Teardrop Axle Placement and the 60/40 Rule

[BellyDoc]

I've been lurking here for a while, reading up as I plan and stockpile materials and parts. My plan is to build a teardrop with light offroad capability, to be towed behind a Bronco 2 door (100.4 inch wheelbase). I'm shooting to come in around 1200-1500 pounds, but it would be nice to be even lighter. Some of my components are already obtained, some are still being decided, and I'm bleeding money at the appropriate rate for a project of this size.

I'll fabricate the trailer frame from 2x2 and potentially some 2x3 .120 wall tube.

My floor plan is 5x9' and I'm going to have a 4' tongue to clear the rear swing gate door, for an overall length of 13'.

As I work the drawings, it's hard to estimate a 60/40 weight distribution. I have been literally counting graph paper squares fore and aft of a candidate axle position and then multiplying by lever arms for comparison, assuming equal density plywood/epoxy/glass cloth. I can throw in some fudge numbers for stove and cooler weights, water, battery, etc., but this departs from reality long before I break out the calculator.

Ideally, I would do a lot of rough building, then actually weigh the project and determine the 60/40 point.

I had actually thought to do that, but my chosen suspension (already obtained) is the Timbren 2000lb HD axle-less. To use that, I'm going to make sure there's a sufficiently beefy build where the arms go.

That means the trailer frame construction should be designed around an axle location, and I'm caught in a logic loop. Can't build without a design, can't design without a 60/40 point, can't find the 60/40 point without a build.

Obviously the cure is to commit/build and then check... and unless I'm egregiously off, accept.

I've seen the 60/40 rule expressed as a weight distribution, but here on the forum, I've also seen it expressed as a distance formula. The latter, I can obviously just do. I've also seen more of a 60+/40-6" thrown around. I would assume that's a fudge factor to accept that the galley is likely more densely weighted than the relatively open sleeper berth.

As an ace in the hole, I've got some moveable weights that can be left off the design till the end such as the spare wheel mount (not trivial - I have 31x10.5's on steelies) and the battery, but I want to be close.

60/40 would put the axle at about 43" from the rear, while -6 would move it back to 37". Both clear the doors nicely, but 3'-ish allows me room to mount my spare ever-so-slightly forward of the axle without interfering with the door or going higher than the roof.

Thoughts?

How far off is a 60/40 by tape measure from a 60/40 by scale weights on a teardrop build with a rear galley?

 

[kipa]

The way I understand it, the 60/40 rule is based primarily on weight, which if one builds a balanced shell is typically going to be close on distance as well.

Because my base utility trailer really doesn't have easy flexibility on axle placement, I've come to the conclusion that it's good to just maintain as much flexibility as possible on where heavier items are carried. So my focus has been to try to keep balance consistent fore and aft with where and how other fixtures are built into the shell based upon that fixed axle.

Other alternative accessory loads can also dramatically shift it as well. My e-bike and rack are about 75 lbs combined. Dangling off a rear mounted receiver at 5' behind the axle reduces the tongue load by half that at 10'. Or I can just put the bike up front to add ~40 lbs to the tongue. (Bad math deleted.)

Long version short, I doubt you can't adjust for that much of a shift on axle placement unless you are already up against a limit on balancing, but extended cargo capacity at either end can help a lot with that.

 

[Tony Latham]

As I work the drawings, it's hard to estimate a 60/40 weight distribution. I have been literally counting graph paper squares fore and aft of a candidate axle position and then multiplying by...

You're overcomplicating it. Just place the axle at the 60/40 point on the chassis frame – disregard the tongue length for this measurement.

It'll tow like a dream.

Tony

 

[reaver]

Mine was a little tail heavy after I built it, making towing on the highway a touch sketchy. I managed it by keep my speed below 65.

I recently extended my tounge by 2 feet, and this has solved my issues, honestly, so even if you end up with the axle not exactly in the right spot, you can adjust for it with an extendable/adjustable tounge.

I'm not saying that's the ideal way to handle it, but it does give a bit more flexibility, and it adds more tounge weight by moving the axle farther from the hitch.

This has made my trailer tow like an absolute dream.

 

[GTS225-TNT]

I'm with Tony.....build it at the 60/40 ratio, and adjust your load-up if there's an issue with weight-and-balance.
After all, once the frame is built, you can't easily adjust placement of the foundation pieces. You CAN adjust where everything else is.

Roger

 

[BellyDoc]

You're overcomplicating it...

Guilty!

Overcomplicating is my MO. I'll just own that right up front!

