Neil. Thinking it through, that lead trailer is set up to run as a set. If the fully laden tail was added I’m pretty sure the weight added to your tri axle would increase, and the fulcrum effect take weight off your drive axles, and possibly add some to your steer. I guess somewhere along the line someone would of worked out king pin weight or % of total weight, king pin to center of axles and possibly bridge weights of combination! Perhaps even a shorter wheel base tractor was originally spec’d adding weight to the steer axle?
Paul