Your bike’s drivetrain is the magic that keeps you rolling, turning your pedaling into power. Whether you’re just cruising around or tackling some trails, knowing what everything is called and how it all works will make you a more knowledgeable cyclist. This information will also help you shop for bicycles and parts, and assist you in communicating with your mechanic when maintenance and repair are required, or if you are looking for upgrades. From what a “dérailleur” really is to how the B-gap influences shifting, here are 30 cool details about your drivetrain to help you ride smarter.

❯❯ 1. The front half of a bicycle’s drivetrain uses a chainring (or chainrings) mounted to the crank. The rear half consists of a cog (or cogs). The chainring (often shortened to “ring”) and cog are linked by a chain that transfers power from the rider’s legs turning the cranks to the rear wheel.

❯❯ 2. If a bike has more than one chainring or cog, it has a derailleur. Dérailleur is a French word that means de-rail. When you shift, you are de-railing the chain from its current cog or chainring, which allows it to move to the next “rail.”

Trevor Raab

❯❯ 3. The meaning of “upshift” and “downshift” has nothing to do with the chain’s vertical position. A “downshift” means shifting to a lower or easier gear, while an “upshift” refers to shifting to a higher or harder gear.

❯❯ 4. When you upshift, you select a higher (harder) gear ratio, while downshifting involves engaging a lower (easier) gear ratio. In front, these shifts are literally up (small to big ring, higher ratio=upshift) and down (big to small ring, lower ratio=downshift). However, and confusingly, this reverses at the rear derailleur. An upshift means moving the chain down the cassette to a smaller cog, while a downshift results from moving the chain up the cassette to a larger cog.

❯❯ 5. Many derailleurs have limit screws—marked High and Low or H and L—that adjust their total range of motion. To remember which end of the range each screw affects: Low is slow (easier gear), high is fly (harder gear).

❯❯ 6. Modern drivetrains are designated as 1x (or one-by) or 2x (two-by). This indicates the number of chainrings on the crank: 1x or one-by signifies one chainring, while 2x represents two. Although 3x, or triple, cranks exist, they are rarely seen on modern bicycles.

❯❯ 7. If a bike features a 2x or 3x drivetrain, it includes a front derailleur and a rear derailleur. A 1x bike has only a rear derailleur.

❯❯ 8. Bicycle drivetrains are designated by speed. Years ago, a bike with two chainrings and five cogs was called a “ten-speed.” But in modern terminology, speed does not refer to the total number of gear combinations provided by a bike’s chainrings and cogs; instead, it simply denotes the number of cogs. Therefore, the current iteration of Shimano 105 is referred to as a 12-speed drivetrain, even though there are 24 possible combinations (2 chainrings and 12 cogs).

Trevor Raab

❯❯ 9. The size of a chainring or cog is indicated by the number of teeth (sometimes abbreviated as T).

❯❯ 10. Because bicycles have only a few chainrings, they are usually listed individually in a product’s information. An example of a bike with two rings is 50/34T.

❯❯ 11. Regardless of speeds, bicycle product information typically lists only the highest and lowest cogs instead of detailing every cog’s tooth count. For example, a Specialized Tarmac SL8 Expert with Shimano Ultegra specifies an 11-30 cassette in its specifications. If you want to determine the tooth count of each cog, you can usually find that information on the drivetrain manufacturer’s website. On Shimano’s site, you will see that the Ultegra 12-speed 11-30 cassette includes the following cog sizes: 11-12-13-14-15-16-17-19-21-24-27-30T

❯❯ 12. A chain and derailleur drivetrain has all its parts exposed. While it has drawbacks, the chain and derailleur system is lightweight, highly efficient, and easy to maintain. This is why it remains the dominant form of multi-speed drivetrain. Other styles of bicycle drivetrains include gearboxes and internally geared hubs, which enclose their gear-shifting mechanisms in a sealed container, protecting them from the elements. These systems are generally very durable, weather-resistant, and have excellent longevity; however, they typically experience more friction and weigh significantly more than a chain and derailleur system.

❯❯ 13. Throughout their history, derailleurs have taken many forms, but the parallelogram serves as the foundation for modern derailleur design. Rear derailleurs designed to accommodate a drivetrain with a front derailleur use a slant-parallelogram design. A rear derailleur specifically built for use with one chainring and no front derailleur often employs a straight parallelogram.

Trevor Raab

❯❯ 14. A rear derailleur’s parallelogram consists of two parallel plates that connect the B-knuckle (the rear or upper knuckle, which also includes the bolt used to attach the derailleur to the frame) and the P-knuckle (the front or lower knuckle).

❯❯ 15. Beneath the parallelogram is a cage with two pulleys (sometimes called jockey wheels). The cage rotates on a spring-loaded pivot within the P-knuckle. The chain wraps around the pulleys, and the cage spring provides tension on the chain.

