Bicycle Mechanics - What are these parts and what's the best way to adjust them?

These pics are from my Trek FX 7.2. My chain has been rattling and when I brought it in to have it looked at, the guy said he just made a minor adjustment and it's fixed. When I asked what he did, he told me about these knobs and that they just need a small quarter turn to adjust.

I'd like a little more insight as to what these are and more details about how to adjust them.

Thanks!

260997
260998

They are barrel adjusters. They essentially will release or add tension to the derailleur cable to adjust the shifting & to compensate for cable stretch.

Google derailleur adjustment - Sheldonbrown and Parktool sites are popular but try a few. It's helpful to know more than just what the barrel adjusters do - you need an overall understanding of how the derailleurs operate.

Ah nice, thanks. What's the best way to tell if they're adjusted correctly?

Too much tension and you'll experience hesitation shifting to higher gears; too little and you'll have hesitation or missed shifts trying to go to lower gears. But before you adust the cable tension you first want set the high and low limits of the derailleur travel, so go to the websites listed above to learn how the various adjustments interact.

When you have everthing adjusted properly you'll get smooth, quick shifts with little noise.

Yes, it's very important to learn this stuff! I've got two left thumbs when it comes to mechanical stuff but I've learned to adjust my derailleurs and brakes. The Sheldon Brown site is very good.

http://www.parktool.com/blog/repair-help

Ah nice, thanks. What's the best way to tell if they're adjusted correctly?

By following the directions properly and quiet operation.

I'm nitpicking, but "tension" isn't really the right word. They actually change the effective length of the cable's path. The tension on the cables is provided by the return springs in the derailleurs, and with a few exceptions, that's not adjustable.

mechBgon: Thanks for nitpicking; distinguishing cable tension versus length is an important distinction to make in having a clear understanding how the shifting system works and what the adjustments actually do.