Bicycle gearing may look fairly complicated but in reality, when you get down to the nitty-gritty, gearing is fairly simple. Basically, a bicycle has multiple parts that make up a whole to propel your bike forward at varying cadences to keep a certain pace and speed. Bicycle gearing is an art form in a way and works quite eloquently. But it does take some knowledge to understand completely.
The cassette, or cluster of gears on the rear wheel, is concentric to the rear axle. The cassette is made up of individual chainrings that run closely together, and are often pinned together or machined from one piece of aluminum, steel, or even titanium. These smaller chainrings have machined-in ramps and specific teeth to catch and release the chain as the rear derailleur moves the chain over them. The cassette lets you fine-tune your gears as you ride, allowing you to hold a steady pace in varying terrain.
Crank Arms and Chainrings:
These two parts function together. They are mounted in the lower middle of the frame and are concentric to the bottom bracket. Chain rings come in varying sizes depending on your gear configuration. Crank arms are the driver that connect the feet and pedals to the chainrings that ultimately propel your bike forward. These also come in a range of sizes that depend on the leverage the rider is looking for, as well as ground clearance in varying terrain.
The rear derailleur is a loaded spring mechanism that guides the chain over the rear cassette. The rear derailleur has a spring-loaded cage and two pulleys that hold the chain taut as it moves from larger to smaller chainrings.
Similar to the rear derailleur, this component mounts above the bottom bracket of the bike frame and guides the chain over the chainrings. It is also responsible for keeping the chain from coming off and when ideally setup, will not rub on the chain.
The chain connects the rear derailleur, cassette, and front chainrings together and is the final driving piece of a bicycle’s gearing system. Made from machined inner and outer links and pins, chains are specific to the amount of gears in the cassette. So if you are running a 9-speed cassette, you need to run a 9-speed chain. A bike chain needs consistent lubrication and sometimes cleaning.
Shifters are handlebar-mounted pods that drive the derailleur by either two finger-activated paddles or via a twisting grip. Shifters come in all shapes and sizes and, like chains, are very specific to the gearing you are running.
The sum of all the parts listed above as a whole. The drivetrain makes your bike go forward and can be thought of as one piece where all the parts work in unison. If one piece of the drivetrain is damaged or worn out, it will affect how the rest of the system works.
As you switch gears and the derailleur moves the chain from larger to smaller rings, the chain becomes slack due to less cassette and chain ring teeth retaining the chain. This slack is called chain growth and unattended can cause the chain to either break or fall off. In the past, it has been up to the spring-loaded cage of the rear derailleur to take up that slack. This extra function of the rear derailleur impeded its performance as a shifting mechanism.
Okay, phew, got that out of the way. Now we just have to apply what we have learned to two main classes of bicycles: road bikes and mountain bikes. Generally, most geared bikes will fall into these two categories in some way or another.
Mountain Bike Drivetrains
Mountain bikes in the past suffered from the 3x9 drivetrain. Meaning three chainrings in the front and a 9-speed cassette in the rear. This style of drivetrain was the standard for a long time until more contemporary bike designs opted for a lighter and simpler 2x10 drivetrain that offers a similar range of gears.
3x9 drivetrains were limited in that you could not use the gears in the opposite extremes. Meaning, you could not run your chain in the big ring in the front and back at the same time, and vice versa for the little rings. This would strain the chain and make it rub on the front derailleur. The result was an annoying noise and the potential to break the chain.
By adopting the contemporary and simpler 2x10 drivetrain and optimizing the size of the rings for 26 and 29-inch wheels, manufacturers now have a light and simple drivetrain that is amazingly efficient. You can still find 3x9 or 3x10 drivetrains on less expensive mountain bikes, but they’ll still come with the same disadvantages mentioned above.
1x9, 1x10, and 1x11 drivetrains are just now being widely implemented on mountain bikes. The 1x drivetrain eliminates the front derailleur from the equation and creates a cleaner, lighter system. Also, with a 1x drivetrain, the rear derailleur does not have to accommodate for as much chain growth created by having such vast size differences between the front chainrings. As a result, rear derailleurs on 1x9, 1x10, and 1x11 drivetrains can shift faster and smoother than ever before.
Road Bike Drivetrains
Road bikes have used the 2x10 drivetrain for a long time now. And some component manufacturers are branching out to 2x11 drivetrains for an even larger gear range. 3x drivetrains have not been adopted for road bikes due to excessive weight penalties. And with the advent of electronic bicycle shifting from Campagnolo and Shimano, road bike drivetrains have never been as fast, or more precise, than they are currently.
Compact road gearing uses smaller front chainrings to accomplish lower gearing for climbing hills. Compact gearing was a concept brought around for less-than-fit riders looking for an easier way to get up the hills. Touring and cyclocross riders have also adopted compact gearing. The easier pedaling lets cyclocross riders pedal up and over obstacles easier, while touring riders aren’t always out to break records, but may need to hold an easier pace over vast distances.