Bicycle Anatomy

A diagram of bicycle parts courtesy of Wikipedia, which is a great resource on this topic. However, the information in Wikipedia is scattered across many articles and it takes some searching to find it all. I was motivated to write this page to cover the topics most relevant to my blog and using an organization focused on understanding my blog, but Wikipedia covers a much broader range of topics in much greater detail than I do here. There are many other excellent sources of information about this topic on the web, most importantly, the web pages created by the late Sheldon Brown.

NOTE: This page is a work in progress and is incomplete.

Table of Contents

  • Introduction
  • Gears
  • Chain Guards
  • Cranks and Pedals
  • Rims, Wheels, and Tires
  • Frames
  • Forks, stems, and handle bars


I talk about the parts of a bicycle on my blog a lot, and one bit of feedback I have gotten from my readers is that it would be useful to have pictures or diagrams illustrating what those parts are, and that is the purpose of this page. I expect to be updating this page as required by new references on the blog.

To start, I felt I needed to define what I mean by bicycle; to constrain that definition to something manageable. As Bike Snob notes, doing so can be quite controversial.
  • As a scientist, I have a lot of interest in and respect for recumbent bikes, but they are not included on this page because I don't ride a recumbent. 
  • At the end of the 19th century the safety bicycle was invented, a bicycle with similarly sized  front and rear wheels, roughly ⅔ of a meter or 26 inches in diameter, and a chain drive linking the pedals to the rear wheel in a way to allow the rear wheel to rotate faster than the pedals. This kind of bicycle, currently referred to as an upright bicycle is the kind of bicycle I am describing here. 
  • For most of what I write about, I am interested in a subset of upright bicycles, those commonly known as "road bikes". These are bicycles with relatively skinny tires designed to ride fairly quickly on paved roads or bike paths, usually with "dropped" handlebars:

A picture of drop handlebars on a road bicycle courtesy of Wikipedia

  • Types of bicycles other than "road bikes" are "mountain bikes" with fatter tires and flat handlebars designed for off-road use, "hybrid bikes" which look a lot like mountain bikes but which have a few changes to make them perform better on paved roads, "commuter bikes" with conventional handlebars, fenders, chain guards, lights, and other features to make them as practical as possible for day to day utility use, "bmx bikes" with small wheels and frames used for stunts, and others. On this page, I will mostly focus on road bikes but will occasionally refer to one of these other types.
The rest of this page will focus on the different parts of a bicycle.


The major innovation that made the development of the Safety Bicycle possible was the use of a chain drive between the pedals and the back wheel with a large sprocket attached to the pedals via the crank arm and a small sprocket attached to the back wheel. There is a narrow range of speeds at which people can efficiently turn the pedals of a bicycle. If you were to directly attach the pedals of a bicycle to its wheels using a standard sized wheel of approximately ⅔ of a meter (26 inches) in diameter, if you pedaled as fast as most people can, the bicycle would move so slowly it would be hard to keep it balanced, much less to have it move at a reasonable speed. The predecessor to the safety bicycle was the penny-farthing:

Courtesy of Wikipedia

The penny-farthing solved this problem by making the front wheel large. A single turn of the pedals makes a bicycle with a large wheel go faster. However, because the wheel is so large, it is difficult and un-safe to get on and off of the bicycle, thus the name of the safety bicycle, a safer alternative to the penny-farthing.  A safety bicycle with the proper ratio of front to rear gears is quite rideable under many common circumstances and single speed bicycles of various kinds remain quite popular. That said, one would ideally want lower gears (easier to pedal) if one is riding up hill or into the wind and higher gears (bicycle goes faster) if one is racing or is riding with a tail-wind, and so the ability to change the gears from higher to lower while one rides is a feature that many cyclists appreciate very much.

There are a couple of different ways the rear gear to the rear wheel of a bicycle:

  • It can be directly attached. If you pedal forward, you go forward. If you pedal backwards, you go backwards. If you stop pedaling, the wheel stops moving (and fairly quickly, so does the bicycle.) In short, you cannot coast. Most people have never been exposed to this mechanism and would find it awkward and dangerous, but it is found on bicycles used to race on a track and on road bikes called "fixies" used by bicycle messengers, hipsters, and bicycle racers who are training to improve their rhythm. Because you can halt the motion of the bicycle by using your legs to resist the motion of the pedals, brakes are less necessary with this mechanism, though most experts advise using a brake as well when you ride these on the road as opposed to on a track (where brakes are forbidden.)
  • It can be attached via a ratchet. If you pedal forward, you go forward. If you stop pedaling or pedal backwards, nothing happens (you coast) and usually there is a clicking sound. This is one of the most common mechanisms for attaching rear gears to rear wheels and is found on single speed bikes, on bikes with derailleurs, and on bikes with multi-gear rear hubs (explained below.)
  • It can be attached via a coaster brake (explained below, under brakes). If you pedal forward, you go forward. If you stop pedaling, you coast (usually without a clicking sound). If you pedal backwards, a brake is engaged which stops the rear wheel much more effectively and with much less effort than with a fixie. Such a coaster brake is usually considered sufficient braking all by itself.


