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I could probably bugger an iPhone in no time flat on my RE 500 (Enfield, not Suzuki!). My fillings try to depart their housings at about 65mph...

There's also the obvious but-not-yet-mentioned: smartphones are not waterproof if they're not ruggidised for that particular application. I was on a ferry yesterday with four multis and a V-twin (Ducati) in front of me and all had various smartphones exposed on the handlebars. The sky was getting a bit black and on that route you either have to go across or around a very, very large block of granite. I did wonder how the phones would survive. As it happened, I got home across the mountains before the Gods of Water decided to take a leak, but I had also passed the Ducati rider stopped at the side of the road where she was carefully packing away the smartphone.

I would never consider doing that. A GoPro plus motorcycle-specific GPS is the only way to be sure that they will survive. Or a cheap Muvi as I found out some years back crossing the Himalayas. The two days of monsoon we hit would have killed a smartphone stone-dead within minutes.