Hi Tom,
I had always simply taken a trailer's "tongue weight" to be the difference between the total weight and the weight borne by the axle(s). That's certainly the way RV manufacturers use it. However, your question made me think about it a little more and I'm not so sure if that's the best definition in the context of our discussions. As soon as we mention "borne by the axle(s)", we're talking about the ground's reaction through the axles, which, as we've clearly illustrated in this thread, is changed by a WD system.
I think instead that the unchanging "tongue weight" that we've discussed would have to be described in terms of gravitational mass, but I'm not sure that there's a simple way to characterize the remaining mass that's not centered over the axle(s).
Care to take a stab at it?
Thanks for the brain ponder. :E
