Uber Ultra-light tear

[brian_bp]

... In order to save some more weight I’m thinking of fabricating my own wood/carbon/glass tongue. I have the luxury of being able to do a finite element model of the trailer and tongue...

That would be great to see, and almost worth doing just for the novelty!

[brian_bp]

...
Anybody have an input on the tongue weight? I was thinking 50 lb minimum (that’s 10%) with the ice chest, water and cook stove/propane on board. With any of those missing or the A/C omitted, the tongue just gets heavier...

With any traditionally laid out teardrop, if there is no significant mass riding up front (like a big propane tank or a battery on the tongue), then the trailer's mass is pretty well concentrated (in the back half), reducing the polar moment of inertia. Due to the rear-set mass distribution and corresponding rear-set axle, the axle to coupler distance is quite long for the size of the trailer. These are both good factors for sway resistance, so there should be no need for a high tongue weight... and by European standards for even boxy travel trailers, 10% is high. I don't think it will be a big deal.

I suppose for stability the tongue weight can go as high as the tug happily allows, but too high will required the tongue structure to be stronger and therefore heavier.

[brian_bp]

...Michelin used to make an actual 8" car tire, but I don't think you can get them any more.

Probably not... they don't sell trailer-specific tires, and in the North American market (see Michelin US and MichelinMan) they don't seem to have any small diameter products other than scooter tires (which are for stuff which leans, and thus not conventional trailers).

Also, there is a heavier duty 8" with a higher load rating -- think that would help?

They might be more puncture-resistant, but I doubt they would help much, at a guess.

I wonder if the car radial wouldn't be better as some have suggested -- it's almost the same weight as the trailer tire, but probably better built (and a bit more expensive). The problem there is I don't know if the mini rims are the same 4-bolt pattern (I got to go to bed tonight -- we're 14 hours out-of-sync with y'all over here) -- I'll have to check tomorrow. Still for the same weight as the 10" set-up you could run 12s in either a trailer tire or a road tire. I'm not sure what the implication is to running one verses the other.

I couldn't find the real Mini pattern quickly the other day, which is why I didn't quote it. I wouldn't bet on a match.

I can't think of any reason to run a trailer-specific tire, rather than a normal tire, other than size availability. There are lots of trailers operated on normal tires, and in the commercial world of big trucks the only difference between a trailer tire and drive axle tire appears to be tread pattern... and sometimes they just run the same tire in all positions on the truck and trailer.

[kennyrayandersen]

I've been working on the analysis and I completed the meshing of the body. I included two doors in case I want to add one later or decide to add another before the build starts and a cutout for an slide-out AC unit. At least this will give me an idea of the stresses (strains) in the fiberglass. Initially, the floor will be 2 plies in and out as will the front, for the bottom half. Shelves will be two plies top (compression side -- stability failure of the top facesheet would be expected) and 1 ply on the bottom. I'll start light and add structure as needed. Next week I should be able to get some results and also complete the composite tongue design and analysis.

[angib]

Here's a drawing I posted a long time ago, for the tongue of a frameless teardrop. It was designed to be made in plywood, but composite would be better as the laminate thickness could be progressively increased all the way to the coupler.



Andrew

[Trackstriper]

Regarding coupler weights Brian wrote:

I bought a coupler for a utility trailer a while ago, and noticed that the weight difference was a significant fraction of the total coupler weight. I don't think it's the socket size difference; instead, it is the thicker steel used for higher load capacity in the 2" unit, since a 2" coupler is normally rated for at least the Class 2 limit of 3500 lb (the one on my travel trailer is good to 5000 lb), and 1-7/8" couplers are routinely rated for only the Class 1 limit of 2000 lb (but some are much higher).

Yep. He's right. I happened to be at Harbor Freight today and weighed their simple couplers. The 1-7/8" weighed in at 43 ounces while the 2" came in at 67 ounces, a pound and a half more. The weight difference was primarily in the gauge of the stamped parts as Brian mentioned.

[kennyrayandersen]

Andrew,
I remember that and there is every indication that this would not only work, but be very light-weight. I'm trying aesthetically, though, to keep the front end unobstructed (so far no tongue box etc.). I'll post some pics of the composite tongue in a couple of days and see what you think of it. I'd like to keep it simple enough that someone else could reproduce it if they would like to.

Does anyone think that the will be any practical problems with the radii in the corners of the door opening. It might make the hinge a little more difficult to install, but it should really reduce the stresses at the corners of the door cutout.

Also, I got an e-mail back from the Dexter folk who said the #8 full axle was approximately 50 Lb. so either they are guessing high, or the #9 sounds like it is cinsiderably heavier.

[angib]

I got an e-mail back from the Dexter folk who said the #8 full axle was approximately 50 Lb. so either they are guessing high, or the #9 sounds like it is cinsiderably heavier.

Here are some axle weights supplied by Dexter:

Axle #8, no brakes, 1100lb capacity, 60" over hubfaces - 53 lb
Axle #8, electric brakes, 1100lb capacity, 60" over hubfaces - 75 lb
Axle #9, no brakes, 2200lb capacity, 60" over hubfaces - 68 lb
Axle #9, electric brakes, 2200lb capacity, 60" over hubfaces - 89 lb

Andrew

[Arne]

originally, I was going to radius my doors... they are 1/4 inside and out with 1.5" foam sandwiched in between.... I gave up. I could not get an accurate cut that deep with the sawz-all.... it would look better, though. I'm not worried about stress..... with the box on a metal frame, it ain't going no where.

[kennyrayandersen]

Andrew
My trailer design is around 58 inches wide, though I recently thought of a way to push it out to 60", so I might go a little wider (I was thinking 58" wide and use the 60 inch material to overlap the top and sides, but I could go to 60 and do that with some tape -- that would also look a little like molding as well). Don't they cut down a little when the de-rate them to 500-600 Lb rather than the 1100 Lb? I'm not sure how that works, but that might explain the 3 Lb difference in the quote, or he's giving it to me in round numbers. Regardless there is quite a weight-saving to go to the #8 axle verses the #9 it appears.

Arne,
I don't have a metal frame, so the shell is going to be loading up. Sharp corners can cause, depending on the radius, very high stress concentrations so it's definitely a concern in this application. In the case where you have a frame that can carry the loads, like in your build, it's not so important (I see that a lot of doors are square, at least on the hinge side) and evidently it hasn't been a problem.

The walls are a total of 1 inch thick. I'm thinking to lay-up the door cutout and door frame together (built as one piece), then just cutting the door out. Then I can wrap the edge of the door and wrap the door jam after they are separated. I could also make each separately and then just cut the glass skins, but I think epoxy would seep into the gap and glue them together making separation more difficult than if I just cut them out of a single whole piece. One inch is not so deep that I cant use a router to cut all of the parts out before they go in and then I can use a fine jigsaw to cut them out (since the edges will be sanded and wrapped).

[kennyrayandersen]

I'm now designing the tongue. Looking at couplers, they all seem to be 2.5 inches -- is that an inside dimension for the tongue? If you use 2" square for instance, do you shim the sides?

Thanks

[Trackstriper]

Try this one for 2" channel/tube:

http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200331131_200331131

[kennyrayandersen]

When they say 2" thats the OD of the Steel and the ID of the coupler, right?

[Trackstriper]

Yes. You could check with Northern to verify the dimension, but generally that is the case.

[kennyrayandersen]

to calculate the bearing load I need to know the dimension between the ball center, and the bolts that attach the coupler to the tongue. Plus, is there a standard bolt size?