By Johan Bornman

A worn chain will not damage a chainring, although it will damage a cassette.

The big difference between a cassette and a chainring is that the former is a driven sprocket and the latter a driving sprocket.  The difference lies in the fact that the chain enters the sprocket either under tension, or slack.

Let’s picture the chain rolling over the two sets of sprockets whilst a rider is pedalling. Marked links are now right at the bottom of the loops between the chainring and jockey.

• Jockey wheels are just 'idlers' and can take an inordinate amount of wear before they need replacing. Even then they don't bring about any type of problems with either shifting or chain longevity. The links are not under load within the idlers - the load is spread over the engaged sprocket - and they only experience light tension from the derailleur spring. The links zigzag through the idlers to the rear sprocket where, still under no load, they load onto cogs and enter thanks to their perfect alignment set up by the idlers.

• Each subsequent link is ‘loaded’ onto a corresponding cog as the chain enters the rear sprocket. The load is distributed evenly amongst all the links making sprocket contact ,and the position of the derailleur ensures that there is enough wrap (engaging almost three quarters of the sprocket at once) so that the chain doesn’t ‘jump up’ and skip.

• Our marked links now move around the sprocket towards their exit at the front from where they will approach the chainwheel. Looking from the side, we can see how the links articulate to accommodate the sprocket shape / wrap.

• Whilst on the sprocket the contact angle (as seen from the top) between chain roller and sprocket is perfectly parallel. The instant the link exits the rear sprocket things change (unless the rider is now in the only gear that perfectly aligns the front and back sprockets and therefore keeps the chain in a straight line). Assuming the bike is in any other than the perfectly-aligned gear, the links now suddenly bend laterally to accommodate the chain’s diagonal path to the front. This does two things; i) it loads only the one side of the cogs and ii) forces a similar shift in force inside the chain (to one side of the pin and sideplate) as well. The length of chain between these links is perfectly straight, the bending is experienced at the two ends. The worse the cross-chaining, the worse these angles are and the more abuse you’re inflicting on your chain.

• Our links are just about to enter the chainring. The way the links enter the chainring is different from the way they entered the sprockets because they’re entering under tension (whereas they entered the sprockets freely). If there is cross-chaining, there is another lateral bending motion in the chain and a skew loading of the first cog, just like that at the link now exiting the back sprocket. 

• Our links now move around the chainwheel and again articulate to accommodate the ring’s shape.

• The links exit the chainring under no tension because both the chain and sprockets are in perfect synch, all engaging teeth and links are loaded equally. 

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