With that said, though, thinking is what I enjoy. Calculating. Solving puzzles. Projects are just one of the many ways I can dig into something and satisfy the itch to think. This isn't just about getting done and having a serviceable camper - although there's that... it's also about target practice for my brain. The more I think about this, the more I want to come up with a good prediction of weight distribution.

I obtained a postal scale and I'm starting to tabulate actual weights of components.

I found this resource/article, which summarizes my brain itch nicely:

Trailer Axle Placement Calculator | Find Your 60/40 Balance Point - Overland Teardrop Trailer

Use our free Trailer Axle Placement Calculator to find the exact spindle position for your DIY teardrop build. Tows safely and track true.

overlandtrailer.com

And I found some chat about an "inch per foot" rule of thumb, which came from a fabricators forum. Basically it was a way to do a (bad) approximation of 60/40 by length. They were suggesting to offset the axle from trailer midpoint (excluding tongue) by one inch per foot. If they had suggested to offset by 1.2" per foot it would be exactly 60/40 by length. Why not just multiply by .6/.4? ... it's all good I suppose. Unfortunately, that's no help because they're really just talking about flat bed utility trailers without a heavy rear galley problem. It's all in how you load a flatbed.

This is actually a cool puzzle. It's one of several I'm doing pencil and paper work on, right now.

I really feel like I've got to make a best possible effort at getting the weight distribution right, before the build. If I try and I fail, I'll be relocating the axle. If I don't try... and I fail... I'll call it a missed opportunity, kick myself a few times... and then relocate the axle!!!

Mine was a little tail heavy after I built it, making towing on the highway a touch sketchy. I managed it by keep my speed below 65...

This is what I'm thinking about. ^^^

I had a sporty misadventure in college when I was working construction. I was volunteered for the dump run from a construction site, but unbeknownst to me a huge amount of demolished concrete slab had been loaded at the tail of the trailer. Tongue weight was profoundly negative. The rear tires on the truck were barely on the ground. I didn't know any better. I just did what I was told. Jack knifing repeatedly going downhill, it was a school lesson on why this was a colossally bad plan.

My teardrop is going to do offroad/overland. I have no problem driving moderately on the highway, but this may be sketchy even at low speed when steeply ascending or descending on dirt and rocks. If anything I want a little MORE tongue weight than standard. I've also located my brake controller where I can stab at the button and activate the trailer brakes without having to take my eyes off the trail.

The way I understand it, the 60/40 rule is based primarily on weight, which if one builds a balanced shell is typically going to be close on distance as well...

The "balanced shell" is exactly what I'm hoping to assess using graph paper tech.

While I haven't firmly decided on my material, at this point the standard baltic birch choice is favored based on availability locally as well as the option to obtain 5'x5's as well as 4'x8's. Other options include marine grade meranti (only in 4'x8 ) and marine grade okoume (significantly lighter, only in 4x8', not locally available).

Material choice aside, though, I think the graph paper exercise is conveniently constrained.

First, the total wood input looks like it's going to be within 5 sheets of half inch, 3 of 3/4, and 5 of 1/8. Even with the heaviest choice (birch vs. meranti depending on info source), that's a max under 500 pounds before epoxy and fiberglass, ignoring insulation. Maybe this part of the balance problem isn't overwhelming.

Second, I think I can assume a relatively uniform density for the walls. For my purposes, I can count graph paper squares in front of/behind the axle and multiply by a midpoint distance each for a reasonable estimation of lever arm. This isn't rocket science.

I'll have an idea - shortly - of where 60/40 sits for the shell. Also the frame... Then I can consider some of the fixed load elements and see if they booger it all up.

Postal scale for the win!

More soon.

 

[Tony Latham]

With our old teardrop, we did a 2,000 mile (round) trip the South West with two bikes on the rear with a heavy rack. It never started bobbing and weaving.

With our new teardrop, I built this removable double rack on the tongue.

In the end, both rigs towed fine.

Tony

 

[BellyDoc]

Absolutely gorgeous!!!

Not only are your camper builds above reproach but your camper photography makes me want to quit work and go camping.

 

[BellyDoc]

Ok... so this is going deep nerd but I'm really into this, so please be patient.

I've done the graph paper math. The results are very interesting... at least to me. Let me walk you through my thought process, regardless of the overthink.

First, I started with one of my rough sketches regarding teardrop layout:

This is on graph paper with a 5-per-inch rule. My scale is 1"=1', so 5 squares per foot = 1.2" per square.

