Chain Stretch Explained

Whenever a shop mechanic checks your bike in for an estimate it's customary for him or her to measure your chain for "chain stretch". The key term here is a bit of a misnomer because chains don't actually stretch, though they do lengthen over time.

To understand what's going on you have to know that a chain is composed of pins that hold a set of inner and outer plates with a roller in the middle. If you are looking at the chain from the side, the plates are the flat things shaped like the number 8, the pins are in either of the holes of the 8, and the rollers are held in place by the pin and plates. When you pedal, the plates are pulled in one direction by the forward mechanical thrust of your legs and another direction by the resistance of the road, the gears, the wheel, friction, gravity, etc. The plates themselves do not deform in any visible way but the connecting points where the pins meet the plates are subject to the forces of the plates pull, which in effect squeezes the pin on either side. (In the case of the modern bushingless chain the forces are directed to the inner plates which are raised around the hole so that they can hold the roller instead of the pin, giving the chain more flexibility and easier lubrication flow - but for the sake of this article I'll focus on the pin as the main joint because it's an easier image to think about. See this link for a more in-depth explanation of different chain types)

Along with the squeezing plates, the joints are treated to stress whenever the chain rounds a sprocket. In this event the plates cut into the pin around its connecting surface. Think of an apple spinning on a stick, and a knife being held against it as it spins, cutting in deeper as it rotates. This cutting and squeezing of the pins occurs on a minute scale to each joint on the chain every time you turn the cranks, but it usually takes thousands of cycles before the gradual wear of metal-on-metal becomes noticeable. At this point the plates have worn grooves into the joints, impairing the joint's flexibility and lengthening the chain overall.

What's the real enemy here? Who's really wearing away the metal joints? Friction. The friction of metal pieces constantly rubbing against each other under pressure causes heat and heat makes metal deform more easily when stressed.

Overcoming friction is simple: oil your chain. Chain oil, wax, or any other kind of bike chain specific lubricant penetrates into the joints of a chain and creates a slick buffer between the metal parts so that they can't create as much friction when they rub under pressure.

But why why does chain stretch matter? It matters because eventually the repeated forces on the joints will cause it to break and then it can get caught in the spokes of the rear wheel or against the cassette and hub or around your leg or in the wheel of the guy behind you. The other issue is that an elongated chain won't fit perfectly into the sprocket, giving you poor shifting or even causing the chain to skip. A worn chain will wear out a sprocket so it's best to replace both parts together and check the chain regularly so that you can replace it before it gets to the point where its damaging the cassette.

For more information visit Sheldon Brown's website or pick up a copy of Zinn and the Art of Road Bike Maintenance. 

If your bike is giving you shifting problems, you may want to check out this site and then this site.