I hardly see single axle trailers have much spring flex or movement. Trailer pivots of hitch mount.
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That's because too many people have overly stiff springs and over inflated tires on their trailers. I've watched out the side view mirror looking at my trailer's tire in relation to the fender. Even when going down the highway I could watch the distance between the tire and fender constantly changing just a bit. That is the suspension working. I'm running 1150lbs for the pair (575lb rating for each spring) on my trailer.
My trailer weighs about 1250lbs wet. I'm also only running 20psi in my 235/75/15" tires. So between properly chosen springs and lower tire psi, I've got decent flex.
You could bolt the trailer's axle to the frame solidly, and put 100lbs of air in the tires, and the trailer tongue would still pivot at the hitch. But then you've got no trailer suspension, and your trailer's contents (and your hitch) are taking a beating. And the trailer's probably going airborne with every little bump.