Basically, I then treat the plan like a CT scan. I'm thinking in slices from left to right. If this were a boat build, I'd be considering the cross sections at 45 separate stations. That's not an unmanageable number.

Next, I lined up a scatch sheet under the drawing and started tallying the number of boxes inside the drawing, at that station. Here's some of my raw data:

Working from back to front, it only took a few minutes to count all the squares. I then added in other data rows that reflect the presence of the roof material at a given station, or the presence of the cabinetry carcass in the galley space. I accounted for the layers of the floor.

I considered half-inch birch ply as my standard, and adjusted the square count up or down proportionately when I was accounting for 3/4" or 1/8". Initially, I *did not* add in any drawers, a stove, a cooler, water containers, propane tanks, fire extinguisher, although I now have those numbers to add at any location.

My calculations included an estimate that a half inch birch ply 4x8' sheet weighs 45 pounds. I divided by 32 square feet per sheet, and then again by 25 (graph paper squares per square foot), I got 0.056 pounds per graph paper square. Really, the actual number doesn't matter for this because the model is about balance and unless theres non-uniformity of material, I could plug into the model whatever weight per square I want and it would result the same.

I put the numbers into Excel. I have one column for a wall, another for roof, for the floor, the cabinet carcass, etc. Each is a column of 45 values which represent how much material from that layer of the build is at THAT PARTICULAR station.

Here's a single wall layer graphed out, showing immediately how intuitive this is:

Notice that it parallels the shape of the wall. The scale on the left is numbers of squares counted.

Here's the cabinet carcass. Note that it's contribution is concentrated to the rear:

The big tall spikey thing is the bulkhead which concentrates a lot of graph squares at that station.

Roof and floor are comparatively boring but they add a fair amount of mass so when you put it all together, much of the curvature of the side is lost.

Finally, after adding the columns and converting from number of squares to pounds, I had it account for a running total from rear to front, then express this as a percent of the total weight.

Please note that this doesn't include insulation, epoxy/glasscloth, paint, drawers, cabinet hardware, appliances, water, propane, matress pad or any camping gear whatsoever. It assumes that the weight of the galley hatch is uniform with the rest of the roof (likely very false). Also the steel frame is not included. This is just the shell.

Mathematically, 40% of a 9 foot trailer frame is 3.6' from the rear.

What I find is that the 60/40 point in my shell model is at 3' from the rear, back from the linear estimate by 0.6 feet (7.2"). Here's the math:

Adding in the larger bits like stove, cooler, and water/ice moves the 60/40 point back significantly from station 14/15 (at 3 feet from the rear) to station 9 (less than 2 feet from the rear!!). Further, when the 60/40 point is at 2', the balance point (50%) is very close to the intended axle location.

While there are clearly many examples of safe and sane teardrop towing, my conclusion is that this issue is NONTRIVIAL. A significant amount of the safety must arise from the as-yet unaccounted for variables. I suspect a major contributor is the trailer frame and more specifically the tongue/hitch itself which is a reasonably large mass with a long lever arm pointing the correct direction. I'll incorporate some additional values into the model in order to test this, after some more doodles.

But at this point, based on the above, I'm inclined to focus on more of a 67/33 position.

 

[philpom]

Many folks that build off road rigs actually move the axle forward a bit to get better tongue clearance. Going further back than 60/40 makes the clearance issue worse. I understand your goal but wanted to approach from a different angle of concern you may have.

 

[BellyDoc]

Many folks that build off road rigs actually move the axle forward a bit to get better tongue clearance. Going further back than 60/40 makes the clearance issue worse. I understand your goal but wanted to approach from a different angle of concern you may have.

Interesting thought! Thank you!

I think you're talking about ground clearance under the tongue. True?

Shortening tongue length does improve the trailer's breakover angle!

For this build, I think I'll have to prioritize that much lower, though, than weight distribution. Breakover angle matters when I'm on rocks going 5mph. Weight distribution matters when I'm doing 70. Design failures at 5 mph are addressed with angle grinders and rattle cans. Design failures at 70 are addressed by morticians!!

Between better breakover and better anti-jackknife, if they're at odds... I'll take the latter.

 

[KCStudly]

Some offroad tongue designs have adjustable telescoping tongues. Extended gives towing stability; ease of backing up without jack knifing or having to pull forward more often; and potentially better weight balance. Shorter equals more maneuverable on tight trails and better breakover angle.

Good that you are putting thought into this, but wholly cow I am thankful for CAD (computer aided design) design software, in lieu CAD (cardboard... or in your case graph paper and pencil... aided design).