If you’re new to cycling, you might’ve overheard more experienced cyclists chatting about replacing or upgrading parts of their bike’s “drivetrain”.
But what is a bike drivetrain, and what different bike parts does it include?
To help you get to grips with your bicycle drivetrain, we’ll be covering:
- What Is The Bike Drivetrain
- Each Of The Drivetrain Bike Parts Explained
Ready for the lowdown on drivetrain bike parts?
Let’s get started!
What is the bike drivetrain?
The drivetrain of a bike is all the parts involved in converting power from your legs into energy to propel the bike forwards.
In other words, the bike drivetrain is the collection of mechanical parts that make the bike move when you pedal.
The bicycle drivetrain has evolved greatly throughout its history. From the direct-drive simplicity of penny-farthings in the 19th century to the complexity of modern derailleur-based systems, the innovations and progress made in the design of drivetrains have been central to the enduring popularity of the bicycle.
Each of the drivetrain bike parts explained
The parts of the bike drivetrain can vary depending on the type of bike (fixies and other single-speed bikes won’t have derailleurs, for example), but the following are all typical drivetrain bike parts found on most geared bicycles:
- Crank Arms
- Front and Rear Derailleurs
- Bike Chain
Let’s take a look at each of the drivetrain bike parts in more detail.
The pedals provide a contact surface for your legs to rotate the chainset (the name given to the crank arms and chainrings as a unit).
A bike pedal typically consists of a spindle that attaches to the crank arm, the main body of the pedal that the foot sits on, and a set of bearings acting as the interface between them to allow the pedal body to rotate freely.
Bike pedals come in all different shapes and sizes, but most can be broadly split into clipless pedals (especially common on road bikes) and flat pedals (found on BMXs and preferred by some mountain bikers).
- Check out our Complete Guide To Bike Pedal Types here!
Crank arms – sometimes simply referred to as cranks – are the levers that connect the pedals to the chainrings.
Like wheels and frames, crank arms come in a range of sizes. Longer crank arms provide greater leverage, amplifying your pedaling force, but require more torque to turn (similar to using a bigger gear) and put extra stress on the joints.
Most manufacturers consider 170-175 mm to be the “Goldilocks Zone” of crank arm length, providing a compromise between leverage, ground clearance, and joint stress – although particularly tall or short cyclists might want to experiment with crank lengths outside of that range.
The chainring is the toothed metal disc that transfers power from the pedals to the rear wheel via the chain.
When a bike has multiple chainrings, they act as the front set of gears. The front derailleur moves the chain between the different chainrings to make pedaling easier or harder. The larger the chainring, the harder it is to pedal, and the further each rotation will propel you forward.
Though the standard for road bikes is to have a dual-chainring setup (and sometimes even a 3x chainring system), single chainrings are becoming increasingly common for mountain bikes. A single chainring helps shed weight and reduce mechanical complexity, while still allowing for a broad range of gear ratios when paired with a wide-range cassette.
- Check out our Complete Guide To Bike Chainrings here!
The cassette is the component that contains the rear set of gears. It’s connected to the hub of the rear wheel – when the chain turns the cassette, the cassette turns the wheel.
The cassette began to appear in the 1970s as an evolution of the old freewheel (also known as a block) mechanism. Since then they’ve become more popular and cheaper to produce, and are now the standard for bikes with derailleur-based drivetrains.
Cassettes typically contain between 5 and 12 sprockets (the name for each gear ring).
- Check out our Complete Guide To Bike Cassettes here!
Derailleurs are devices that “de-rail” the chain from one sprocket or chainring to another, allowing the rider to change gear.
Rudimentary derailleur systems have existed since the 19th century, but only came to prominence in the 1930s.
Frustrated by the existing process of changing gears (which involved removing the rear wheel and flipping it over to a differently-sized sprocket on the other side), bike racer Tullio Campagnolo designed an innovative new road-actuated derailleur system that would allow the rider to change gears without getting off the bike: the Cambio Corsa.
The derailleur’s design soon evolved to be cable-actuated so that cyclists no longer had to reach back to the seat stays to change gear. This helped the derailleur system become much more popular, and by the 1950s they were commonplace.
Friction shifters remained the norm until the introduction of indexed gears by Shimano in 1985. Indexed gears meant that the derailleur would move to a pre-determined position for each gear, improving precision and ease of use. Indexed gears are now standard on almost all geared bikes – so they’re probably what you’re already used to using!
The front derailleur is responsible for moving the chain between the different chainrings. A bike drivetrain will only feature a front derailleur if it has multiple chainrings.
The rear derailleur serves the same purpose as the front derailleur, but moves the chain between the sprockets of the cassette on the back wheel instead.
Additionally, the rear derailleur is also responsible for maintaining tension in the chain (pulling it tight to remove slack).
It achieves this with a pair of pulleys. The upper pulley – called the “guide pulley” – is responsible for guiding the chain onto its intended sprocket. The lower pulley – called the “tension pulley” – is spring-loaded. This allows it to pull backward on the chain, keeping it taught regardless of the gear ration being used.
- Check out our Complete Guide To Bike Derailleurs here!
The bike’s chain is the final piece in the bike drivetrain that allows all the other components to work together.
The chain is responsible for transferring energy from the chainset to the sprocket, providing power to the rear wheel.
The chain is made up of individual links that hook onto the teeth of the chainrings and cassette sprockets. Chains of different widths are compatible with different cassette ranges: cassettes with more sprockets (e.g. a 12-speed cassette) require narrower chains, and vice versa.
- Check out our complete guides to Removing and Replacing a Bike Chain and How To Put a Bike Chain Back On here!