i was looking over here: http://www.surlybikes.com/spew3.html and reading point #7 made me really nervous about my drivetrain. i dont like to replace stuff until it causes me to crash and die, but i have been having problems with my chainring getting all eliptical and egg-shaped, making chain-tension a highly variable thing, depending on where my pedals are at. so, i was wondering, how often do y'all replace your chainrings/rear cogs/chains? i replace my chain pretty frequently, and i've changed my rear cog once (cuz i had to) in the year+ that i've been riding my fix. am i do for new stuff?