I was told the same thing. They under rate to give a good amount of safety margin.
Last year at the PA show, I spent a good hour talking to one of the reps (we talked about racing and campers more than anything) and he told me the weight they really test them to and at what point they started to see failures. Believe me, from what he said, at 18k you won't have anything to worry about. He told me that the engineers kept trying to get the owner to at least rate them to 20k, but he wouldn't. His thinking is, if they rated at 20, some people would try and haul 22 or 24 and he wanted a large margin for safety especially for when people don't install them properly.... (Like the one fellow in another post said you don't need to loosen the U-bolts to remove or install.
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Just my experience on the subject...