The drivetrain is an essential aspect of any bike. Designed much in the same way as a car transmission, a drivetrain allows you to climb and descend hills. Though it may seem like a complicated mechanism, it is relatively simple, and understanding its workings will make riding and maintenance more attainable to you.
The bottom bracket serves as the connection between the bike frame and the crank set. Most bottom brackets consist of a pair of sealed cartridge bearings that are either threaded or pressed into the bottom bracket shell of the bike frame. On the inside of the circular bearings is a smooth cylindrical opening that the spindle of the crankset sits inside of. This snug, freely-rotating connection is what allows the cranks to spin smoothly, but without excess play or creaking.
A crankset consists of two crank arms and the spindle that connects them. Cranksets are typically made of aluminum, though some feature carbon fiber. At the apex of each arm is a threaded hole where the pedals are attached. Cranksets come in varying widths and spindle diameters to accommodate different sized bottom brackets and bike frames. The length of the crank arms also varies, to suit riders of different heights and inseam lengths.
A chainring refers to the circular drive mechanism that is attached to the crankset and carries the chain. Chainrings are typically made of aluminum, and feature evenly spaced teeth that fit into the links of the chain. Newer 1X chainrings sometimes feature alternating narrow and wide teeth, which adhere to the shape of the chain links and help prevent the chain from dropping off the ring during rides. Chainrings come in different tooth counts that affect the bicycle’s acceleration. Larger tooth counts propel the bike at a higher rate of speed, but are more difficult to pedal. Smaller rings don’t provide the same amount of distance per pedal stroke, but are easier to pedal. Chainrings are also unique to each drivetrain system. The two most popular mounting types are bolt-on and spider— the former features a symmetrical bolt pattern of a specific bolt center diameter (BCD), while the latter uses a splined hole at the center to lock into the complementary shape of the spindle.
The chain connects the cassette to the chainring, allowing the rider to propel the bike forward when they pedal. Chains are made of steel or alloy, and are comprised of alternating inner and outer links connected by small cylindrical pins. Chains are specific to certain drivetrains, and vary in width depending on the amount of gears in the cassette. Proper function of the drivetrain requires that all links of the chain are tight yet flexible.
A cassette refers to the cone-shaped stack of chainrings that are assembled in order from small to large. The bicycle’s acceleration and distance traveled per pedal stroke is dictated by which ring in the cassette the chain is riding on. The smaller (high) gears are used when descending or traveling at a high rate of speed. High gears make the bike more difficult to pedal, but propel the bike a far distance with each pedal stroke. The larger (low) gears are used when climbing or riding slowly. Low gears make the bike easy to pedal, but only propel the bike a short distance with each pedal stroke. Most mountain bike cassettes feature somewhere between 8 and 12 speeds in the cassette. The highest rings feature 12 or fewer teeth, while the low gears can have up to 50 teeth.
A derailleur is a metal, spring-loaded mechanism that’s fixed to the bike frame and sits below the cassette. It is comprised of a cage and two pulleys through which the chain runs. As the rider shifts gears, the derailleur carries the chain over different gears in the cassette, changing the bike’s rate of acceleration. The movement of the derailleur is actuated by the tension of a steel shift cable that runs from the shifter to a clamp on the derailleur. Some more modern incarnations, such as Shimano’s Di2 derailleur, are electronically controlled. Derailleurs are adjusted by: cable tension, which assures the pulleys sit precisely below the assigned ring in the cassette; limit screws, which prevent the derailleur from pushing the chain off either end of the cassette; and the b-adjustment screw, which determines the amount of space between the bottom of the cassette and the top of the upper pulley.
The shifter is the handlebar-mounted mechanism which allows the rider to change gears. Most shifters feature two triggers, which can be controlled by the thumb and index finger. One trigger shifts the chain into a higher gear, while the other trigger shifts the chain into a lower gear. Typically, shifters use a spring and pawl system, which, when fixed to a shifter cable, holds the derailleur in place below the desired gear.