Torsion axle location and rating questions.

[RockyMountainTeardrops]

About ready to order my torsion axle for the "Test Mule" and I have run into a problem that I'm hoping somebody can help me with....

Camper body calculates to 250lbs. Mattress (full innerspring with pillow top) comes in at 100lbs. Cooler mounted to tongue will weigh 100lbs.
Add another 300lbs for battery, spare tire, frame and camping equipment.

Total 750lbs.

My axle is pretty far back, so there will be a large tongue weight (especially with the cooler all the way forward).

My question....what affect does weight distribution have on the axle rating?

Trying to decide if I should order a 550# or a 935#
Either one will have an adjustable "start" angle.

Thanks,
Lee

[jstrubberg]

I don't think you're calculating nearly enough weight for your frame, tire and camping equipment. The typical frame is several hundred pounds and a full size spare is 30-50 pounds. Don't forget people weight, too.

[angib]

If (If) you have a 750lb weight and the axle position means that the coupler will be carrying (wild guess) 30% of that, then the axle will only be carrying 70%, or 525lb.

So that's right at the limit for a 550lb axle, but nicely in the sweet spot of a 935lb axle.

The 225lb tongue weight will be OK for most tow vehicles, but too much for most cars.

[jonw]

If it helps:

My TD weighed in around 700 lbs with a tongue weight of 65 lbs. After I had much of the TD built on a rolling platform I experimented with different positions by using a pipe from a pipe clamp as a fulcrum and measuring the tongue weight. The ideal spot came out to exactly where the calculations and plans said it should be. Click on the build journal link in my signature to see what this all looked like...

I ordered a Flexride 2000 lb. axle and had it de-tuned to 1400 lbs, and am running it at a downwards 22.5 angle. I went that route so I didn't have any issues with separate 550 lb. half axles and possible alignment issues.

The TD tows and rides great.

YMMV...

[RockyMountainTeardrops]

I don't think you're calculating nearly enough weight for your frame, tire and camping equipment. The typical frame is several hundred pounds and a full size spare is 30-50 pounds. Don't forget people weight, too.

Frame will be approx 100lbs (main members calculate at 72 lbs, which leaves 28lbs for connectors and what-not)
Planning on a "space-saving" spare tire from a passenger car.
"People weight" is not needed in this calculation, I am only worried about "driving weight"

then the axle will only be carrying 70%, or 525lb.

So that's right at the limit for a 550lb axle

So I'm right in thinking that it is only the axle load that matters when the manufactures rate their axles, not the total trailer load.

nicely in the sweet spot of a 935lb axle

Ah, that makes sense. I was worried about not enough compliance in the suspension with a 935#.... but if I had spent a little time researching.......I could have answered that question myself!

I blame "stress" leading to a lack of sleep lately.


THANKS!

Lee

[angib]

So I'm right in thinking that it is only the axle load that matters when the manufactures rate their axles, not the total trailer load.

More than a few commercially-built trailers have an axle rating that is less than the GVWR - because some of the weight is carried on the coupler.

I was worried about not enough compliance in the suspension with a 935#

I'm just repeating what Cary of Camp-Inn trailers said - that rubber torsion axles have sufficient compliance from around 50% of their rated load upwards.

[Cary Winch]

RockyMountainTeardrops,

I really don't think you have the frame weight fully accounted for. A torsion axle alone will weigh 70-75lb. If you are putting the cooler on the tongue and truly having tongue weight that is close to 30% of the trailer's total weight the strength calculations for this will be a pretty heavy frame tongue section. Assuming you are not using the frame for any body structure the frame is going to have to weigh at least 210-225lb, if designed very sparse. If building a camper design that requires a solid frame for structure that number could grow by at least 50lb easily.

Tongue weight. A trailer needs to have about 10% minimum of the total weight on the tongue for it to trailer safely. We target about 12.5% on ours so that we have a cushion for loading variances. I see this as a 925lb or so trailer at least, assuming a average load of gear and no built-ins like stove, gas, sink or water. That means you need to have (using our numbers) about 116lb of tongue weight. It is true that a portion of this comes off the axle weight. But, not all. The hitch height angle will affect the tongue weight far more than most realize. Just going up a grade will take a good deal of weight off the tongue and apply it to the axle. So, for the sake of real world numbers I would only use about 75% or so of the tongue weight for this. That means a axle load weight of 838lb. With that load I would like to see about a 1200lb axle minimum and it should not exceed a 1600lb axle. A 2000lb axle with a trailer this light would ride a bit rough.

