Rotax 912 startup vibrations
As has been said, the startup vibrations from the 912 is pretty horrible, and the 912s even worse. Rotax' solution to this is a "slipper clutch", which seems to make a huge difference to startup and low-RPM vibrations. I've never done one myself, but I believe that it adds about 1½kg and can be retro-fitted, presumably Skydrive can give you chapter and verse.
Incidentally, the analysis I did on this particular aeroplane showed that it was the final approach (float!) that was most extended rather than the ground roll - so if you are really determined turning the switches off at about 100ft would be the way ahead.
Would I have the nerves to do that? Nope, I'd go to an airfield with a longer runway!
G