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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.


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  • 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

89 Answers 89

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Tyre Clearance or Tire Clearance

Many bikes will state the maximum tire size the frame and fork have room to fit. For example, 2020-2021 gravel bikes can typically fit at least 40mm tires, and many 2020-2021 road bikes can fit as much as a 32mm tire. There is often some wiggle room, because tires' actual width can vary slightly from their nominal size (i.e. the size printed on the sidewall). This can be due to manufacturing variations, but tires will get larger as the rim's internal width grows, and bike manufacturers do not know what rim size their bikes will be used with.

On road bikes, manufacturers typically aim for at least 4mm clearance on all sides of the tire, measured at the narrowest part of the fork or the rear of the bike. Off-road bikes should aim for more clearance than this. The photograph below illustrates insufficient tire clearance between the arch of the brake caliper and the fork crown, i.e. there isn't enough vertical clearance. You also need to check horizontal clearance.

enter image description here

If you have too little clearance, then if dirt or mud accumulates, it will abrade through your paint and possibly through the frame itself. A better, longer, and illustrated discussion is here. Note that if you don't have measuring calipers, you can use allen wrenches to check your current tire clearance, i.e. try to fit a 4mm allen wrench between your tire and frame.

Some aerodynamic road or time trial bikes are designed for minimal clearance between the back tire and the frame. They may have cutouts in the seat tube, or a curved seat tube. A similar arrangement is also possible in front.

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  • 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
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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 and other manufacturers are an exception. While they have BCDs, the crankarms are not spaced evenly. If you are looking for a third-party chainring for these cranks, the manufacturer will state which brand the rings are compatible with (and sometimes, which models, e.g. Shimano R9100 and R8000 vs 9000 and 6800). Alternatively, some cranks are direct mount, i.e. the chainring is made with the spider and bolts to a splined interface shared with the crankarm. These chainrings will state which interfaces they are compatible with, e.g. Praxis or SRAM/Quarq.

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

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

Power meters measure your power output. This StackExchange question has more in depth discussion, but briefly, they enable structured training (where you do repeated efforts at various intensity levels). They aid in pacing an effort. They are ubiquitous in professional road racing at the time of writing. Very few amateur cyclists need one to progress satisfactorily, as it is possible to do un- or informally-structured training or to train with heart rate. However, power meters can benefit many riders. Power is better than heart rate for pacing an effort because heart rate will take several seconds to respond when you start an interval, and it will likewise take several seconds to come down when you stop an interval.

For structured training, riders estimate their functional threshold power (FTP). This is the maximum power you can sustain for 45-60 minutes, when your body is relying mainly on aerobic (as opposed to anaerobic) processes to generate energy. You can then base intervals off your FTP, e.g. warm up, ride 15 minutes at 93-100% of FTP, recover, repeat once, cool down. Riders typically re-estimate their FTP several times during the season. There are several protocols to do this. A common one is to warm up, then ride a 20 minute maximum effort, then take 95% of the average power during this effort.

A brief history and basic technical information

Power meters require a strain gauge, which measures the amount of deflection in a bike component, as well as a cadence sensor. This answer has more information on how power meters work physically. They transmit this information wirelessly to a computer, typically over Bluetooth or a sport-specific protocol called ANT+. The data files can be analyzed by a coach and uploaded to a training site like Strava.

The first power meters were made by SRM (Schoberer Rad Metrik, a German company), and they were very expensive. As with many electronic goods, they have declined substantially in price in recent years. SRM power meters replaced the chainring spider, and several other power meters are still mounted here. Later power meters were mounted in hubs (e.g. PowerTap, now owned by SRAM, pictured below), one or both crankarms (i.e. without replacing the spider entirely, Stages was probably the first), or the pedal spindles. Less commonly, companies have mounted power meters in the crank spindle. E-bikes also measure power to determine how much assistance to provide.

