This question and its answers list the names of bike parts and cycling concepts.

Some Rules

  • Make sure you only put one term per answer!
  • Try to include an image if applicable
  • Include sources that contain detailed information
  • Add a link to the index in this question using edit.

Also, I made this a community wiki, so that anyone will be able to edit it, and to stop rep-hoarding

There's a handy reference at the Park Tool Co. website, a bike repair map; it's a diagram of a bike with all the parts labeled, and is very handy! At the moment, the diagram is up at parktool.com/blog/repair-help. (They've changed the URL in the past, so this link may break.)

A road bike has the following parts (source):

enter image description here

A mountain bike has the following parts (source): enter image description here

Edit: This page is meant to identify what things or concepts are (as per this thread in meta). If you want to recommend an accessory or a specific product you've found handy, please use the accessories page.

  • 7
    One term per answer please - would be beneficial.
    – dotjoe
    Aug 26 '10 at 13:51
  • 1
    Is there a way to link to a specific answer, so that in future questions you can use one of these terms and link to it for reference?
    – Kevin
    Aug 26 '10 at 16:13
  • 2
    Kevin: Under the bulk of the answer, there is a 'Link' hyperlink, which will link to the answer (its right above comment) Aug 26 '10 at 17:12
  • 7
    @MarkIngram: useful things that aren't actually questions are what community wikis are for.
    – freiheit
    Sep 12 '10 at 17:47
  • 2
    This page could use some more love. Maybe if people keep linking to individual terms here, more people will know about this page. Jun 27 '11 at 2:39

81 Answers 81

1 2

Chain guard/Chain cover

It's a frame, usually made of plastic or metal, that covers the entire length of the chain or only the upper part, mainly for protecting the rider from the dirt and lubricant on the chain, but can also protect the chain itself.





This is the distance (in millimetres) between the outside faces of your crank arms. Effectively, a larger Q factor means your pedals are further away from the bike's center line, and therefore your feet are further apart.

A narrow Q-Factor is considered more aerodynamic, but increases the risk of chaffing the inner thigh on the nose of your saddle. Likewise a wide Q factor means your knees and thighs are opened up, which may help joint comfort. Going too wide will make joints painful.

Q-factor can be increased by adding extenders, or using pedals that have longer shafts.

enter image description here

A pedal with an extender added. Imagine that screwed into your crank - the pedal will end up further "outboard" from the bike's centerline.

Q-factor can only be reduced by changing the cranks for narrower ones, which increases the possibility of heel/shoe strike on the crankset or chain or front derailleur.

Note - q factor is NOT measured from the centerline because it could be different on each side.

Edited version of previous pic from https://electricbikereview.com/forum/attachments/bicycle-q-factor-jpg.21791/

  • 1
    I'm not sure I get this correctly: "Q-factor can be increased by adding extenders, or using wider pedals." -- But in the diagram I see that Q factor is not influenced by pedal width.
    – Robert Lee
    Sep 24 '19 at 6:58
  • 1
    @RobertLee fair point - how's that edit? Basically the extender is a short bolt, with another thread in the end that effectively makes the pedal's shaft longer / have more stick-out. Downside, more leverage on the threads because its now a longer lever. I only drew one extender, normally they'd be fitted in pairs, but variation in people might make fitting a single appropriate, perhaps their hips are not symmetrical. A bike fit would show up differences like that.
    – Criggie
    Sep 24 '19 at 9:08
  • To @RobertLee's point, perhaps the title should be changed to Q-factor and Stance Width. The first sentence says Q-factor is the distance between the outer faces of each crankarm. The 3rd sentence says it can be increased by adding pedal spacers. Those two contradict, actually - if we take the first sentence, the only way to get a different Q-factor is different crankarms.
    – Weiwen Ng
    Sep 25 '19 at 10:10
  • 1
    "Q-factor can only be reduced by changing the cranks for narrower ones, which increases the possibility of heel/shoe strike on the crankset or chain or front derailleur." Can't it also be reduced by a shorter BB spindle (square tapered type for example)?
    – Robert Lee
    Sep 25 '19 at 17:14
  • 1
    @Criggie I feel this is missing one of the most vital pieces of information about Q-factor: The "Q" stands for "quack", a reference to the wide stance and waddling gait of ducks. Something I was told many years ago and assumed was probably true, now validated on Wikipedia with a proper source - en.wikipedia.org/wiki/Q_factor_(bicycles)#cite_note-3
    – DecSim
    Oct 9 '19 at 21:59


Play is a technical descriptive word for sloppiness in joints:

Play is movement in a mechanism that results from one or more moving mechanical parts where the fit is less than perfect.

