My Trailer Build Plan W/Questions

[LFD2037]

This morning leaving work I looked at 13 different trailers. Several utility, dump & enclosed versions. My new plan will be stronger than all of their designs except 1. It was a 12' enclosed heavy duty trailer. It had an A frame with 1/4" 2x4' tubing and the A beams went all the way back to the front of the springs. Besides that one, mine will be far superior to the rest and quite a few of those are old and have been beat on for a long time with no failures.

 

[greentruck]

If you were going to build a 100 of them, you could worry more about shaving a few pounds off. As it is, not a lot of extra weight that's unneeded, whatever that amounts to.

The drawbar on the M101 CDN is just two pieces of channel, no center section, that sandwich a casting at the front that carries the pintle and drop leg mounts. I suspect it's over built a bit, but the military likes some things that way. Nothing wrong with a little extra margin when you're handing something over to the lowest paid and ranking personnel to bash around in the woods.

Since you, not that taxpayers, are funding this, having that extra margin it looks like you do should be sufficient.

 

[LFD2037]

If you were going to build a 100 of them, you could worry more about shaving a few pounds off. As it is, not a lot of extra weight that's unneeded, whatever that amounts to.

The drawbar on the M101 CDN is just two pieces of channel, no center section, that sandwich a casting at the front that carries the pintle and drop leg mounts. I suspect it's over built a bit, but the military likes some things that way. Nothing wrong with a little extra margin when you're handing something over to the lowest paid and ranking personnel to bash around in the woods.

Since you, not that taxpayers, are funding this, having that extra margin it looks like you do should be sufficient.

Thanks. I tend too waaaaay overthink things and go back and forth. I feel confident I shall have zero issues with my plan. Looking forward to more welding on this trailer and it all coming to an actual useable unit! Still trying to find/order a Dexter 3,500lb axle with 6 on 5.5 hubs that's also 64" hub surface to hub surface. IBe found one but its $178 with no springs or any other hardware. Also some ~36-44" leafs. I'm also searching for 2 285 or 295/75/16 tires for it. The search continues!

 

[greentruck]

SNIP
Still trying to find/order a Dexter 3,500lb axle with 6 on 5.5 hubs that's also 64" hub surface to hub surface. IBe found one but its $178 with no springs or any other hardware. Also some ~36-44" leafs. I'm also searching for 2 285 or 295/75/16 tires for it. The search continues!

That's not a bad price.

Something to consider, especially since you're planning for a roadable weight of around 1,500 lb, is whether you'd be better off just going ahead and spec-ing the Dexter with brakes. I looked again recently, because that's what I have planned for ours, but IIRC it was still under $400. I started looking at just going with a whole new braked axles when I looked into the cost of adding brakes to the M101 CDN. Turns out the parts to add the brakes, bought separately, considerably exceeded the difference between the unbraked axle and the one with brakes installed. Why fight all those old parts when I could Craigslist the old one whole (a more desirable buy) and have a nice clean axle to weld spring mounts to and install?

Of course, since you don't have an old axle to come out of the deal as trading material, the calculations are a bit different financially. Brakes are all added cost to what you want to do.

Performance wise, it starts looking better, if you can afford the extra cost of brakes. A 1/4 ton trailer isn't much to tow -- until you get it loaded up. I think if you planning on hauling water and fuel, as I am, then 1,500 lb ready to go isn't excessive, but darn close to what you'll be hauling, if not a little low when full up. And that much weight will push you around in the mountains if it's unbraked. California requires brakes on all trailers 1,500 lb and up. Texas ain't Cali, but it's something to consider, not so much to worry about future resale there, but because a trailer that heavy is really on the edge about needing brakes for best performance and safety. And it's not only on the road, as they definitely help on the trail, too, perhaps even more than on the hardtop.

 

[LFD2037]

It'd be about $350 for a braked axle minus springs, hangers, etc. I'm heavily contemplating that route.

 

[greentruck]

It'd be about $350 for a braked axle minus springs, hangers, etc. I'm heavily contemplating that route.

You'll be kicking yourself if you decide you need them later. Not just the relatively small difference in price, but the fact you'll have to piece it all together with the axle already on the trailer will seem like a real timesuck compared to bolting it all on whole the first time.

 

[canman]

The Canadian M101 is well built (rust issues aside). We had one of the three Mercedes-Benz G-Wagons pull the trailer fully loaded for the entire length of the RAMD trails we conducted down at the Aberdeen Proving Grounds before awarding the contract. There were no issues I recall with the trailer, and each vehicle logged over 20,000 kms over a course that was comprised of a of mix of highway, secondary and off-road sections. The off-road section was so difficult that the bracket which supported the pintle on the rear of the G-Wagon had to be redesigned 3 times by Mercedes-Benz because of catastrophic failures. The MB engineers just could not believe how much force was being transmitted from the trailer tongue to the pintle bracket. Again, there were no issues with the trailer, so I would try to mimic that design if you plan on a lot of off-roading with the trailer. The twisting of the tongue is something to really consider for a trailer designed for off-road use.

