Chain Stretch

In the interest of bike maintenance...

Let's take a look at how to check the chain stretch.

Bicycle links are based on a 1" link length.  This is handy because it gives a nice easy metric for measuring the amount of stretch that a chain has using a simple measuring tape or ruler (I prefer the former for reasons to be explained).

It is possible to buy tools (really nice pretty expensive tools or way cheaper versions of the same thing) that will measure your chain for you, it's easy enough to do on your own.

The technique I learned is to measure a length of chain that is 12 links long, which depending on your gearing can is probably in big ring up front.

I generally measure from the back end of 1 link to the back end of a link 12 links away:

In this case, I only have two hands, so the tape measure is taped in place, but in practice all one has to do is find the length and look, no tape needed.  Because each link is 1" long ideally, it would be nice to see the 12th link align with the 12" mark on the ruler (as shown).  In reality, wear in the chain will, over time, cause that length to grow.  I go by the following guidelines; If the length of 12 links is...

• < 1/16" past the 12" mark, go about your business.  Your chain is fine.
• > 1/16" but < 1/8", replace your chain.
• > 1/8", you need a new chain and it's worth looking at your crank and cassette teeth to see if you've drastically worn the tooth spacing.

Pretty simple.  Chains aren't cheap, but they're cheaper than replacing your crank sprocket or cassette and your chain, so they're completely worth replacing when it's time.

So, what causes stretch?

Modern bicycle chains are constructed with inner plates, outer plates and (possibly) roller/bushings with a connecting pin between the two that allow the system to pivot:

Over time, tension on the chain chain under load (pedaling) causes the inner plates and outer plates to dig into the pin as force is transferred between each consecutive link.  Eventually, this force gouges the each pin a little and over the space of an entire chain length there is a measurable length increase: chain stretch.