Modern bicycles are amazing examples of engineering with remarkable strength to weight ratios. While parts like super light wheels get all the attention, the chain is a vital component whose design and performance are often under appriciated. To accomdate 10 or 11 sprokets on the rear wheel chains have been put on a diet. Nominally 3/32" or around 7.5mm wide, current chains can be as narrow as 5.5mm. With 25% less material than traditional designs, modern chains still transmit 100% of the energy that turns the rear wheel. Chains have the most moving parts of any part of the bicycle. Chains average 110 links and each link has 8 moving parts. Not suprisingly, these parts wear each other down during the tens of thousands of cycles they go through during their service life. Eventually this wear effects the performance of the chain. If the chain is used beyond its service life it will damage the other drivetrain parts.
In the good old days, the better 8-speed chains would, with good maintenance, occasionally last 7000 miles or more. The narrower 9 speed chains do well to last over 1500 miles. It is not uncommon for the newest 10 speed chains to reach the end of their service life in as little as 800 miles. A worn chain is not going up in flames if you continue to use it, but, because the wear manifests as gaps between the moving parts, the chain will elongate (stretch is the mechanic jargon for this phenomenon). Thus the worn the chain will wear an elongated and—when matched with a new chain—unusable pattern into the cogs and chainrings (see picture below).
Below are two photos of the chain being measured for wear. In these photos we are using the Park CC-2 chain wear checker. The black piece swings on a pivot and moves a pin further away from the fixed end when you push it. The further it moves, the more worn or elongated the chain is. You will notice that
the new chain still has some swing to the measuring tool. The worn
chain will allow the tool to swing the full distance. The second chain
is ready for replacement. If the chain tool makes its full swing
easily, or if the tool can move back and forth after the full swing, then
the new chain is likely to skip on the current cogs. The bottom line is, chains are cheap compared to
cogsets and chainrings. Measure often and replace if in doubt. The new
Shimano Dura-Ace 10-speed cassette is around $200. Replace your chain before the end of its service life and you can get tens of thousands of miles from a single cassette.
Again, if the cogs or chainrings are
worn, you won't find out until you have to install a new chain. The
new, unworn chain will "skip" or override a
set of teeth that have worn by a chain used beyond its service life.
Rear cogs that are worn will give the rider little popcorn pops or skips
as the chain releases, but catches on again
quickly. Chainrings that are worn will result in chain skips that feel
like the bottom dropping out of your pedaling world.
On mountain bikes with the new ultra wide range of gears on the rear
wheel, many riders stay in the middle chainring nearly all of the time.
Luke! Use the little ring! Running one ring most of the time means that
it wears out much sooner than the unused ones!
The three rings have overlapping gear ratios, so, if you are on the
middle ring and one of the innermost cogs, shift down to the small ring
and one or two cogs smaller in the rear. The result is that you will
have the same gear ratio, meaning the same effort and cadence, but in a
kinder and gentler combination.
Worn chainrings are also the reason that “chain-suck” happens.
Notice in the picture below the flaring at the leading edge of the
teeth. When the front derailleur shifts to a smaller chain ring there
typically is a bounce to the lower chain path and the bouncing chain can
snag on the flared tooth and get pulled up into the frame's chainstay.
The only reliable solution is to replace the chainring.