Ruth, whether it is mounted to the bumper or the frame, I do not think makes
a difference. It is still 30-50#s per bike, added to the rear of the
trailer, directly or indirectly supported by the frame and located 5-10 feet
from the rear axles (think of it as a lever/fulcrum). To much weight to take
a chance on.
I have the same problem, generally with 6 bikes to tote. My solution is put
as many as possible on the tow vehicle, and the rest in the Stream. Or, I
just take a second vehicle.
Phil Renner
63 Overlander and too much stuff to haul.