Originally Posted by
DaveSSS
There are many different minimum insertion length that range from 7-10cm. Thre's no way to know what's driving those differences.
I have three frames that all have 27.2mm reducing bushings built into the frame, that are no more than 6cm in length. From an insertion standpoint, anything over 6cm just hangs in the open space below the bushing and does nothing.
The other side of the issue is maximum extension. If you can see 2.5cm of the post that's supposed to be inside the frame, then you've gone beyond the maximum extension. Whether that's an issue or not is hard to say. You'll never get a manufacturer to say it's OK, but it may cause no problem. How badly do you abuse your seatpost? Do you slam yourself onto the saddle a lot? That might be a problem with too much extension.
I'm thinking that sloping top tube bikes might be a driving factor. The seat post manufacturer doesn't know what kind of bike it's going into so they're going to want to be covered for the worst case.