By Johan Bornman 

The extent of chain wear is determined by measurement; not 'miles travelled', not seasons in the sun and definitely not by psychic bike mechanics.

Chain wear results from the chain’s ‘bending’ as it rolls over a sprocket. Rear sprockets cause more wear then front chainrings to the chain simply because a chain has to bend more to accommodate the smaller diameter / number of teeth. For instance, a chain being driven around a typical 52 tooth chainring undergoes a 7-degree bend at each line. For a 12-tooth sprocket, the bending angle - and therefore internal chain abrasion - is much more.

Although the jockey pulley wheels (called idler wheels) are smaller than the smallest rear sprocket and subjects the chain to more articulation, they have very little effect on the chain’s lifespan because the chain negotiates their chicane bends whilst unloaded.

An ‘idler’ is a pulley or roller - on bicycles applied as a 'jockey wheel' - that does neither produce any mechanical advantage, nor transmit power to a shaft. Idlers are used to lead a chain around a bend (as in short wheel base recumbents), or to take up slack in a drive chain (as in a tandem chain tensioner, or the rollers in a derailleur.)

A chainring can become very worn - to the point where the teeth represent sloped stairs (way past the sharkfin stage) before it will skip.  Even at this stage it does no damage to the chain.
It is very seldom that chainrings are so worn that they warrant replacement. When maintaining general good chain hygiene, they last for dozens of chains before wearing down. The sole exception is perhaps a MTB's middle chainring; this should actually be made from steel (and NOT carbon!)

"Chain wear" arises from the play created within the chain as link pins and their sideplates erode (by effectively enlarging the sideplate’s hole and reducing the pin’s diameter). As a result chain pitch lengthens, elongating the chain. Cog pitch - even on worn teeth – remains static at a half-inch. The loading of cogs into links are no longer perfectly synchronised, and the chain applies more load on the prominent teeth of engagement within the loaded sprocket. This uneven loading increase wear.

"Pivot wear" means the link pins can both move outwards under tension, which allows the two inner links attached to one outer link, to move apart. The rollers inside the inner links always retain their half-inch pitch regardless of wear since they float inside their retaining flanges in anyway, and have built-in play.
It means that only every second link elongates and when a worn chain rolls over a sprocket, only every second link no longer meshes perfectly. The only way for the chain to compensate for this imperfect match is to ride higher up the teeth which in turn then load only every second tooth. (It is quite easy to see such a worn chain by looking for light shining through between the chain and sprocket.) This represents a 50% reduction in engagement surface and a 100% increase in load on the teeth that do engage. It leads to rapid wear of the sprocket teeth.

Wear causes visible pockets on the rear of each tooth. These pockets are clearly visible if you compare a good tooth from a worn one. You’ll also notice that the wear is not at the bottom of the tooth but slightly higher up – the chain rides up, remember? Tooth pitch remains exactly ½ inch even on worn sprockets.
Wear pockets form slowly at first, but as soon as the case-hardened outer sprocket is worn through, the chain rapidly eats into the softer steel inside. The chain and sprockets now wear each other down (and thus remains synchronised in pitch). The problem only arises when you put on a new chain. The new chain no longer matches the distorted pitch of the sprocket and the chain starts to skip under load. It does this because the load forces the rollers up along the teeth into the wear pocket. The next incoming roller now cannot engage because it catches on the next tooth and up it rides, over the tooth and you get the familiar skipping action.

On a bike stand, a new chain on a worn sprocket will not be evident. It is only when you ride the bike and apply force to the chain that the skipping starts. Therefore, when you replace a chain, do a short test ride and make sure you apply force in all gears before accepting the job as successful.

When to replace a chain? 

(The 12 inch-link rule.)

The modern bicycle roller-chain has a pitch of 1 double-link per inch. (A link comprises both an inner - and outer –sideplate section.) New chains will skip when used with worn sprockets. You prevent sprocket wear by regularly changing your chain, however, most people allow their bike shops to replace their chains too often, which is in the end as expensive as a new cassette.

Replace the chain only when it has elongated by 1%, or more. This is easier to determine than how it  sounds. Regularly measure your chain with a ruler calibrated in inches. These are hard to come by in ‘metric countries’, but it is worth the search. I found a cheap carpenter’s square with inch markings at my local hardware shop and it works fine – the short end of the square even gives me a good handle on things and the fact that the measurements start at the edge of the rule means I can hook it against a pin or sideplate when measuring and only focus on the 12-inch side of the rule.

A half percent elongation works out as 1/16th of an inch per foot of chain (24 links). You don’t even have to count the links, you simply lay the end of the rule against any point on the chain – I like to jam my steel rule against a pin, pull the chain straight and look at the 12-inch mark where you’ll quickly see whether the end point lines up, or runs over. Your inch ruler will almost certainly be marked in 1/16ths. Chain sections up to just below the 12 and 1/16th inch mark the chain is still good, on and above it should be replaced. If it has already elongated by 1/8th the sprocket has been ruined. Replace it.

It is interesting to note that the wear (per link) is in fact twice what you measure, since only every second link (the ones with ‘outer ‘sideplates) wears. It is useful to remember that a worn chain will run on new sprockets without skipping, but not vice versa. Running an old chain on a new set of sprockets is not a good idea. This will accelerate wear in the sprockets since it will now only load the last, and perhaps second-last cog and not distribute the load evenly amongst all the available cogs.

If a cassette is worn, a new chain will quite possibly skip in your favourite gears. This simply means these sprockets are worn and need replacement. It is expensive but can be contained by buying chain and cassettes from lower groupsets, i.e. ‘105’ instead of ‘Dura Ace’. Their functionality is the same, only weight and shine differ.

What about commercial chain wear indicators?

Don’t bother, get a yardstick. Most commercial indicators engage on the rollers, which have built-in float and remain at a perfect ½ inch pitch on inner links. These therefore measure an unknown and variable entity – roller clearance is not the same for different brands of chain. The only standard you can rely on is the half-inch pitch from link pin to link pin. Compounding this problem is the size of the measured area. These chain wear indicators only measure across a couple of links, compounding any reading or measurement error.

And remember: if it ain't broke, fix it!