I want to make a tow bar and need some help with calculations to make sure I choose suitable steel profiles. The tow bar won't be for towing, but only for attaching a bicycle carrier. The current idea is as follows: 1) two pieces of channel pointing front to rear. These will have the open side downwards, so they look like a "n". They would be about 80 cm in length and the overhang at rear would be about 40 cm. 2) resting on these two channels will be a piece of square or rectangular hollow section, that is transverse mounted. The hollow section will be 129cm in length, supported only on its ends by the two channels. The hollow section will be bolted to the channels. A flange tow ball will be bolted to the rear face of the hollow section. I estimate that the dead weight of bicycles, bicycle rack, hollow section and tow ball to be no more than 90kg; ie 45kg resting on each channel. (Plus more due to bouncing while driving). So the big question is what size profiles would be needed? 1) channel: how about 51x38x5? 2) hollow section: 60x60x5? I have no idea how to do the necessary calculations, so any help would be appreciated.