The seatpost -- or more accurately the seat tube -- is the one part that lets the Carbolite bikes down. The bike needs a 24 mm seat post, and those are sometimes hard to find. I found a Kalloy post for my P4 above but for the latest Carbolite bike i worked on, i was lucky to find a plain old candle seatpost. That bike was a mixte by the way and was the nicest mixte i have ever ridden. Nice and sturdy, no flex at the wrong times.