Hi Fellows,
Local fab shops hesitate to talk with me cause of liability issues - but I need some help!
The eight ton RV has two parallel I beams 66" apart under the aft section. The I beams are 3" x 12" x 0.25". The simplest solution might be to overlap and weld a three or four foot 2" x 2" receiver under each I beam. The receivers would extend 12" beyond the I beams to clear the body.
Pinning a two foot 2" x 2" draw bar into each receiver, then welding a cross piece of 2" x 6" C channel to hold the bike at the 18" mark starts the platform. Probably need a front cross and maybe some "X" bracing to keep sway down.
The bike weights in at about 375 lbs. At 30" the 375 lbs would result in a moment of less than 1000 ft-lbs. But that is a static number. What happens to the moment when we hit a pothole? Does it jump to 20,000 ft-lbs? If so will the weld hold? Would a three foot weld hold better than a two foot weld? Does it just start to rip at the end of the I beam?
I would love to hear some ideas. Thanks, Ash
To facilitate some discussion, Let me say that one can go to Tractor Supply and buy a cargo carrier that claims to hold 500 lbs. It is designed to fit into a 2" receiver already mounted on a bumper. My hope was that two receivers would safely support 400 lbs or so.
I would first go back to the fifth wheel manufacturer and determine if there was a trailer receiver or utility carrier option. If there was such option see what the OEM recommended design looks like and the rated capacity.
Moving on - every time I see a question like yours all I see is a bunch of engineering and design unknowns.
First let's examine your mass estimates in more detail. The total mass hanging off the back of the fifth wheel is equal to the motorcycle + the carrier you are suggesting. This is likely to be in the 500 lbs + range.
No doubt that a carrier can be engineered to accommodate the motorcycle however there is the issue of the fifth wheel structure being able to handle the modifications (carrier + motorcycle + extra-whatever) which is a big unknown without some analysis knowledge of the existing frame structure. The peak moment and loading is going to be somewhere forward in the frame. So now we’re saying:
carrier + motorcycle + extra-whatever + trailer mass + cargo, etc…
Also, have you considered the weight and balance effects while towing?
Tell me and I forget. Teach me and I remember. Involve me and I learn.
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