after a couple of rides my knee is really hurting.. I'm letting it rest for some time, while I look for a proper seatpost.. the bike is a 51 and I should use something between 53-55. I think the top-tube+stem distance is not that bad, I'm not looking for a totally flat back, but if I could put my saddle a bit to the back I would get better knee alignment. By buying one of those seatposts that go a bit further back I should be able to compensate for the smaller size right? (btw what's the name of those seatpost types?)