There is no doubt that the material is very light. I've built many racks for the back of trailers, and I've yet to have any of them tear, break or otherwise fail... I usually carry a full size generator and gear box on them. I'm chompin at the bits wanting to reinforce or replace the skimpy bike rack on the back of my new trailer, but can't do it until the warranty is up. westxsrt10 gussets are a good start on adding some strength.... however, there is nothing that will replace using a peice of metal that has some substance to it. The thin wall stuff that they use isn't really made to carry a load. If the bikes are attached to a post in the center of the bumper, it is likely that it bounces back and forth. This leverage can tear the light metal. I would suggest that you add a support from the center of the bumper back to the rear cross member of the Chassis. This would help to stabilize the twisting effect that any bike hitching post might have.
Snakebite