Search and

Sheldon Brown photo

Sheldon Brown's Bicycle Glossary Bo-Bz

This glossary is also available in pdb format for your Palm OS PDA Palm


Up to previous page

Bolt Circle Diameter (B.C.D.)
On a crankset, the diameter of the circle formed by the stack bolts.

Common 5-bolt double-chainwheel sets use 130 mm or 110 mm diameters.

Modern triple chainwheel sets have two diameters, a large one for the two outer chainrings, and a smaller diameter, with a separate set of bolts, for the granny ring.

Full-size triples usually use 110 mm/74 mm, or 130 mm/74 mm for newer "road triples." Campagnolo uses 135 mm/74 mm. Compact triples commonly use 94 mm/58 mm.

Some crank sizes:

"C-C" is the distance between adjacent stack bolts.
This is easier to measure directly than the actual Bolt Circle Diameter.

Measuring Bolt Circle Diameter

Measuring BCD: This 110 mm BCD ring measures 64.7 mm center-to-center
(or, from left side to left side, or right side to right side --easier to measure).

Multiplying this dimension by 1.701 will give you the actual BCD for a 5-bolt chainring. Use 1.155 for 3-bolt chainrings.

If there is an even number of holes, diameter can be measured directly unless something is in the way -- usually, the crank. Then measure at two adjacent bolt holes, and multiply by 1.414 for 4-bolt chainrings and 2.000 for 6-bolt chainrings.

For dimensions of specific models, see my Bicycle Crank/Chainring Bolt Circle Diameter Crib Sheet.

A fastener with male threads, usually with a hexagonal head. Strictly speaking, it's only a bolt if the threads do not extend the full length of the shaft.

If the threads do extend full length, technically it is a "screw " but only pedants insist on this distinction.

Slang term for a non-quick release hub or axle. More properly, these should be referred to as "nutted" or "solid-axle" units, since conventional hubs use nuts, not bolts.

A very few high-end hubs actually do use bolts, usually with an Allen head. Among these are American Classic, Bullseye, Phil Wood and White Industries.

The earliest type of bicycle, from the mid 19th century. A boneshaker had wheels of roughly equal size, like a modern "safety" bicycle, but the pedals were directly connected to the front hub, as with a high wheeler.

Boneshakers existed before the invention of Dunlop's pneumatic tire, and had wood or iron wheels, with iron or (later) solid rubber tires. This caused them to provide a very harsh ride, hence the name.

The boneshaker evolved over time into the high wheeler, with the drive wheel getting larger and larger, the rear wheel shrinking down.

Boob tube
A television set. (Boob: "A stupid or foolish person; a dolt" (who watches too much TV); Tube: the cathode ray tube of a TV receiver.)

Some misguided souls use this term to describe the keel tube of a tandem frame. This makes no sense semantically. It probably arose because somebody couldn't think of one of the several more descriptive terms, and picked something cute0sounding out of the air. It is to be hoped that this foolish term will disappear into the dustbin of history.

Boom tube
Keel tube. Also, the tube (often adjustable by telescoping) that holds the "bottom bracket" on a short-wheelbase recumbent.
The bosses of cantilever brakes and centerpull brakes can flex outward when the brake is applied hard. This contributes to a "spongy" feeling to the brake lever, and, in some cases, the extra flex may permit the brake lever to bottom out against the handlebar.

A "brake booster" is a metal bridge that connects the outer ends of the two cantilever bosses, holding them in position and greatly reducing their tendency to flex under load. A booster resembles an inverted "U", as it curves up and over the tire.

  • A temporary patch to cover a large hole or cut in a tire. A boot does not need to be air-tight, but needs to be sufficiently strong and stiff to prevent the inner tube from bulging through the hole in the tire.

    Many things may be used to boot tires, including leather, currency, food wrappers, etc. Patches intended for tube patching will not usually do a good job, because they are stretchy and will blow through the hole just as an inner tube would.

    The best material for boots is a piece cut from an old thin-tread tire, preferably a tubular. Many cyclists carry such a piece of tire for this purpose.

    A boot does not need to be glued in place. It will be held in position by the pressure of the inner tube.

  • V brake bootAn accordion-like rubber sealing device to cover the connection where one part slides into another, preventing dirt from entering.

