EH52ARH said:
Prof, I can only see, the nose weight changinging, with height of the tow hitch , if you have a very high C of G directly over the axle. And normally only on a caravaan with a short wheel base.
Can explain to me why , the hcitch hight changes the nose weight, please.
Hutch.
Hello Hutch,
At last, someone else who has taken the time to really look at the forces involved. Yes it is precisely the position of the COG and it's relation to the effective pivot of the road wheels that gives rise to this very real effect. The complication is the fact that as soon as you change the position of any of the items in the caravan it affects the three dimensional location of the COG, and if the trailer has change in attitude, then the arc through which the COG moves affects it's horizontal displacement to the turning point.
In terms of nose load we can effectively ignore any lateral displacement and are only concerned with longitudinal and elevation.
As you suggest the height of the COG is a significant factor that affects the degree of nose load variation, and that is an aspect that cannot easily be seen or measured by a caravanner. There is no simple way of measuring the height, but through a wide experience of towing, and observed effects, coupled to the disposition of components found in caravans, I guess the COG will usually reside somewhere between about 1 to 1.5M above ground. Crucially above the caravan's axle. This of course changes when payload is added. In a normally set up caravan there will be a nose load that pushes the nose down. This is the result of the COG being trimmed to just forward (and higher) of the axle.
The effect changing the height of the hitch is to cause the caravan to rotate about its axle, this also rotates the COG and it's loci will described an arc about the axle. Relatively speaking nothing changes, but in reference to gravity, the COG moves horizontally relative to the axle the closer the COG moves to the axle the smaller nose load it will produce. If the cog moves to be directly above the axle, the nose load becomes zero, and if the COG moves behind the axle the nose load becomes a lifting force. It's reasonably easy to see how this affects a Single Axle (SA) trailer, but the effect on a TA is far more complex and dramatic.
Whilst the processes involved are not necessarily easy to comprehend (especially the TA) fortunately in the real world we only really need to know the result which is the nose load, and that is a practical feature for us to assess. All we need to do is to measure it. But if we are to assess it properly, it needs to be measured correctly and height is therefore a variable which must be controlled.
One of the characteristics of a tow vehicle is the height of the hitch will change when a load ( such as a caravan ) So the true nose load can only be assessed with the hitch at its towing height, and that is how the authorities would have to measure it as that is the way the relevant regulations describe it.
The issue is even more important for TAs because depending on the geometry of the caravan, the COG may reside between the axle's, and that changes the whole dynamic of the load change with hitch height. The rate of change is much faster and bigger, and it is possible the for the load to go negative within the normal range of high heights. So TA nose load is more critical to get right.