OFF ROAD Caravan wheels

If the wheels, bearing, brakes, axles, springs and tyres are rated correctly, I think you can run any weight you want on a single axle. The real issues are suspension design and weight distribution.

Modern caravans all seem to go for independent trailing arm suspension which gives excellent tracking, but a very low roll centre. This causes additional body roll which can lead to instability if you get a pendulum thing happening. It also proves very poor load sharing for dual axles, unless they are running connected air bags on each side, like on the Bruder and the Kimberley Cruiser.

Putting a lot of weight at the ends of the caravan increases the pendulum effect (called yaw or polar inertia) which, with a low roll centre, can cause real stability problems. Kimberley (bless their liquidator) used to put anti-roll bars onto their caravans for this reason.

A single axle frees us a lot of space in the caravan and will reduce weight by maybe 100 kg. It also does less track damage because there is no scuffing of the tyres on tight bends. The down side is increased ground pressure, which can be overcome by using larger tyres and dropping pressures when on sand.

IMHO, the best suspension for a single axle is a solid axle with either long, well designed leaves properly damped, or a four or five link setup, like on the back of most 4WDs. This gives a much higher roll centre. Add good dampers and bags for a great ride.

For what it's worth, here's what I've put on my build:





Keith

Excuse me as I'm a bit confused, but why would you be running Free Wheeling hubs on a trailer?

Ian, not to many people notice this. I didn't want to use the hydraulic disks as a long termparking brake. The disks and stub axles are off the front of a 79 series, so a drum brake wasn't an option. If you look just inside the brake backing plate below, you can see the a bolt passing through the centre of the beam axle.



This holds a sawn off axle which has been drilled out to take the bolt and to hold the cut down axle in place. By locking the hub, the wheel can't rotate. It also means you can lock one wheel to manouvre the caravan.

Keith

Thanks for that keith, I'm old and easially confused, just seamed like overkill but with your explanation I can now understand how it works

Keith
Isn't the roll centre of your design quite low, lower than a trailing arm type? Just seems to me the highs and lows are back to front.

There is a good strength in your axle setup but will the chassis setup, riser sections allow flex and fracture if any significant weight is in the vehicle?
Is the FWH there to be a park brake? How can it be registered without a manual park brake? it isn't just a 6 x 4 trailer.

RMD, I think (hope) I have my roll centres right. The lower the roll centre, the more body roll you get. A bit like a lollypop on a stick. Their further down the stick you hold it, the easier it is to push the lolly left and right.

The upsets in the chassis are 4mm steel rather than the 100x50 x 3mm RHS used everywhere else and are increased to 150mm depth above the air bags. There is additional reinforcement in there as well.



It's also bonded to the composite body, which adds a lot of stiffness to the structure. We'll see when it's on the road.

On the emergency brake thing, I am going to talk to the inspector next week and see if an over-ride button or switch on the hydraulic over hydraulic brake unit will suffice. I didn't realise that I had a problem until recently when I re-read the rules. I've made a dozen blunders like that.

Sorry to have hijacked this thread. It wasn't my intention.

Keith

Looks a bit to professional for my liking. :)
Dave.

One of the best designed and engineered set ups I have seen not being a fan of the populist trailer arm systems. I think you will find that emergency brakes have to activate automatically should a de-couple occur. Quoting from VSB1: "The brake system must cause immediate application of the trailer brakes in the event of the trailer becoming detached from the towing vehicle." Not sure how a 'press button' would do this or if you would be in a condition to activate the button should the de-couple occur resulting from a MVA. I have seen this done though by activating the electric motor on the hydraulic (power pack) over hydraulic set up using a cable attached to the tug (a la electric emergency brakes) & a battery (that you would have in the van anyway) that will keep the emergency brakes activated for the mandatory 15 minutes. Also over 2000kg GTM the service brakes must be able to be operated from the tow vehicle.

Thanks Blown4by for the kind words. I don't like the popular independent trailing arm systems either. They seem to be more a fashion statement that anything else. Why you'd need independent suspension on a single axle trailer is a mystery to me.

Just on the brakes, I have a Hydrastar electric over hydraulic unit in the system with a breakaway switch and have just added a manual switch to the outside of the van so that the brakes can be applied by one person when it's being manhandled. The locking hubs are for when it's stationary.

Apparently the requirement for an in-car brake battery monitor in NSW has just been removed. My WiTi setup flashes all the lights on the van if the voltage drops below 11.5. The brakes are powered by the 400AH lithium house batteries with 990 watts of solar on the roof, so the 15 minute requirement should not be a problem.

Keith

"You can't by road knobby tyres for vans."
Whether you would want to run them or not is one thing but given the correct diameter/width rims to suit the tyres, correct offset & correct stud pattern/PCD to suit the hubs, you can fit any type of tread pattern tyre you want to a van.

The size of the tyre will affect the trailers load carrying rating.

There is a lot of space between "dirt road" and your definition of "off road".
Beaches, for instance, and most of us go there. Fraser is classed as a "road".
Cheers,
Peter
OKA196 motorhome

Peter,
If it is "classified" as a road, then it's a road. driving there is driving on a road, no?

Agree that beaches aren't paddocks, but most are not classed as roads, and would qualify as "off-road". Not many people would take their caravan onto a beach, would they? I must admit, I'm not a beach person, so would not be bothered - I prefer my water frozen and on the side of a hill.