Further to my earlier postings regarding an off-road equalising hitch, David Pentreath has now agreed to my describing his most interesting new mechanism.
Briefly, an equalising hitch is essential on any but a very light trailer if it is attached to a towing vehicle such that its weight is carried behind that vehicle's rear axle.
The problem is not that the trailer depresses that vehicle's rear suspension (because that can readily be fixed by stiffer springs etc), but that its weight acts as a lever to lift the towing vehicle's front wheels - thus reducing their grip on the road. If the vehicle sways at the same time, front wheel grip may be lost and the rig is likely to jack knife..
To see how an equalising hitch works, visualise a rigid beam that is rigidly attached along the whole length of the rig - from the rear of the van to the front of the towing vehicle. Now, no matter how much weight is placed on the front of the 'van - and thus the rear of the towing vehicle, the front of the towing vehicle cannot lift.
This would in fact be a good equalising hitch except the rig is likely to break in half over bumps, and will only move in a straight line..
So we insert a pivot (i,e, the tow ball) to allow the rig to bend in the middle for cornering. We also make the beam springy (to allow the rig to flex over bumps etc).
This removes the problems of a rigid beam but still ensures that weight pressing the towbar down does not lift the front of the towing vehicle.
This is how a normal equalising hitch works - and it's fine for normal road use.
There is however a big problem off-road in that if the towing vehicle is traversing (say) a deep creek crossing, the towing vehicle may be climbing up the far side wgilst the 'van is still going into the gulley.
The towing vehicle and the 'van are now at one considerable angle - and this causes the springy hitch bars to have to deflect to a hugely greater extent than was intended.
The greater the spring bars are deflected the greater the loads imposed on whatever they are attached to - and in the above scenario the bars may break, or the towbar be torn off.
Until now there has been no solution to this - except to continually connect and disconnect the bars. This is quite a problem with tracks such as the Cape Leveque Road (my home patch!) that has many sections that enable a comfortable and safe 60 km/hr plus, but also deep dips etc. Many outback tracks are like this.
What David has done is to retain the existing equalising hitch principle, but has cleverly modified it such the hitch maintains constant (but readily adjustable) 'lift' on the equalising bars such that the stress on the equalising bars remains constant regardless of the angle between the towing vehicle and the 'van.
The hitch works by having a smallish airbag that is connected to a larger air reservoir. The small airbag expands and contracts vertically to raise/lower the 'van ends of the equalising hitch bars. (The rig is levelled by adjusting the air pressure.)
As the rig traverses dips and rises, the corresponding upward and downward movement of the bars will vary the pressure on the air bag.
This will cause it to expand or contract accordingly and as this air bag is interconnected to a much larger air reservoir, the overall system pressure will remain substantially constant.
The overall effect is that the lift transmitted via the equalising hitch remains more or less constant regardless (within very wide limits) of the angle between the towing vehicle and the ''van. This is in total contrast to a conventional hitch in this situation - where the lift off the bars, at the more extreme angles may even lift the rear wheels of the towing vehicle off the ground (assuming nothing breaks beforehand).
David's hitch also has an ingeniously simple but very effective way of allowing independent movement of the arms, (but maintaining correction) during roll.
This hitch apears to be a very elegant but eminently practical approach to what has previously been quite a major problem when towing heavy trailers over rough going. I wish I'd developed it myself!
I need to add that this is perforce a theoretical assessment, but as the operating principle is so simple there seems no reason at all why it would not be highly effective in practice.
David has been using his own BT as a test bed and has some very dramatic pix of the hitch in action.
I need to stress I have financial interest of any kind in this project, nor am I accepting payment for this assessment.. I'm simply very pleased to see such a long-standing problem apparantly solved.
Finally, to those hopefully few remaining people who believe that one of Newton's more dramatic Laws are circumvented by buying several tonnes of BT - please do think again - because others have to share the road with you.
I will attempt to post a few pix of the hitch on this site.
Trusting this is of interest
Collyn Rivers