I have a Specialized Hardrock 29er with the original seatpost. On rides the seat slips down and I have to stop and pull it up. With the QR as tight as possible I can turn the seatpost. My vernier calipers say that the seatpost diameter is 30.75mm. Will buying a 30.9mm seatpost solve the problem? Is 30.9mm the outside diameter of the seat post or the inside diameter of the seat tube? What is the actual diameter of a 30.9mm seatpost?
I tried shimming the seatpost with a beer can but the seatpost got stuck. I had to lock the bike to a tree with a cable and use all my strength to pull the seatpost out, over about 45 minutes.
Any other suggestions? Shim it with aluminum foil?