The drivetrain is the system of chain and cogs that transmits power from the cranks to your back wheel. The drivetrain wears like any other part and if you ride regularly it will need replacing at some point. A full drivetrain replacement is a decent-sized job that is often required after about 5000km of riding but there are ways to extend the life of these parts.
What comprises the drivetrain?
On a derailleur-geared bike the drivetrain is made up of the cogs on the crankset (known as chainrings), the cogs on the rear wheel (known either as the freewheel or cassette) and the chain that runs between them. Often the chainrings are separate pieces that can be removed individually from the cranks, but sometimes they are peened together into one piece. Freewheels and cassettes are both complete stacks of cogs which are replaced as a whole. Freewheels are an older style that has the freewheel mechanism (that allows you stop pedaling while coasting) built in, where cassettes are a newer style that slides onto a freewheel mechanism that is built into the hub.
How does a drivetrain wear out?
All of the parts of a drivetrain are metal and as they move against each other they create friction which wears their surfaces. The chain wears against the inside edges of the teeth on the cogs and each of the moving parts of the chain wears against adjoining parts of the chain. As the chain wears internally with each rotation, it effectively elongates. As the chain elongates, it no longer sits perfectly between the teeth and starts to ride up higher and higher on the edge of the teeth. This in turn wears the teeth since the chain is now grinding onto and off of them as you pedal.
If you catch the wear of the chain at the point at which it starts to accelerate the wear on the cogs, you are often able to replace just the chain with no ill effect. Leave it longer and the cogs will be worn to a point that a new chain will no longer mesh well with them.
If the wear of a drivetrain progresses too far, the chain will ride so high that it can slip over a tooth on the cassette. This skipping is most likely to happen under load, like when taking off from the lights or getting out of the saddle to pedal hard up a hill. It will jolt you forward and can result in an accident. A well maintained bike will never experience this. A bike that isn’t serviced at all might do this after 2 or 3 years, depending on many factors.
What does a drivetrain cost?
Drivetrain components vary widely in quality, weight and cost. For a 9 speed commuter bike the chain and cassette are likely to be $45 each (give or take) and the chainrings could be around $50 each. Different setups will vary greatly, but we will be able to advise you about your individual setup. Drivetrain replacement is rarely done in isolation and is usually part of a service or overhaul which is between $100 and $200 in labour.
How long should a drivetrain last?
This is a difficult question to answer as there are so many factors influencing the life of a drivetrain. These include condition, cleanliness, lubrication, range of gears used, pedalling style and quality of components. A chain is likely to last anywhere from 2000km – 5000km. Catch it at this point and you might be able to replace it for another 2000km – 5000km on the same cogs. Leave it longer and you’re likely to be up for a whole new drivetrain. Someone who rides 5km each way to work, 5 days a week, 48 weeks a year, can easily clock up 2500km a year just in commuting. On a 9 or 10 speed geared bike, that sort of person is likely to wear through a chain in a year or potentially even less.
What can be done to increase the life of a drivetrain?
The life of a drivetrain can be increased by keeping it clean, lubricated and staying on top of chain wear. These are all things you can do yourself and to varying degrees.
To properly clean a drivetrain we remove it from the bike and wash it out in a special microbial wash that eats away at the oil and muck all over it. We will sometimes recommend this as part of a service to maximise the life of your drivetrain. At home, something as simple as wiping down your chain with a rag and digging out some of the larger pieces of gunk from around the cogs and derailleur pulleys can help without requiring removal or solvents.
Lubricating your drivetrain with chain oil reduces the friction between the metal parts, which in turn reduces the wear. Chain oils are light to penetrate the chain and contain additives to help them stick around longer. The goal is to get the lube inside each of the chain links where it can reduce friction inside the chain. Too much on the outside will only serve to attract dirt.
Here’s some popular drivetrain products…
Shimano BB-UN300 Bottom Bracket$35.00
Surly Single Cassette Cog 3/32″ Splined$40.00 – $60.00
White Industries Freewheel 3/32″$150.00 – $160.00
KMC Chain Link 10sp$5.00
Shimano Chain HG71 6/7/8 Speed$40.00
KMC Chain Link 10CR – Reusable Campagnolo 10 Speed$6.00
Surly Hurdy Gurdy Chain Tensioner$50.00
Shimano SM-CN900-11 Speed Quick Link Master Link$10.00
KMC Chain Link 9R for Shimano, SRAM 9 Spd$7.50
Gates CDX Centretrack Belt Black$120.00 – $150.00
Surly Tuggnut (single)$50.00
Single Speed Chainring Bolts Steel$12.50 – $15.00