Bottom bracket

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Outside screwed-on bearing (Hollowtech II)

Inner bearings , also known as bottom brackets , are the bearings on the bike in which the bottom bracket shaft is mounted, to which the cranks are attached. The term "inner bearing" was established in the 1980s by the bicycle mail order company Brügelmann . A bottom bracket always consists of at least two bearings that support the bottom bracket shaft on both sides of the bottom bracket shell.

To reduce the friction contains the bottom bracket bearing , as well as the bicycle hub and rate earlier than conical mounts were formed. In the meantime, cartridge chambers are mainly used for simple wheels . Better bottom brackets contain replaceable bearing units, which in the sports field are known as industrial bearings . With these, the inner and outer running surface with balls (more rarely cylindrical rollers or needles) and cage form an inseparable unit.

Attachment in the bicycle frame

Bottom brackets are pressed or screwed into the bottom bracket shell of the bicycle frame . The following table gives an overview of the common dimensions of the fits or threads and the width of the bottom bracket shell.

Technical name Outside-Ø shells or bearings [mm] Short name Housing width Explanation used axes
BSA 1,370 ″ × 24 tpi (rarely ISO 1,375 ″ × 24) 34.8; Thread 34.8 × 1.058 CLOSELY; BSA; BC 1.37 mostly 68 or 73 (oversized),

