i dont understand why you push back on the saddle while pedaling up hill :S I don'think i do this, i just sit on it and spin up the hills, if your pushing back on the saddle, my guess is your mashing too high of a gear, but that still shouldnt lead to a failure like this in your post, it was possibly just due to a faulty seat post, but if i were you, i'd stick with the alu until u learn to go up hills spinning