Thread: Roller Rocker or Airbags with Trailing Arms - What's best?

Hi Guys,

I'm new to the forum and have read with interest about a number of really cool trailer builds along with the do's and don't of trailer design/build. One area that I've not been able to find any real information on though relates to the various version of suspension and pro's and cons of each type.

I looking to commission the build of a tandem axle 8m enclosed trailer to carry a race car. I should weigh about 1.5T unloaded and needs to be rated to 3.5T all up. It is never going to go road and will be towed by a VW Touareg Diesel. I'm getting lots of conflicting advice in relation to the suspension and the why trailing arms with airbags are better than roller rocker and if I'm honest I'm now lost.

So my question is this. If you were going to build the trailer described above what suspension would you opt for and why?

Thanks in advance

Simon

If you buy the pre-made airbag trailing arm setup there pretty dear in comparison to the roller rockers.

Hi Simon and Welcome aboard.
In some cases it's like asking to compare Apples with Oranges... let me explain.

There are two types of axles available for small trailers, either a beam axle or an independent arm.
The beam axle is found on most trailers as it is cost effective and simple. Beam axles are mounted to trailers on leaf spring suspension systems in single, tandem, tri-axle and much less commonly here in Australia quad-axle.
Multiple axle suspensions then split into load sharing and non load sharing groups. Slipper springs are cheap, but do not share the load between all axles. Load sharing suspensions can then be split into Shackle or Rocker Roller systems. The Rocker Roller system is less prone to any sideways movement when it is worn, whereas the Shackle system will have quite a bit of sloppy movement once the shackles and bushes have worn.

Independent Arm type axles have a few options too. The Knee type suspension that Melbourne Trailer & Caravan Supplies produce is the only non-trailing type as it is a wishbone arm that pivots near the centre of the trailer.
Trailing Arm suspensions then have options such as Coil spring, Leaf spring, Rubber Torsion or Air Bag suspended. Independent Arm axles are also just that... Independent, so by their nature, non-load sharing in multiple axle applications and therefore require rating as such.

Air Bag suspension is a relative newcomer to the industry so there's not as big a choice of products. Also, you need an air supply, so some sort of air compressor, and if you want self leveling, then some sort of sensor setup. I can see it would be the bees knees as far as ride quality goes, and you would get a true load sharing independent suspension, but I don't know of too many race cars that complain about the ride quality of their ride when in the trailer.


So you see, there's more to it than meets the eye. The best compromise for cost, simplicity and longevity would be beam axle with either 2-1/2" or 4" drop and Rocker Roller suspension. If you have a slightly bigger budget, then have a look at the Dexter or Al-Ko rubber torsion axles, or the MTCS Knee suspension.

Air bag suspension and beam axles are the current norm on heavy transport, rapidly overtaking metalic springs on all axles other than steer axles on trucks and on all axles on busses.

The reason being that they are self leveling, load sharing and "road friendly"...this means on certain routes that heavy transports can carry more load than a metalic sprung vehicle.

The other huge advantage is that they ride a hell of a lot better, laden or unladen.....this is because they are self leveling and the airbags only run as hard as need to maintain the nominal ride height.

They also have the advantage that they can be set up to allow the suspension to squat or lift for loading purposes.

I'm about to begin a rebuild on a 2 tonne single axle trailer & had been toying with the idea of an air bag system with the exsiting rigid axe.....its never going to happen because it will be too expensive....I think.

But on a car trailer it would have the advantage that the trailer could be made to squat, thus either minimising the length of the ramp required or eliminating it completly.

Yes there is a compressed air source needed.

how much need is interesting.

To run a live self leveling system a constant source of air is required.....this is not a problem in heavy transport because every truck has an air system that runs many things, brakes suspension..hell even the windscreen wipers on some models.

There are some air bags used...particulary on 4wds that are manual inflate once systems....where the air bags can be inflated, adjusted and run with out further need for compressed air till they are adjusted next.

Another thaught is a system with all the self leveling gear, that is inflated once using automatic self leveling, then locked off....thus only requiring a source of compressed air at the time of loading.

But some how somewhere compressed air is needed.

All of the above could be operated with a dump valve to lower the suspension onto the bump stops.

The truck rear ends are mostly 5 link systems with upper and lower traileing links and either a panhard rod or a Whats link...some of them have lowrer trailing links and a wishbone to the top of the diff housing or to the middle of the axle.

For a light trailer, it would be reasonable to run a beam axle, with a pair of rigid lower trailing links ( replacing the leaf springs) and a pahard rod.....some sort of fairly large bush would be needed to accomodate the changing alignment......but this is nothing new.

Because the whole light trailer market is so cheap scate, I doubt that there are any systems like this on the market off the shelf.

But if you are keen, it could be either fabbed up or a suspension system of some vehicle pressed into place.

cheers

A mate has a tandem version of one of these.

http://www.tiltatrailer.com.au/ and it is simple to use an doesn't need ramps.

I've towed it many thousands of K's and the only downside is it bounces around unladen if you don't drop the air pressure.

The on board air needs an additional 2 volt supply to the trailer but is pretty handy when inflating tyres.

One point to consider with air bags is the fitment of effective shock absorbers.
As has already been posted, many trucks have adopted airbags for suspension, (although a good steel suspension kills them off road tor traction). Their road friendly status is really only preserved while their shocks are in good repair. When worn they are worse than any steel suspension.
I personally would rate airbags as overkill for your application unless you want to make a show trailer.

For good or bad, airbags are a fast acting suspension.....without good shockies..bouncy bouncy.

The other thing with airbags is limiting travel, there has to be something stoppin them from over excersing.

The good old prehistoric leaf spring....been in use since the romans....does a lot of things for its self....it limits it sown top travel and provides some damping in its self.

As for off road traction...oh hell...Amen brother.

I drove tippers for a short while, my truck was an old mack with a walking spring rear end......put that in one of its 5 reverse gears and there were't much it would not climb......the guys with their new trucks, with their soft seats, airconditioning, hot and cold running everything and "road friendly" air bag rear ends realy had to be carefull not to get hung up or crossed up.

Those fancy new trucks where great laying bitumen with a nice flat bed to run on.....but when ya gota walk the truck into a hole, be first up the stock pile or play in the mud......a walking spring or walking beam rear end is what you need.

cheers

I built this trailer, but got the chassis built by my by someone who has built a lot of trailers from the ground up. Not just small trailers, but also truck trailers. His experiences led him to go with tandem slipper axles, as he says it tows the best and does not sway. I only towed it around for a bit after I built it, not loaded up. Empty, it towed well anyway.