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BC Guide: Finding the right bottom bracket for your bike
There are so many standards for bicycle bottom brackets that it is easy to lose track of them all. We’ll help you find the right one for your frame.
The bottom bracket is literally a central component on every bicycle. Only when the bearings rotate smoothly can you generate propulsion via the pedals and cranks. However, this component, which is itself quite simple, comes in so many variants and dimensions that it is easy to lose track of everything. The term "bottom bracket" comes from the fact that in the past most bottom brackets were placed in the frame. Today, the bearings are often located in bearing cups outside the bottom bracket shell. The bottom bracket takes on the difficult task of mediating between frame and cranks, which don’t usually come from the same manufacturer. The list of interfaces to the frame on the one hand is diverse – whether threaded or not. On the other hand, it is important to consider the compatibility with the crank. Whether you want to replace a worn-out bearing or build a custom bike, our guide will help you find the right bottom bracket in just a few steps.
Find the Right Bottom Bracket Standard via Search Filters The Shell Width General Notes on Assembly Types Assembly Types: Threaded Bottom Brackets Assembly Types: Threadless Bottom Brackets The Axle Type Axle Types for Two-Piece Cranks Axle Types as Part of the Bottom Bracket Bearing Type and Quality Bottom Bracket Installation and Tools
The bottom bracket functions as a mediator between the frame and the crank.
The decisive parameters when it comes to finding the right bottom bracket for your bike:
Shell width, installation type & axle type
Find the Right Bottom Bracket Standard via Search Filters
Our shop’s search filters can help you narrow down the selection of suitable bottom brackets. You can decide whether to start with the shell width and the assembly type or with the axle type (filter: axle type). Regardless of the order in which you set the filters, you should always end up with the appropriate bottom brackets. We’ll explain what the individual filter options mean and which ones will lead you to the right results. It would be handy to have a foldable measuring stick, ruler or tape measure to measure the bottom bracket shell on your frame – a caliper gauge would be even better. To be able to measure all dimensions, there should be no bottom bracket installed in the frame. Only very few bottom bracket standards can be determined while they’re installed. If you have a data sheet or the documentation of your frame or bike from the manufacturer, you may be able to find the correct measurements without taking them yourself.
The Shell Width
The bottom bracket shell – the part of your frame in which the bottom bracket is installed – has a certain width. It is relatively easy to measure, often even while a bottom bracket is installed. You only have to measure the frame from right to left, without the bearing cups or spacers. With the help of spacers, some bottom brackets can also be adapted to different shell widths. The filters will usually take this into account. Once you have selected the shell width, the number of other filters is reduced to a more manageable level.
The bottom bracket shell width can be determined quickly with the help of a caliper gauge. © bc GmbH
General Notes on Assembly Types
In addition to the shell width, the bottom bracket shells of bicycle frames differ in other aspects that you can narrow down with the “Type” filter. Since manufacturers have always introduced new installation dimensions with their own names onto the market, there is unfortunately no uniformity in terms of designations. Letters are mostly abbreviations. Numbers refer in some cases to the diameter of the bottom bracket, in others to the axle type and in still others to the bottom bracket shell width. The decisive factor is compatibility with the bottom bracket shell of the frame. The standards differ essentially in two respects: in the assembly type (screwed in with threading or pressed in without threading) and in the diameter.
When it comes to the type of mount, a distinction is made between bottom brackets that are screwed in…
... and bottom brackets that have to be pressed in (Press Fit).
Assembly Types: Threaded Bottom Brackets
Bottom brackets for threaded shells can be recognised during assembly by the fact that the inner or outer bearing cups have some kind of gearing that allows them to be gripped and screwed in with a tool. They are currently only available in three different versions, which you can distinguish relatively easily by the inner diameter of the bottom bracket shell and the direction of the threading when the bearings are removed:
| Assembly Type | Inner Diameter | Thread Direction |
| BSA | about 34 mm | Left-handed threading on the drive side Right-handed threading on the non-drive side |
| ITA | about 35 mm | Right-handed threading on both sides (rarer than BSA) |
| T45, also called Colnago Threadfit82.5 | about 44 mm | Left-handed threading on the drive side Right-handed threading on the non-drive side |
| T47 | about 46 mm | Left-handed threading on the drive side Right-handed threading on the non-drive side
|
| T47A (Asymmetrical) | same threading and diameter as T47, but bottom bracket shell is offset laterally |
Assembly Types: Threadless Bottom Brackets
In the case of threadless bottom bracket shells, the ball bearings are pressed directly or with bearing cups made of aluminium or plastic into the frame. The principle is similar to that of headsets. The term "press fit" or the corresponding abbreviation "PF" often appears in this context. You can distinguish the shell types by their inner diameter and width. The best way to measure the diameter is with a caliper gauge directly on the frame with the bottom bracket removed. Alternatively, you can also measure with a ruler or similar tool. You can then assign the following inner diameters to various types :
The various types with the same diameter differ in shell width. If you have set the appropriate filters, the selection will be reduced accordingly.