Gear weight. Our experience has been gear will average about 250lb. This does not include mattress, battery, water, stove, gas system and cooler (with load in cooler). We figure the cooler separate and all the other items are a fixed part of the trailer in our designs. Don't know what size battery you are planning to run but a small battery will be about a 32ah and a more typical one would be a 75ah. Figure close to a 1lb per ah for battery weight. A spare tire will be about 25lb (the 12" we use is 24lb).

I highly recommend doing a accurate bill of material of everything used in the construction of the trailer and all gear that will be packed in it (don't forget your camp chairs, awnings, welcome mats, wheel chocks, extension cords, party lights, flashlights, etc etc etc). Estimate the weights of all and work from there.

As for figuring axle location. This is much trickier. Once you have the bill of material with weights assigned you would need to do a weight and balance diagram. Much like is done with a airplane. Take your design drawings and place the items where you think they would go on paper. Measure all from a set datum line (probably the wheel spindle) and use these moment arms to determine the tongue weight. Once you have this you can shift the axle forward and backwards in the diagram until you are happy with the tongue weight.

You haven't mentioned what the tow vehicle is yet. As Angib pointed out before you will want to keep the tongue weight in a range that works with the tow vehicle. Don't build a teardrop that can only be towed by a 3/4ton truck because of a crazy high tongue weight.

Trailing arm angle? This will be determined by three things. What size tires you are running, how high off the ground you are wanting the trailer and how you want to mount the axle into the frame. Our standard street height is 12" off the ground and our off road package is 15". They also run different sized tires. My recommendation is to get a Flexiride axle because the angle is not set and you can move it up and down to fit. Eliminates the question then.

I hope this helps. Seemed a great opportunity to go through some of the elements of trailer design. Can the trailer be done lighter than this? Of course it can, heck we just built a nice 5x8 that came in at 420lb. Just going with the info given here, the fixed torsion axle, rearward axle location, included spare tire and large cooler on the tongue will make this a little heavier design.

Cary

[RockyMountainTeardrops]

Okay, you folks made me put pencil to paper and come up with some actual numbers based on the current design.
Please note: all weights were actually measured, or calculated from reliable sources, nothing was "guess-tamated"

Tongue length, 41" (this is the midway point of the cooler load, which is directly in front of the body)(actually the cooler is inset into the body by 7", but that's not important)
Figuring the weight of the cooler, filled to the top with water, 82% of the body (current axle location), 100% of the mattress and a little bit for clothes in the inside front storage.
This comes to 495 lbs.

Rear galley, including 12% of the body, battery, kitchen items, chairs, screen tent, propane, spare tire, and dry food.
This comes to 230 lbs.

Total weight of the body and assortments is 725#
Current axle location then sets the hitch weight at 320 lbs (44%) (Remember, this is with the cooler filled to the top with water)
(clearly the axle will have to be moved forward to get the hitch weight closer to 200 lbs)

No frame weight yet, we're still figuring out what we need to build it.

Running the calcs (if I did it right) says that I will have a maximum moment of 16,470 in/lbs.

If I use 2"x2"x.120 square tube in an "A" frame design, I should be allowed 32,200 in/lbs (using Andrew's "cheat sheet")

This gives me not quite 2X the allowed stresses on his chart (which I'm assuming already have a 4X safety factor)

Now we should talk about the body.
I'm using foam core with bi and tri directional fabric.
The floor construction is designed for 1psi (144lb/sf), and I have a safety factor of almost 3X that.
So I don't need the frame to carry anything more than the tongue load. Everything else will be supported by the body.
(My biggest worry is the frame flexing and breaking the mounting points because the body is so stiff)

So with all of that said, I'm still planning on approx 100lbs for the frame.
Two 10' tongues will come to 60 lbs, two 1/2 axles are 7 lbs each.
Which leaves me still to support the spare tire and the cooler.

Worse case scenario is 825 lbs total weight. (more likely it will come in under 800 lbs)

Any thoughts?
Lee

[madjack]

Lee, if you take in your 800#s and allow for 50% overage for an axle rating(pretty standard for industry practices) you have an axle need of 1200#s...my figures for total weight of my trailer are similar and I ordered a 2000# axle and had it derated to 1500#s...it rides like a dream, tows like a dream and my gear generally is sitting right where I left it when I get where I am going...I can't strongly enough recommend you do similar and get the 2000#axle derated to 1200-1500#s...you'll be glad you did...plus you will get heavier duty bearings and such with the upgraded axle...by the way...a donut spare probably won't fit on a trailer axle...the offset and pilot hole will be wrong(as I could be)...........
madjack

p.s. you have 3 of the foremost trailer experts in the whole world, Cary of Camp-Inn, Andrew of Angib and Jack of the Swamps recommending virtually the same thing...think about that...the choice is your to make...........mj

[RockyMountainTeardrops]

I hope I'm not coming across like I'm not listening to anybody...
I'm only posting my calculations so somebody can poke holes in them.