 

[greentruck]

SNIP
The twisting of the tongue is something to really consider for a trailer designed for off-road use.

That's good stuff to know. I think the channel that forms the A frame for the tongue/drawbar is a big part of this resistance to twisting. It's gotta be pretty stiff and resistant to twisting. The heavy tube stock that LFD2037 is using is similarly resistant to twisting distortion.

Kinda interesting that they used a G-wagen for testing. With the trailer designed for the Iltis, that's certainly a step up in terms of capabilities of the prime mover. The drawbar does have two positions for the lunette. And just about anything they would hitch it to is going to be stronger that an Iltis, so I expect it was designed heavier than it would have been if it had been restricted to use with it. The Canadians wanted a universally useful trailer that could hold up to whatever it was likely to be hitched to.

 

[canman]

You are correct in the the Iltis was the prime mover for the 1/4 ton trailer. At the time, early 2000's, we were in the process of testing the G-Wagon which was going to replace the Iltis. Also, a few years later, one of regiments out west wanted to use the G-Wagon to tow their 105 mm light howitzer because they were losing their prime mover, the AVGP (an early 6x6 version of the Stryker 8x8 vehicle). They did all their own testing and were convinced it was up to the task, however we could not approve of the use of the G-Wagon because the tongue weight was 50 kgs above the rating for the pintle. MB stated that it would be a complete redesign to certify another pintle and pintle bracket for the additional 50 kgs. I couldn't believe it, and my boss was not willing to spend the money to undertake the redesign.

 

[ntsqd]

Until the coupler limits out in rotational travel I don't see how the trailer tongue can get any torsion in it.

That potential for torsion is why the lunette assembly that I built can rotate. What looks like a stack of various diameter washers is actually a bronze thrust washer, the flange of a flanged bronze bushing, two heavy flat washers, and a Belleville Spring. Which is why the plate bolts on like it does, access to the big nut for tuning the minimum torque required to rotate the lunette ring. There isn't enough room on any of my vehicles to use one of the rotating pintle hitches.

 

[greentruck]

Using "torque" very generically here. I'm not an engineer. But there are certainly a variety of forces acting on the hitch. The M101 CDN lunette rotates and incorporates an adjustment mechanism. I rather doubt anyone touched it much after it left the factory. Mine's been adjusted, because I moved the lunette to the upper position.

Much of the potential forces applied to a trailer drawbar are from backing up, jacknifing, etc. While not pure torque loads, there's elements of that depending on how free the lunette is to swivel or the binding that might occur in off-kilter situations.

Then there is military stuff. Jamming a bunch of things together in the hold of a ship, you don't want it bending. Air drops (the manual for that is available) aren't something most civvy trailers experience, but which need to be designed in here. Then there's simple buffing forces from towing, whcih can also result in torque.

 

[ntsqd]

In my head I'm defining torque as an attempt to twist the tongue about an axis parallel to the longitudinal axis of the trailer. Which is why my statement that until the coupler runs out of rotational play there can't be any torque.

All other forces acting on the tongue are bending (a big one at the first cross-member), compression, tension, or any two or more combinations of the 4. Easiest to look at each as an isolated case, at least at first. Chances are good that in doing so you'll have a design that meets any possible combination so long as appropriate Factors of Safety are employed. If you're the military, a Mfg., or particularly anal then you'll need to evaluate the design against any reasonable combination of those forces. The rest of us can probably stop at the isolated forces analysis. The M.M. Smith book that I've recommended in the past walks you thru all of this.

 

[greentruck]

In my head I'm defining torque as an attempt to twist the tongue about an axis parallel to the longitudinal axis of the trailer. Which is why my statement that until the coupler runs out of rotational play there can't be any torque.SNIP

Well, friction in the lunette mount will start loading torque, but it won't be much, granted.

As I noted, was in no sense trying to use "torque" other than generically for forces applied to the drawbar. Your objection to my imprecise word use duly noted.

Now if I was building a trailer and was concerned my Mark I Eyeball was leading my astray, breaking out the book would be comforting. I haven't work much with metal myself compared to wood, but have been getting more experience with it in recent years. Sure, there's a tendency to overbuild, but rarely do I see that done to the point where it's a negative as it gets expensive and beyond home tools pretty quickly. Plenty enough, on the other hand, is comforting, if perhaps not strictly necessary.

 

[greentruck]

Well, friction in the lunette mount will start loading torque, but it won't be much, granted.

SNIP

Was doing some more thinking about this and realized I was certainly wrong about this aspect of the drawbar design being limited to torque from friction from the lunette's resistance to turning, which as I noted due to maintenance shortfall, corrosion, or other neglect can be substantially more than zero.

