A carbon seat post – how should it actually fit into a carbon seat tube? Back in the steel and aluminium days, the seat tube was reamed out to the required diameter. How should I proceed with carbon material in these days? Background: Only by wrapping the carbon seat post with a 0.04 millimeter thick film it fits snugly with a little pressure into the seat tube. Without film it wobbles, slides through and cannot be tightened.
Reply by Dirk Zedler, TOUR technology expert and bicycle expert
Your description of the facts is correct in general: A seat post should slide without play and without applying force into the frame and fit snugly. It is also correct that in former days the seat tube was reamed out to create an accurate fitting. What we do today, is nothing else. However, you cannot do that by hand and with the usual tools.
Your description that a 0.04 millimetre thick film makes the difference between "the seat post which is untight" and "the seat post slides in without play" is not plausible for me. Most probably you have measured only in the top area and the seat post is much wider in the bottom area. In this case you should refrain from using any further the seat post, as there is the risk that the seat post clamping is affected by cracks in this area. But have it measured once again to be on the safe side.
In addition, seat posts may be also subject to tolerances. You should therefore try various seat posts. Another tip is to use carbon assembly paste which increases friction significantly due to its granular plastic particles thus ensuring a safe clamping.
