Help. What would cause a bike to start shaking (has happened on new, old and many) when coasting at least 12mph, riding with no hands, sitting up straight. Shaking to a point where I must grab the handlebars or shake out of control. Appling pressure to the side of the top tube with a knee helps.
I have been told everything from geometry, size, seat too high, etc
Any help, ideas, thoughts or solutions greatly appreciated.