Sprockets, Derailleurs and Bicycle Chainrings

Shifting gears can make riding a bike up a hill easier, but exactly how do the gears shift? To answer that question, you need to understand such bicycle terms as "derailleurs," "sprockets," "bike cogs" and "chainrings." In addition to the chain, all of these elements are essential to a bike's transmission system.
 

Sprockets

Sprockets are toothed wheels that are part of a chain drive system. Although the term "sprocket" can refer to the front or back toothed wheel, it is usually used to refer to the rear wheel gears. Individual rear sprockets are also called bike cogs or gears, while a group of sprockets can be referred to as:
  • a bike cassette
  • a block
  • a cluster
  • a freewheel.
Technically, bike sprockets are not gears, since gears mesh with each other and sprockets mesh with a chain. The term "gear," however, is often used interchangeably with "sprocket."

Bicycle Chainrings

Bicycle chainrings are the front sprockets that are mounted to a crank (the arm that attaches to a bike's pedals). Bicycle chainrings are also referred to as chainwheels, CRs or rings.
 
The two specifications of bicycle chainrings are the bolt circle diameter (BCD) and the number of teeth on the chainring.
 
The number of teeth on a chainring simply refers to the number of uniform projections on the sprocket. Several riders have shown interest in a 10-tooth cog. However, the 10-tooth cog is not as readily available as other dimensions. The optimal number of teeth on chainrings or other rear sprockets will vary based on the chain size.

Bicycle Derailleurs

Derailleurs, or deraillers, move the chain from various size sprockets, or cogs, to shift gears. The components of derailleurs are located by both the rear sprockets and chainrings. The entire system of the chainrings, sprockets and derailleurs is called a derailleur gear system.
 
When a rider needs to change gears to match the riding conditions (lower gear ratios are typically used for uphill climbs and higher gear ratios are used to increase speed on a flat street or path), the derailleurs derail the chain from one chainring to another. This shift optimizes the bike's performance for a given terrain.
 
The front and rear derailleurs are remotely operated by a shift lever that is usually located on or near the handlebars. The lever controls a shift guide that moves the chain.
 
The components of derailleurs are:
  • derailleur cage (front)
  • derailleur plate (rear)
  • high and low adjusters (both front and rear)
  • jockey pulley system with guide and tension pulleys (back)
  • outer arm or lever (front).

Maintaining Derailleurs and Bike Cassettes

It is important to regularly check your bike's chain, derailleurs, chainrings and sprockets for wear. Riding a bicycle with compromised cogs, worn or stretched derailleur parts or a defective chain could not only hinder your bike's performance but can also be dangerous.
 
Here are some tips for maintaining the condition of your bike's cassette, derailleurs and chain:
  • Avoid using the smallest sprocket with the smallest chainring, as this will cause them both to wear down quickly.
  • Be sure to have the proper chain for the derailleurs and sprockets on the bike.
  • Pedal backward slowly and smoothly and review the profile of the bike cog's teeth. (A misaligned tooth should be repaired.)
  • When shifting loses some of its smoothness, check the derailleur cables for tightness. (Loose cables can be quickly tightened.)

If you experience a problem with your bike's cogs, chain or derailleur that you cannot fix, take your bicycle to a bike shop.

Resources
 
Answers.com (n.d.). Derailleur. Retrieved December 7, 2006, from the Answers.com Web site.
 
Brown, S. (1996). Sheldon Brown's bicycle glossary. Retrieved December 7, 2006, from the SheldonBrown.com Web site.
 
KHurramWeb.com (n.d.). Front and rear derailleurs. Retrieved December 7, 2006, from the Khurramweb Web site.
 
EasyStreetRecumbents.com (n.d.). Chainrings. Retrieved December 7, 2006, from
Easy Street Recumbents site.
 
eHow (n.d.). How to fine-tune a bike's rear index shifting. Retrieved December 7, 2006, from the eHow Web site.