Join Date: Mar 2005
Location: Washington, DC
Bikes: Redline Conquest, Cannonday, Specialized, RANS
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What the nice guy at the bike shop didn't want to tell you is that your "24-speed" bike doesn't really have 24 usable gears. You can only use about 4 or 5 of the rear speeds on each of the front rings. The chain can only move so much side to side and if you put it in an extreme position like the biggest ring on the front and the biggest on the rear it puts abnormal stress that will cause the chain and the gears to wear quickly, and it will not run smoothly.
Also, even if you have 24 usable gears, you don't have 24 unique combinations -- the ratios available from the middle front ring overlap those available from the other two rings. What the guy was getting at when he said just use the middle is that you can get to most of the usable ratios from the middle. The only ones you can't get are the extremes, about two in each direction, and for those you need the other rings.
Also, on every bike I've ever had, there are two front derailleur positions on the middle ring, one for the low gears and one for the high. If I run from lowest to highest on the rear without adjusting the front it makes a terrible racket.
Here's a good article on gears and shifting: http://sheldonbrown.com/gears.html