I had the 'flap about a bit' experience going over the mountain on my Street Triple, the wind caught in the suit and completely ripped the suit open at the front zip - the suit was trailing behind me in shreds but I didn't even realise until I got back to my B&B. * the fab thing about the IoM is that you don't need to be coy about higher speeds Also, leathers get very heavy when wet, and are probably harder to get off, so an undersuit even more important.
