I had a spring failure on the OK turnpike. One broke in two on the rear axle and it caused the other one on that axle to break in two. As it turns out, one of them had been cracked a long time based on the rust in the broken crack. I replaced all 4 springs for my 3,500 lbs axles with OEM spec 1,750 lbs rated springs. When I got home I looked at the new ones and the there was little arch in the springs and nearly a reverse curve on them. A trip to the scale verified the trailer was not overloaded. I then changed the PR4b springs to PR4 (from 1,750 to 2,400 lbs for 4,800 lbs axles) and the arch was normal and raised the frame by 2". No sign of a rougher ride over the last 1-1/2 years. My conclusion is that the springs originally on the trailer and the first replacements did not meet their rated specs. Get the heavier one for safety's sake.