    This type of "boot" is commonly seen on the legs of suspension forks, and on the cables of direct-pull cantilever brakes.

A bump or protrusion. This term has several specific bicycle usages:
  • Cantilever bosses are the brazed-on pivots attached to frames and forks for cantilever brakes.
  • Shift lever bosses are brazed-on pivots for down-tube-mounted shift levers. Most newer "road" bicycles have the shifters mounted on the handlebars, so they use the old-style lever bosses as attachment points for housing stops
  • Older style freewheel pullers had two (or four) protruding bosses that engaged the notches of the freewheel body. This system was prone to failure when removing freewheels that were unusually tight. The development of splined freewheel removal systems made this obsolete
Boston Montréal Boston
Boston Montréal Boston is North America's premiere randonnée, the North American equivalent of Paris Brest Paris.
Bottom bracket
The part of the frame around which the pedal cranks revolve, also the bearings and axle assembly that runs through the bottom bracket shell of the frame.
A conventional cup-and-cone bottom bracket. The lockring wrench is about to loosen the lockring, the pin wrench is engaging two of the holes in the adjustable cup A cartridge bottom bracket. The splined tool is shown above the bottom bracket. The crank would need to be removed to actually use the tool. Old American-style "Ashtabula" One-piece Crank Bottom Bracket
The lock nut and the cone behind it have a left-hand thread .
Three piece adjustable cup-and-cone bottom bracket Three piece sealed cartridge bottom bracket American Ashtabula one-piece crank bottom bracket
An old American term for "bottom bracket" is "hanger". This is usually used in connection with one-piece cranks.

Bottom brackets come in different sizes, according to the nationality of the frame:

Standard Threading Adjustable
(left) cup
(right) cup
1.370" X 24 tpi
1.375" X 24 tpi
right left Standard 68 mm
O.S. 73 mm
The overwhelming majority of bicycles in current production.
British and I.S.O. are interchangeable.
ISIS Overdrive 48 x 1.5 mm right left 68mm
100 mm
New proposed standard oversized system.
Italian 36 mm X 24 tpi right right
70 mm Italian and some high-end French bicycles.
Prone to problems due to the right-threaded fixed cup,
which tends to unscrew itself in use.
French 35 mm X 1mm (25.4 tpi) right right
68 mm Obsolete, used on older French bicycles.
Prone to problems due to the right-threaded fixed cup,
which tends to unscrew itself in use.
Swiss 35 mm X 1mm (25.4 tpi) right left 68 mm Same thread as French, but fixed cup is left-threaded for reliability.
Raleigh 1 3/8" X 26 tpi right left 71mm
76 mm
Older British-made Raleighs, especially 3 speeds.
Male threads
on crank
24 tpi (most)
28 tpi

68 mm (2.68") wide
51.3 mm (2.02") i.d.
Older U.S. bikes, BMX, Juvenile bikes,
Department store bikes.
24 tpi cranks use #66 retainers, with 10 5/16" balls.
28 tpi cranks use #64 retainers, with 9 5/16" balls.

What happens if you try to mix different sizes:

Bottom Bracket
Shell Threading
1.37/1.375" x 24 tpi CUPS R-L
(34.8/34.9 x 1.06 mm)
36 mm X 24 tpi CUPS R-R
(1.417" x 1.06 mm)
35 mm X 1mm CUPS R-R
(1.378 x 25.4 tpi)
35 mm X 1mm CUPS R-L
(1.378 x 25.4 tpi)
1 3/8" X 26 tpi CUPS R-L
(34.9 x 1.06 mm)
1.37/1.375" x 24 tpi
(34.8/34.9 x 1.06 mm)
Made to Fit 36 mm Cup diameter is too large, thread won't even start. Cup diameter is slightly too large, usually won't start.
Right (fixed) cup threaded in the opposite direction.
Cup diameter is slightly too large, usually won't start. Diameter matches, but thread pitch does not.

Will bind after only a few threads are engaged.