Lee

[madjack]

...and my p.s. was being at least a little sarcastic...at least to my part .......................

[Cary Winch]

Lee,

Clearly you are listening very well! Nice work. Jack is right though, both Jack and Andrew have been two of the top tech "go to" guys on the forum for a long long time now.

Your numbers are looking pretty good for sure. I still think your frame will be a tad heavier than estimated (though not nearly as much as I first indicated). The use of torsion stubs will require a substantial cross member of some sort to tie them together. On our lightweight experiment we use a aluminum hat shaped section so it didn't weigh much. Other cross member are up to you, you could use the body of the trailer to tie the 10' tongue pieces together without any crossmembers, which is what I think you are planning. This would make for a pretty light frame. Nice!

The downside to the stub axles is the lack of ability to adjust ride height. Might just be worth the extra weight of a full Flexiride axle to be able to tune your set angle.

I do have to ask. Why the cooler up front? That seems to be a big factor that is really throwing your weight and balance off. That seems a bit of a odd design element making this challenging. If nothing else a cooler out in the sun is always a struggle to keep ice in. Always best to get inside.

If it will help any, you are right about the cooler weight estimate. I believe the 100lb is way on the high side. We use a 54qt cooler in our trailers. Full to the top with water that would be 108lb of water, not including the weight of the cooler. Our experience has shown that the average 54qt cooler weight loaded is about 60lb. You could use a lower number than the 100lb for your figures, say maybe 75lb?

Cary

[RockyMountainTeardrops]

The ultimate tow vehicle will be a jeep, so that's the reason for the cooler mounted at the front.
Also making a "1/2 galley" similar to Little Guy designs, so no room there either.
We use a 75qt Yeti, which is where I came up with the weight calcs for that. Empty cooler is ~35lbs.
I think actual cooler weight (loaded) will not exceed 75lbs, but I used 185 lbs (18.75 gallons * 8lbs/gal)

Current plan is to add a 1/4" x 4" flat across the width to tie the axles together. Then two lightweight cross tubes under the cooler.
100 lbs may not be possible, but I don't think I will over-shoot that by more than 30 lbs.

Flexride offers a 935 or 1400 half axle with adjustable start angles.
Right now I'm leaning toward the 935#
If actual camper weight is 750lbs with frame, and I'm going to move the axle to get approx 190 lbs hitch weight (25%), that will leave 560 lbs axle weight.
That should put it pretty close to ideal....yes?

Thanks!
Lee

[Cary Winch]

Lee,

Yes, your larger cooler weights make sense.

A flat stock will likely not provide enough strength for a stub axle tie member. You would be far better off with a channel. Steel channel will add over 30lb alone. With your box tubing frame rails, two members for cooler, stubs and a steel tie channel you are over 150lb of frame weight. Add the A-frame coupler, a tongue jack, chains, etc and this will start hitting over 175lb fast. I really think you be better off figuring 200lb for frame weight estimation.

Now you are at 850lb. Figure your 250lb gear load and you are 1100lb. Your 190lb tongue weight target should still be derated for towing angles which would reduce this amount to 142lb or so. This puts you at 958lb. Really looks like you need the 1400lb axle stubs.

Awesome by the way that Flexiride has adjustable stubs in the larger weights like that. You will like that setup.

Cary

[angib]

Take your design drawings and place the items where you think they would go on paper. Measure all from a set datum line (probably the wheel spindle) and use these moment arms to determine the tongue weight.

It always seems tempting (or just plain 'right') to use the distance to the spindle position to work out balance but there are two reason not to do that:
- There will be positive (in front of spindle) and negative (behind spindle) measurements and, as sure as eggs is eggs, you will give some of them the wrong sign, or add up the negatives as positives, or some such.
- As soon as you find that axle needs to be adjusted a bit, all the measurements you've recorded become wrong, because the spindle has moved. You can try to stick with the old measurements, but if you want to add or move something, you will get in a right pickle.

So go for some Poka-yoke and keep all the measurements one sign by using a datum point that is either behind or in front of the whole trailer. I like the back end of the trailer but you could use the hitch ball too.