Originally Posted by w7ts
A I have not made any adustment in my hitch going from 15" to 16 " wheels and tires.
Nor have I. The difference was so small it was not worth re-adjusting the hitch. One of things I wonder about is whether it is possible to find someplace that is actually flat. Small differences in elevation from the front of the tow vehicle to the trailer wheels can be impossible to see. I could look for a 40' level but I don't think I can find one. Parking lots are advised if you can find a level one, but most are not level so runoff can go toward the edges to drains or surrounding grass. In the Rockies, level is something you hear about somewhere else.
When I adjusted my hitch, I drove the truck into the shop where the concrete floor is pretty close to level, but the trailer doesn't fit through the door. So I built up wood scraps outside to be level with the shop floor and pulled the trailer onto them. Since I don't have a 25' level, this was dicey and I used an 8' piece of straight aluminum (how much deflection was there?) and a 4' level to get the wood to the proper height. Was this process accurate? Was the level accurate? No, but was it within reasonable expectations? Maybe.
My leach field for our septic system is pretty flat, but it is not perfectly level—I checked it as it would have been easier than what I ended up doing. I couldn't think of any parking lot anywhere nearby that was level and often parking lots are full of cars anyway. You don't actually need a level surface, but you do need one where the entire length of the trailer and tow vehicle are on the same grade. Then when you use a level, you have to make sure you have the bubble in the same place for both truck and trailer—on a slight grade, that is very hard. Making it even harder is that there is no where on the truck that is perfectly parallel to the tires—and I found the one side of the truck is a little lower than the other side. I ignored that. I assumed the truck was level in the shop and measured the wheel wheels as I went through the adjustment process. But did I measure at the exact same spot in the wheel wells each time? Probably not, though I could have put a paint spot on the paint—I wasn't going to do that. The trailer is easier to check for level—put a level on the belt line assuming it is parallel to the ground. I hope it is, but I had no way to check that.
My point is that when adjusting the hitch, lots of small errors creep into the process. As time goes on and tires wear, more changes. With lots of miles, Airstream axles change their position and the trailer gets closer to the ground, further increasing (or decreasing) error.
So I did the best I could. It was a lot better than the dealer did and the trailer towed better. There are several places to adjust an Equalizer, each one affecting the others. I experimented with each until I got the trailer level and the truck close—less than one inch difference between front and back wheel wells. A difference in height of the trailer axles of 1/2" to 3/4" may not make a difference that can be adjusted. It could make the trailer just as off level in the opposite direction as it was beforehand and thus no meaningful change.
I don't mean to say this is not important. But unless you have a testing lab with well calibrated tools, you'll be off a bit more often than not. It is possible the trailer was slightly lower than level at the tongue and will become slightly higher. That would be good luck. Bad luck would be if you were too high before and become even higher.
One more point—does a change in the height of the axles translate to the exact same change at the tongue? It seems so, but is that affected by the distance from the axles to the tongue? This may require a knowledge of geometry that I have had 55 years to forget. And are both axles mounted perfectly so they are each the same distance from the ground? We assume these things are correct, but I didn't expect the truck to be lower on one side than the other either (the shop floor could be off level, but I think I checked that).
There are too many variables to predict this. About the only thing you can be sure of beforehand is to make sure the new tires will fit in the wheel wells and then see what happens.
A lot of words, but better than what I was doing, loading up the trailer. I guess I'd better get back to work.