less often 83 or 100 mm

Also referred to as "English"; mostly right-hand thread on the left and left-hand thread on the right; very rare right-hand thread with forged cone or smooth or stepped for use with industrial bearings or as part of a cartridge chamber ; Ø with wedge or square connection to the crank: ~ 17 mm; Ø with multi-tooth or clamp connection: Octalink and ISIS 22 mm, Hollowtech II / MegaExo (FSA) / X-Drive (Race Face) / Easton 24 mm, GXP (SRAM / Truvativ) 22 mm left and 24 mm right, Campagnolo Ultra -Torque 25 mm; Length: 103-132 mm, mostly 107-113 mm (Octalink double chainring 109.5 mm, triple 118.5 mm)
ISIS Overdrive 48 × 1.5 48; M 48 × 1.5 thread ISIS 68 or 100 mm proposed as the new standard with oversized smooth or stepped for use with industrial bearings; Ø 22 mm; Multi-tooth connection to the crank; Length of racing bike 108 mm (also 118 mm for triple chainring) / MTB 113 mm (also 128 mm for DH or 138 mm for external bearings)
ITA 36 × 24 tpi 35.9; Thread M36 × 1.058 or 1.42 ″ x 24 tpi ITA; "Italian" 70 mm right-hand thread on both sides; often recognizable by the designation "36 × 24" on the outer surface of the right bearing shell like BSA
FRA 1.378 ″ × 25.4 tpi 34.8; M 35 × 1 thread FRA; "French" like BSA right-hand thread on both sides; was used until the 1980s; Until the 1970s, rarely also as a “Swiss thread” with a left-hand thread on the right like BSA
Bell bearing; Thompson 34.7 or 35 or 40; rarely 30, 34, 35.9 or 37.5 (French) or 38 mostly 70 mm Impact shells sit in the threadless bottom bracket housing; Cones sit on the shaft (screwed on both sides or only on the left); hardly in use anymore; was z. B. used in touring bikes and cargo bikes on the right mostly with a pressed external cone, on the left with a thread for screwing on the cone or smooth or offset on both sides for use with industrial bearings or as part of a cartridge chamber (repair warehouse); Wedge or square connection like BSA
Fauber; Engl .: One piece cranks or Ashtabula 51.3-51.5 (2.02 "); possibly also 45 Fauber; Engl .: OPC or Pro Size z. B. 70 mm Type of bearing like Thompson (instead of impact shells occasionally also screwed shells); with older American or Scandinavian as well as BMX bikes (with one-piece crank-shaft element) and various threads The cranks and axle are forged from one piece and have to be threaded through the bottom bracket shell , which therefore has a larger outer diameter
Mavic cone conical 34 - approx. 38 Cone cartridge; Repair bearings for use in housings with damaged threads
BB30 42 BB30 68, 73 or 83 mm Ball bearings are pressed directly into the frame. Open Standard (originally introduced by Cannondale) Hollow axle Ø 30 mm
Specialized OS-BB 42 OS-BB 84.5 mm Ball bearings are pressed directly into the frame. Hollow axle Ø 30 mm, larger support width than BB30 cranks
BB90; BB95 37 BB90 90.5 or 95.5 mm Ball bearings are pressed directly into the frame. Hollow axle Ø 24 mm, originally intended for use with Shimano Hollowtech II cranks
BB86; BB89.5; BB92 41 Pressfit (also: Shimano Pressfit) 86.5 mm (racing bike); 91.5 (or 89.5 mm with 2.5 mm wide washer) (MTB); The ball bearings are in shells (mostly made of glass fiber reinforced polyamide ( nylon )) Hollow axle Ø 24 mm, smooth or stepped for use with industrial bearings; Originally intended for use with Shimano Hollowtech II cranks or similar, bottom bracket for Truvativ / SRAM GXP and Campagnolo also available
Pressfit 30 46 PF30 68 mm (racing bike), 73 mm or 83 mm (MTB) The ball bearings are in shells (mostly made of glass fiber reinforced polyamide (nylon)) Hollow axle Ø 30 mm (like BB30)
386 EVO 46 386 EVO 86.5 mm (MTB) a variant of the Pressfit 30 bearing, uses the same bearing shells Hollow axle Ø 30 mm (like BB30)
Square inner bearing with BSA thread and ISO square shaft. Without counter ring for adjusting the bearing shell, therefore probably an industrial or cartridge chamber.
Square inner bearing with ITA thread, counter ring and ISO square shaft. After loosening the outer lock ring, the bearing play can be adjusted by screwing the bearing shell in or out of the four holes with a special key.
  • The types of fastening in the first half of the table up to and including the Fauber bearing were originally used for classic cone bearings (see below). Today, bottom bracket housings of this type are usually equipped with cartridge bearings or, less often, with industrial bearings.
    The mounting types in the second half of the table are not intended for use with conical bearings. These are generally equipped with industrial bearings.
  • ENG and FRA can sometimes not be clearly distinguished. The only thing that helps here is careful trying.
  • Bell bearings had a square, more rarely a hexagon socket for the cranks and the conspicuously curved, mostly chrome-plated dust caps that gave them their name. They were replaced by Thompson bearings, initially with wedge fastening and later also with square mounts for the cranks and angular dust caps. Balls with a 1/4 inch (6.35 mm) diameter are used. Visually, Thompson bearings differ from the common cone and cartridge bearings in that they have a smaller lock nut that sits on the shaft (instead of on the outer bearing shells) and thus, like the dust cap, rotates when pedaling.
  • Right-hand bearings with a right-hand thread should be secured with medium-strength screw locking or a lock nut.
  • Usual shaft lengths for inner bearings with an integrated shaft (cartridge bearing): 103 - 107 - 110 - 113 - 116 - 119 - 122 - 132; Intermediate sizes possible.
  • BB30 and PF30 housings are intended for BB30 cranks, but can be reduced for other systems.
  • OS-BB bottom bracket shells are not compatible with BB30 cranks, but can also be reduced.

Installation and maintenance

Most bottom brackets were previously attached to the bike frame with a fine thread. During installation, internal and external threads should be thoroughly greased, otherwise it is often difficult to loosen the screw connection again later. If the bearing is only replaced after many years, it is often no longer possible to unscrew it with simple means. If an extended lever arm or the use of an impact wrench is unsuccessful, it is a good idea to clamp the bottom bracket tool in a vice so that the bike is horizontally above the vice. Two people can then grab the frame of the bike to rotate it around the bottom bracket. If this does not help either, you can try to heat the bottom bracket shell with a flame. Rapid heating leads to thermal expansion of the housing, while the bearing shell remains relatively cool, so that the clamp that exists between the two is released. After this, the burned paint usually has to be removed in order to apply rust preventive and a new coating.

