Tri Axle Trailer Suspensions

[sae8425]

I'm contemplating building a trailer because I want to combine all the features I need into a single unit which will then allow me to go cold turkey on my CTD (compulsive trailer disorder as another forum member called the addiction) and clear some badly needed space around the property.

This trailer would need to be capable of carrying around 18K lbs., and would have a 16-18 ft deck.

It would be a bumper pull design.

This trailer would spend much of it's time on farm lanes and on poorly maintained dirt/gravel rural roads.

For a number of reasons I want no part of a dual wheel setup (ie. 4 tires per axle).

So, I'm considering a tri-axle design.

Finding 6 x 16 inch radial tires able to carry 22,500 lbs total (or thereabouts) load is no problem.

I'm also no fan of leaf springs either (since my primary tow vehicle doesn't use leaf springs, I would rather not use such antiquated technology for a trailer of my own design/construction).

So, setting aside a custom bell crank operated horizontal coil spring setup (not considering, only mentioned in passing), that would seem to leave either rubber torsion, or air bag suspensions.

However there seems to be a marked reluctance to embrace torsion sprung tri-axle trailers here in America.

But the German military has a wonderful torsion sprung tri-axle multi-body trailer that, even in places like Afghanistan, functions superbly irrespective of whatever abuse it's subjected to. Far as I know they've never had one of the suspension units fail there, and I've been led to believe that they don't even keep spare axles 'in country'.

However, I've talked with Dexter at some length about this and they contend that torsion sprung tri-axle designs are problematic and prone to overload failure (especially if used in conditions that lead to repeated single wheel/axle loads).

So; do any forum members have any experience with non leaf sprung tri-axle trailers?

Does any forum member know of any torsion sprung tri-axle trailers being made anywhere in North America?

I've also talked with Firestone and AirLift about air suspensions and both tell me that they have 'primary' (as opposed to the secondary 'over-load' type) air springs that would be suitable for such use. And both have told me that if I wanted to have fail safe redundancy I could just use 4 airbags per axle.

I would obviously have to design and fabricate the axle locating linkages and mounting brackets necessary to incorporate an air bag suspension system into this trailer.

Both Firestone & AirLift contend that an air bag suspension system will live no matter what terrain I use the trailer in, or however often each tire/axle is singly loaded. They also say the cargo will have a much better cushioned ride.

Do any forum members have air suspended trailers? Are any of them used off-road? Such air suspensions are fairly popular in the Australian outback, so I'm not trying to reinvent the wheel here (other than for the weight capacity perhaps).

Many thanks to any forum members who take the time to respond.

 

[tcartwri]

I pulled a triple axle GN everyday for five years, and it ate tires, broke rims, broke an axle stub off, killed bearings... should I go on. Yearly inspection turned into a yearly rebuild of all hubs, and usually two or more tires.

With respect to overloading a particular axle, this is how it happens; As soon as you get into any uneven terrain, one axle ( usually the forward ) will have all of the trailers weight transfered to it, which obviously is two to three times its rated design.

Stay away from triples.

 

[Diamondpilot]

I have a tri axle boat trailer with 3 6,000# axles and many of my friends/customers and family members also have tri-axle trailers ranging from 10,500# all the way up to 25,000#. Non of us have had a single tire, wheel, bearing issue. A quality trailer will pull just fine.

I am with you and despise the tandems. I had a 25,000# 25' GN trailer with them and hated it from day one. I always wanted to convert it to 3 8,000# axles but never did and sold it before it caused me any more grief.

As for torsion axles, they stink. We had lots of boat trailers set up with them in the early 2000's late 1990's and we saw way more tire failures. When we started digging into the issue it came down to load balance. Many times we would see 70% on one axle and 30% on the other as when you check a tandem or tri axle with leaf springs and load levelers they were all in the +-5% range. Skip the torsion axles and go with what works.

My last concern is why only 18'? That seems short for a tri-axle trailer with that much capacity. Look at your future needs and make sure it will be what you need down the road. I no I am like you and in the small percentage that do not care for GN so I agree, stay with bumper pull.

Chris

 

[PoorPlowboy]

What is this "primary tow vehicle" suspended on coil springs that is capable of towing 22.5k?

 

[tcartwri]

I have a tri axle boat trailer with 3 6,000# axles and many of my friends/customers and family members also have tri-axle trailers ranging from 10,500# all the way up to 25,000#. Non of us have had a single tire, wheel, bearing issue. A quality trailer will pull just fine.

I guess if you don't put any miles on them they're fine. :thumbsup:

Brand in question was a Load Trail. Heard of them?

 

[Diamondpilot]

I guess if you don't put any miles on them they're fine. :thumbsup:

Brand in question was a Load Trail. Heard of them?

I pull about 10,000 miles a year.

Never heard of Load Trail.

Chris

 

[radman1]

I pull about 10,000 miles a year.

Never heard of Load Trail.

Chris

Don't you mainly pull customer's boat trailers? Lots of different trailers for those 10K miles? Most people never pull the same boat trailer for 10K miles/year.
Most people tend to avoid triple axle trailers. Better to go tandem axles or dual tandem axles. Triple axles are just cheaper than dual tandem axles. I can easily see my tandem axle trailers scuff the tires in tight turns and heavy loads. A triple will really scuff.
In the boat trailer world, triples are probably not too bad. If the boat is big enough for a triple, there are not a lot of options. Dual tandem boat trailer is not very practical.

