I only need half of it so I cut off one side with the torch. It will be suspended with an adjustable motorcycle monoshock.

[mezmo]

Hi WeirdDogGuy,

Welcome to the forum.

Could you give more general details on your overall build approach or plans ?
[The "how" you plan to proceed.]

It's a very singular design, and I have some potential comments
but don't see any need to bother you with them if you have already
covered them in your planning so far.

Cheers,
Norm/mezmo

[WeirdDogGuy]

Sure. Form follows function. The design was originally a little longer b/c the longest item was my windsurfer mast but I decided to house the mast in a piece of pvc pipe underneath the subframe so that allowed me to get the length down to 16 feet. I like my Soob since it gets 32 mpg and is 4wd but it couldn't pull a greased string out of a cat's behind so it has to be a foamie ie lightweight. Serendipidity ruled the rest of the design. I'm trying to build it out of "stuff" that I already have. The hinge on the swing axle will be offset 4 inches from the centerline of the long axis. I had to offset it in order to have the centerline of the tire track inline with the centerline of the long axis. It's just the way the geometry worked out with the VW axle. Because the mounting point of the swing arm is offset, it will always impart a torsional moment on the structure. The best cross section to resist torsion is circular, but, oddly, I didn't have enough of the correct size round stock to use for the subframe. I do have a lot of 2-1/2 inch galvanized square tubing (14 ga) and that's the next best thing to resist torsion that I have available. But this will be a stressed skin structure so really, the fiberglass composite skin of the body will be working in conjunction with the subframe to carry the loading. So the body is a big torque tube in bending. Most (but not all) modern cars are built this way. The body is a space frame/truss with a smaller/lighter subframe assembly so really, this is nothing new. I'm using homemade casein as a resin for my composite. I started a thread in the foamie subforum about using casein and fiberglass as a composite material. It's common for foamie builders to use titebond+canvas as a composite. I've been experimenting with casein+Tyvek as the fabric but it's pretty lightweight stuff-1.85 oz/yard-and I'm not particularly impressed with its strength. I can tear Tyvek with my fingers. I have some other leftover fabric from a couple of years ago. I think it's 6 oz Xynole or some similar polyester fabric.Not sure, I'll have to check my records. Not sure yet which fabric I'll use, hemp is in the running, but I have decided on DIY casein as the resin. I'm also considering using more than one layer of fabric. That way I can orient the weft and the weave to its greatest potential. So for torsion, I'd lay the first layer of fabric 45 degrees to the long axis, then the second skin would be parallel to the long axis. I'm still welding the subframe so I have plenty of time to stew of this as I am not close to glassing yet. Two heads are better than one and I'd be happy to hear your ideas. Thx.Andy

[working on it]

Because the mounting point of the swing arm is offset, it will always impart a torsional moment on the structure.

Why does it have to be a single wheel? If you have the complete axle on hand, use both sides, and make it track true without the twisting bias imparted by a single wheel. And you could weld-in a very short axle/mounting point in-between, and then another piece between the wheels for a single spring/shock, or an airbag or airspring mount. Plus, the added benefit of a second tire in case of a flat or load carrying capacity.

rabbit dually.png (127.16 KiB) Viewed 2730 times

[WeirdDogGuy]

It doesn't have to be a single wheel. Your suggestion would work fine. But why do it? One wheel has more than enough strength for my little foamy. So if I go to 2 wheels, I have added weight, and I have lost interior space. If you look at the top of my drawing, you can see that even my single wheel just barely fits in it's allotted space. Yes, I can put two wheels outside the body like a conventional build, but then I would have more aerodynamic drag. So, more weight, more wasted space, more expense and if I needed the extra carrying capacity, I would do it. I simply don't need a second wheel. My structure can handle the torsion so why worry about it? And I already have a spare. I machined the VW hub to take the Subaru 140mmX4 bolt pattern. So my trailer wheel will be a Soob wheel. If I get a flat, I'll just use the car spare. I'm trying to build as lightly and efficiently as I can. Remember, I only have around 60 HP. Thx for your ideas. Andy

[KCStudly]

But why do it?

You said it yourself, because 16 ft of sail behind your light car is going to steer it all over the road, especially with a rigid hitch. All of the uni-wheel trailer setups I have seen have a double hitch at the TV bumper and a caster provision at the trailing wheel. The are generally very short by necessity to keep the trailer in (or close to) the track of the TV. I see none of these features in your drawing. How do you plan for articulation at the TV and turning? How will the rear of the trailer not scrub tire or change lanes while turning?

[WeirdDogGuy]

But why do it?

You said it yourself, because 16 ft of sail behind your light car is going to steer it all over the road, especially with a rigid hitch. All of the uni-wheel trailer setups I have seen have a double hitch at the TV bumper and a caster provision at the trailing wheel. The are generally very short by necessity to keep the trailer in (or close to) the track of the TV. I see none of these features in your drawing. How to you plan for articulation at the TV and turning? How will the rear of the trailer not scrub tire or change lanes while turning?

It doesn't have a rigid hitch, it has a standard universal joint. It's not in the drawing and that's why you can't see it. Here I am building it:

[WeirdDogGuy]

Test fitting it into the mounting block. The assembly will be welded into the center spine which is 2-1/2 inch galvi square tubing. :

[rowerwet]

even limited to a mountain bike, kayak and windsurf board and rig, you have a 100-200 pounds of gear and, while a foamie is light, you are still looking at close to 800 lbs of structure with that length, the torque applied on that hitch and whatever it is attached to on the tear and car just going around a corner at speed will be high. With that "sail" area of the side of the tear in a cross wind or on-coming 18 wheeler the force it will exert on the car/hitch/tongue will be amazing . I would be very concerned about the trailer just twisting over on it's side going down the road, or having the trailer try to weather-vane on you
hope you get all this worked out, but I would go with three points determining the plane of the trailer rather than just two (two wheel axle over just one)

[WeirdDogGuy]

I didn't make this clear before but I don't need to carry all my gear at once. So I meant the design must be capable of fitting my different toys at different times. When I'm going windsurfing, I won't need to carry my mbike. I didn't do a spreadsheet so I don't know where I going to come in weight-wise. I was guessing between 500 and 600 pounds but that's just eyeballing things. I'm not sure what you mean about flipping the trailer? It's attached to the TV with a universal joint so in order for the trailer to flip on it's side, that means the TV would have to flip as well. They are attached. I can't see that happenning unless I'm doing some extreme four wheeling-and I won't be doing that. Thx for your thoughts.

[eggsalad]

I'm completely lost. If I understand what a universal joint is and how it works, what's to keep this from tipping over when stationary, say, at a traffic light? Sorry, maybe "tipping over" isn't the correct phrase... what I envision is the TV stopped at a light, and the trailer leaning to one side or the other until the frame rail touches the ground.

[KCStudly]

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.