Image of power meter

Power meters mounted at the crank spider or in the hub will measure a rider's total power, but they do not directly measure the amount produced by each leg. Power meters mounted on the crankarms or pedal spindles are capable of actual dual-sided measurement, but manufacturers commonly offer left-only versions of these power meters as a low cost option. Left-only meters measure power at the left leg and double it. This is presumably the weaker leg for most riders, and hence it produces a conservative measure of power. The advantage to true dual-sided power is that you can determine if you have a muscular imbalance, but it isn't clear how critical this is.

Power is key to virtual cycling environments like Zwift, where you simulate a rider's progress through a course and interactions between multiple riders. Virtual cycling programs can accept readings from a power meter. A traditional trainer can be used to estimate power as well, although this is less accurate than an actual power meter. Last, smart trainers measure power at the trainer's flywheel, and they can vary the flywheel resistance to simulate changes in gradient on the course.

Reference: Wikipedia

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  • @DanK I was thinking that SRM was a German company, and some wires got crossed in my head, and I made the rather amusing typo about Metrigear being a country that you caught. Thanks!
    – Weiwen Ng
    Sep 27 '19 at 14:04
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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.

http://commons.wikimedia.org/wiki/File:Bike_chain_guard_full.JPG

http://upload.wikimedia.org/wikipedia/commons/thumb/6/69/Bike_chain_guard_part.JPG/320px-Bike_chain_guard_part.JPG

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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

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Single-speed

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.

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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.

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Axle

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.

axle

The image above is a hub axle. This hub takes cup and cone bearings, and the balls are in contact with the cones. The locknuts adjust the preload, I.e. how ‘tight’ the hub is. Too much or too little preload will damage the bearings. The other common axle on bikes is attached to the crank or bottom bracket, although this is often called a spindle by convention.

See questions about axle

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Play

Play means unwanted movement in a system. It can be caused when one or more moving mechanical parts do not fit as specified. This can be caused when manufactured parts are out of the specified tolerances, e.g. one part is too small.

In some cases, play can develop if parts were in tolerance to begin with but they wear excessively. For example, older rear derailleurs may wear to the point where the pivots are sloppy, which decreases shifting quality.

Alternatively, some systems need to be tightened down to avoid play. For example, headsets need to have their top caps tightened, or the steerer tube will rock fore to aft. Some cranksets and hubs will have lateral play if not tightened. This is called adjusting the preload of the system, and is discussed under bearings.

Opposites: Tight/stiff/fine +tolerances, precise.
Synonyms or similar concepts: Slop, wear, wobbly, loose, janky, worn, worn out.

enter image description here

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Q-Factor

Q-factor is the distance (in millimetres) between the outside faces of your crank arms. It is one component of your stance width. The term may originate as a contraction of “quack factor”, originating with Grant Peterson, an engineer formerly at Bridgestone Cycles. A larger Q-factor, all else equal, means that your pedals are further away from the bike's center line, and therefore your feet are further apart. The diagram below illustrates.

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

Ultimately, when you are pedaling, you want your feet to be moving straight up and down. A stance width that is too wide or too narrow can make your knees depart from this ideal path, e.g. they might wander outwards at the top of the pedal stroke. This can eventually cause knee problems.

Q-factor is an inherent property of your crankset. Generally, road cranksets have Qs around 146mm and MTB cranksets have Qs at least 160mm (wider for Boost versions). Within drop bar bikes, gravel-specific groups (e.g. Shimano GRX or SRAM AXS Wide) have slightly wider Qs than the road-focused groups.

There are many options to adjust stance width. The picture below shows a pedal extender, which usually adds 10mm on each side to the stance width. You may be able to add 1-2mm washers to a standard pedal. Most pedal manufacturers use 52-53mm as their default pedal width, and many offer axles that are 4mm longer than standard. Last, you can move your cleats laterally on their mounting points, usually by about 2-3mm from the centerline.

enter image description here

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

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  • 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
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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.

http://www.parktool.com/blog/repair-help/fork-column-length-and-sizing

http://sheldonbrown.com/gloss_st-z.html

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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.