The Axle Type
The “axle type” filter lets you narrow down the selection of bottom brackets according to crank compatibility. You have the option of measuring your crank yourself or obtaining the parameters from the data sheet or the item’s description. We can sort axle types into two groups: systems where the axle is part of the crank (often: two-piece cranks) and systems where the crank axle is part of the bottom bracket. There is not a bottom bracket for every crank and frame combination. In some cases, however, bottom brackets can themselves act as adapters to mediate between systems or can be installed with the help of adapters.
Axle Types for Two-Piece Cranks
On most cranks on high-quality bikes, the axle is firmly connected to a crank arm. The dimensions from the shop filters and in the data sheet usually correspond to the diameter of the axle, which you can measure with a caliper at the point where it would rest in the bottom bracket. Alternatively, you can also measure the inner diameter of the bottom bracket – i.e. the hole through which the crank axle is inserted. The following systems are currently available:
| Inner Diameter | System |
| 19 mm | FSA MegaExo |
| 24 mm | Shimano Hollowtech II and compatible products from other manufacturers |
| 24 / 22 mm | the axle on GXP cranks from SRAM measures 24 mm on the drive side and 22 mm on the non-drive side |
| 25 mm | Campagnolo Ultra Torque and Power Torque |
| 28,99 mm | SRAM DUB, also Truvativ |
| 30 mm | various manufacturers |
| 30 / 28 mm | Praxis Works M30, the axle measures 30 mm on the drive side and 28 mm on the non-drive side |
The second important dimension is the length of the axle. If you only want to change the bottom bracket, and the crank and frame already work together successfully, then you don't need to worry about this. If you want to install a new crank, make sure that the axle is not too long or too short for the bottom bracket standard and the width of your frame. Notes in the product description or the manufacturer's documentation will help here.
When choosing a bottom bracket, the inner diameter of the bearing and the crankshaft diameter must be matched. © bc GmbH
Axle Types as Part of the Bottom Bracket
On older or cheaper bikes you will often find systems where the crank axle is part of the bottom bracket. These are usually sealed cartridges, i.e. self-contained units that disappear almost completely into the frame. The two crank arms are individually screwed onto the axle. To transmit forces, the interface between the crank arm and axle is either a tapered square profile or a geared profile. The usual systems are:
• Square (JIS) – found predominantly among Asian manufacturers (e.g. Shimano)
• Square (ISO) – found predominantly among European manufacturers
• Octalink MTB – multi-tooth with eight teeth, gearing longer than with Octalink Road
• Octalink Road – multi-tooth with eight teeth, gearing shorter than Octalink MTB
• ISIS – multi-tooth with ten teeth
• Power Spline – multi-tooth with twelve teeth
The two square variants are not compatible with each other. Having the wrong bottom bracket could damage your crank. Since they are hard to differentiate, you should check with the manufacturer of your crank to find out which square fits. In addition to the installation width of the bottom bracket itself (see above), the length of axle must be selected to match the crank. It determines the chainline, the Q-factor and ensures that your crank does not collide with the chainstays. You can find out which axle length is required for your crank either in the manufacturer’s documentation or by measuring the old axle with the crank disassembled.
Pay Attention to Bottom Bracket Options
If you have narrowed down the selection of bottom brackets via filters, you may still have different options to choose from. During the process, the characteristics and dimensions described so far may come up again. However, even if you have selected a bottom bracket without options, you should check the installation width, the assembly type and axle type again in the product description. The similarities of some names can easily lead to confusion.
Bearing Type and Quality
Varying price points of compatible bottom brackets usually differ in terms of manufacturing quality and the type of bearings used. Nowadays, encapsulated ball bearings are mostly used – often referred to as cartridge bearings. In some cases, you can replace them individually when they wear out. This saves time and money. Please also follow the manufacturer's care instructions! You can lubricate and adjust some bottom brackets yourself to get the best performance and durability. Rolling element bearings are made of either classic bearing steel, stainless steel or ceramic. While normal bearing steel is very durable when well cared-for, stainless steel has the advantage of being more resistant to corrosion (the simplified term is rust-free). If you ride a lot in the rain or wash your bike often and don’t have time to inspect and re-grease the bearings regularly, stainless steel is a good choice. Ceramic bearings are also corrosion-resistant, lighter and particularly smooth-running.
Depending on the bottom bracket, you will need different specialist tools for installing and removing.
In order not to damage the frame or bottom bracket, you should always pay attention to the torque specified by the manufacturer.
Bottom Bracket Installation and Tools
In order to work on bottom brackets and cranks, you’re going to need some special tools. There are various wrenches for assembling threaded bottom brackets and bearing cups. For threadless bottom brackets you need a press tool. Some headset tools can also be used for press fit bottom brackets with the help of adapters. For disassembling pressed-in bottom brackets, an extractor, knock-out tool or puller is required. A special case are bottom brackets for threadless bottom bracket shells where the two halves are screwed together. You don’t need a press tool for them, but a wrench like one for threaded bottom brackets. In any case, we recommend that you find out which type of tool fits your new bottom bracket before ordering. During assembly, you should pay attention to the torques and whether the manufacturer recommends using assembly grease or a screw lock. In the case of BSA and T47, it is essential to consider the left-handed threading on the right-hand side. With a new frame, you should check that the bearing seats are properly prepared. It may be necessary to prepare them with milling tools or to re-cut the threads. If you are unsure, please feel free to contact us!
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