Also known as "backlash" though that term is more common in a drivetrain or chain system.

Play also increases over time as mechanical parts like axles, shafts and pins wear and bed into their mountings.

On a bike:

  • play in a rear derailleur would lead to decreased shifting quality.
  • play in a wheel bearing would allow the rim to move sideways, affecting the ride and braking, and wearing bearing surfaces faster.
  • play in a headset means steering is worse, and hard front braking can lead to judder.

Opposites: Tight/stiff/fine +tolerances, precise.
Synonoms: Slop, wear, wobbly, loose, janky, worn, worn out.

enter image description here

Curiously, an elongated chain (stretched) is a fine example of play on a bicycle, but that's the one thing rarely referred to as play or backlash.

Further info (not bicycle specific) https://en.wikipedia.org/wiki/Backlash_(engineering)



This means a bike that has exactly one rear cog, and cannot change gear. Very similar to a fixed-gear bike except a single-speed has a freewheel mechanism to allow coasting, i.e. riding along without pedalling.

Compare with Fixed-Gear.


Rim Tape

For clincher type rims and tires with inner tubes: tape applied to the inside of the wheel rim to protect the inner tube from sharp edges and the ends of the spokes and spoke nipples, which would otherwise abrade the tube and cause punctures.

For tubeless clincher rims and tires: tubeless rim tape covers the spoke holes and seals the inside of the rim. The tape aids in keeping the system airtight. Thus, rim tapes made specifically for tubed clinchers, like cloth tape, will not work as tubeless rim tape. However, tubeless rims with tubeless tape can still accept inner tubes. In fact, riders with tubeless tires will often carry a tube for emergencies.

Tubular rims do not use rim tape in the same manner as clincher rims. Tubular tires themselves are an airtight unit. However, some cyclists use tubular glue tape in place of liquid glue, or as a supplement to glue.


Dropper Posts

Dropper posts are more commonly found on mountain bikes and allow the rider to quickly lower or raise the saddle height using a remote lever (mounted on the bars). Cable-operated and hydraulic-operated designs exist and the cable or hose can be routed either internally or externally to the frame (as the frame allows). A small release lever is fitted to the handlebar. Wireless designs also exist, including SRAM AXS where the left shift paddle can be set up as the dropper controller in a 1x configuration.

dropper post
(source: evanscycles.com)

There are many brands of dropper posts, not limited to:

  • RockShox
  • Fox
  • Thomson
  • 1
    Maybe we don't need to list manufacturers in these wikis, we can't list them all and makes can go obsolete, we want the entries to be as enduring as possible
    – Swifty
    Sep 25 '19 at 10:31
  • Excellent! Want to add an entry for "seatpost" too? We don't have one listed. Could be worth adding why a MTB rider might want to lower their saddle, without stopping.
    – Criggie
    Sep 25 '19 at 10:32

Stack Height

The stack height of a headset is the vertical space taken up by a headset, and the stem when using a threadless headset. It's the difference between the headtube length and the fork steerer length needed to be able to use that headset with that fork and headtube.




BCD (Bolt Circle Diameter)

"Bolt Circle Diameter or BCD is the diameter of the circle that goes through the center of all of the bolts on your chainring. On bicycle chainring this dimension is usually measured in millimeters. It is critical to know the BCD of your crankset when you are selecting a new chainring for your bike."

BCD illustration

If the BCD is not written on the chainring, it must be found another way. One method is to compare the chainring directly with another one of known BCD, or against a scaled reference drawing.

Alternatively, the distance between the centres of adjacent holes/bolts is measured and this value read into a look-up table to find out the BCD. This can be done for 3, 4 and 5 bolt designs, meaning the BCD can be measured without removing the chain ring.

For a symmetrical, four bolt arrangement, measuring BCD is trivial with the use of a caliper, as seen below. The BCD of a chainring can similarly be measured directly with a ruler when the chainring is removed and the crank is not an obstruction.

Measuring BCD on a symmetrical four bolt design

More recent, asymmetric 4 bolt designs from Shimano are an exception not included in look-up tables, because the bolts are not spaced evenly around the BCD.