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  • 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
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Chainsuck

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?

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Bidon. Aka water bottle.

A typical bidon.

Specifically designed to securely fit into bottle cages on the frame. Generally hold 500-750ml (22-26oz) of water, electrolyte solution or similar depending on rider preference to provide hydration during a ride. Bidon is the French term and is likely to be understood by road cyclists.

See here for more, Cycling Weekly article.

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Derailleur or derailer

Derailers enable bicycles to use multiple gears. The word's origin is French, and it is frequently spelled derailleur in English. Before derailers, 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.

Almost all derailers are actuated by shift cables. Because cables stretch a bit over time, new bikes require a minor adjustment after a few weeks' riding. While there is a bit of a learning curve, it is possible to adjust your derailers yourself, a process outlined later. Last, the rear derailer is mounted to the frame with a derailer hangar, usually replaceable and made of aluminum. Even a minor drop can bend the hangar, which will affect shifting. However, the hangar is designed to break before anything more valuable, i.e. the derailer itself or the frame, does.

The shift cables and housing are the main wear parts in derailer drivetrains, as they will accumulate contamination over time. Very active riders may replace these as often as annually. In many cases, poor shifting can be rectified by changing the cables. The Delrin jockey wheels are the other common wear part on the rear derailer, but the replacement interval is much longer. If you can't find an OEM replacement, you should be able to find an aftermarket one.

Internally geared hubs as the main alternative

Internally geared hubs (IGHs) are an alternative system. The rear hub contains all the gears and shifting mechanisms. These are much less vulnerable to contamination than traditional drivetrains, but they have slightly greater drivetrain friction. Some IGHs may use drive belts rather than chains. Note that drive belts are not compatible with traditional derailers. IGHs are sealed from the elements, although some main require periodic lubrication through a port. This answer discusses more.

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).

Recent developments

The number of cogs on the rear wheel has increased with time, and the gear ranges available have become a lot wider. On road bikes, this has mostly led to the demise of the triple crankset.

Modern mountain bikes have switched to 1x (pronounced one-by) drivetrains, dropping the front derailer entirely. This has enabled wider tires and improvements in the rear suspension. 1x systems are becoming more common on gravel bikes, but they are not widespread on road bikes as of late 2021. On a 2x drivetrain, if you drop the chainring to the inside, you can often recover by shifting to the big ring and pedaling (and the reverse if you drop the chain to the outside). 1x drivetrains do not have this option. They use 1x-specific chainrings with narrow-wide teeth, i.e. the teeth alternate between narrow and wide teeth to retain the chain more firmly. They also incorporate clutches in the rear derailers, which reduce the amount of chain slap on rough terrain.

Electronically actuated drivetrains have become popular as a high-end option as well. These do away with metal shift cables entirely. They also have potential for adaptive cycling, as you can actuate shifts with a lot less finger force and you could set up the system for one-handed operation. However, they are considerably more expensive than mechanical shifting.

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

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  • 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
3

Suspension Systems

Suspension allows the fork and/or the rear triangle to compress when a rider goes over a bump. This helps the wheels maintain traction in rough terrain and improves rider comfort. Suspension seatposts and stems are also available for rider comfort, but they don't assist with traction.

On road and most gravel bikes, the tires deliver sufficient suspension for the terrain these bikes traverse. In contrast, mountain bikes are designed to excel on rougher terrain. Almost all MTBs have at least front suspension, aside from those who elect to run rigid MTBs. The image below shows a sample of suspension forks.

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.

Some key concepts 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.

Some key terms are:

  • Travel: the maximum distance which the suspension can travel. Very roughly speaking, more enables you to handle rougher terrain.

