Hello everybody,
I just got a new bike, http://www.masciaghi.it/catalogo/gir...1u26721bap.jpg

Everything was fine, and then as I was coming around a corner, the seat twisted around. I have tightened the seat really tightly, but it keeps on moving left and right. It doesn't slide up or down when I have it tightenend, it just twists. I never had this problem on my old bike.

What is the trick to stop it from doing that? Should I slide something like a piece of paper in there?

thanks