Quotation and photograph from: https://www.wolftoothcomponents.com/pages/how-to-measure-bolt-circle-diameter-bcd

  • Typical/common sizes would also be helpful.
    – TLW
    May 6 at 1:26


Chainsuck happens when the bicycle chain fails to disengage from the teeth of a chainring—usually during a shift—and wraps back up and around the chainring.

enter image description here

Citation and image from: http://reviews.mtbr.com/workbench-how-to-un-suck-your-chainsuck
See also: What causes chain suck?


Derailleur or derailer

Derailleurs enable bicycles to use multiple gears. Before derailleurs, bicycles often had one gear mounted to either side of the rear hub, and riders would stop and then flip their rear wheel around to change gears. Derailleurs enabled riders to change gears while riding by "de railing" the chain from one cog to the next. By enabling multiple gears, they significantly increased the range of terrain that bicycles could cover.

Derailleurs were first invented in the late 19th century. Early derailleurs used rods to push the chain onto different cogs. Tullio Campagnolo played a major role in the development of modern derailleurs, having commercialized a successful version in 1949.

At the rear wheel, derailleurs have steadily increased the number of cogs. At the time of writing, 11-speed systems are common on performance bicycles of all types, and 12-speed systems are being introduced for both road and mountain high performance bikes. Cheaper bicycles tend to have 8 to 10 rear cogs. Increasingly, rear derailleurs designed for off-road use incorporate a clutch, which prevents the derailleur cage from moving while the bike is bouncing on rough terrain. This prevents the chain from slapping the chainstay, and makes it less likely that the chain will come off the front chainring.

At the front, derailleurs can handle two or three chainrings. As the number of speeds at the rear increased, most road bikes settled on two chainrings. However, an increasing number of bicycles, especially mountain, cyclocross, and gravel bikes, come with single front chainrings and no front derailleur. This is called a 1x, pronounced "one-by", drivetrain. On rough terrain, it is possible for a derailleur to drop the front chainring, possibly damaging the frame and interrupting pedaling. A well designed 1x system will eliminate this problem. 1x chainrings have specially designed teeth to ensure chain retention, and will use wider-range rear cassettes. 1x systems simplify shifting and remove the need to adjust the front derailleur. They do lead to greater gaps between gears.

Some research estimated that a 1x drivetrain should have just under 3 watts more drivetrain friction than a 2x drivetrain at a rider output of 250W. This increase is small is partly counterbalanced by the decreased aerodynamic drag from eliminating the front derailleur.

Derailleurs have been actuated by cables for most of their existence. As alternatives to cables, Mavic pioneered the Zap and Mektronic groups in the early 1990s. These were operated electrically, but were deemed too unreliable for regular use. In 2009, Shimano introduced Dura Ace Di2, which was the first commercially successful electronically-operated groupset, and other manufacturers followed suit. These systems are not in general use due to high cost. They do require the bike to mount one or more batteries. Alternatively, Rotor introduced a groupset where the derailleurs are actuated by hydraulic lines instead of cables, but this system has yet to see widespread use.

General alternatives to derailleurs are hub gears, which contain the gearing mechanism inside the rear hub. As another alternative, one can ride a single speed or fixed gear bicycle, both of which have only one or two gears (the second gear, if present, would be mounted on the opposite side of the rear hub).

You can adjust your derailers yourself!

Both front and rear derailleurs are adjustable by high (H) and low (L) limit screws. The limit screws control how far the derailleur can move, so that it does not shift the chain off the large cogs into the spokes or off one of the front chainrings.

In addition, rear derailleurs have a cable tension adjuster, which accounts for the fact that cables stretch after some use. They also have a screw (the B-tension screw) which moves the derailleur angle in relation to the rear cassette. Rear derailleur operation can be compromised if the derailleur hangar, or the plate that a rear derailleur bolts to on the frame, is bent. Hangars can be replaced entirely or bent back, depending on material.