 

[Diamondpilot]

Don't you mainly pull customer's boat trailers? Lots of different trailers for those 10K miles? Most people never pull the same boat trailer for 10K miles/year.
Most people tend to avoid triple axle trailers. Better to go tandem axles or dual tandem axles. Triple axles are just cheaper than dual tandem axles. I can easily see my tandem axle trailers scuff the tires in tight turns and heavy loads. A triple will really scuff.
In the boat trailer world, triples are probably not too bad. If the boat is big enough for a triple, there are not a lot of options. Dual tandem boat trailer is not very practical.

Yes and no. I have pulled my personal boat just over 4,000 miles this year. Just got back from a 850 mile trip last weekend alone. Most everything else I deal with, maybe 2/3 rds, are triples and in the 12,000# or more range.

I have only ever seen one boat trailer with dual tandems.

What I am saying here is I have never had a issue related to it being a triple and as far as scuffing goes they seem to pivot around the center wheel and the forward wheel goes one way and the rear goes the other. No worse than any other tandem.

Chris

 

[crazyal]

You may not like leaf springs but for a tri axle they really are the best option. With anything other than leafs if your trailer is not level it's going to quickly overload one of the axles. Also there is a lot of sideways force applied to the axles when turning hard with a 18k load on it on pavement.

 

[sae8425]

Diamondpilot,

Thanks for providing the axle load balance/imbalance figures none of the manufactures I've been talking to have been willing to cough up such information. Now at least I have some baseline figures to work with.

Obviously with torsion axles being rigidly mounted there can't be any load sharing/balancing between them.

The ability to distribute loads much more evenly between multi-axle leaf spring setups is due to the walking beam utilized between the leaf springs, it has absolutely nothing to do with the springing medium used.

A spring is a spring is a spring. Rubber, steel, air, etc. None have any magical properties/abilities.

The magic is in the linkages/geometries used to attach said springing mediums to/between axles and frames.

So why not just incorporate a walking beam into a torsion type multi-axle setup?

Now I don't know of any manufacturer offering such a design. And, for all I know (I should have crawled under one of those German tri-axle torsion sprung trailers I mentioned in the OP when I had the chance) this is exactly the type of tri-axle torsion design being used by the German military.

If we could send men to the moon in the early 1970's, surely we can design a better trailer suspension in 2011.

While the German military can evidently make tri-axle torsion suspensions work, here in the US we evidently can't, and your own experience is yet another example of the ongoing situation that I mentioned in the OP?

What are trailer builders in America apparently missing regarding tri-axle torsion trailer suspension design?


PoorPlowboy,

You had to ask didn't you!

The vehicle is a Mercedes-Benz Unimog.

The Unimogs began life shortly after WWII as a multi-purpose vehicle that German farmers could use in their fields to plow/harvest, and then drive on the roads to take their produce to market.

Today, they (or at least some variants) are perhaps best thought of as very-very fast tractors with 3 seat cabs.

Portal axles, portal hubs, 16 inch (min) clearance under the differentials, 24 inch (min) clearance under the transmission/transfer cases, fully locking (at any speed) 4WD, and tires that start at 12.5R20 (43 inch OD) and go up (you can fit 14.00R20 48 inch OD Oshkosh FMTV tires/rims with only minimal modifications).

They have suspension travel and axle articulation measured in feet not inches.

The M-B straight 6 engine used began life at around 95 hp, but with turbocharging & intercooling & better cylinder head designs the later variants could be had with up to about 240 hp. Mine only has about 150 hp.

Mine (a 1984 U1250 Ag spec model) does about 63 mph @ 2800 rpm in 8 th gear, and about 150 yards per hour @ 1200 rpm (peak torque) in 1st gear.

The 8 speed main transmission has two 8 speed auxiliary reduction gearboxes fitted.

It's 1 st gear, low, low ratio is 4,739:1 (no, not a misprint).

My Ag spec model has both a 2 speed live PTO and a 3 point linkage at the front and rear.

It weighs around 12,750 lbs.

The main hydraulic system is v-belt driven by the engine, and a separate higher volume hydraulic system that is powered through the transmission/transfer case has also been fitted.

Some torque converter variants (only the clutch is replaced, so all the manual transmission gears remain) will tow upwards of 600 tons (again, not a misprint) and they are regularly used to move railcars in Europe.

Mine has both an air and hydraulic trailer braking system (hydraulic trailer brake systems are quite common for agricultural use in Europe still).

Mine (in Europe at least) is rated to tow 48,400 lbs using either the air or hydraulic trailer brake system.

One Unimog variant was sold by Case in America back in the 1980's and a few municipalities and airports still use them. I gather that many, many hundreds (Buffalo NY bought at least 50) were sold during this period.

Freightliner sold the most recent Unimog variant in America, but stopped in 2007.

The US Army has a number of the 1970's air-droppable variant fitted with a FEL and a backhoe (these have the slickest reinforced frame I've ever seen on any vehicle), with rigid links to lock out the suspension during parachuting.

Unimogs are hugely popular for using as a basis for 4WD expedition campers destined for Africa (there are a huge number of European firms offering such conversions.

Unimogs are also the 2.5 - 5 ton 4WD vehicle of choice for most European municipalities (there are literally hundreds of attachments which allow them to be used year round for a vast range of activities).

No, you don't get anywhere particularly fast in a Unimog, but you are able to (almost) get absolutely anywhere!