Originally Posted by skanking biker
Why do I bounce in my saddle when I spin at a high cadence in a low gear?
practice spinning on each foot(unclip the other foot) - kick across/push down/scrape back/pull up - now make it smooth, ideally a circlular motion rather than 4 separate motions.
do this for a couple of minutes on each leg at the beginning of a ride.
Then combine both legs and focus on smooth circular motion. Just be warned it's hard work and don't do the single leg thing for more than a couple of minutes - you may damage yourself otherwise.
Also - relax part of the bouncing thing is being stiff in the saddle.