Storage types

All bottom brackets contain one roller bearing on the right and one on the left side of the bottom bracket shaft. The technical development went from the cone bearing to the deep groove ball bearing (industrial bearing) and then to the cartridge chamber. Since the service life of industrial and cartridge bearings was often unsatisfactory, the bearings on sporty bikes are increasingly being placed outside the actual bottom bracket housing. By increasing the distance between the bearings, the forces acting on the balls are reduced. The modern bearings are also easier to assemble.

Simple cone bearing
Bottom bracket shell. The threads cut on both sides are visible (on the left a normal right-hand thread and on the right mostly a left-hand thread), the ventilation openings to the seat and down tube and the underside hole for drainage or for fastening the brake and shift cable guide.

Classic cone bearing

The cone bearing is an angular contact ball bearing that is traditionally also used in bicycle hubs and in the headset . The running surfaces for the balls are formed directly on the bottom bracket shaft in the form of ring-shaped elevations during forging . The surface is then hardened (so that no additional hardened cones are necessary as with Thompson bearings or with wheel axles ). Bottom bracket shells serve as the outer running surface and are screwed into the bottom bracket shell. The balls are often held together by a sheet metal cage (bearing cage) and run directly between the bottom bracket axle and the bearing shell.

Cone bearings are often not as well sealed as modern bearing variants, but they usually have a longer service life than industrial or cartridge bearings because they are adjustable, contain larger balls and are usually less heavily loaded due to a slightly larger bearing distance. With sufficient material quality and occasional adjustment of the bearing play , cone bearings can achieve a significantly higher mileage than cartridge bearings and deep groove ball bearings. Due to the additional work step for setting the bearing, cone bearings are no longer used in large-scale production. Also, cone bearings are not sealed from the inside of the frame. Water that runs through the seat tube into the frame, for example, can mix with the bearing grease and lead to corrosion if the bottom bracket does not have a drainage opening. A cracking bottom bracket indicates that maintenance is required or the bearing is defective.

With the cone bearing, the bearing play is usually set on the left bearing shell. The bearing shell is screwed into the bottom bracket housing until the axle just has no more play. When turning the cranks, no increased resistance should be felt.
The right bearing shell often has a collar that serves as a stop when screwing into the bottom bracket shell.

In most cases the right bearing shell has a left-hand thread. If the thread has too much play, the rotating rolling movement (flexing movement) of the threads of the bearing shells could otherwise cause the lock ring to loosen and the threaded shell to gradually unscrew (against the direction of rotation of the cranks ). Due to the large thread diameter and the fine threads of the bottom bracket cups, however, the risk of unscrewing them is lower than with the pedals, which must be provided with a left-hand thread on the left side. It happens much more often that a bearing shell that is insufficiently greased when screwing in can no longer be loosened at all after the lock nut has been removed.

If there are signs of improper running, the bearing must be serviced by dismantling it and cleaning the grease from the cone and balls. If the surface of the running surface is visibly damaged, the bearing shells or bottom bracket shaft should be replaced. If the balls have an uneven surface, a completely new set of balls should be used. Otherwise, only the bearing grease is renewed and the bearing play adjusted.

Bearing shells and their circlips often only have narrow, flattened or grooved side surfaces to allow the assembly tool to be used. If the threads were not sufficiently greased during assembly, the shells and rings will often sit so tight after a few years that they can only be loosened with special tools or special aids. Bearing shells made of steel with flattened side surfaces can be clamped in a high-quality vice with sharp-edged, parallel jaws in order to be able to loosen the bearing shells by turning the entire frame.

Traditional cone bearings can be replaced by cartridge bearings or bearing sets with industrial bearings. The following are to be compared: the housing width (68 mm or more), right-hand or left-hand thread on the right-hand side of the frame, the length of the shaft, the type of crank mounting (wedge, square, spline) and the outer diameter of the bearing shells (BSA and FRA- Bearings with a left-hand thread on the right-hand side can only be differentiated with a thread gauge or by screwing in the bearing shell on a trial basis).