  • Bottom out and top out: When you hit a bump, if the suspension system reaches the limit of its travel, it has bottomed out. If you have too many harsh bottom outs, you need more suspension and you also risk damaging the suspension. Conversely, a suspension system will rebound after taking a hit, and top outs occur when it hits its travel limit on the rebound. Suspensions may have rubber bumpers inside the suspension chambers to prevent damage if it bottoms out.

  • Sag: When you sit on the bike, your weight compresses the suspension. This is called sag. Suspension systems can be tuned for riders of different weights. Notably, suspension manufacturers may recommend that you set the damping such that the sag is a certain percent of the total travel.

  • Compression damping and rebound damping: You can adjust suspension, e.g. by adding air pressure, to adjust its feel while compressing. More compression damping means more resistance as the fork compresses, which feels firmer. You can similar adjust the amount of rebound damping.

Maintenance

Suspension systems require periodic maintenance. Riders should wipe their fork stanchions, or the metal legs, after each ride, and inspect for scratches. Periodic higher-level maintenance may involve replacing the fork seals, which keep dirt out of the suspension chamber, lubrication, possibly rebuilding springs and replacing bushings.

Newer cyclists riding only on paved paths should consider foregoing a suspension system. The prime reason is that suspension is not necessary for this use case, and furthermore a suspension fork requires maintenance and will eventually seize without maintenance - at which point it is dead weight that you paid extra for.


2

BSD (Bead Seat Diameter)

A measure of the diameter of a wheel rim, for the purposes of tyre sizing. This is the vertical distance from the center of the hub to the shelf in the rim where the tire beads sit.

Common modern BSD sizes include 622 mm (700c road or 29er MTB), 584 mm (27.5" MTB or 650B road), and 559 mm (26" MTB). 571mm (650c wheels) is less common; it is used on some older triathlon bikes and some road bikes for smaller riders.

The European Tyre and Rim Technical Organization (ETRTO) and the International Organization for Standardization (ISO) defined this measure and how to measure it, and played some role in standardizing wheel sizes.

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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 a one-time fee of $20 USD to establish your membership, and no renewal fee.

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/

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    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
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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.

Variations

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

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  • 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
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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.

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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.

https://tadpolerider2.files.wordpress.com/2015/03/anatomy-ice-trike.jpg

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.

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Mudguard Stays / Fender Stays

Holders/supports to attach the mudguard/fender to the frame, as pictured.

enter image description here

A single stay has two legs with one on either side of the bike, and is secured to eyelets with a bolt, or bolted into a tapped hole or rivnut. If the frame lacks mounts it is possible to use P clips or zip ties, at the cost of rigidity.

Normally made of metal for durability and impact resistance. Plastic ones exist but are uncommon.

A full rear mudguard would have two stays normally, and a front guard would have one.

Most stays have some kind of adjustment to help center the guard to avoid rub. Older stays may need bending/straightening to achieve the same effect. Modern stays would have breakaway features as well, to reduce injury if a body part got in the way.

See also "skirt guard"

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MIPS helmet

A helmet with a low friction layer that allows the helmet to slide relative to the head following an angled impact. This reduces the the rotational motion of the head that may otherwise be transmitted to the brain. Our community have discussed MIPS here and here.

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Lights

Units that provide illumination while you ride your bike. Energy will come via electricity in any modern light, but early lights may have been fuelled by kerosene or other combustibles. Modern lights will be LED-based, and older ones could have an incandescent filament lamp inside.

Vintage Kerosene lamp Early liquid lights

80s Eveready lights 1980s style, incandescent

Ravemen modern LED light Modern LED rechargable light

Most lights are detachable in some way, but a few bikes will have lights designed into the frame. These are unusual, and will likely only be LED based.

Internal lights


Seeing Lights

Bright lights for illuminating a dark road or track, for the rider's benefit.

