A beginners’ guide to … a cycle’s drivetrain

You’ve almost certainly heard the term ‘drivetrain’ bandied about, but as a new or returnee cyclist, you may well be wondering what it means and what this mysterious thing does.

The drivetrain is a pretty important part of a cycle: it’s what, as the name suggests, drives it forwards. Here, we explain what it does, how it works and the different parts that make it up.

What the drivetrain does

Put very simply, a cycle’s drivetrain is what allows it to move. It is the mechanical system that transfers the power from the rider’s pedalling motion to the rear wheel so the bike can move forward. It also provides the rider with a range of gears, allowing speed control and efficient gear shifting.

This system is comprised of several key components. These are: pedals, cranks and chainrings, which together make up the chainset, bottom bracket, chain, derailleurs and cassette. More on these later.

A drivetrain’s exact setup will vary according to type of bike and type of cycling. There are, of course, several different options available.

The components will be made of the usual triumvirate of aluminium (in most cases, for its low cost, light weight and durability), carbon fibre (usually in high-end road bikes, for its extra lightness and durability) or steel (not as common these days due to being heavy, though durable).

How the drivetrain works

When you put your feet on your bike’s pedals and apply pressure, they start to turn. Known as torque, this rotational motion or force is transferred via the cranks, or crank arms, to the chainrings which in turn transfer the movement to the chain.

As the chain turns it powers the cassette which, finally, turns the rear wheel and so the bike moves forward. As you pedal harder, this whole mechanism speeds up so your bike goes faster.

The drivetrain also allows you to change gears. As you pedal the chain feeds through the derailleurs. Using the gear shifters, which are usually located on your handlebar, moves the derailleurs which move the chain – derailing it, hence the name. To better understand how your gears work, read our beginners’ guide.

The rear derailleur can be shifted left or right to move the chain up and down the sprockets of the cassette, which is located on the hub of the rear wheel. The front derailleur moves the chain up and down the chainrings, part of the chainset.

Bigger sprockets mean lower gears and easier pedalling. This is because the bigger sprocket moves the rear wheel further with every rotation of the pedal. Conversely, smaller ones are your higher gears for harder pedalling, because the rear wheel doesn’t move as far with each rotation.

Choose the former for going uphill, for example, and the latter for faster riding on the flat.

Anatomy of a drivetrain

The drivetrain is made up of several different components that all have to work well together in order to make your bike ride smoothly.

Pedals

These are the flat(ish) platforms that you rest your feet on when riding. They are the power transfer points between you and your bike. As you pump your legs, turning your feet and making the pedals go round, you cause the cranks to turn.

There are many different styles of pedal to suit all kinds of riding. Flat pedals are preferred by most riders, especially new ones. More experienced cyclists or those who prioritise speed might go for clipless pedals.

Cranks or crank arms

The two crank arms are levers that connect the pedals to the bottom bracket and hold the chainrings.

Cranks come in a variety of sizes, depending on the type of bike, size of the bike and height of the rider. Lengths typically range from 165mm to 175mm. Longer lengths give more leverage, but might result in pedal strike on very uneven terrain.

Most cranks are made from aluminium, though high-end bikes might have carbon fibre ones. Some cheaper cranks might be steel.

Chainrings

These are the large sprockets in the centre of the drivetrain. These are your front gears. The chainrings hold the chain in place, connecting the front of the drivetrain to the cassette on the rear wheel.

A bike will have between one and three chainrings, depending on the type of cycling it’s designed for. These will be described as 1x, 2x or 3x. The number of chainrings determines how many front gears your bike has and will affect gear range, weight and simplicity.

Chainrings also come in a range of sizes, again depending on the riding discipline. Size is determined by the number of teeth the sprocket has. So you might see ‘53/39’, meaning the outer chainring has 53 teeth and the inner one has 39. If your bike has a single chainring, you’ll see just one number: ‘36t’, for example.

Road and touring bikes tend to have two or three chainrings, giving a higher range of gears. In mountain and gravel bikes there has been a move towards 1x chainrings, making the setup lighter and simpler. These bikes tend to have more sprockets on the cassette.

Together, the chainrings and cranks are referred to as the chainset or crankset. The latter is more commonly used in the US.

Bottom bracket

This sealed bearing system is housed within the bike frame, providing the connection between the frame and the chainset. It consists of a pair of bearings and an axle that support the chainset and allow it to rotate smoothly, with minimal friction and loss of energy.

Chain

This is what transfers the power from the chainset to the cassette. It feeds through the derailleurs and around the chainrings and cassette. It’s made up of a series of metal – usually steel or alloy – inner and outer links joined together by small circular pins to make a flexible loop.

Chains come in a variety of lengths, which have different speeds. You need the right chain for the number of speeds your groupset – the drivetrain and shifters – has.

Derailleurs

These are the mechanisms that move the chain between the chainrings and sprockets of the cassette. The front derailleur controls the chain at the chainrings; the rear one moves the chain over the cassette.

Bikes with a 1x drivetrain – just one chainring – don’t have a front derailleur, saving on weight and simplifying the setup, while fixed-wheel or single-speed cycles don’t have either derailleur. They also don’t have a cassette on the rear wheel – needing only one sprocket. This gives them the advantages of being lighter and easier and cheaper to maintain.

The derailleurs are operated using shifters which are usually located on the bike’s handlebar. The right shifter moves the rear derailleur, with the left controlling the front. This allows the rider to select the correct gear to maximise pedalling efficiency for acceleration (high gear), climbing (low gear) or maintaining speed.

The rear derailleur features a jockey wheel. This small, rotating wheel guides the chain as it moves between different cassette sprockets, aiding in gear shifting. It also helps to tension the chain and keep it aligned.

Cassette

This is the set of sprockets that decrease in size and are mounted to the bike’s rear hub in the centre of the back wheel. If you want to know more about a bike’s wheels, read our beginners’ guide. It is essentially your rear gears, determining the number of gears your bike has.

Cassettes come in a range of sizes, which is usually expressed by quoting the number of teeth on the smallest and largest sprockets. So you might see 11-32t, meaning the smallest sprocket has 11 teeth and the biggest one has 32.

Cassettes have between seven and 13 sprockets, with bikes with just one chainring – usually mountain and gravel bikes – tending to have wider-ranging cassettes than bikes with 2x or 3x drivetrains – most often road and touring bikes.

A cassette is designed to be used with a specific type of chain. The wrong chain won’t ride on the sprockets’ teeth properly, resulting in a very poor ride and potentially damaging the components.

Looking after your drivetrain

The drivetrain is about the hardest-working part of your bike – with the possible exception of your legs. The whole mechanism is moving while you’re pedalling, with constant contact of metal against metal.

Added to this, the drivetrain is close to ground so is first in line for dirt and debris from the wet roads and trails. This means the parts wear down and can get damaged. One of the easiest ways to keep your bike running smoothly is to maintain its drivetrain.

You should clean the drivetrain regularly in all weathers and more so in winter when the terrain you’re riding over is wet and muddy. This will keep dirt and grit out of all those moving parts and ward off rust. Read our beginners’ guide to cleaning your bike for everything you need to know about keeping it sparkling.

The chain is the drivetrain’s weakest link – pun intended – so keep an eye on it. Riding with a damaged chain risks damaging the cassette and chainrings – both of which are much more expensive to replace. Check it regularly for wear and replace it when needed.

Our friends at GCN have some advice on when to replace your chain, cassette and chainrings: