Firstly, I am not an engineer but I am a practical thinker and I hope the following might make other people think about what I am about to say and give their opinion.
Let's say your vehicle has a 300kg ball weight limit. For the sake of argument, you hook up and you are well within 'legal' limits at 250kg. You set off along nice a smooth road surface and at any given time you probably still have 250kg on the ball. The 'system' says that you leave home with 250kg on the ball and it won't alter until you arrive home. Wrong! We have all travelled along some of the goat tracks we call highways and regional roads around Australia and hit a few causeways, rough areas onto or off a bridge and your vehicle and van is jumping up and down like a bucking bronco. Is anyone going to tell me that there is still a lovely, even 250kg on that ball that never changes? I think not. I suggest that the ball weight could go from 250k to almost nothing to perhaps several tonnes and back again in an instant.
I do try to drive to conditions but some times you can't avoid a dip in the road and the van and vehicle both jump up and down. On one occasion, I had a 2kVa Yamaha generator stowed in a rear compartment of my van. They are reasonably heavy. On arrival at my destination, I found that the generator had jumped so high that it came down on its side. I think my ball weight would have been exceeded perhaps by several tonnes when the rear of my van jumped high enough to flip a heavy generator onto its side. It would be interesting to know if any research has been done in this area because I could never be convinced that a ball weight does not alter considerably both up and down on a long trip. Obviously vehicles and towbars are designed to withstand many tonnes more than their rated ball weight. If this is the case, why all the hype about getting the ball weight so accurate when towing.
Of coarse you are absolutely right. This is why we are saying that it is the vehicle manufacturer, and the tow bar/ball manufacturer that stipulates the maximum weight imposed on that area. As you say, there is a large safety margin engineered into those parts, but those figures are what they have calculated to be safe.
There is also a huge difference in the strength of parts being pulled (tensile), as opposed to cutting action (sheared), and most of the bolts holding a tow bar on are in shear, and off course the tow ball itself. Add onto that leverage, for instance extending the tow ball back, i.e. a WDH, and the stresses become even worse.
Another factor is called stress reversal, i.e. the number of times the load is applied in one direction, and then reversed. bend a piece of wire backwards and forwards, it gets hot and then what?
We have all seen pictures of utes with a broken back.
The whole thing is as one of our members says, "a can of worms".
An excessively heavy tow ball weight will tend to lift the front wheels of the tow vehicle off the ground and a light tow ball weight will tend to cause a lever up condition and tend to lift the rear wheels of the tow vehicle off the ground. This of course, will depend on the loading of the caravan.
A pig trailer (caravan) configuration is always a compromise to achieve the most suitable and safest weight distribution but if we were all really fair dinkum we would all have fifth wheelers. Unfortunately that is not the way we have gone in Australia and to do so is generally much more expensive, hence the problem of weight distribution and safety with our caravans.
As iana has mentioned as well, the mechanical stresses are quite excessive when a pig trailer configuration is experiencing any thing but good road conditions and when the tow ball weight is not within that safe specification.
Again, in Australia unfortunately, our roads in many cases are less than conducive for smooth travel.
-- Edited by Dickodownunder on Tuesday 8th of May 2018 08:39:53 AM
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"Seek the truth or bury you head in the sand, both require some digging"
Back about 20 years ago a car club I belong to was invited to a towbar engineering manufacturer night at Tingalpa/Wakerly in Brisbane, at first we were introduced to the oowners and there chiefs who told us about there plans and achievements, all very interesting stuff, thinking back the cars we all drove would rarely see even a small trailer behind them.
They had jointly developed a machine with the assistance of a Qld Uni department, replicating a car with a trailer wiggling and vibrating at a great rate of knotts to work out how long the towbar would last, the stress points , weaknesses, at the time the only one in the world and by these exhaustive test they landed the supply of towbars to a large car manufacturer which was a real big thing for a small Australian company
For the love of me can not remember the names of the people but I do remember they said that they were under threat by a larger manufacturer. A few years later I was delivering steel products to this factory and they were manufacturing a totally different products. No towbars in site.
What I did gleam from this night was that the towbar was possible a faction stronger then the chassis its attached to but not to strong, the bar must flex a little, all this was way over my head. At the end of the night, I do remember the supper was excellent.
So myself I go down the road feeling very safe knowing that we have an approved Australia designed towbar. Please don't spoil my dream.
Ps, I wonder who owns that forward thinking business now.
