Marc, I was A. There in Greenville doing tech. B. have a GSA in the garage. Dale always has the final say. I'll certainly go read the '19 rules. I wasn't given them since I wasn't a rider in '19. They should be online for public view now. I certainly could be wrong. I know I wouldn't pass a cell mounted to pillion seat mounting points only. I've see fuel cells depart bikes in a crash. And I stand by my point that the latch and mounting for the seat is not designed for the kind of weight a fuel cell presents.
Just read the '19 rules and I see no provision for pillion seat mounted tanks. I see the following applicable sections:
3. Any auxiliary tank(s) shall be mounted in a secure manner so as to minimize the chance of its becoming separated from the motorcycle.17 Approval of attachment systems rests at the discretion of the rallymaster or the president.
17. When pressure is applied to the fuel cell, limited visible movement (i.e., 1/2") relative to the frame of the motorcycles will be permitted only if it is due to the compression of padding on which the tank ispositioned. However, all tanks must be secured to the motorcycle with straps or fasteners that are securely connected to a bracket or structural member that does not flex when pressure is applied to the auxiliary tank. No bungee, shock cord, or similar attaching device will be permitted.
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If you see something I'm missing, please bring it to my attention.
It's completely moot if StormRider just wants this for his normal use and is not planning on rally use. If he plans on rally use, it will fall to the individual rallymaster to allow it or not. For the IBR, I don't recommend it for the reasons I noted.