I am contemplating getting or making a utility trailer and wanted to throw some ideas around and ask some questions to others that may have had the same thoughts or that have had experience that will be able to speak to some of what I am asking about... So lets see where the discussion goes.

First off - my trailer idea on the surface is a bare frame. I am not sure if I want an enclosed trailer or a an open deck trailer. If I made one in the back of my mind I think I would start with a deck, but with the design implemented to have walls and a roof put on.

Secondly - I want to have around a 7,000lb gvwr trailer - not what your run of the mill wire deck utility trailer with a 2000-3500lb axle is.

So my first question is what everyone thinks the differences are between a single axle and tandem axle. To ground my question - there are axles you can get up to 10,000lbs+ (for a single axle). So for the weight class I am asking about (7,000lbs) there is no problem there of getting a single 7,000lb axle or a pair of 3500lb axles. See the link below for an example:
https://bendtrailers.com/dexter-12-7000lb-torflex-axle/

Along with the discussion on tandem vs single - there are more parts and more tires for a tandem. The conventional wisdom with a tandem axle is they generally balance a load better and keep tongue weight down a bit as compared to a single axle (load placement in relation to the axle makes less of a change in handling and tongue weight on a tandem axle vs single).

Something I have noticed, and don't like the thought of (on the surface, but I tend to nit-pick and look at the details) on tandem axle torsion axle trailers, specifically, is that the suspension side-to-side is independent, however there is no load transferring mechanics to balance the load between axles. If you look at a leaf spring tandem there is usually a pivot triangle between the axles that lets the springs float up and down independent of each other, thus allowing the weight of the trailer to more evenly rest on the pair of axles. If you take the torsion axle example - if you are on uneven terrain, lets say you are backing down a hill from a level surface to a downward slope for this example, the weight on the rear axle is reduced and the weight on the front axle is increased to get the tires to blend in with the slope. In the leaf spring example the weight remains the same on both axles.

However, taking the above theory in to consideration - the "fixed" torsion axles are also what make the load placement less critical because the tipping of the trailer is held back by the suspension.

As was the case of a trailer I used to have, a tandem axle, only one axle had brakes on it. That meant if I locked up the brakes the axle without brakes never broke traction. In the winter time or on slick roads that would be a plus, but I never ran the brakes that hard to be worried about it.

I would think in either case of a tandem axle or a single if a tire blows a tire blows. If the axle(s) was (were) dually(s) then this would be a different story - unless a pair of tires blew the load of the pair would be transferred to the good tire. That doesn't mean you could keep going for another 500 miles, but it may mean the difference between sitting on the shoulder of the highway and being able to get off the next exit and to a gas station. For single tires - tandem axle or single axle - if a tire blows you're riding on a rim, period.

As far as balancing a load between axles - it wouldn't be too hard to get a torsion axle tandem set up on a pivot like leaf springs. That would allow the weight to be shared evenly between the axles, but it would make the weight distribution of how the trailer holds the weight act like a single axle (because now there is a single pivot for all the weight, not a spread "platform" as wide as the axle spread).

About 20-25 years ago we had a 21ft fiberglass boat and tandem axle trailer. One of the leaf springs on the forward axle broke going down the road. The axle went backwards, pivoting on the opposite spring mount, and locked up the rear axle also. So my point is having a second axle in case something breaks is a moot point because if something goes on one axle - tire, spring, hub/bearing, etc - you're still SOL.

From driving semis (flatbed, stepdeck, and curtainside - all 10ft spread axles) I understand the weight capacity of the axles and how that is different than a traditional "tandem" set up on a box trailer. Commercial regulations and the bridge formula have to do with those numbers, though, and not the physics - in the same way I am trying to figure out on a smaller trailer. Weight capacity, for example, isn't going to go up by 15% (just a random number to illustrate) by taking a tandem axle (2x 3500lb axles) from a 36" spread to a 84" spread. 2x 3500lb axles can hold 7000lbs at either 36" or 84". What will change, though, is tongue weight and the balance of the trailer going down the road.

In thinking about trailers in the past, I would generally say, given the choice between a single axle and tandem axle, that a tandem axle would be a better option due to weight capacity. However, weight capacity is a moot point if given the option of comparable, or even higher, rated axles as a single. So that is where my core question is. A single, to me, is more simple.

Can you come up with a good reason why a tandem is better than a single? Do you have any experience that will highlight why either is better?

This is somewhat a different subject than I am asking about, but does anyone have experience with spread axle box trailers? As in bumper pull or gooseneck trailers with a 6ft or more spread between axles, specifically torsion axles? Aside from the tires skidding more in sharp turns (during backing maneuvers, for example), how do they ride going down the road?

 

Lots of questions, Steve! LOL. I can’t help you with the spread axle with utility trailers, but I di have a 38’ enclosed gooseneck with triple 6000 lb torsion axles. And yup on the skidding tires when making tight turns, but you understand that concept. It had a “Shocker hitch” ( https://shockerhitch.com/?gclid=Cj0K...EaArKIEALw_wcB ) on it. That combined with the torsion axles it rode and pulled great. The down side was that it had to be absolutely level when going down the road, as you know, there were no equalizers between the axles like on a spring ride trailer.
As far as the question of single vs tandem axle, personally I like the redundancy of the tandem. But that may be just me.
Good luck.

 

That is speaking to my questions. I don't think redundancy is an accurate term. If anything goes on any of your 2 axles you're still up against a road-side repair or being towed somewhere for repair. If redundancy did play in to things you'd be able to keep going, at least for a short period, without repair. That was my point about the dually axles. If 1 tire goes of a dually pair you may be able to limp on the 1 good tire of the pair to at least get off the road. If anything else goes, though - hub/bearing, spring, etc - you're in the same roadside repair or towing to get repaired situation.

Something else hit me about the leaf sprung tandems that is a plus - because the load is shared better between axles it also means that the axles articulate over bumps in the road better. That in and of itself would mean a smoother ride compared to a single axle. So that would be a plus. Though, I am not sure how the theory would work with fixed tandem torsion axles because I think the impact of a bump in the road is not absorbed - each axle transfers the bump 100% to the trailer, whereas in the leaf spring example the articulation of the axles over the bump takes out some of the bump.

Edit - I reread the above paragraph and I can see where that wouldn't make a whole lot of sense. Yes, in either a torsion or leaf sprung design the "springs" absorb bumps in the road and allow "articulation". That is what they're for. However, a tandem axle leaf sprung axle is mechanically different as the shock absorption of the springs isn't the articulation I am referring to - it is the mechanical linkage between the springs, axles, and frame. See the picture below. The red box shows the linkage that allows the articulation of the axles, separate from the spring compression.

 

Yes. The equalizer may help to absorb road shock in some instances, but on the other hand, the independent torsion may “float” each axle individually over potholes, etc. in the road. Especially if it is a triple axle like the one I had.

As far as redundancy is concerned, I have in times past, chained up one end of an axle with a tandem axle to get off the road, or to get where repairs can be
made. Well, I guess I did that one time with a single axle by finding an old wood fence post along the road and chaining it under the axle to make a skid to get it off the road! Just have to make sure one breaks down close to and old fence post!!!!!!!!!!! LOL

Edit: PS. I like the last line in your signature!
Something I have posted on other forums I will share with you. LOL

Internet diagnostics..........unless you know exactly what to describe, and how to describe it, no one knows what exactly what to tell you. And if you DO know exactly what to describe and how to describe it.........you likely don’t need the internet answer!!!!!!! LOL

 

Don -

All good food for thought.