Problem solved! I had a stem
"Problem solved! I had a VCLS canyon double-bladed carbon seatpost whose main known problem was that it slipped so much that overtightening the seat clamp made it impossible to loosen; after sawing out the screw and fitting a new clamp, I applied a miracle paste to the seatpost and the inside of the clamp and screwed 0. 5 to 1 n below the recommended setting, it doesn't slip any more, even over big bumps; at the same time, I bought a BBB torque wrench that does the job."