36 mm X 24 tpi
(1.417" x 1.06 mm)
British/I.S.O. cups fall through Made to Fit Italian shells are larger diameter, all other size cups fall right through, threads will not engage.
35 mm X 1mm
(1.378 x 25.4 tpi)
35 mm = 1.378". Shell is slightly larger, thread pitch slightly finer.
Left side may seem to fit, but will be loose.
36 mm Cup diameter is too large, thread won't even start. Made to Fit Left (adjustable) side is interchangeable.
Right (fixed) side is threaded in the opposite direction, won't fit.
35 mm = 1.378". Shell is slightly larger, thread pitch slightly coarser.
Left side may seem to fit, but will be loose.
35 mm X 1mm R
(1.378 x 25.4 tpi)
35 mm = 1.378". Shell is slightly larger, thread pitch slightly finer. May seem to fit, but will be loose. 36 mm Cup diameter is too large, thread won't even start. Left (adjustable) side is interchangeable.
Right (fixed) side is threaded in the opposite direction, won't fit.
Made to Fit 35 mm = 1.378". Shell is slightly larger, thread pitch slightly coarser.
May seem to fit, but will be loose.
1 3/8" X 26 tpi
(34.9 x 1.06 mm)
Diameter matches, but thread pitch does not.

Will bind after only a few threads are engaged.

36 mm Cup diameter is too large, thread won't even start. Cup diameter is slightly too large, usually won't start.
Right (fixed) up threaded in the opposite direction.
Cup diameter is slightly too large, usually won't start. Made to Fit

Chainline Standards:

Application Dimension Notes
Road Double 43.5 Shimano spec, measured to the midpoint between the rings.
with typical 5 mm chainring spacing, this puts the inner at 41 mm, the outer at 46 mm.
Road Triple 45 Shimano spec, measured to the middle ring.
MTB Triple 47.5-50 mm Shimano spec, measured to the middle ring.
47.5 preferred, but for frames with oversized seat tubes, the larger dimension may be needed, because the fat tube places the derailer mechanism farther to the right.
Track/Coaster Brake
Traditional One-Speed
Most internal gear hubs
40.5-42 mm Older bikes with 110 mm spacing would be on the smaller end of this range
Newer bikes with 120 mm spacing normally use 42 mm
Singlespeed MTB 52 mm Wider chainline need for chainstay clearance on MTBs.

This is close to the chainline of the outer ring of a typical MTB triple.

Rohloff Speedhub 54 mm
(58 mm w/13 tooth)
Singlespeed MTB
47.5 mm White Industries ENO hubs use this chainline, which lines up with the middle position of a typical MTB triple.
It's also fairly close to the outer position of a typical "road" double.

Standard bottom-bracket bearing assemblies are specified in terms of width of the bottom bracket shell (generally 68, 70 or 73 mm) and the length of the spindle (102 - 130 mm.)

Current usage generally assumes that 68 and 73 mm bottom bracket units are for British/ISO threaded frames, and that 70 mm units are for Italian threaded frames.

The spindle length mainly depends on what kind of crankset you'll be using, and doesn't have much to do with the frame. New cranks come with a spec sheet that lists what length spindle they are intended to be used with. Sometimes two lengths are listed. When this is the case, the longer size is for use on frames with fat seat tubes, because such frames put the front derailer mechanism farther to the right than on a frame with a standard seat tube.

See also my article on Bottom Bracket Sizes for information on matching spindles to cranks.

See also my article on Bottom Bracket Tapers.

My Tool Tips series includes related articles on Cottered, and Cotterless crank removal, as well as tools for disassembly and adjustment of cup-and-cone bottom brackets.

Bottom Pull
See Top pull.
"Boutique" Parts
Most of the best bicycle parts are made by a few major companies, who provide excellent value thanks to good design and economies of scale resulting from large-scale mass-production.

In the bicycle industry, the best frames have traditionally been made by very small-scale, custom or semi-custom framebuilders.

There are also small-scale makers of premium or super-premium parts, companies such as Chris King, E.A.I., Phil Wood , White Industries and a few others. These small, high-prestige manufacturers often (but not always) make products superior to those of the big manufacturers. For instance, Chris King headsets are the very best, and Phil Wood hubs and bottom brackets are unequalled.

Unfortunately, the "boutique" category also includes a lot of other companies who produce overpriced, inferior products, or sometimes just slap a prestigious name on generic Asian imported stuff.

"Boutique" hubs

When a company wants to get started making bike parts, hubs, especially front hubs or singlespeed/track hubs are the easiest thing to make. There are many boutique and off-brand hubs out there.