The following shows how to adjust cable-operated front and rear derailleurs and is intended for general information. Adjustment may seem intimidating at first, but it can be learned easily. NB - Before any rear derailleur adjustment, first ensure the derailleur hanger is aligned correctly

Chain Gap Adjustment (B screw)

  1. Shift the rear derailleur to the largest cog.
  2. Ideally, using a chain gap gauge, adjust the B screw until the top derailleur jockey wheel aligns with the teeth of the largest cassette cog. If you do not have a chain gap alignment gauge handy, aim for the gap to be between 10-15mm* (*refer to your groupset provider)

Rear Derailleur Adjustment

  1. Shift to the smallest cog on the rear cassette (this is the natural resting position for the derailleur when no cable tension is applied)
  2. For cable operated derailleurs, remove all tension from the rear derailleur by loosening and/or removing the cable
  3. Use this opportunity to wind the barrel adjuster on the rear derailleur shifter all the way in, then back it out 1 - 2 turns.
  4. Adjust the high limit (H) screw so that the centre of the top pulley wheel aligns with the outboard edge of the smallest cassette cog
  5. For cable operated derailleurs, re-attach the rear derailleur cable, pulling taught, but not under extreme pressure, and tighten the cable bolt.
  6. Shift to the largest cog on the rear cassette
  7. Adjust the low limit (L) screw so that the centre of the top pulley wheel aligns with the outboard edge of the largest cassette cog
  8. Fine adjustment can now be made via the barrel adjust on the shifter

Front Derailleur Adjustment

  1. Adjust the derailleur at its mounting bolt so that there is a clearance of 1 - 3mm between the derailleur outer plate and the largest chainring
  2. Tighten the clamp bolt, but do not torque to spec
  3. Shift to smallest cog on the rear cassette the largest chainring
  4. Adjust the high (H) adjustment bolt and align the front end of the derailleur outer plate parallel to the surface of the largest chainring
  5. Adjust the derailleur by rotating the mount so that the rear portion of the outer plates is 0.5 - 1mm inside the outer chainring
  6. Tighten the derailleur clamp bolt and torque to spec
  7. Shift to the largest cog on the rear cassette and the smallest chainring
  8. Adjust the low (L) adjustment bolt so that the clearance between the skid plate of the derailleur and the chain is 0 - 0.5mm

Shimano Dura Ace Front Derailleur Deore XT Rear Derailleur

  • Can you source the contentions in your Considerations section? I don't see how 1x necessarily provides greater gear range than a 2x - you can certainly get equivalent high and low gears on 1x and 2x, but 1x doesn't unconditionally mean a wider range. The average chainline on a 1x system could be worse than on a 2x, because your high and low gears are more cross chained. You'd do better to say that 1x should have fewer chain drops (from the front chainring(s)) on rough terrain than 2x.
    – Weiwen Ng
    Oct 8 '19 at 17:53
  • A typical 11 or 12 speed cassette on a specific 1x drivetrain is as narrow, or narrower than a 10spd/11spd cassette used on 2x. In addition, the crank chainring is spaced in a manner that would that would fit where a traditional middle ring would be, eliminating cross chain even further. SRAM 1x systems typically offer a 500% gear range and I was noting that the flexibility in that range comes from not having to shift to a larger front cog.
    – Lucero79
    Oct 9 '19 at 9:16
  • In a 1x system's lowest gear, the chain angle is larger than in a 2x system's lowest gear, because of the change in the chainring placement that you noted.
    – Weiwen Ng
    Oct 9 '19 at 15:46
  • Except, a modern rear hub on a 1x would be greater than 135mm often found on 2x systems, meaning the lowest gear is further outboard on a 142mm or 148mm hub. That would mean lowest gear would align far better with the front chainring than on a 2x system.BB shell width would also play a factor. There are too many variables to generalise that chain angle on a 1x is in any way adverse to a 2x system
    – Lucero79
    Oct 10 '19 at 12:02
  • If there are too many variables to generalize that a 1x system's chain angle is worse than a 2x, then I'd argue that the converse is true. One also can't argue that 1x leads to better chain angles. I have seen no literature making that statement. Also, aren't modern road bikes moving to 142mm at the rear anyway?
    – Weiwen Ng
    Oct 10 '19 at 13:36

Tyre Clearance or Tire Clearance

A measurement of the gap between the outer surface of your tyre/tire and the non-wheel parts of your bike.

Smaller gaps are better for aerodynamics, but larger gaps are needed for mud clearance and MTB/cross riding.

Narrow (or negative!) gaps will impact the life of your tyre and will slow you down, potentially wearing through your frame's paint and worst case wearing away your bike frame structure. The photograph below illustrates insufficient tire clearance.

