Road bike. Prresumably drop handlebars already fully furnished. There is also the worry about vibrations. I would put the film in a small bag off the seat post and rails and sling the M6 across the shoulder and hanging beneath your torso. I do this for leisurely rides in good weather. It does not work with the two lug M5. I have a case for an M4-2 which is a bit bulky, but I have had the M6 in that too. It adds some protection. I have never come off my road bike, a tourer. If I had regular 'offs' I wouldn't take a Leica. A back pack might be OK for a bit but with a rock like a Leica in there I suspect it would get uncomfortable very quickly. The same rock will be what breaks a spinous process of a vertebra when you do come off......
I will often take one pannier with the Leica atop a lot of padding beneath, but this is way beyond what most would have on their road bike.
