i actually own two Dahons, three Bannards, a FnHon, two custom bambu jobs, and a FSIR, and a Surly is on order so I am not speaking from a Dahon universe. In the universe of bicycles are 31.8mm seatposts rare? What's half a million Bromptons against the wide range of bikes and the common seatpost sizes? On the Chain Reaction, for example, 31.8mm shows one item, a shim. 31.8 is closer to rare than commonplace, and speaks to the more 'specialized' sizes and parts of the Brompton bicycle.
Originally Posted by
berlinonaut
Not really, just in the Dahon universe. 31.8 is what i.e. the Brompton uses and there are more than 500.000 out there. Seat posts with this diameter are available from Brompton as well as a huge amount of 3rd party suppliers in steel, aluminum, titanium and carbon from cheap to very expensive. 600mm is a standard length on the Brompton - it is called the extended seat-post. You get it in this length from a Brompton-shop in steel or i.e. a cheap titanium or carbon post from China via ebay or a aluminium one with built in rear lights from Lightskin. Or you look at the various Brompton tuning shops on the net if you are willing to spend more money.