on this subject of which I have very little knowledge we have just bought a Bailey 640 the previous owner paid £65 to Bailey to get an upgraded weight plate, taking its max weight up by nearly 100kg how’s that work when it’s exactly as it was when it left the factory seem a bit odd that it’s this simple
It means your axle is capable of taking the increased loading. So the caravans MTPLM was less than the maximum load it was able to take. This makes it more attractive to car owners whose cars were not heavy enough if the 85% guidance was used. There was a period when cars were getting lighter with each successive model iteration. In some cases 200 kg or more removed from a typical mainstream C class car. So you can see how this might have influenced peoples decision on what MTPLM caravan they could tow. The makers considered this preferable to letting owners having a higher MTPLM but not towing it fully laden. It also helped those that did not have the full BE licence.
In the same period caravan payloads reduced in some instances I’d see payloads on some caravans down around 125kg. Which is pitifully low considering a battery and mover could take 60kg. Hence to my mind it led to a situation where you would see a couple putting a roof box on the car to aid carrying equipment that I would have carried in the caravan. It put me off changing one van at five years and decided to keep it for another four years. It had a 250 kg payload when upgraded by Bailey.
One thing ensure your tyres load index is sufficient to carry the upgraded payload and increased MTPLM. When I had my Bailey upgraded the new MTPLM was within 20 kg of the combined tyre LI. So I had new higher LI tyres fitted.