Lights in this category are powerful enough (upwards of a thousand lumens or so) to let the rider clearly see the terrain ahead for several meters, even while riding at speed. These are best used when avoiding glare in other road users' eyes is a secondary concern, such as when mountain biking at night, or when riding on deserted roads. An analogy could be drawn to the high beams on a car. Inappropriate usage of seeing-type lights — for example on crowded roadways — can be counterproductive as other road users are likely to be dazzled by the blindingly intense light, obfuscating their vision.

Be-Seen Lights

Smaller lights with a wider FOV so that other road users can see the rider.

Like the low beams on a car, be-seen lights are primarily intended to delineate where the bike is on the road by being just powerful enough to be seen without risking dazzling other road users. Their purpose is not to illuminate the road surface, although that may be a secondary benefit. These are best used when street lighting is adequate to spot road markings and hazards, but inadequate for other road users to easily see the cyclist. They are not recommended for off-road use as their light output may be too dim to see trail features.

Daytime Running Light (DRL)

Physically small lights used on a bike during daylight hours, to help with being seen during the day. Long runtime and bright output requirements means these have only been a thing since this century. Studies have proven that cars and trucks benefit from DRLs especially in bad and marginal weather, so any road user would gain visibility by having lights.

There is not a lot of difference between a DRL and a "Be-Seen" light, other than the time of day they are in use. Laws differ by jurisdiction regarding the use of flashing or other non-steady light patterns.

(a somewhat `shopped) scene of riders in a shadowed gully, with DRLs.

Passive lights, aka Reflectors

TBC Unpowered passive optical devices designed to bounce light back toward a source.

Corner reflectors, Tape, increased performance over time, colours

Rear lights

TBC

Other notes

TBC Batteries, Lithium/Dry cell/dynamo/other?

Flashing vs steady, and halfway-throbbing ones. Mandates in EU about cutoff and dynamo.

Laser lines on the road.

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Tolerances

Every manufacturing process has variance. That is, for each dimension specified (e.g. diameter of a bore, length of a part, how round a bearing seat is, how parallel two bearing seats are), parts will vary slightly around the mean. In engineering and manufacturing, tolerance (link to Wikipedia) is the amount by which dimensions are allowed to vary. That is, official specifications will show a dimension and a tolerance.

In common speech, people say a manufacturer has “poor tolerances” when they mean that a company has high variances in whatever important dimension. Poor manufacturing tolerances can cause play in components. As one example, cheap hex keys frequently fit sloppily in bolts because they may be a bit undersized.

As an example of tolerances in official specifications, the picture below is one manufacturer’s (FSA) specification for a BB386 EVO shell. The shell must be 86.50mm wide, with a symmetrical tolerance of 0.25mm. Note that tolerances can be asymmetric, as is the tolerance for the width of the bearing seats (46.00mm, +0mm, -0.50mm). Some manufacturers may run poorer tolerances than others, i.e. their parts have higher variance in their important dimensions. Manufacturers presumably try to catch parts that are out of tolerance, but especially in high volume production, they cannot test every single part (they likely test batches for items out of tolerance, and they may reject a whole batch and investigate if enough items were found).

enter image description here

A side note: Tolerances/variances stack

In general, bottom brackets creak if they fit loosely in the frame. Tubeless tires may blow off the rim if they fit too loosely. "Too loosely" involves the tolerances of both the bottom bracket shell and the BB cup, or the rim and tire. That is, a brand of rim may work with many tires, but if you have a rim that's relatively small combined with a tire that's relatively large, you can have a loosely fitting tire.

Formally, the variance of the sum of two random variables (e.g. the mean diameter of a BB shell and the mean diameter of a BB cup) is the sum of their individual variances. Thus, the system has greater variance than a single component. (NB: technically, one also has to add 2* the covariance (similar to correlation) of the two quantities, but in the above scenarios the covariance can be assumed to be 0.)

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Frame Sizing

Bicycles may have nominal sizes, e.g. extra small through extra large, or they may have a centimeter size (e.g. Specialized's road frames in 2021 have 44cm, 49cm, 52 to 58cm in 2cm increments, and 61cm). Manufacturers usually recommend a range of heights for each size.