Cone bearing with hammered-in bearing shell and screwed cone (bell or Thompson bearing)

The previously used bell bearings and their successors, the Thompson bottom bracket, can be recognized by a thread on the bottom bracket shaft and a rotating hood or bell that covers the bearing. During assembly, the bearing shells are first hammered into the bottom bracket shell before the shaft and ball bearings are inserted. The right cone is then either screwed onto the bottom bracket shaft with a right-hand thread or it is firmly connected to the shaft. The left cone is screwed onto the shaft, which has a left-hand thread on it. This is followed by the rotating dust cap, an intermediate ring, which is usually secured against twisting by an internal tooth, and a lock nut .

The dust caps of the older bearings of this type are rounded, chrome-plated and have the shape of the eponymous “bell” that grips from the outside over the bottom bracket shell and rotates with the bottom bracket shaft. With the newer Thompson bearings, the dust cap is flat, much less noticeable and partly not visible on the right side. The lock nut is significantly smaller than that of conical bearings with a screwed-on bearing shell, as it sits on the shaft and not on the outside of the bearing shell. Bell bearings were already connected to the crank with a square on the shaft. Since there was no internal thread in the crank for attaching a crank puller, special pullers had to be placed between the cone and crank to push the crank off the shaft. Bell and Thompson bearings can be replaced by cartridge bearings that do not have a thread on the outside and are hammered or pressed into the bottom bracket shell. It is removed by hitting the bottom bracket shaft.

Industrial ball bearings

Key from the manufacturer FAG for screwing in the bearing shells
Cutaway model of an early chamber. Visible from the outside to the inside are:
- the bottom bracket housing of the frame,
- the screwed-in lower threaded shell (the upper one is missing) for fixing the bearing cartridge (the protruding edge necessary for attaching the assembly tool cannot be seen),
- the housing of the prefabricated compact bearing, which so-called cartridge ,
- the hardened inner bearing shell inserted into the bearing cartridge , - the
actual ball bearing that runs on a cone that is formed on
- the forged and hardened bottom bracket shaft.

Instead of the conical running surfaces for the balls, there are two fits on the shaft , onto which the deep groove ball bearings are pushed. The outer races of the two ball bearings are fixed by the screwed-in bearing shells in the bottom bracket shell.

Due to the precise manufacture of the industrial ball bearings, a reduced resistance to rotation is to be expected. However, the significantly smaller ball diameter generally results in a shorter service life, especially since the ball bearings, in contrast to conical bearings, cannot be readjusted.

Industrial ball bearings (roller bearings) sealed on both sides can be used, which, in contrast to classic cone bearings, are also waterproof to the inside of the bottom bracket. The sliding lip seals, however, in turn lead to a slightly increased resistance to rotation.

The internal threads of the bottom bracket shell should be aligned with each other so as not to stress the deep groove ball bearings when screwing in the bearing shells. If a rigid spacer sleeve is not installed between the outer rings of the two industrial ball bearings, the bearing shells may only be screwed in carefully and just enough that the smooth running of the bearings is not impaired. As with the cone bearing, the threaded cups are secured with lock rings.

Cartridge chamber

Cartridge chambers are the most common type of bearings used on bicycles today. The two roller bearings are inserted into a cylinder (the cartridge ) together with the bottom bracket shaft , adjusted and pressed by the manufacturer. This means that the bearings are tight towards the inside. The bearing does not have to be adjusted during assembly. Counter rings as with conical and industrial bearings are no longer necessary, as the two retaining shells can be clamped or pressed against each other. Cartridge chambers cannot be serviced. The size of the balls is between that of the conical bearing and that of the industrial ball bearing. The resistance to rotation is usually higher than with the other bearings due to the less precise manufacturing and the lack of adjustment options. The service life depends on the manufacturing and material quality, but is always less than that of a conical bearing that can be cleaned, re-tensioned and re-greased.

