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?
Over misheard as drivechain, the word drivetrain comes from automotive mechanics where it is used to describe all of the components between the engine and the driven wheels. It does the job of transferring power from the engine to the wheels. The bicycle equivalent is essentially the same. It is the parts between the rider and the wheel that transmit power from the riders legs to the rear wheel. These parts are 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 on both of 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-the distance between each of the chain pins increases ever so slightly. 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 accelerates the wearing of the teeth as 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. The greater the number of teeth on a cog, the better the load is spread and the longer the cog is likely to last. Hence, chainrings typically last longer than the cogs on the cassette.
The wearing of both chain and cogs isn’t very noticeable initially as the wearing of the cogs opens up the gap between the teeth so the elongated chain can still sit in place. Eventually though, the chain gets really worn and it will ride so high on the edge of each progressive tooth that it will sit right on top of a tooth and then clunk down into place on the other side, skipping a gap altogether. This skipping is most likely to happen under load which, in practice, means at the least convenient times. 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 typical commuter bike the chain and cassette are likely to be $45 each (give or take) and the chainrings could be anywhere from $25 to $150 with most averaging about $50. The work itself is rarely done in isolation and is usually part of a service or overhaul which is between $90 and $180 in labour. With a few other parts such as brake pads and cables it can easily turn into a $300 job.
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, pedaling 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. If left unchecked, it might not be problematic for twice as long, excepting a slow deterioration in shifting (because the chain is less stiff laterally).
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. This is something we will sometime recommend as part of a service to maximise the life of your drivetrain. In the old days, people used solvents such as kerosene to achieve the same outcome. Kerosene works fine but is pretty nasty to use daily and for the environment, so we have spent the big dollars on a microbial wash set up. At home, something as simple as wiping down your chain with a rag and maybe digging out some of the larger pieces of gunk from around the cogs and derailleur pulleys can help without requiring removal or solvents. Cleaning the drivetrain removes particles of dirt and sand which can wear the metal components at a rate even faster than if they were dry.
Lubricating your drivetrain with chain oil reduces the friction between the metal parts, which in turn reduces the wear. Unlike a car, your bikes drivetrain is all exposed to the elements which means the lubricant (chain oil) can be washed off if ridden in the rain. Cars have casings around chains and cogs and circulate large volumes of heavy oil through them to make sure they stay lubricated. On a bicycle, there is no such system so you need to regularly reapply chain lube, and more so if you ride in the rain. 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 the dirt and sand you’ve wiped off. Thicker oils are sometimes used in winter when rain is more likely. These are not too thick though, as the drivetrain on a bike is not moving fast enough to heat up and thin the oil like it does in a car. Provided it is kept oiled, a chain will not rust too. Components that are left to rust will wear at an increased rate as the rust (Iron Oxide) is more brittle than the original steel and chips off more easily.
Staying on top of chain wear can result in only needing to replace the chain and clean the (not-too-worn) cogs. It is not unusual to get 2 or 3 chains to a cassette and 2 cassettes to a set of chainrings. A new chain can often be fitted to lightly used cogs, but new cogs will never mesh well with a worn (and elongated) chain. A new chain on old cogs will feel really rough under pedaling. An old chain on new cogs will be even more likely to skip than it was with the old cogs. To measure the wear on a chain we have tools which do it quickly, but it can be done with no more than a steel rule if you’re careful. We are happy to check a chain anytime you’d like to roll it in.