In the past, road frames typically had equal length seat and top tubes, and their top tubes were parallel to the ground (as in the picture in the header). The frame's centimeter sizing corresponded to the length of the seat tube. Mountain bike frames had sloping top tubes (i.e. not parallel to the ground) from the beginning, and most road frames adopted a similar design. Modern road bikes' centimeter measurements do not typically correspond to their seat tube lengths. Frames with sloping top tubes may be called compact frames, and they look like the picture below.

Height ranges are an approximate guide. People can have relatively long legs for their height, or the reverse. Your local bike store will be able to help you determine the correct size in person.

Measuring frame sizes - x- and y- dimensions

If you are sizing yourself, there are two schemes to measure frame size. It is important to know how long the bike is, and how tall it is. Once you know a range of bicycle sizes in either measurement scheme that work for you, you can select potential bikes more easily. Keep in mind that you can also adjust the stem length and its vertical height to alter the position of your handlebars.

The effective top tube length (diagram below) measures the former. This is what the top tube length would be if the bike had a level top tube; most new bikes these days have sloping top tubes, and the degree of slope can vary. The bike's seat tube length is the other parameter to know in this scheme. Beware that seat tube length can be measured to the center of the seat tube/top tube junction, or to the very top of the seat tube. There can be as much as a 20mm difference in length between the two types of measurement.

enter image description here

Now, manufacturers also provide stack and reach dimensions. These measure the bike's horizontal and vertical dimensions from the center of the bottom bracket to a point at the top of the head tube.

enter image description here

Keep in mind that typical ranges of x- and y-dimensions for a given size will vary by bicycle discipline (e.g. endurance road bikes have lower reach and greater stack than road race bikes; many modern MTBs are designed for short stems and flat bars, so their reach dimensions will be long compared to road bikes) and by manufacturer.


Secondary measures

Seat tube angle and seatpost setback/offset

Bicycles typically come in a small range of seat tube angles (STAs) for a given size range. Frames for average men usually have STAs around 73 degrees, with the angles steepening on smaller bike sizes and slackening on larger sizes. Some bikes, particularly mountain and triathlon bikes, may give a virtual STA. This is the STA if the bike had a straight seat tube. The steeper the STA, the further forward you sit on the bike, although remember that you can adjust your saddle position fore and aft.

Seatpost setback or offset describes if the saddle rail clamp is in line with the post (i.e. 0 offset or straight, commonly seen on mountain bikes), or if it is to the aft of the centerline of the seatpost (15-20mm rear offset is common on road bikes, although there may be some posts with more). Seatposts with setback position your saddle further aft by default.

Most riders should be able to achieve their ideal saddle fore-aft position on most combinations of STA and offset. As always, there may be exceptions for unusual body proportions and/or preferences. For example, if you prefer a very far forward position (e.g. you have short legs or you just prefer it), you may want a zero offset seatpost regardless of frame seat angle. It is possible that you may not be able to get your saddle far forward enough on some frames with slacker STAs. The reverse is true if you prefer to sit further aft; you may want to default to a setback seatpost, and avoid frames with steep STAs. Secondarily, the size of the seatpost's rail clamp can reduce the amount of fore-aft adjustment available.

Do note that STA does interact with top tube length but not with reach. If you fix the top tube length, a steeper STA will give you longer reach.

Head tube length

This affects how much vertical adjustment you can make to your handlebar position. This is less of a concern if your frame size is correct. It is possible that you might have to use excessive spacers (e.g. over 40mm, but see your bike manufacturer's guidelines) to get your handlebars high enough to be comfortable. However, if you need to do this, it may be a sign that you're on the wrong size of bike or on the wrong type of bike (e.g. you should be on a endurance road bike rather than a performance road bike).