Boutique hubs are often sold on the basis of light weight, but that's often a bit of a scam. Boutique hubs are generally sold without quick release skewers, while major-brand hubs come with skewers. This has two negative effects:

  • The boutique hubs are effectively more expensive than they seem, because you still need to buy a skewer.

  • The weight specifications you'll find in catalogues are the hubs as supplied. Thus the boutique hubs will have a significantly lower listed weight, since they are weighed without quick release skewers!
Boutique hubs are commonly CNC machined out of bar stock, so the "grain" of the metal is perpendicular to the hub flanges.

Major brand hubs have forged bodies, making the spoke flanges considerably stronger.

"Boutique" hubs are often marketed as if they were an upgrade from, say, run-of-the-mill Shimano hubs, but in fact they are generally inferior in quality to genuine Shimano hubs! (The only hubs that I consider actually superior to Shimano are Phil Wood, and they typically cost at least three times as much as Shimano.)

"Boutique" wheels

Many newer bikes come with "premium" wheels with reduced numbers of spokes. Basically anything fewer than 32 or 28 spokes per wheel would fall into this category. These wheels are touted as being faster due to improved aerodyamics, and many people assume that the reduced number of spokes makes them lighter. In practice, these wheels are actually heavier than good traditional wheels! The reason is that when you use fewer spokes, you need to use a stronger, stiffer, heavier rim to make an acceptably strong wheel.

These wheels are basically throw-away items, intended to be used for a season or until they get damaged, then replaced with a new set.

Bowden Cable
The type of concentric cable used for brakes and gears was once known by the term "Bowden cables", after its inventor.

There was a famous legal case arising from Bowden's cables. He licensed many manufacturers to make cables based on his patent, but failed to exercise quality control over the licensed products that bore his name. As a result, the patent and trademark were deemed to have been abandoned, and became public domain. (I am not a lawyer, I heard this from a lawyer some time ago, I don't vouch for the accuracy of the legal information...)

Caliper, Cantilever, Centerpull, Coaster, Direct Pull, Disc, Double pivot, Drum, Dual pivot, Roller, Rollercam, Roller lever, Sidepull, Single pivot, Spoon, "V-Brake" &reg:
Brake bridge
The short length of tubing connecting the seatstay just above the tire. This is the usual mounting point for a rear caliper brake.
Brake Lever Types
Brake levers (handles) for use with cable-operated brakes come in 4 basic types.

The differences depend on the handlebar type they are to be used with, and on the amount of cable travel required:

Handlebar Type

Levers for drop ("road" "racing") handlebars curve toward the handlebar, to follow the curvature of the handlebar. These levers have mounting clamps to fit the 23.8 mm (15/16") diameter of almost all drop handlebars.

Levers for upright ("mountain" "cruiser" "BMX") handlebars mount on a straight section of the handlebar, and the lever curves away from the bar for better clearance. These levers have mounting clamps to fit the 22.2 mm (7/8") diameter of almost all upright handlebars.

Cable Pull

Standard pull levers are designed to work with caliper brakes or traditional centerpull cantilever brakes.

Long pull levers are designed to work with "direct-pull" cantilever brakes, such as Shimano "V-Brakes" ®

Direct pull cantilevers have twice as much mechanical advantage as traditional brakes, so they require a lever with half as much mechanical advantage. Long pull levers pull the cable twice as far, but only half as hard.

Mismatched Lever Issues:

Old short-pull lever, new direct pull ("V type") cantilever

The excessive mechanical advantage of this combination will make it difficult to modulate the brake, and it may be all too easy to lock up the wheel.

The lever feel will be very soft and mushy.

The lever will travel too far before engaging the brake, and it may bottom out against the handlebar. Thus, the brake may be super-powerful at first, but as the brake shoes wear, the lever's bumping up against the handlebar will prevent full application. This is likely to be a particular problem in wet conditions.

New long-pull lever, old caliper or traditional cantilever

The reduced mechanical advantage will require unusually high hand strength to get barely adequate braking force.

Paradoxically, the lever will feel very solid, the brake will engage with a very short amount of lever travel...but won't actually be squeezing very hard on the rim.

Most disc brakes are designed for long-pull levers, but discs designated as "road" models are usually compatible with traditional short-pull levers.

