Hello Dusty,
I have to assume you are referring the the Reich Nose Weight Gauges that are sold to be placed on the tow vehicles ball coupling and the trailer hitch placed on top of the gauge, as opposed to the Reich Portable Caravan Weight Control units that you drive over.
It has to be the case that if you can easily assess your axle and nose loads then that has to be much better than doing nothing or guessing and both types of Reich product are probably some of the better units you can currently get, but they are far from perfect.
In respect of the nose weight gauges, you will be aware that I have banged the drum about how to accurately measure nose weights and how important setting the correct height for the hitch is because the nose load generated will change as you vary the height of the hitch. This is a provable characteristic and should be beyond doubt.
The rate of nose load change with hitch height is not the same for all caravans, it depends on how the particular caravan is loaded, which will be different for everyone.
However what we can say for a Single axle trailer is that the greatest nose load will occur when the hitch is at its lowest point, (i.e. touching the ground) and its smallest load will be when the hitch has be raised until the rear of the trailer dips down and touches the floor. The is the SA's general characteristic, but what we can't tell you is what those loads will be - that depends on how you have loaded the trailer.
The case for the Twin axle is far more complex. This is because the two pairs of suspensions interact and can produce some dramatic nose load changes over small changes in hitch height and even reversal of the nose load its self. Again the actual characteristic is unique for each caravan but again it is dependant on how each caravan has been loaded.
With some loading regimes in TA caravans, if you uncouple the trailer and simply let the hitch go, you may find the hitch does not fall to the floor. it may float. If it does float then it means at its resting height the nose load is zero. If you push the hitch lower, it will fight back which is actually a negative nose load! and if you lift it it will produce a positive nose load. It might surprise you how little you need to raise or lower the hitch to find a very significant counter load being produced. This rate of change of load with height is very much greater than for the single axle caravans.
The boffins in the EU who constructed the directive for measuring static vertical coupling loads (nose loads) recognise these trailer characteristics, and thus require the measured value to be taken with the trailers hitch at the same height as the vehicle coupling when it is all hitched and ready to roll. Fundamentally measuring at any other height will give an erroneous reading.
I must stress that the way each caravans will change its nose load with the height of its hitch is a dependant on the way it is loaded, so even the same caravan loaded differently will produce a different nose load to hitch height characteristic. Such matters can be calculated if you can provide accurate answers for all the variables that affect the nose load. Without all the answers, no calculation can provide an accurate result and there is no simple universal table of values that you can use to correct for height differences.
The Reich Nose Weight Gauges add height to the hitch. So they cannot actually measure the correct nose load value because their method is wrong. Reich claim their Nose Weight Gauges compensate for the height variation, but unless you can program the gauge with all the necessary variables for your caravan, it cannot possibly compute the actual nose load. At best it may use a proportioning value based on average caravan dimensions, at worst it may simply add a few kg to the measured result.
Despite their deficiencies, they are definitely better than nothing, but because of their fundamental flaw of adding height to the hitch I would not recommend them especially if you are working close to your upper nose load limit.
All that and then we need to consider the calibration of the devices, which adds a whole new level of uncertainty
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