Springs all the way for me. Every torsion arm type trailer I have ever owned has needed the axles to be replaced. Have just finished converting my dual axle cat trailer over to leaf springs, ride height is similar, but now easily replacebale down the track, much heavier duty. Price was around $1k more than just replacing the torsion axles. Trailer was steel, not ally, but don't think that would make much difference.

Cheers,

BD.

 --torsion axles will do it, pull a tight U-turn when they are getting on and you can drag the whole stub/arm out. As well, they give no warning of failure. They have the advantage of a lower ride height than springs, no cosmetic rust...use multi-leaf springs, you break one on the road, you can get by with various bodges. Have a rubber torsion fail, what can you do? 

The load sharing thing is important, too, IMO. Proper rocker load sharing rides much better than individuals.  You do seem to see a lot more torsions on ali trailers

 Everything mentioned above is absolutely true.

Having said that I have gone with 2500kg torsions on the new trailer I am building now, my first foray into torsion.

Points of note, and my observations

• If you intend doing any offroad or extensive gravel road towing, DON'T use torsions!
• The "lower CofG" with torsions is not really correct, at least on your average V hull. The minimum height the hull can sit is governed by the wheels and mudguard height. To get it down to that point is through the chassis design and axles which can be achieved by either gull wing or drop/overlay conventional if need be. On mine, with 20deg deadrise, gullwing was not needed on either axles or crossmembers making a replacement axle at least a little quicker to source though I still concur with Ranmars comment on availability.
• Torsions-advantage is that the axle cross beam is actually a structural member rather than unsprung weight as on leaf.
• On ANY dual axle set up the axles should be mounted as close together as practical (825 on 15" rims from memory or thereabouts). I cringe when I see a home built trailer with the axles spread out noticably further, the turning stresses Ranmar mentions increase exponentially the further apart they are.
• ADR's require that non load share such as dual torsion, dual slipper etc must be 120% rated on all components on the axle, for the exact reason Darren mentions (curb etc). So for example my 2500kg axles combined can only be rated at GTM 4160 (but most take it as 4000) + towball download of say 350kg
• Galvanised leaf springs are virtually unavailable nowadays, most of what you see is actually Dacromet (or simply silver Galmet paint in some cases). This is due to "hydrogen embrittlement) that occurs during galvanising. The surface metalurgy changes making galvanised springs much more prone to cracking and breaking

During the boat (and trailer) import boom 10 years back, I noticed that many trailer importers (and many private imports as well) are actually not legally rated correct in Aus as the US afaik does not have the 120% rule.

It is quite easily noticable, the US standard 3500 pound axle uses Ford 5 stud hubs, 10" discs and 14" rims.

3500 = 1590kg  x 2 @120% is only around 2700kg-many are rated here as 3.5T. I guess mainly because here they have just used the US, or the company importing can simply tag them as whatever they want as manufacturers only need to get something like 1 in 20 (or 50?) actually inspected.

 This may also be contributing to the numbers of failures people observe given our roads are generally narrower and rougher than US

Long post I know sorry 

 Hey mate, have you had a look at the parabolic springs. Had some fitted to my hilux and the ride is great. I presume they make them for trailers etc. Look like they would be easier to wash the salt off too. Just a it more initial outlay.

 After a catastrophic failure on the Ningaloo station road. It's all very well to carry a spare spring , but you had better carry a spare axle as well.   My 600 Quintrex CC had tandem parabolics, which of course are non-load share.  On the way home one year, about 10.30 am,  one of the parabolics ( rear axle)  snapped about halfway out to the highway. You have absolutely no backup with parabolics--they snap through the locating pin hole , then you have nothing securing the axle on that side. So the wheel/tyre assembly on that side goes back with great force, bending the axle and doing other damage such as ripping the tyre open on any hardware it encounters and taking your mudguard with it.  

This all happened to me, I was taking it steady, 15km to the bitumen, then bang. Disaster. What a mess. I won't go into details on the fix, but was lucky enough to pick up spares in Exmouth . After leaving the boat on the side of the road. We got back to Kalbarri about 6.30 next morning.  And I have seen it happen to at least one other vehicle on that track. I donated a set to Phil next year to put in the spares bin, and still have some lying around here, if anyone wants them.

yes, galvanising springs certainly can cause embrittlement. My current set, just on 3yo, was on the Easytow trailer I ordered with the boat--appear to be galvanised, but may be some other finish, no rust. 

 Had torsion arms on my 685 Cruise Craft trailer for 7 years. Towed back and forth to Coral Bay every year (almost) without any issues and all over Perth/Mandurah. I know that Ace Trailers ensured that the torsion bars were compliant with the 3.5t capacity.

Preferred torsion suspension over springs for smoother and quieter ride but then I was very gentle with my gear when I towed.

 I have just had a new trailer built to replace a tri-axle cat trailer with torsion axles, we went to a dual axle with leaf springs and landcruiser hubs, I will never have torsion axles again, they are impossible to repair.