How many teeth are you missing?

Can you spot anything unusual with this Shimano 105 chainset? The owner said that the chain was slipping! With 5 teeth completely missing I am not surprised. Chains, cassettes and chainrings wear out, in that order. There are too many variables to be precise about when these should be changed. However, a good rule of thumb is 3 chains per cassette. Chainrings should last years but it is possible to wear them out fairly quickly on high mileage bikes ridden in all weathers and not cleaned as often as they should be. A worn cassette will often cause the chain to jump, usually on the most used gears in the middle of the cassette. A worn chain can often do the same but nearly always leads to degraded shifting. Worn chain rings can be lethal. Unlike a worn chain or cassette that cause poor shifting a worn chainring will cause the chain to slip, as in this example, or to come off. If that happens when under load you will more than likely go over the handlebars. The lesson here is to keep the chain (cassette and chainrings) clean and change it as soon as wear reaches 75% if not a little before. Remember that putting a new chain on an old worn cassette will not work.. It is far more cost effective to change the chain often than to wait for something to break or until you have coaxed the last bit of serviceable life out of it. Any questions, just ask.