If there is a larger bearing play , in contrast to the other designs, the complete cartridge chamber including the axle must be replaced.

The cartridge is usually screwed into the bottom bracket shell with two threaded cups and a tightening torque of 35 to 45 Nm. A thread is often integrated on the right side of the cartridge so that the right threaded cup is not required. The threaded shells are often made of plastic and are fitted with special keys. To prevent the key from slipping if the shell is stuck, the tool should be held firmly in place, for example by putting the crank back on and screwing it on slightly. If the shaft is completely hollow, a quick release can be used instead of a screw.

Cartridge bearings are also available with shafts of different lengths so that the chain line - the distance between the chain and the center of the bicycle frame - can be varied as the rider wishes.

A variant whose outer retaining shells have an internal thread instead of an external thread or are simply hammered in or pressed in is used to replace bell or Thompson bottom brackets.

In the cartridge chamber there are either classic cone bearings (see picture) or a pressed industrial ball bearing (deep groove ball bearing) on ​​both sides, usually sealed with a protective ring. The axis usually has a diameter of 16 mm. As a result, the following ball bearings are installed: BB 163110 - 2RS with an outside diameter of 31 mm, an inside diameter of 16 mm and a width of 10 mm.

Since the cartridge completely encloses the bearing in the cartridge chamber, the accuracy of the seat in the bottom bracket housing is not significant. The correct alignment of the ball bearings and the shaft to one another is guaranteed by the cartridge. This type of bearing is therefore the one most used in mass production, even if the service life of a classic cone bearing is not reached due to the lack of adjustment options.

External bearings, screwed

In this type, introduced by Shimano in 2004 , the bearings are located outside the bottom bracket shell - the term “bottom bracket” is therefore inappropriate for this. The large distance between the two bearings is intended to improve the stability-to-weight ratio of the construction. Such bearings are available for different housing threads, including BSA and ITA.

In contrast to other systems, the bottom bracket shaft is not a unit with the bottom bracket; rather, the bottom bracket shaft is inserted through the two bearings. Therefore, when installing, you have to make sure that the two bearings are exactly parallel. Otherwise the service life of the bearings will be reduced considerably. It is therefore essential to mill the bottom bracket shell flat at both ends.

Shimano has this type of bearing, the term Hollowtech II or HT II introduced in order to clarify that both the cranks (and the pedal bearing shaft hollow engl. Hollow ). In the cheaper groups, the cranks are not hollow, so that these sets are not called Hollowtech II , although the same bearing technology is used. Colloquially, the bearing type is nevertheless referred to as Hollowtech II . Truvativ calls this storage type GXP , as an abbreviation for Giga X-Pipe . At FSA this system is called Mega Exo .

With external bearings, the right crank arm is usually firmly connected to the axle, while the left crank is attached and then locked by tightening two screws (see section Fixed connection to a crank arm ).

Modern bottom bracket shafts of 22, 24 or 24.07 mm in diameter

Although the Hollowtech II , GXP and Mega Exo variants require bottom bracket shafts with a diameter of 24 mm, the standards are not mutually compatible. The diameter of the Mega Exo bottom bracket shafts is 24.07 mm and therefore does not fit into GXP and Hollowtech II bearings.

GXP bearings have an opening with a diameter of 24 mm on the drive side and 22 mm on the non-drive side. For this reason, Hollowtech II and Mega Exo cranksets are not compatible here either. With the Hollowtech II format from Shimano, both bearings have an opening of 24 mm, but this creates a clearance of 2 mm on the non-drive side using a GXP crankset, while the Mega Exo crankset because of the larger diameter of the Wave generally does not fit.

The manufacturers of the different Shimano , FSA and Truvativ ( SRAM ) cranksets always refer to the use of the appropriate bearings from their own brands in order to guarantee full compatibility. Due to the discrepancies and incompatibilities, the 24 × 37 standard for BSA crankcases cannot be called a standard in practice.