Standover clearance

This is the distance from the ground to the top tube. On bikes with sloping top tubes (i.e. most of them), clearance is usually stated from the ground to the center of the top tube. When wearing cycling shoes, you can measure the distance from the floor to your crotch and compare.

In the past when road bikes had level top tubes, if you had about an inch of standover clearance, that was a potential indicator that the frame was the correct size. This is not as important as the other basic frame measures.

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Rim brake, e.g. cantilever, U- or V-brake, single or double pivot brake, side-pull brake

Brakes help you control your speed and stop. Rim brakes include all types of brakes that push pads onto a rim's brake track. Park Tools has a page dedicated to identifying rim brake types. Rim brakes may also be called caliper brakes. Alternatively, "brake caliper" is a generic term for an item that squeezes the sides of the rim, and it houses the brake pads and bolts to an attachment on the frame. This usage stems from the measuring device, but the term is also used to refer to disc brake calipers.

Drop bar bikes have typically mounted the front caliper in the middle of the fork crown, and the rear one in the middle of the chainstay bridge. Types have included center pull and side pull brakes. Single pivot and dual pivot brakes are types of side pull brakes, with dual pivots being stronger. Cyclocross bikes have historically used cantilever brakes, or sometimes mini-V brakes with cable pull suitable for road levers. Notably, road and MTB brake levers have different cable pull ratios, so one does not typically use MTB V-brakes with a drop bar brake lever (although adapters exist).

MTBs have used cantilever brakes, later transitioning to V-brakes, and later transitioning to disc brakes. At the time of writing, newer drop bar bikes are generally transitioning to disc brakes, although many older rim brake models exist.

In theory, rim brakes have some disadvantages to disc brakes. Rim brakes don't stop as well in wet weather, and they do eventually wear out the rims. However, they are cheaper and are a lot less sensitive to misalignment than disc brakes. Mountain bikes switched to disc brakes some time ago. Road bikes are in the process of transitioning, and in 2021, many newer bike models do not have rim braked options.

Other types of brakes include disc brakes and hub brakes, e.g. coaster brakes.

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Recumbent cycles.

Recumbent cycles come in two, three and four wheeled versions, in many different shapes and with many different riding characteristics. The only thing they all have in common is that the 'bottom bracket' is not low down between the wheels but farther forward and farther up.
The sitting position is almost never bend over forward but can be anything from straight up to almost flat back, although some people will only call a cycle a recumbent if the seatback leans back by at least a given angle.

Steering can several different ways, most commonly called above seat steering and underseat steering, can be direct or with a rod or more advanced ways as dictated by the geometry of the cycle and the wishes of the builder.

Because of the basic position, not needing to lea on your arms while riding, and the wide variations in models and options, recumbents are often adjusted for use by people who have some limitations. That goes from moving the brake leavers to be on the same side, to replacing the bottom bracket with cranks and pedals to a hand driven power system.
Trikes are more often adjusted as they allow for more limitations, like loss of balance or loss of the use of one arm or leg.

More information in this Wikipedia article.

Flevotrike
This is a Flevotrike which has underseat handlebars but an unusual steering method, in which there is a pivot point under the seat and the whole of the front, including the front wheel drive, tilts. Photo by me, trike owned by me.

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Velomobiles

A special class of recumbent cycles is fully faired, mostly three wheeled (although there are four wheeled versions as well) called Velomobiles, although many people may know them by different names.

The main advantage of a velomobile is higher speed for the same effort, but it comes at the price of being lower and therefor less visible on the road, paying more for the machine and often a bigger turning cicle.

Yellow Quest with blue details
My own Velomobile, a Quest, here parked in Beverwijk, the Netherlands.

There are also two wheeled fully faired cycles, but as far as I know those are not classed as velomobiles, they are the fastest that is out in man powered vehicles, whether single rider or a team of riders. This is a link to a magazine like internet page, a few years old, of an event which gets the fastest cycles.

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