Basically, avoid cross-chaining. That is, the right-most drive chainring with the left-most cogs, or vice-versa. They place excessive sideways torque on the chain (increasing friction and wear) while not prividing any unique gear ratios that a straighter chainline would provide. The rear derailleur may not be able to take up the slack of small-small riding; or your chain length may be sufficiently short that large-large combo will pull the derailleur to far forward, perhaps even damaging it. To illustrate for yourself what crosschaining does, manually shift your bike into the two described extremes then get behind the bike and sight along the top run of the chain. You'll clearly see that it forces two unnecessary sideways bends in the chain. That is really bad for the hardware.
Generally, the large chainring is for high wheel rpm's but low rider torque, so stay toward the same side (medium-smaller cogs) of the cassette in the rear to maintain higher speeds.
Conversely, the granny gear (smallest chainring) is for high torque, low speed conditions and it works best with its same side (medium-larger) cogs on the rear. The middl ring works with all of the cogs in the rear and serves most off-road purposes that don't require extremes of speed or power.
Buy a few bicycle magaizines, visit bicycling.com and read up on shifting and everything else. It's worth it to get it right and ride smarter instead of harder.
Jan 25, 2010 |