External bearings, pressed in

In this case, the bottom bracket shell has no threads, nor are the bearings. These are pressed into the bottom bracket shell. In the press-fit versions, the bearing is located in a shell that is pressed in. Instead of an axle with an outer diameter of 30 mm, an axle with a 24 mm diameter can usually be used with the help of adapter rings.

BB30 warehouse

Case diameter 42 mm, case width 85/83/73/68 mm. Originally for bottom bracket shafts with a diameter of 30 mm.

Pressfit30 (PF30)

Case diameter 46 mm, case width 83/73/68 mm. The bearing shell (with the bearing inside) is pressed in. Bottom bracket shaft diameter 30 mm.

Pressfit BB86

For racing bikes. Case diameter 41 mm, case width 86.5 mm. The bearing shell with bearing is pressed in.

BB90 bearing

Case diameter 37 mm, case width 90.0 mm. Inner bearing shaft diameter 24 mm.

Pressfit BB92

Case diameter 41 mm, case width 92 or 89.5 mm. The bearing shell with bearing is pressed in. Inner bearing shaft diameter originally 24 mm.

Connection of bottom bracket shaft and cranks

The connection between the inner bearing shaft and the pedal cranks must be able to transfer high torques in the direction of rotation of the shaft, but also forces to the side of the bottom bracket shaft. There are numerous systems for connecting shafts and cranks. All systems have in common that the cranks are placed on the shaft of the bottom bracket, so that the lateral forces can be transferred best.

To loosen the connection, most systems require a puller which - after loosening the screw - pulls the crank off the shaft. Some cranks have an integrated puller; in this case, the central fastening screw of the crank pulls the crank off the shaft when it is loosened.


Wedge fastening

The wedge or crank wedge is a cylindrical bolt with an inclined surface on the side that is knocked into a hole in the crank that is transversely to the bottom bracket shaft and presses against a flat on the bottom bracket shaft. A mother secures the wedge. Wedge fastenings can loosen, especially in the case of poor manufacturing and material quality. They have hardly been used since around 1990. Wide trouser legs may get caught on the nut of the wedge.

Wedge bottom bearings are comparatively easy to dismantle. For this purpose, the nut is loosened to such an extent that its outer surface is flush with the thread of the wedge. This means that the thread deforms less when it is knocked out with a hammer. If the wedges are stuck, the bottom bracket shaft should be supported from below. If the wedge cannot be driven out with a few controlled, hard blows, the thread must often be sawed off in order to be able to apply a drift .

It doesn't matter which way around the wedges are inserted; only the nuts should not point to one side, because then the cranks will not be parallel. For the lowest surface pressure on the wedge when pedaling, both nuts would have to be at the bottom, if the left pedal crank (facing away from the chain) points towards the front wheel, then the larger area of ​​the wedge is at the bottom, i.e. has the largest area where the most force acts. The right crank transfers only a small amount of torque to the left crank for lifting the right leg.

The wedges are driven in as far as possible with moderate hammer blows. The nut is only used for additional security and in no way for pulling in the wedge. It should be put on very carefully. Since the wedges are usually made of quite soft steel, the thread can be easily turned over. After driving 50 km, drive in the wedges again with the hammer and retighten the nut.

Conical square

The ends of the shaft are square edges that taper towards the end. The crank has a matching beveled square hole. By tightening an axial screw or nut, the crank is pulled onto the shaft and fixed there. This results at the same time in a form-fitting and a force-fitting connection with frictional engagement.

There are two types that differ both in the slope of the wedge and in the shape of the edges and chamfers, namely the ISO and JIS square. The ISO square is mainly found at European manufacturers such as Campagnolo , Miche, Mavic and sometimes Stronglight. In contrast, Japanese manufacturers such as Shimano in particular manufacture according to the JIS standard. Other manufacturers, such as Sugino, FSA and Phil Wood, build bottom brackets and cranks according to both JIS and ISO standards.

