The bottom bracket (the famous Bottom Bracket or BB in English) is the set of axle, bearings and various parts that joins the rods and thanks to which we can transform our pedals into a turn that moves the transmission of the bicycle hub. Around this term there is some confusion. For example, in some outlet stores online include in this denomination to set crankset. This is because in many modern systems the BB axis is attached to the right crank (in some to the left), so the set of cranks and plates is included in the bottom bracket concept, since the axle is a fundamental part of the same.
Another fundamental term is the bottom bracket shell (Bottom Bracket Shell) , since it largely determines the type of bottom bracket that we can mount on our bike..

It is a short tube of the box inside which is the bottom bracket, in a different configuration according to the system in question, as we will see later. Traditionally, it has internal threads on both sides to mount it, but it has also been extended for some time also bottomless boltless boxes. Therefore, this will be the first criterion to differentiate bottom bracket systems.


It is the group that most interests the cyclist and the urban cyclist, since it is the one that we will normally meet.

According to the width of the box and type of thread we find bicycle hub in three main standards:

BSC (British Standard Cycle, also called BSA or English standard) , which has become an international ISO standard and is the most common. There are two measurements for the width: 68 mm or 73 mm. The internal diameter is 1.37 “(34.8 ≈ 35 mm), with a thread pitch of 24 threads per inch, 1.06 mm per thread and reverse thread (anti-clockwise tightening) on the right side . There is the possibility of mounting a 73 mm bottom bracket in a 68 mm bottom bracket using spacer washers, but not vice versa.

Italian standard, less popular but still used by certain brands. The bottom bracket shell measures 70 mm wide and 36 mm in diameter, with a thread pitch of 24 threads per inch, 1.06 mm per thread and normal tightening on both sides.


French and Swiss standards. They are old, but I mention them for vintage lovers.

They have 68mm width, 35mm diameter and 1mm thread thread, which in the case of the Swiss is reverse on the right side.

There are basically two systems or configurations within the threaded bottom brackets in terms of the shaft / bearings assembly. Systems where the bearings go inside the bottom bracket and systems where the bearings go in external cups.

A) Systems with internal bearings (the oldest but still extended)

1- The old pedals that can be disassembled completely. That is, by means of a specific wrench we can loosen the counter-nut and nuts to extract all the parts (cups, bearings and shaft). To dismantle it does not have much mystery since it is a typical system of anti-nuts, cone nuts or cups, loose bearings or in bearings and shaft. It can be done to perform a maintenance (greased) as they are not perfectly tight or to replace damaged parts.

2- Pedaliera of “cuadradillo”. A name not very technical for the most widespread system of recent decades, falling into disuse but still frequent in low and medium ranges. They are long-lived and there are good quality. The axle / bearings assembly is included in a watertight cartridge, from which the ends of the axle that have a quadrangular shape protrude, hence the name. eye! There are two standards of squared according to their exact form: ISO and JIS . ISO is often used by European manufacturers. The square has a width of 12.5 mm. The JIS, Japanese standard of brands such as Shimano, has a width of 12.63 mm. The difference is negligible, but enough to generate possible compatibility problems with the connecting rods. It is possible to mount cranks for ISO bottoms on a JIS bottom bracket, but not the other way around, because even if the slack is laughable it will end up giving problems.

An advantage of this system is that we can play extensively with the chain line since there is a wide range of shaft length measurements (113mm is a very common one). For its price, universality and variety, it is a pedal that every cyclist must know.

3- Other systems with internal bearings mounted on a cartridge but that are no longer used are Isis and Octalink. Basically, they are evolutions of the previous system with an axis of greater diameter and the ends with different number of grooves or grooves, what increases the solidity of the union of the crank with the axis. Octalink (Shimano, with two different versions depending on the depth of the slots) was more successful than Isis.

4- The novelty introduced by Shimano to remove the bearings out of the bottom bracket box joined the other manufacturers (BSA, Truvativ, Campagnolo) by different standards with different names and that differ in the diameter of the shaft and the shape and number of end grooves where the connecting rod (which can be the right or the left) joins. I do not enter this jungle, in the network you can deepen easily.

Types of Bicycle Bottom Bracket