how often to replace chain?

Gear West folks told me 2500 miles

I agree with measuring the chain.  I'm in there weekly cleaning it, so it's not to hard to grab a ruler and check.

On road bikes, chains break due to poor maintenance, poor shifting, or poor installation.  They don't just happen very often, it's a matter of your habits.  If you have good habits, then look at the installation first, then the manufacturer not installing all the pins properly.  

On a mountain bike it's a different story.  So much crap gets into your drivetrain, branches, rocks, squirrels, etc. they are bound to break.

More than you ever wanted to know about bike chains:
http://sheldonbrown.com/chains.html

Measuring Chain Wear

The standard way to measure chain wear is with a ruler or steel tape measure. This can be done without removing the chain from the bicycle. The normal technique is to measure a one-foot length, placing an inch mark of the ruler exactly in the middle of one rivet, then looking at the corresponding rivet 12 complete links away. On a new, unworn chain, this rivet will also line up exactly with an inch mark. With a worn chain, the rivet will be past the inch mark.

This gives a direct measurement of the wear to the chain, and an indirect measurement of the wear to the sprockets:

 If the rivet is less than 1/16" past the mark, all is well.

 If the rivet is 1/16" past the mark, you should replace the chain, but the sprockets are probably undamaged. 

If the rivet is 1/8" past the mark, you have left it too long, and the sprockets (at least the favorite ones) will be too badly worn. If you replace a chain at the 1/8" point, without replacing the sprockets, it may run OK and not skip, but the worn sprockets will cause the new chain to wear much faster than it should, until it catches up with the wear state of the sprockets.

 If the rivet is past the 1/8" mark, a new chain will almost certainly skip on the worn sprockets, especially the smaller ones.

popsracer - 2009-07-14 6:09 PM

DrPete - 2009-07-14 2:03 PM

popsracer - 2009-07-14 2:54 PM There has been much advice on replacing due to stretch in the chain but I was wondering about broken chains. At an HIM earlier this year, there were several people I saw that had broken a chain. Is that something that can be avoided by replacing after so many miles or do broken chains just happen?

Most broken chains are the result of poor/inattentive mechanic work, honestly--Some chains (Shimano 10 speed especially) are finicky, and if you don't seat the pin just right it can be a setup for breakage.

I once broke a chain during a sprint--the plates splayed out wide enough for the chain to get caught in the rear derailleur, bending the bejeezus out of the derailleur hanger and exploding--yes, exploding into pieces--the Ultegra rear derailleur. The bike was almost new, and I talked to a Jedi Master mechanic about it and he explained the pin issue. I've been doing my own mechanic work ever since.

I currently remove my chain periodically for cleaning. It also makes it easier to clean the rest of the drivetrain. Is it not a good idea to be pressing pins in and out any more than is absolutely necessary?

The other major way to clean chains is with an on-the-bike cleaning machine. These are boxes which clip over the lower run of chain. They contain brushes and rollers that flex the chain and run it through a bath of solvent.

The off-the-bike approach has the advantage that it usually uses more solvent than will fit into an on-the-bike cleaning machine. Thus, it can dilute away more of the scuzz from the chain.

The on-the-bike system has the advantage that the cleaning machine flexes the links and spins the rollers. This scrubbing action may do a better job of cleaning the innards.

popsracer - 2009-07-14 3:09 PM

DrPete - 2009-07-14 2:03 PM

popsracer - 2009-07-14 2:54 PM There has been much advice on replacing due to stretch in the chain but I was wondering about broken chains. At an HIM earlier this year, there were several people I saw that had broken a chain. Is that something that can be avoided by replacing after so many miles or do broken chains just happen?

Most broken chains are the result of poor/inattentive mechanic work, honestly--Some chains (Shimano 10 speed especially) are finicky, and if you don't seat the pin just right it can be a setup for breakage.

I once broke a chain during a sprint--the plates splayed out wide enough for the chain to get caught in the rear derailleur, bending the bejeezus out of the derailleur hanger and exploding--yes, exploding into pieces--the Ultegra rear derailleur. The bike was almost new, and I talked to a Jedi Master mechanic about it and he explained the pin issue. I've been doing my own mechanic work ever since.

I currently remove my chain periodically for cleaning. It also makes it easier to clean the rest of the drivetrain. Is it not a good idea to be pressing pins in and out any more than is absolutely necessary?

Shimano recommends that you never re-use the pins on Shimano 8, 9 and 10 speed chains.  Pushing the pin out makes the hole in the sideplate larger.  Replacement pins are designed to fit this properly.  Never break a chain at a replacement (joiner) pin as there is only one size of replacement pin and this would make the hole too big.  

I'd recommend getting an SRAM or Wipperman connector link or just switching brands to one with a connector link.  Why take the risk of the chain breaking while standing to power up a climb?  Or just clean the chain on the bike.

Shimano recommends that you never re-use the pins on Shimano 8, 9 and 10 speed chains.  Pushing the pin out makes the hole in the sideplate larger.  Replacement pins are designed to fit this properly.  Never break a chain at a replacement (joiner) pin as there is only one size of replacement pin and this would make the hole too big.  

I'd recommend getting an SRAM or Wipperman connector link or just switching brands to one with a connector link.  Why take the risk of the chain breaking while standing to power up a climb?  Or just clean the chain on the bike.