ISO cranks do not slip far enough on JIS bottom brackets, increasing the risk of the crank breaking (for moderate loads, however, such a solution can be sufficient). Conversely, JIS cranks can be placed about 2.2 mm further on inner bearings with ISO square, so that it may no longer be possible to fix the cranks. The chain line changes in both cases.

Shimano and many other manufacturers must not grease the area between the cranks and the square. This is to prevent the crank from being pulled too far onto the square when screwing it on. Otherwise, the resulting high stresses could break the crank. The high-strength screws or nuts with which the cranks are attached to the shaft, on the other hand, should be greased on the thread and on the friction surface and then tightened firmly (as strongly as is possible, for example, with a 200 mm Allen key ).

The square cone was originally used with the bell bottom bracket. In the 1950s, wedge bottom bearings that were easier to dismantle became popular. In the 1980s, a slightly different form of square mounting was introduced again. A special tool can be used in the threads provided in the crank head, which considerably simplifies dismantling.

The square is currently being replaced by the multi-tooth connections that arose in connection with the external bearings. The square cone is rarely used today, especially on very high-quality and sporty bikes.

Hexagonal cone

In bicycles from the 1950s, a hexagonal cone was used together with a bell bearing.


Shimano cartridge chamber BB-5500 with Octalink V.1 crank mount

The shaft end is designed with multiple teeth, sometimes also conical. Eight ( Octalink , originally from Shimano) and ten teeth (International Spline Interface Standard, ISIS for short ) are particularly common . Much less common are Power Spline with twelve and Howitzer with ten teeth and external bearings (both from Truvativ). This type of connection is by no means new, but has only been introduced to the mass market since 2003. Octalink has two different versions that are incompatible with each other. Octalink cranks and Octalink bottom brackets from the Dura Ace, Ultegra, 105SC and XTR groups (all Shimano) and cranks from Ritchey and Alpina from Sugino on the one hand (Octalink V.1) are compatible with each other in terms of the connection between the crank and bottom bracket shaft. and Deore XT , Deore LX, Deore, Tiagra and Sora on the other hand (Octalink V.2). However, the wavelengths differ; there are bottom brackets for Dura Ace, Ultegra and 105 with 109.5 mm and 118.5 mm shaft lengths; XTR is available in lengths of 112.5 and 116 mm.

Fixed connection to a crank arm

A crank arm and the bottom bracket shaft are firmly connected by the manufacturer. The bottom bracket shaft is inserted through the two bearing shells. On the other hand, the second crank arm is screwed onto the multi-toothed end of the shaft.

This technology has been around since the 1980s from small American manufacturers, above all Bullseye. They were originally developed for use on BMX bikes. Shimano introduced this technology in 2004 together with external bearings .

FSA, Race Face and in 2007 Campagnolo followed this trend; the technology also caught on in the cheaper groups. While with Shimano, FSA and SRAM / Truvativ the drive-side crank arm is connected to the shaft, with Race Face the left crank arm forms a unit with the shaft. Campagnolo divides the shaft in the middle so that both crank arms have a fixed shaft stub. For BB30 cranks with a one-sided fixed connection between the shaft and crank arm, this is on the left.


There are cranks with Fauber bearings for BMX bikes. Both cranks and the bottom bracket shaft are formed by a single bent piece. Special bicycle frames with very large bottom bracket housings are required for such storage units, because the unit has to be "threaded" through the bottom bracket housing for assembly. Then the bearing cones are pushed from the outside over the cranks and screwed onto a thread on the shaft. The advantage of such bearings is their extreme stability because the shaft and cranks are made from one piece.


Some manufacturers manufacture their own systems. For example, SRAM builds bottom brackets - and matching cranks - with the name Howitzer under the brand name Truvativ . The manufacturer e * thirteen produces cranks and bottom brackets with a rounded triangular shape (polygon according to DIN 32711-P3).

Eccentric bottom bracket

The front bottom bracket shell of tandems and the bottom bracket shell of a few manufacturers of durable bicycles are made so large that the bottom bracket can be eccentrically mounted in a cylindrical holder. This cylindrical holder can be turned after loosening the clamping screw (s) in order to adjust the chain tension on bicycles without derailleur gears.
In a special design, the bearings and shaft are mounted directly in the eccentric insert (Idworx).