A better, longer, and illustrated answer has been posted here What does 'tire clearance' mean?

enter image description here

  • 1
    I am surprised that a smaller gap should be aerodynamically better, which is also not explained in the linked article. Could someone explain (here or there), there must surely be a lower limit?
    – PJTraill
    Jul 15 '18 at 22:03
  • @PJTraill This might be better in Bicycles Chat, but in brief, the tyre creates a substantial circular airflow while spinning. Test by spinning up the back wheel of your bike while its off the ground, then put a hand near the fast moving tyre. Its much worse on a MTB knobbly, but even a smooth road tyre as pictured will create a draught. The tight clearance helps reduce the two airflows meeting head on - if anything the rear airflow should help push the bike and the airflow from the front should be smoothly diverted to the sides, rather than meeting and being choppy air. Like a splitter on a car.
    – Criggie
    Jul 16 '18 at 9:02
  • @PJTraill the lower limit would be touching - if your tyre rubs your bike then that's wasted watts, and it will wear out the tyre and the rubbed point, which is also bad for your bike. Since all bikes have some amount of flex, the wheel will have some slight movement requiring space to prevent rubbing. That's the lower limit. Anyway - do consider joining the Bicycles Chat.
    – Criggie
    Jul 16 '18 at 9:05

Axle Nuts

An older method of holding a wheel into dropouts.

Compare with Through Axles or with Quick Release/QR

enter image description here

This image also shows a retention washer with a hooked retainer.

enter image description here

Some particularly vintage wheel wingnuts. These could have been on a racer, or used so the rider didn't need a tool.

enter image description here

Vertical view showing the nuts on either side of the wheel, and how the axle and dropouts are organised.

  • 1
    It will take a thief slightly more time, and will need a wrench, to unscrew the nuts and steal your wheel; one small advantage over quick release types, perhaps.
    – Robert Lee
    Jun 15 '19 at 22:29

REI Recreational Equipment Inc is an American chain-store that specialises in outdoor goods, sporting equipment, and outdoor clothing.

The chains is somewhat unusual, because it is a co-operative. There's an annual fee of $20 USD to maintain your membership.

REI may be thought of as a mega-LBS because they have a specialist bike department and staff who ride, and bike mechanics on site.

Contrast that with a "generic box-mover" shop who may not even assemble your bike correctly.

More details at the Wikipedia article

Company website is https://www.rei.com/

  • 3
    You can shop at REI even if you're not a member. you get some rewards and voting abilities if you are a member.
    – Batman
    Aug 6 '16 at 13:20


Quote and image from Sheldon Brown's Bicycle Glossary:

The shaft at the middle of a bearing. There is some controversy as to whether "axle" or "spindle" should be used in particular contexts. The distinction is based on whether the axle/spindle is stationary, as that in a hub, or rotates, as that in a bottom bracket. There have been bitter flame wars fought in magazine letters columns over this point.


See questions about axle


Coaster Brake

(a.k.a foot brake, or pedal brake.)

A coaster brake is a special rear hub for a bicycle, which performs two functions:

  • It allows the bicycle to roll without forcing the pedals to turn. This is the "coaster" part. It is similar in function to a freewheel , but uses a different sort of mechanism to accomplish it.

  • It is also a brake, operated by turning the pedals backwards.

Coaster brakes were invented in the 1890s, and have continued to be popular in some areas to this day.

Source: https://sheldonbrown.com/coaster-brakes.html

coaster brake hub

A video explaining the internal mechanism of a coaster brake:

See questions about coaster brakes


Bidon. Aka water bottle.

A typical bidon.

Specifically designed to securely fit into bottle cages on the frame. Generally hold 500-600ml of water, electrolyte solution or similar depending on rider preference to provide hydration during a ride.

See here for more, Cycling Weekly article.


Suspension Systems

Suspension allows the fork and/or the rear triangle to compress when a rider goes over a bump. This cushions the blow, delivering a more comfortable ride. It can also help the wheels maintain traction in rough terrain. Suspension seatposts and stems are also available. Typically, road bikes lack suspension, although wider tires run at lower pressure do deliver some level of vibration and shock damping. In contrast, mountain bikes operate on rougher terrain, and they have at least front suspension. At the time of writing, most gravel bikes lack traditional suspension.

Suspension forks and rear suspension shocks are available from many different manufacturers including, but not limited to:

  • RockShox
  • DVO
  • FOX
  • MRP

Suspension systems can be sprung by:

  • Coil-sprung shocks use a large metal spring – normally steel – positioned outside the telescopic tubes that make up the shock body.

  • Air-sprung shocks meanwhile feature a compressed air spring inside the body of the shock.