❯❯ 16. Each pulley has a specific role. The upper pulley is referred to as a guide pulley, while the lower one is known as a tension pulley.

❯❯ 17. The tension pulley works with the cage spring to provide constant back pressure on the chain. As the chain moves to larger rings or cogs, the tension pulley rotates forward and spools out chain; shifting to smaller rings or cogs causes the cage spring to pull the tension pulley backward and take up chain.

Trevor Raab

❯❯ 18. The guide pulley initiates shifts. As the derailleur moves, the guide pulley aligns itself under the selected cog, which begins the derailing process. A rear shift starts on the underside of the cassette and finishes at the top. A shift is complete when the chain is fully disengaged from the initial cog and fully engaged with the selected cog.

❯❯ 19. A rear derailleur’s motion is more intricate than that of a front derailleur. As the rear derailleur body moves inboard and outboard, the guide pulley simultaneously moves vertically to maintain a constant distance between itself and the selected cog.

❯❯ 20. The distance between a cog and guide pulley is called the B-gap.

❯❯ 21. Most rear derailleurs feature a screw for adjusting the B-gap. A tighter B-gap typically leads to faster and more precise shifting; however, if it’s too tight, the cogs and guide pulley can make contact, resulting in noise, excessive wear, and problematic shifting. Conversely, a larger gap often quiets a drivetrain, but excessive space can make shifting sluggish and imprecise. The objective is to strike a balance that ensures smooth and precise shifting. Derailleur manufacturers provide B-gap specifications in their setup instructions or technical manuals.

Trevor Raab

❯❯ 22. Though they still incorporate a parallelogram, front derailleurs have a much simpler design than rear derailleurs. Essentially, it consists of a metal hoop (the cage) that pushes against the side of the chain to force it onto the next chainring.

❯❯ 23. In most cases, the vertical position of a modern front derailleur’s cage is fixed, and it only moves left or right. There have been examples of front derailleurs with more intricate cage motion. The Shimano XTR 950 (circa 1995) “differential plate” front derailleur featured a hinged cage that offered more intricate cage articulation for “faster, crisper up-shifts with considerably less hesitation,” Shimano claimed.

❯❯ 24. Modern 2x or 3x chainrings and cogs feature intricate details that facilitate smooth shifting of the chain. Look closely, and you will notice a series of ramps, cutouts, tooth shaping, and pins (for rings only) designed to interact with the chain. These details assist in the transition from one cog or ring to the next by carefully and precisely picking up or releasing the chain. These features explain why modern drivetrains provide much smoother shifting, especially under load, compared to older systems. 1x specific chainrings have alternating thick/thin tooth profiles that keep the chain in place, making it less likely to bounce off the ring.

Trevor Raab

❯❯ 25. Front shifts are generally slower, clunkier, and more prone to hiccups than rear shifts. This is because the tooth difference between rings is usually much larger than the tooth difference between neighboring cogs. Front shifts are also activated on the tensioned top run of the chain, while rear shifts are activated on the un-tensioned lower run.

❯❯ 26. A chain is made of links, and links are made of plates, rollers, and pins. An outer link consists of two plates that connect a series of inner links. An inner link is made up of inner and outer plates with two rollers in the middle. The outer link connects to the inner link by a pin, which travels through the center of the roller. If you turn the chain on its side, it resembles a deli sandwich. From the bottom: outer link, inner link, roller, inner link, outer link, with the pin serving as the toothpick to hold it all together.

❯❯ 27. A freehub features a splined interface known as a driver body, which accepts a cluster of cogs referred to as a cassette. The cassette slides over the driver body and is secured by a lockring.

❯❯ 28. BCD stands for Bolt Circle Diameter and refers to the mounting pattern for attaching chainrings to a crank’s spider. The spider is the multi-armed disc that extends from the center of the right-side crankarm. Some cranks completely eschew spiders and instead feature chainrings that bolt directly to the drive-side crankarm. These are, unsurprisingly, called direct-mount chainrings.

❯❯ 29. Like many elements of a bicycle’s drivetrain, BCDs and spiders were once fairly standardized and designed for easily interchangeable chainrings. Today, spiders and BCDs are often proprietary, which means that before you shop for new chainrings, you must know your crank’s brand and model (Shimano, SRAM, and Campagnolo all have multiple BCD and spider formats).

Bryn Lennon

❯❯ 30. Bicycles have a “drive side” and a “non-drive side.” The drive side—so named because the drivetrain mounts to this side of the frame—is usually the right. However, some bikes use LSD (left-side drive). Some freestyle BMX riders grind on the right side and prefer left-side drive. A few specialty track bikes have drivetrains on the left because track racing is always done counterclockwise—having the drivetrain on the inside of the turns is more aerodynamically advantageous.

READ MORE ON: derailleur drivetrain know-how tech talk