Turning the eccentric is less time-consuming than moving the rear wheel to adjust the chain tension. This is especially true for bicycles with rim brakes, as these have to be readjusted afterwards.

With a tandem, the distance between the front and rear bottom bracket can usually only be changed using an eccentric. The only alternative would therefore be to use a rotating tension wheel.

Eccentric bottom brackets are sometimes abbreviated as EBB (eccentric bottom bracket).

Bottom bracket circuit

Some manufacturers are bringing in (partially enlarged) bottom bracket shell gear for the gear shift under. It is driven by the crank axle. The output takes place coaxially or axially parallel to the sprocket. There are systems that require a specially shaped bottom bracket shell, as well as systems that can be retrofitted to standard bottom bracket shells.

Picture gallery


  • Michael Gressmann, Franz Beck, Rüdiger Bellersheim: specialist knowledge of bicycle technology. 1st edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 2006, ISBN 3-8085-2291-7
  • Richard Hallet: Bicycle Maintenance-Care-Repair. 1st edition, BVA Bielefelder Verlag GmbH & Co. KG, Bielefeld, 2003, ISBN 3-87073-308-X
  • Fritz Winkler, Siegfried Rauch: Bicycle technology repair, construction, production. 10th edition, BVA Bielefelder Verlagsanstalt GmbH & Co. KG, Bielefeld, 1999, ISBN 3-87073-131-1

Web links

Commons : bottom bracket  - collection of images

Individual evidence

  1. Brügelmann established the term inner bearing
  2. See also the very detailed measurements from Sheldon Brown for bottom brackets from different manufacturers.
  3. See also Sheldon Brown's tabular overview of the various Tretlagertypen (English)
  4. See instructions for changing the bottom bracket. ( Memento of October 21, 2012 in the Internet Archive ) In: , accessed on November 30, 2012 (PDF; 362 kB)
  5. Tour special edition “Velowerkstatt” I, page 127 or II, Smolik
  6. ^ Description of the Thompson camp. In:
  7. See Sheldon Brown's Lexicon (English)
  8. Other repair stores also from Stronglight (JP 1000), Point or YST
  9. Sketches of the new, threadless bottom bracket shell (PDF; 142 kB)
  10. a b A description of the modern storage systems can be found on the page Radtechnik für Profis ( Memento from February 21, 2014 in the Internet Archive ), accessed on December 4, 2016.
  11. a b c d e f Overview with the dimensions of modern bottom brackets,, USA, accessed in July 2016
  12. Illustration of a classic cone bearing, here labeled BSA bearing .
  13. See also Sheldon Brown's article on bottom bracket maintenance: Translations on (not all articles have been translated). Retrieved December 4, 2016.
  14. See the article by Sheldon Brown on the dismantling of bearings and retaining rings (English)
  15. a b c d Instructions (English) for installing and removing cranks and screwed bottom brackets (BSA and Hollowtech II) at, accessed in July 2016
  16. Illustration of a Thompson camp .
  17. Further information on bottom brackets from Christian Smolik
  18. a b Tips on bottom brackets / gears , In: Accessed May 2020
  19. A stop edge is often formed on the housing. Then the shaft must be knocked out from the other side, which has a loose bearing shell.
  20. Sheldon Brown's overview of some common cartridge chambers (English)
  21. Tabular overview (in English): Adapter for using various chainring sets (English crankset) with different bottom brackets, Wheels Mfg, accessed in July 2016
  22. See the article by Sheldon Brown about the two variants of the square connection to the crank (English)
  23. Difference between Octalink version 1 and 2
  25. ^ A b Todd Downs: Bicycling Magazine's Complete Guide to Bicycle Maintenance and Repair: For Road and Mountain Bikes . Rodale Inc., 2005, ISBN 978-1-57954-883-4 , pp. 138 .
  26. a b can also be used for threadless housings of Thompson bearings