10 April 2013

Checking for Chain Wear

Last week I checked my chain for wear using the Park Tools CC2 (thanks to SP), and I read up some stuff about chains and wear generally, and here's a jist:

Read Sheldon Brown's article on chain wear before you read further.
Sheldon Brown (SB) recommends that you change your chain after it stretches by 1/16th of an inch over 12 inches - thats about 0.5%.
With a stretch of 1/16th of an inch over 12inches (~1%) he says there is a likelihood that the sprockets have worn out as well.

To measure the stretch you can use a ruler (see machinists' rule) or measuring tape, or a dedicated chain wear measurement tool like the Park Tools CC2 I used for my measurements. There are two problems now:
1. How to correctly (accurately) measure chain wear
2. How much wear should you allow before changing the chain.

Before addressing both questions, heres a very nice explanation of how chains wear, given here. It may take some time to sink in so don't skim through it. Then come back to the two questions:

1. Measurement:

To accurately measure wear you need to eliminate roller wear and only retain pin wear. One reason why chain checkers are not accurate is that they measure roller wear along with the pin wear. Also, when counting over N links such a tool will accumulate pin wear on all intermediate links, and the wear on two rollers (at the two ends). Larger the number of links you count over, smaller is the error from the roller wear. Peter Verdone points out that most chain checkers can't be accurate because the markings can't be equally spaced. I agree in the case of the CC-1 but for the CC-2 the differences in spacing may be small enough to not be noticeable.

2. When to change the chain:

There are multiple opinions here. In general the chain wears fastest, then the cassette and then the chainrings. Among the cassette and chainrings the smaller ones wear faster, and if you frequently use only a few then those wear out the fastest. Some people say you should change chains early before the cassette and chainrings start to go (the rationale being that chains are cheap, cassettes are not). The problem is that if you wait too long and end up installing a new chain on a worn cassette you get really poor performance. So some people will wear everything down together, and then go for a one time replacement of the entire drivetrain - chain, rings and cassette. Again that depends on individual choice, cost and convenience of getting the components in question. Also thinner chains (9 or 10 speed) wear out much faster than thicker (6, 7, 8 speed) chains, simply because of greater stresses involved. I could, however, find little or no documentation indicating variation of acceptable wear across 7, 8, 9 and 10 speed chains.

Time and Mileage:
There are people who prefer to change the chain at a fixed interval (usu. once a year) but that makes little sense without knowing usage patterns. Others watch for mileage, changing after every X thousand kms, which has the same problem. Moreover wear is much too greatly dependent on the type or ride and maintenance condition that there is little sense in trying to "open loop" predict when to change the chain.

Wear and percentage elongation:
S. B. says change after 0.5%. Some people wait till 0.75%, and few wait till 1%. Most people who change at 0.5% get their cassettes to last through three to four chains and cranks lasting anywhere between six to ten chains.

The Bottom Line:
Buy a ruler if you don't have anything else. When possible get a chain checking tool. Keeping in mind the cost and availability of chains, cassettes and chainrings, you should decide at what point you should change your chain. Learn with experience :)

Reference Images from the following locations:
Park Tools CC 1 tool for measuring chain wear and elongation Park Tools CC 2 tool for measuring chain wear and elongation
The Park Tools CC1 http://farm6.staticflickr.com/5083/5346630950_9002ca3652_z.jpg Park Tools CC2 http://pardo.net/bike/pic/fail-004/18164_061544.jpg

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