  • Elastomer-based systems rely on rubber or another elastic material to compress. These typically offer less travel than coil shocks, but are lighter.

Front Suspension Fox Front Forks
Typically, in Cross Country (XC), All Mountain (AM), Enduro or Aggressive Hard Tail frames (ie, those that have no rear suspension), front fork travel can range from 100mm to 160mm travel dependent on the frame geometry.

Modern forks offer a lockout (also available as a remote lever lockout, mounted to the handlebar) which allows the rider to quickly disable the fork travel. This is useful when riding on road to provide a rigid platform so as not to unduly waste rider effort.

Rear Shock

Rear shock

Typical rear shock travel for mountain bikes ranges from 100mm to 200mm depending on the frame discipline with Down Hill (DH) bikes offering the most travel (+/- 200mm front and rear).

Previously, some bikes had a soft-tail suspension, with a shock unit placed where the seat stays attached to the seat tube. These typically had smaller travel.

Similarly to front suspension, some rear shocks offer a travel lock out which limits the rear shock travel, useful for riding on tarmac or more predictable surfaces.


BSD (Bead Seat Diameter)

The working diameter of a wheel rim, for the purposes of tyre sizing.

If a bike has a BSD of 622 mm then the required tyre will have an inner diameter of 622 mm, and a suitable inner tube will be sized to fit 622 mm tyres.

Common modern BSD sizes include 622 mm (road, 29er mtb), 584 mm (27.5" mtb) and 559 mm (26" mtb)

(Explain how ETRTO makes life simple?)


Lateral Tube (on a frame)

In a tandem, the Lateral Tube is an optional reinforcing tube that is not the top tube, the downtube, nor the keel tube.

From https://www.rodbikes.com/articles/tandem-designs/direct-internal.gif

For tandems the lateral tube is normally a single. However a frame may have two smaller tubes, with one on each side. This allows a continuous tube to go around the seat tube rather than two ends being welded to the seat tube.


Some tandems lack a Lateral tube, and simply have a quadrilateral space under the stoker. Some have a Lateral tube and no top tube. Some may even use a single large tube to provide an easier step-through design.

The Lateral tube has a parallel on a Mixte frame where the tube or tubes continue down to the rear axle.

By Rwendland - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=21142240

  • This answer could be improved by providing the correct name for the twin tubes on the mixte frame, either as its own entry or inserted into this one. I don't know whether they're top tubes or a pair of laterals or some other name.
    – Criggie
    Dec 28 '20 at 9:31

Keel Tube

In a tandem, the Keel Tube is approximately horizontal, and joins the front bottom bracket to the rear bottom bracket.

This part does not exist on a double-diamond bike frame.

From https://www.rodbikes.com/articles/tandem-designs/direct-internal.gif

One might consider a recumbent bicycle frame to have a keel tube as well, but generally recumbent frames have only one main tube doing all the work, so it doesn't get a special name.


Boom / Boom Tube

Any frame part of a bike that is in front of the head tube, excluding carriers or decks for cargo.

A Boom is almost always found on a recumbent or semi-recumbent frame where the bottom bracket is in front of the head tube. Pictured is a Tricycle recumbent, though the same terminology applies to a two-wheeler recumbent bicycle as well.


A Boom allows the frame to be adjusted for a rider's leg length. As such, boom adjustments will almost always require a chain length adjustment unless very minor, as well as front derailleur tweaks. A device called a "chain gobbler" can be fitted to recumbents who change riders a lot, like rentals, though they add about as much drag as a chain tensioner or derailleur.

The Boom Tube is normally a round, occasionally a square or profiled tube of 2-4 inches (50-100mm) diameter. There is a slot underneath and a couple of pinch bolts, exactly like a seatpost clamp from the 80s or earlier. A QR probably won't generate enough clamping force here.

The Boom itself is a T shaped tube that is just small enough to slide into the Boom Tube and be clamped onto by the pinch bolts. At the other end is a conventional bottom bracket ready to accept a BB axle or cartridge, and a crankset.
The boom will probably have a short 6" or 150mm stub tube that replicates the mounting for a conventional front derailleur, and may provide a mount for a front light.

The Front Derailleur control cable may be internal though the boom, or externally routed. For booms that get adjusted a lot, an external housing makes sense.

A crank-forward bike or Semi-recumbent generally still has its cranks behind the front wheel, so does not have a boom.

1 2

Not the answer you're looking for? Browse other questions tagged or ask your own question.