Well, sort of. I was going to say this was wrong, then stopped and did the math before answering. Turns out the answer is yeah, you’re increasing the load on the trailer axle over a trailer without WD, but this extra load is probably considerably less than the trailer tongue weight if you set up the WD hitch correctly.
See my attached math, including free body diagrams. Pardon my sloppy handwriting and somewhat disorganized work, it’s been a while since I took Statics in college and I had to clear some cobwebs from my brain to do this. On my example I used a 4000 lb towing vehicle that has 2000 lbs on each axle (no trailer), with a 1000 lb trailer set up with 10% (100 lbs) of tongue weight. For distances I used 10’ from the trailer axle to the hitch ball, 5’ from ball to rear axle, and a 15’ wheelbase on the tow vehicle. I used the assumption of maximum recommended weight distribution, which is making the front axle of the tow vehicle weigh the same with the trailer as it was with the vehicle unloaded.
With these weights and distances, a vehicle towing a trailer with no weight distribution would have 900 lbs on the trailer axle, 2133 lbs on the towing vehicle rear axle and 1967 lbs on the front axle.
Using weight distribution to bring the front axle back to 2000 lbs, the trailer axle would be 933 lbs and the rear axle of the towing vehicle would be 2067 lbs. So the towing vehicle would not be level, since it has extra load in the rear, but as mentioned you do NOT want to use WD to bring the vehicle completely level, since to do so would load the front axle more that it is loaded with no trailer.
You can then use airbags or stiffer rear springs to keep the rear of the towing vehicle closer to the non-trailer ride height. But the WD hitch would help considerably on its own.
You would have to severely improperly overuse WD to actually make the rear of the vehicle lighter than it is without a trailer.
Summary is that the appropriate use of a WD hitch will greatly help with maintaining tow vehicle angle, but not completely eliminate it, you need airbags or different springs for that. But the primary purpose of a WD hitch is to put the weight back onto the front axle to maintain traction and steering, and doing so will add some weight to the trailer axle but not anything that’s concerning, at least in this example.
As @floridanative said, it doesn’t matter how soft the springs are, using WD will still help considerably, because you will always have less weight on the rear axle with a WD setup than without. Spring stiffness has nothing to do with the weight that are applied to the axles, only the vehicle sag. And if you have half the weight difference on the rear axle with linear springs, you have half the sag (assuming the WD is adjusted to keep the front axle at the weight it was without a trailer).
If anyone has any questions on my math, let me know. Statics is fun, and pretty straightforward. I started doing this and initially couldn’t remember how to do it, before my teachers voice came into my head, repeating what he drilled into our brains during Statics: “the sum of the forces in the Y direction equals zero. The sum of the forces in the X direction equals zero. The sum of the forces in the Z direction equals zero. The sum of the moments about any point equals zero.” After I remembered that it was super easy, especially as we only have 1 direction the forces are acting in for this calculation. Dynamics is where it gets really interesting though, because for accelerating objects the sum of the forces and/or moments does not equal zero…luckily we don’t need to worry about that here. 😂
