One of the most important parts of your bike (and anything else) is the bottom bracket. This is a very important element: the bottom bracket is a metal shaft with two bearings. Cranks are placed at each end, which absorb all the strength that we exert on the pedals. The shaft allows the cranks to rotate with little friction. Thus it is easy to understand that the bottom bracket is a set of parts subjected to great mechanical stress, so it is important to always keep it in perfect condition. There are several types and sizes of bottom bracket. In this article we will explain their types and characteristics, and how to know which bottom bracket you need according to the size of its shell.
Threaded bottom bracket measurements according to bottom bracket shell
The bottom bracket shell has a cylindrical shape, and according to the measurements of its inner diameter (A), its length (B) and the direction of the thread we must choose one type of bottom bracket or another. These are the most common standards of threaded bottom brackets:
BSA (also called BSC or Standard English):
This is the most common. There are two sizes for the width, 68 mm or 73 mm, and the inner diameter is 34.8 mm. Both the left and right threads go anti-clockwise. A 73 mm bottom bracket can be mounted in a 68 mm box using spacers, but it is not possible to mount a 68mm bottom bracket in a 73mm case.
Less and less used, but still used in some brands. It has a diameter of 36 mm and a width of 70 mm with the threads going clockwise.
Common on classic bikes, the thread is like the Italian standard, clockwise, and the dimensions are 34.8 mm in diameter and 68 mm wide.
Types of threaded bottom bracket
Squared bottom bracket
They are the most common on mid-range bikes. They are good quality and last a long time. The shaft/bearing assembly is located in a closed cartridge from which the ends of the shaft, which are square in shape, stand out.
There are two types of square bottom bracket depending on the shaft shape: ISO and JIS. The ISO, with a width of 12.5 mm which is used by European manufacturers, and the JIS, the Japanese standard used by Shimano with a width of 12.63 mm.
Threaded bottom bracket with external bearings
These bottom brackets are easy to identify, as the bearings are located on the outside of the frame. Most have 34.7 mm x 24 English thread (BSA type). The right vessel is screwed into the bottom bracket case clockwise, and the left is screwed clockwise. As for the bottom bracket shaft, the Hollowtech II system is used, consisting of an axel attached to the right crank.
Detachable bottom bracket
Now hardly ever used, the bottom bracket is removable by means of counter-nut wrenches. You can remove all parts and replace them individually
Other threaded bottom bracket systems with internal bearings
The Shimano Octalink or ISIS systems were the evolution between the detachable bottom bracket and the square bottom bracket. They were already mounted on a cartridge, but the diameter was larger and there were ends with different numbers of gaps or grooves, improving the friction between shaft and crank.
Types of direct mount bottom bracket
For some years now, medium and high-end road and mountain bikes have already got threadless bottom bracket axels. Internal bearings press on the bottom bracket. The advantage over the threaded brackets is that their assembly is simpler, in addition to providing more rigidity and saving a few grams of weight.
On direct-mounted bottom brackets, the shaft is usually mounted on one of the cranks, in a system similar to Shimano’s Hollotech II. It is a system which is more sensitive to small intolerances, so the adjustment of the bottom bracket shell on the frame must be accurate to avoid the insertion of dust, water or dirt that causes annoying noises and creaking when pedaling.
There are several types of systems. The most common are:
Also called “true BB30”, it has no vessels. Bearings are mounted directly on the frame shaft and are subjected by circlips or by press-inserting them with a special hammer that does not damage the square.
Standard Shimano shaft in which the bottom bracket shaft stays fastened by the bearings, which are placed with greater separation from each other.
The Press-fit 30 system combines the best of the BB30 and Pressfit into a single bottom bracket. The bearings, in this case, are mounted inside the bottom bracket box, as in the BB30 system, but are housed in nylon vessels that are pressed into the bottom bracket, maintaining the rigidity of the Press-fit system.
The SRAM bottom bracket has recently been introduced with the DUB (Durable Unifying Bottom Bracket) system, which consists of unifying the best parts of the two most commonly used systems today: the rigidity of the 24 mm diameter steel shaft and the lightness of the 30 mm aluminum shaft. And it does so by means of an aluminum shaft of 28.9 mm diameter, which according to SRAM is the perfect measure.
To achieve this, it has created specific bottom bracket bearings which are compatible with any frame with threaded BSA bottom bracket, PressFit, BB30 or Pressfit30. This joint promotes the durability of the components by being more airtight. Also if you change the frame of your bike, and it has a different bottom bracket, by simply changing the bearings, you can keep the cranks.
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