This is the most general term, one that is used to mean a whole range of different things. Beside the fact that it can refer to how fast the bicycle is going, which is only somewhat determined by the gear ratio which is the topic of this decision, it can also refer to a unique combination of front and rear sprockets or a specific gear ratio which are not quite the same thing. Where possible, I will use a more precise term.
This can refer to the same things speed does (e.g. a gear might be a specific combination of front and back sprockets or a specific mechanical advantage, e.g. gear ratio) but also can refer to the sprockets on the front or rear of the bicycle or the pieces inside a hub that provide different ratios.
The gears on the outside of a bike, the front gears attached to the crank arm and pedals or the rear gears attached to the rear wheel.
Chain Ring
A front sprocket.
A rear sprocket.
A freewheel or cassette (see below).
A collection of cogs attached to a ratchet mechanism designed to thread onto the threaded rear hub of a rear wheel. These are largely obsolete.
A collection of cogs designed to be placed onto a freehub, a rear hub containing a ratchet mechanism. This is the modern replacement for a freewheel.

Kinds of Gears

There are three ways to change the gear ratio that are in common use today. These three ways are independent, which means they can be combined to further increase the range and number of different gears. The first is to have larger and smaller sprockets on the rear wheel of the bicycle (the rear sprockets) and a device named a "derailleur" to move the chain from one to another. The second is similar; to have larger or smaller sprockets attached to the crank arms and pedals of a bicycle (the front sprockets) and a derailleur to move the chain from one to another. A derailleur must accomplish two tasks, it must move the chain from gear to gear and it has to adjust the chain length to compensate for the change in the size of the gears. If there are front and rear derailleurs, only one of them needs to adjust the chain length. Currently, this is always done by the rear derailleur, the front derailleur moves the chain, relying on the rear derailleur to adjust the chain length. As a result, it is possible to have multiple gears on the back of a bicycle along with a rear derailleur with but a single gear on the front of the bicycle with no front derailleur, and this arrangement is commonly seen. On the other hand, using currently available components, multiple gears on the front and a front derailleur only work if there is a rear derailleur attached. Thus, I have never seen a bicycle with multiple gears on the front and a single gear on the back.

Diagram of a front and rear derailleur gear system, courtesy of Wikipedia.

The third way used to change the gears on modern bicycle is to have "planetary" gears internal to the rear hub. In this arrangement, the rear sprocket is not attached directly to the rear wheel, but transfers its force via these internal gears which can be used to change the gear ratio, or the "speed". When I started riding back in the mid-60's, hubs with two, three, or four different "speeds" were available, with three being by far the most common. This was so common that it transferred its name to the bike, a "three speed", in the same way road bikes with derailleurs were called "ten speeds." Since the 1990's, tremendous progress has been made in internally geared hubs which are now commonly available in 3, 7, 8, 11, and 14 speed versions. Back in the day of the three speed, the difference between the highest gear and the lowest gear was not very great; it was much less than the range of gear ratios on a derailleur bicycle. Modern 14 speed hubs have almost as wide a range between the highest gear and the lowest gear as modern derailleur bicycles, making them an expensive but reasonable alternative to derailleur bikes for long distance bicycle riding. That said, the greater cost and somewhat lower efficiency (higher "drag") of these hubs have prevented them from being very common for long distance cycling, but internally geared hubs are very popular for commuting and other forms of utility cycling due to their lower maintenance and higher reliability.

The inside of a three speed hub, courtesy of Wikipedia. There is one additional set of gears that makes this work which is on the outer case of the hub which has been removed in this photograph to make the insides visible.