There are stepped pulley devices to permit mixing otherwise incompatible levers/brakes. The best known is the QBP Travel Agent ®.

An alloy of copper and zinc, in the proportion of about two parts of copper to one part zinc. The zinc makes brass stronger and harder than copper is alone. It is easy to work, and doesn't rust. Most spoke nipples are made of brass, because it is easy to thread and will not corrode onto the spoke threads.

Brass is also easy to cast, and has been used for derailer parts and quick-release skewer acorn nuts, and for thrust washers in caliper brakes. Various special brass alloys are used as filler material for brazing.

A small fitting permanently attached to a frame. On traditional steel frames these attachments are held on by brazing, but the term "braze-on" is also used for fittings that are welded, glued, riveted or molded onto frames of other materials. Typical braze-ons include cable stops and guides, water bottle cage mounts, shift lever bosses, cantilever brake bosses and cable stops, pump pegs, etc.
Brazing is the joining of metal parts by melting a different metal (of lower melting point) which bonds the parts together. Typically, brazing involves joining steel parts with molten brass. Soldering is a similar operation, using lower temperatures and different filler metals (with a lower melting point). In brazing and soldering, the filler metal penetrates inaccessible areas of the joint by capillary action.

See also welding, fillet brazing.

Breaker Bar
An extension handle for a socket wrench, used to achieve high leverage and to loosen parts which have become corroded or otherwise are too tight to be loosened with an ordinary wrench. A breaker bar can be merely a length of pipe placed over the handle of a wrench, but only a non-ratcheting wrench should be used due to the risk of damage to the ratchet mechanism. There is still the risk of breaking a bolt, stripping a wench flats, or cracking the socket.
A randonnée to qualify a rider to enter a longer randonnée.

This is a French word, variously translatable as "diploma," "certificate" or "patent."

Bridgestone is an enormous multinational company, one of the largest tire companies in the world...and a fairly small bicycle company, with its own factory in Japan. In the late 1980s and early'90s, its U.S. bicycle division was run by Grant Petersen, a brilliant, talented and idiosyncratic designer.

Petersen, a hard-core cyclist, marched to a "different drummer" than most of the industry. He introduced many innovations to the market, and also strongly resisted other trends and innovations that he didn't approve of. He is now the head of Rivendell Bicycle Works.

Bridgestone bicycles are something of a cult item now. This site includes a whole subsection on Bridgestone bikes.

A combination brake/shift lever, such as a Campagnolo Ergo or Shimano S.T.I. unit. This term was coined by Bruce Frech.
The dents that sometimes develop in headset bearing races are often colloquially referred to as "brinelling" from the resemblance to the dents made by the Brinell Hardness Test.

Sometimes the term is (somewhat inappropriately) applied to damaged headset races.

See Jobst Brandt on "Indexed Steering"

Brinell Hardness Test
At the beginning of the 20th century. Johan August Brinell, a Swedish Metallurgist, proposed a rapid, standard method of measuring the hardness of metallic materials. The method as most commonly applied consists of indenting a surface with a 10 mm hard steel ball with a 3000 kg load for 30 seconds. The Brinell Hardness Number (BHN) is then measured by measuring the diameter of the indentation using a straightforward formula (or usually plucked off of a chart).

The load is reduced to 500kg for very soft materials, and the steel ball is replaced with tungsten carbide for very hard materials. Substantial section thicknesses are required to ensure that the test is "valid", that is, edge effects or gross section deformation remain negligible.

As a standardized test, the procedure is very useful in industrial settings, primarily as a quality-control tool. In common steels, as a rule-of-thumb, BHN 515 is approximately equal to the ultimate tensile strength.

The test is useless in the context of bicycles since almost no part of a modern bicycle would survive intact the effect of a 10 mm ball at 3000 kg and no part of a bicycle is sufficiently massive to ensure a "valid" test.

The BHN number, albeit a very useful tool in manufacturing, is not a satisfactory physical constant nor is it applied to design. Also, the BHN number is not reproducible under nonstandard ball diameters and loads unless geometric similitude is maintained.

Other hardness tests have largely replaced the Brinell test.

