The safest place on a motorcycle for cameras is the middle of the bike, on the tank. This part of the motorcycle is where the least amount of jolting is felt from the suspension, plus, with some insulation the vibrations which occur at around 4,250-4,500 rpm on the flat twin motorcycle will not effect the cameras much, if at all.
I have crashed 9 times in a big way over the years and with over 2,000,000 kilometres under my belt on bikes, I have seen some interesting things happen to cameras.
In the late seventies and through to the nineties, the Elefantboy tank bag from Germany was possibly the best. This was due to the size of the tank bag itself, plus the thick construction and the closing over the top flap with map carrier on the top.
One of the German manufacturers, KArl HEinz DOrn, "Kahedo" has a great range of tank bags, they supply Touratech with a Touratech badged tank bag system so their quality is right up there.
I was in Germany this January and met up with a friend who has family ties with this company, he showed me an experimental bag for the modern motrocycle that is a modern version of the Elefantboy tank bag He wasn't sure whether this type of bag would sell. If that bag is around in Germany this summer, I would look at this as a contender.
When carrying cameras in the tank bag, wrapped in some covering, sometimes a case, they will usually survive virtually anything except for when you cartwheel a bike and it happens to land on the tank first.
I have had three tank bags rip off in crashes and go flying or sliding all over the place, but the cameras inside survived.
If you have your cameras sitting directly on something hard that transmits vibrations, then they will be shaken to pieces. If a Nikon F2 shakes to pieces that way, then I'm sure any other camera will also shake to pieces.
A hasselblad with a WLF folds down to a very small square box, with a film back on, a small rectangular box. Pack your lenses on their own, that way they are each self contained in a personal sleeping bag.
Film can be carried almost anywhere.
The greatest advantage of carrying your cameras in the tank bag, is that you take the bag off the bike for lunch or whatever, thereby reducing the chance of theft greatly.
Mick.