For the lunette to turn as freely as possible, the casting that holds it must remain held in position, not whimppy or floppy where you'd end up with binding (itself a form of torque in some cases.) So there is a significant amount of torque that must be resisted to accomplish that. It'll be momentary, true. However, it will occur in backing the trailer, as well as making one's way down the trail. There, any place the drawbar reaches a physical limit it will likely impose at least some torque to the drawbar.

I rather doubt any single equation would define this, especially if oriented toward constant force type equations (dunno, haven't read the book), rather than the trickier task of catching and accounting for momentary forces. All I know for sure is if you built a lunette and its mount so that it only needed to be strong enough to resist the torque necessary to turn the lunette, your drawbar would quickly crumble from other forces. How exactly this fits the textbook definition of torque I dunno, but there's something going on there that needs to underline resistance to twisting or the lunette will not only not rotate as designed, but will bind or otherwise cause problems.

 

[ntsqd]

FWIW I wasn't objecting to your use, I was clarifying my thinking since I know mind-reading over the net is near impossible.

I suspect that you're looking at a military trailer with a rotating lunette and I'm thinking in terms of a home-built which isn't very likely to have that option w/o doing something like below:

One of the big challenges in designing such a coupler is getting enough weld bead length to have enough Factor of Safety. Minimum is 2:1 and I wouldn't want one that low. I am not a fan of putting simple butt-welds in tension. Wrap it, fish-plate it, do something but don't leave it like that! I also find welding on nuts & bolts to be abhorrent when they are structural. There are times when there's no other simple choice, but to start out with that as a basic part of the design is poor practice.

Found the rotating lunette pic. Note that those seemingly small bolts are 9/16" G8's. Nuts are prevailing torque lock-nuts Lock-tited and torqued.

[EDIT] Those "Washers" stacked up on either side of the 1/2" plate are bronze thrust washers, steel heavy washers, and a stout Belleville spring. The lunette will rotate. I need about 2' of leverage to make it move. The 2" X .250" wall tongue tube does not appreciably twist when I do this, not even when at full extension.[/EDIT]

What typically happens is that you evaluate a design for each of the possible singular loadings and tweak it until it's Factor of Safety for the greatest loading, what ever that is, is high enough to live and work. Then you look at combined stresses to insure that nothing unexpected is going to happen.

Then you can look at dynamic loadings if so desired or it is expected that those will greatly exceed the static loadings. Dynamics is far from my forte', so what I usually do is factor the static loads by at least 3 and re-apply them to a static analysis of the design. Note that this is for my own stuff, not what I do at work.

 

[ntsqd]

BTW, Engineers call this kind of torque a "Moment". NFC why......

 

[greentruck]

BTW, Engineers call this kind of torque a "Moment". NFC why......

Or as I called them in non-engineer speak, momentary forces.

I agree, those are certainly more complex, as they involve something other than typical forces encountered just rolling down the trail. Obviously, their influence depends on how one defines the potential for them to occur and their assumed peak values. For a civvy trailer, no need to worry about what happens if you air-dropped it, for example. Your method of taking static forces and using a 3x factor to get potential to resist momentary forces is most likely a good rule of thumb for all but exceptional cases.

 

[LFD2037]

Today I welded the 'caps' onto the open ends of the frame tubing. I first tried 22ga. Had my welder on the lowest volt setting & WFS turned waaaaay down & still blew thru it pretty damn easy. Switched to 16 gauge 'caps' & that worked much better. I've never tried welding 22ga sheet before but it sure surprised me how difficult it was! Even on the lowest setting, I had to haul butt to keep from over-penetrating. No luck so far finding some used YJ, FJ40, CJ, Samurai or whatever ~36-45" used springs for a decent price. Cheapest used has been $130 & new for $150. Found a pair of 30" rated @ 1K each for $102 delivered but I'm afraid I'll regret going that short. I work about 70hrs. this week so not much happening until Sunday probably. More to come!

 

[LandCruiserPhil]

If you are set on leaf springs for your set up I would not overlook parabolic springs as an option.

 

[greentruck]

Phil,
IIRC, one of the Dexter pages that showed the axle with leaf spring options did show parabolic as one of the options. Did not inquire further, but IIRC it's one of those previously cited here.

One reason I didn't look further was I've always seen them used in apps where one or both of thse applied, cheap or space limits. I presume if you're recommending, they're up to trail use, but do you need to buy carefully to ensure that's the case?

Asking the question that way, because WYSIWYG with a leaf spring, while things like parabolics, torsion bars, etc are a bit too arcane to judge by just looking at them. In other words, I'd have no problem taking the Dexter leafs if they were long enough, which IIRC they weren't being like 26"? Where longer sounds better to me per my experience with our 1/4 ton (36" spring length and IIRC most others are similar length?) But I'd be wary of the other option. Then again, maybe they're all pretty bullet proof nowadays?