(This definition contributed by John Fox)

In the early days of bicycle manufacture, every industrialized country had its own system of thread sizes and standard dimensions for bicycle parts. The British system is the one that has, by-and-large, prevailed. British dimensions have mostly been adopted by the I.S.O. These standards include:
A maneuver consisting of locking the rear brake while turning, so that the rear wheel oversteers and the bike comes to a stop in a cloud of dust, while turning up to 180 degrees. Usually performed with the inboard foot off the pedal and extended inward to act as an outrigger.
The world's foremost maker of leather saddles. Brooks is based in England, and is one of the oldest companies in the bicycle industry. It was formerly also the leading maker of English-style touring bags.
Balanced Response System. Dia Compe's system of adding a weak return spring to the brake lever. Since the cable was being pushed at one end and pulled at the other, a positive return function could be attained with a much lower overall spring tension. This greatly improved the "feel" and sensitivity of the brake.
B.S.A. (Birmingham Small Arms) was a major British manufacturer of firearms, and later of bicycles and motorcycles. It reached its peak in the 1920's. The standard thread sizes that B. S. A developed for its bicycles were ultimately adopted as the standard British (B.S.C.) sizes, which, in turn, were mostly adopted by the I.S.O.
British Standard Cycle. The standard dimensions pioneered by B.S.A. were ultimately adopted by the bulk of the British cycle industry under this designation.
Bead Seat Diameter
BSX / Bicycle Supercross
Bicycle Supercross, an adult version of BMX, using mountain bikes on a downhill course similar to a BMX track.

Also known as "Four cross" or "4X" Mountain bike riders compete on a specially designed highly-challenging course. The races last between 25 seconds and one minute and are usually fast and frenzied. The courses are a mix of natural and man-made obstacles covering a steep descent.

The array of obstacles includes triples, doubles, table tops, step ups, drop offs, bermed or off-camber corners and gap jumps. The difficulty of getting over these obstacles at high-speed whilst being jostled by three other competitors means that there are plenty of crashes. The 4X competition starts with a limited number of riders competing in knock-out rounds.

The knock-out rounds can be decided by a series of heats called 'Motos' with riders competing three times before moving on to quarters, semis and final. The final consists of the last four riders left in the competition.

Matt Andrews

Bull-Horn Handlebar
See "Cow Horn."

Bull Moose Handlebar

A style of handlebar popular on early mountain bikes. The handlebar and stem are a single unit, with the stem splitting in two at the quill and attaching to the handlebar at two points, forming a triangle.
Bungee, Bunji, Bungie, Bungy...nobody knows how to spell it
An elastic shock cord with hooks on the end, commonly used for securing baggage to a luggage carrier. Also known as a "Sandow".
A manufacturing co-op, based in Eugene, Oregon. Burley has been a leading manufacturer of tandems, trailers and rain gear.
Busch & Müller
A German manufacturer best known for its "Nabendynamo" generator hub.
A hollow cylindrical part that connects two other parts, usually serving as a simple bearing. A bushing may have a stepped outer circumference to locate the outer part axially.
Thicker at the ends. Said of spokes and frame tubing. (Butted spokes are also called "swaged") Butted tubing is usually made with a constant outside diameter, but thicker walls at the ends. The idea is to make the part stronger at the ends, where the stresses are greatest, and lighter in the long middle section, where stresses are less.

Some writers have objected to this term being applied to spokes, and maintain that "swaged" is more correct, since the operation that produces a butted/swaged spoke is one of thinning the middle, not thickening the ends. For some reason, these writers don't generally object to the use of "butted" in reference to tubing, though the process is also one of thinning the middle, not thickening the end. This objection is based on a misunderstanding of the origin of the origin of the term "butted." "Butted" means having a butt, i.e. a thick end, and has no reference to the means of fabrication.

Down to next page


Spoke Divider

Articles by Sheldon Brown and others
Beginners Brakes Commuting
Fixed Gear
Frames Gears &
Tandems Touring What's
Wheels Sheldon

Accessories Bicycles Parts Specials Tools

Copyright © 1996, 2008 Sheldon Brown

Harris Cyclery Home Page

If you would like to make a link or bookmark to this page, the URL is:

If you would like to make a link or bookmark to this glossary, the URL is:

If you would like to make a link or bookmark to a specific definition, that's fine too.
I am committed to keeping the urls stable, so I won't be breaking your link.

Last Updated: by Harriet Fell