I'm posting this in the commuting forum, because commuting last winter was the cause of this. That, and laziness on my part.
About two weeks ago, I thought I'd try a different seat on my commuter, an old ''91 Bianchi Nyala. To make things easier, I figured I just pull the post out and pop in a different one. Flip the quick release, pull . . .nothing. Totally frozen! Quite surprising to me, since for the last 15 years, the biggest problem I had with the seatpost was that it constantly slipped.
Anyway, I put the bike in the stand, loaded up the seat tube with pb blaster and let it sit two weeks. Nothing. A six foot cheater bar only succeeded in twisting the pressed on seat mount.
So I resigned myself to sawing it out. An hour with the hacksaw pretty much completed the first cut. The problem is that the bike is big for me, so there was a lot of seatpost in the seat tube - and the hacksaw blade was just barely long enough to get to the bottom of the post. Took a vise grips, and still nothing. So I made the second cut. It wasn't until I was actually able to pry and twist the piece and break it out of the seatpost that it was loose enough to twist free.
I know I adjusted my seat sometime in late April, AFTER the snow was gone; I should have gotten off my butt and done a full overhaul then. But since it is my "beater' bike, I figured I could let it slide. Paid the price. New seatpost will be getting LOTS of grease!
That's bizarre. I ride the same bike in the winter (and in the same town!) and like you, all I've ever had with my seatpost on the Nyala is slippage. I was just thinking about getting it ready for the winter. Thanks for a motivating story. Now I'm wondering what I'll find... Undoubtedly a somewhat neglected chain and gears.