Given the research that I have done, I would advise that you deal with the rear frame of your bike in some way before mounting heavy bags. As Grant has indicated in the write up on building his bike, the rear frames on the G/S series was weak and will cause evil handling and break if loaded. From what I can tell having owned both bikes (R100, and R80 G/S) the main differerence in dimension is that the R100 frame is longer. While I suspect that you could bolt on the newer sub-frame with little or no modifications (except to your lighting, which is mounted differently), You might want to consider adding some sub frame supports in the way of an extension, thus alowing fitment of the standard rear racks. I am planning this for my bike and do not anticipate it to be a difficult job. Even if you chose to swap sub-frames, the newer rear frame still benefits from triangle bracing from the swingarm up to behind the seat (see Grant's diagrams). I can attest that an average load swinging on a standard rack from the rear of a R100 GS can cause some interesting lateral swaying in certain conditions. This is partly caused by the general design of BMW and aftermarket bags, which tend to be mounted well out to clear the muffler and shock/ rear drive. Good luck with whatever you decide
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