Hello Colin,
I am sorry to be so late responding, but I am currently in Canada on holiday so access to teh internet is limited.
I can understand what you are trying to say. So to try and clarify: - a little!
The law regarding MPTLM and vertical imposed nose load is crystal clear and uses the limits specified by manufactures for these elements to set a defining value against which an outfit can be judged.
However and where the law falls down is that it is not specific about how these values should be measured, and that is the part that is open to interpretation. So this is where the specification for level ground should be made, as should the method and specified accuracy of the measuring tool.
The science of measuring is actually far more involved than most people understand. There is a division of the British Standards Institute that shows industry how to evaluate the measuring processes they use and how take account and quantify levels of uncertainty. What most of this thread has developed into is identifying factors that increase the uncertainty of the measured value.
I think you know by now how I interpret the meaning of level ground, though to be more specific - the caravan road wheels and the reaction point for the nose weight gauge should he on the same horizontal plane. The height of the ball hitch above that plane should be the same as when the caravan is attached to the loaded tow vehicle.
Given that set of criteria, (and I know I have not given flatness limits etc) your first example would fail to give an accurate evaluation of the nose load.
For reasons that have been explained in other threads, the jockey wheel will give a different reading to the true value of the nose load. So both examples would fail to give an accurate reading.
With regard to your MPTLM figure, that is a matter for the caravan manufacturer to explain. I can only postulate that perhaps that after some time of manufacture they found that the original figure was causing some concern, so they opted to reduce it on later production, and when replating.
As for the maximum towed weight for a vehicle, Lutz has established that the MPTLM does not include the nose load, as that is carried by the car not the caravan wheels, so in theory you can load up to MPTLM plus the nose load. However, and here I might disagree with Lutz, is that as the nose load is an imposed force, rather than a mass on the hitch, derived by the loading of the caravan so it is in my view dangerous to make a simple addition of the two, so I opt to consider them as separate entities.
You are right to be skeptical about the abilities of the enforcement agencies to unravel the intricacies of measurement techniques, I think it is highly probable that their measurements methods could be called in to question where a case is only just over limit, but that would mean calling expert witnesses at quite a high cost, so rather than be on the limit aim just a bit lower to remove any chance of appearing to be over the limit.
I am sorry to be so late responding, but I am currently in Canada on holiday so access to teh internet is limited.
I can understand what you are trying to say. So to try and clarify: - a little!
The law regarding MPTLM and vertical imposed nose load is crystal clear and uses the limits specified by manufactures for these elements to set a defining value against which an outfit can be judged.
However and where the law falls down is that it is not specific about how these values should be measured, and that is the part that is open to interpretation. So this is where the specification for level ground should be made, as should the method and specified accuracy of the measuring tool.
The science of measuring is actually far more involved than most people understand. There is a division of the British Standards Institute that shows industry how to evaluate the measuring processes they use and how take account and quantify levels of uncertainty. What most of this thread has developed into is identifying factors that increase the uncertainty of the measured value.
I think you know by now how I interpret the meaning of level ground, though to be more specific - the caravan road wheels and the reaction point for the nose weight gauge should he on the same horizontal plane. The height of the ball hitch above that plane should be the same as when the caravan is attached to the loaded tow vehicle.
Given that set of criteria, (and I know I have not given flatness limits etc) your first example would fail to give an accurate evaluation of the nose load.
For reasons that have been explained in other threads, the jockey wheel will give a different reading to the true value of the nose load. So both examples would fail to give an accurate reading.
With regard to your MPTLM figure, that is a matter for the caravan manufacturer to explain. I can only postulate that perhaps that after some time of manufacture they found that the original figure was causing some concern, so they opted to reduce it on later production, and when replating.
As for the maximum towed weight for a vehicle, Lutz has established that the MPTLM does not include the nose load, as that is carried by the car not the caravan wheels, so in theory you can load up to MPTLM plus the nose load. However, and here I might disagree with Lutz, is that as the nose load is an imposed force, rather than a mass on the hitch, derived by the loading of the caravan so it is in my view dangerous to make a simple addition of the two, so I opt to consider them as separate entities.
You are right to be skeptical about the abilities of the enforcement agencies to unravel the intricacies of measurement techniques, I think it is highly probable that their measurements methods could be called in to question where a case is only just over limit, but that would mean calling expert witnesses at quite a high cost, so rather than be on the limit aim just a bit lower to remove any chance of appearing to be over the limit.
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