Because the multi-speed hub is independent of derailleurs, it is possible to combine all three systems to produce a bicycle with a mind-numbing 14 x 11 x 3 = 462 different gear combinations! Such bicycles actually exist. However, few cyclists find them useful, so I would classify this configuration as more of a novelty than anything else. The four most common configurations of gears are:
  1. One speed with no variable gears.
  2. A single sprocket in front, multiple sprockets in the rear, no internal gears in the hub.
  3. Multiple sprockets in front, multiple sprockets in the rear,  no internal gears in the hub.
  4. A single sprocket in the front and rear attached to a hub with internal gears.


After all the discussion above, we still need to talk about how all these shifting mechanisms are actuated; what does the rider actually do to change from one gear to another? The first thing to realize is that there are two fundamental kinds of shift mechanisms, indexed and friction. Indexed shifters click into discrete positions where the number of discrete positions corresponds to the number of gears being controlled. Hub gears require precise positioning of the internal gears and so have always used indexed shifters. Friction shifters move continuously over a range of positions that covers the range of the gears to be controlled; one adjusts the position of the shifter until the gears do what you want them to. When I started cycling in the mid-1960s, all derailleurs used friction shifters. There is some logic to this.   Consider the 10 speed discussed in the previous section; the correct position of the front derailleur will not be the same for 53x14 as it will for 53x28. Experienced cyclists have little trouble adjusting friction shifters, they learn to find the correct position by ear and by feel. Some cyclists, especially inexperienced ones, tend to find them frustrating; the gears never seem to do what they want. As a result, index shifters for derailleurs were developed in the 1980s. The issue of adjusting for different gear combinations, discussed above, was dealt with by using compromise positions and redesigning the derailleurs to be less sensitive to this issue. Despite the availability of index shifters, friction shifters are still preferred by some and are still sold and used. Friction shifters are more versatile and are less likely to get out of adjustment and thus are more reliable and for these reasons many randonneurs use friction shifters. Finally, some shifters have friction and index modes you can switch between.Very recently, with the introduction of electronic shifting, sophisticated logic in the derailleur can handle the adjustment needed for different gear combinations automatically. Electronic shifters are quite new and expensive and at present are only available on the most expensive bicycles and are less repairable on the road, a definite disadvantage for the randonneur.

Somewhat independent of index versus friction shifters is the question of where the shifter is located. Back in the mid-1960s, almost all derailleur road bikes mounted friction shifters on the down tube:

Down Tube Shifters

Photo courtesy of Wikipedia

Shifters On Stem

Photo courtesy of Wikipedia

Bar End Shifters

Photo courtesy of Wikipedia

Trigger Shifter (for three speed hub)

Photo courtesy of Wikipedia

Twist Grip Shifter (for derailleur)

Photo courtesy of Wikipedia

Shifters built into the brake levers (a.k.a. Brifters)

Photo courtesy of Wikipedia


This Section Coming Soon

Chain Guards, Cranks, and Pedals

Full Chain Guard a.k.a. Chain Case

Used courtesy of Wikipedia

Chain Guard to be used with a Front Derailleur & One Piece Crank

Photo courtesy of Wikipedia

Other kinds of cranks, photos and descriptions coming soon:

  • 3 piece
  • Cottered (steel)
  • Cotterless (aluminum alloy or carbon)

Rims, Wheels, and Tires

Picture from the Wikipedia article on bicycle wheels. I added the arrows and labels.

The most common kind of bicycle wheel by far consists of a hub, containing the bearings that allow the wheel to spin connected to a rim on which a tire is mounted with the rim connected to the hub via spokes. There are exceptions to this, but they are sufficiently rare that for the purposes of this blog I will ignore them. The flexibility provided by spokes allows pretty much any hub to be connected to pretty much any rim except that the number of spoke holes in each must be the same, and there is considerable variability in the number of spoke holes provided.

Clincher vs. Tubular Tires

The first panel is from
The second and third panels are from Wikipedia articles on tires and tubular tires.

There are two general kinds of tires I use which I refer to as clinchers and sew-ups. Sew-ups are more properly known as tubular tires. The difference between them is illustrated above. Tubular tires are held onto the rim and the air pressure contained by a metal or kevlar bead which is held against a rim. The middle diagram shows a clincher being used with a tube, but tubeless versions of clinchers are now available. If we look at the parts of that diagram, a tubular tire has no bead, the inner tube and air pressure are contained because the edges of the tire where a clincher has a bead are sewn together and the "rim" strip is pre-glued to the tire rather than being put on the rim. The rim has a simple concave surface to which the tire is glued; it is this glue is all that holds the tire to the rim.


This section coming soon.

Forks, stems, and handle bars

This section coming soon.

Seats and Seat Posts

This section coming soon.

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