How much do you know about the Bottom Bracket Shell and Bottom Bracket with so many specifications?

If you want to know the specifications and adaptations of various parts of the bike, the bottom bracket shell and bottom bracket may give you a headache. 

For most riders, it’s confusing to have a clear understanding of the bottom bracket and bottom bracket shell, especially for some new amateurs. In this article, we have compiled the relevant specifications and assembling of the bottom bracket and bottom bracket shell. 

The bottom bracket shell is a short and large diameter tube, which is commonly located at the intersection of the lower end of the front triangle and the rear triangle of the frame. It connects the seat tube, down tube and chain stays of the frame, which runs side to side and holds the bottom bracket. 

The bottom bracket connects the crankset to the bike and allows the crankset to rotate freely. It contains a spindle to which the crankset attaches. The bearings allow the spindle and crankset to rotate. Bottom bracket bearings fit inside the bottom bracket shell. 

1. Common bottom bracket shell standards

External bearing bottom bracket

(1) British threaded bottom bracket shell (BSA) 

One of the most traditional bottom bracket shell standards, it is more common on road and mountain bike frames, with a width of 68mm or 73mm (DH frames may have a width of 83mm). It uses right-hand threading for the left side and left-hand threading for the right side.

(2) Italian thread bottom bracket shell (ITA) 

It is specially used for road bike frames and needs to be equipped with an Italian bottom bracket, which is generally found on old Italian road bikes. It uses the right-hand thread on both sides. The outer diameter of the thread is 36 mm and the pitch is 1.058mm. 

(3) Press-fit type BB90/BB92 bottom bracket shell

Both shells use the press-fit method to install. They have the same inner diameter which is 41mm. The width of BB90 and BB92 are 89.5mm and 92mm respectively, and gaskets are generally required for BB90 during installation. 

(4) Press-fit BB30 bottom bracket shell

It does not have the thread. The width of the shell is 68mm and the inner diameter is 42mm. The crankshaft axis diameter is 30mm. It is jointly promoted by FAS and Cannondale.

(5) Press-fit BB86 type bottom bracket shell

This shell is developed by Giant based on the BB90 bottom bracket shell. The width is 86.5 mm and the inner diameter is 41 mm. 

2. Types of bottom bracket 

Bottom brackets can be divided into threaded and press-fit types based on installation standards. Threaded bottom brackets include square taper bottom brackets, splined bottom brackets, and some external bearing bottom brackets. Press-in bottom brackets are all external-bearing bottom brackets. 

(1) Square taper bottom brackets

Splined bottom bracket

The interface consists of a spindle with square tapered ends that fit into square tapered holes in each crank. Not all square taper crank and bottom bracket combinations are compatible. Although nearly all spindles use a 2-degree taper, there are two competing standards, defined by the measurement across the flat at the end of the spindle. The JIS size is used by Shimano and most other Asian manufacturers. The ISO size is primarily used by Campagnolo and other European manufacturers. 

(2) Splined bottom bracket

Square taper bottom bracket

The connection position between the splined bottom bracket and the crank is a multi-tooth structure. Compared with the square taper bottom bracket, it can provide more contact surfaces to achieve better bite efficiency. The common splined bottom bracket has 8 teeth or 10 teeth.

(3) External bearing bottom bracket

Bottom bracket shell

External bearing bottom brackets are also called integrated bottom brackets. They are generally used with an integrated crankset in the upscale market. The bottom bracket and the crankset are a hollow one-piece structure. It not only can provide powerful bite force but also is lighter and has better stiffness compared with the square taper and spined BB, which can greatly improve the rider’s cadence. It is also easy to maintain and has high reliability. 

3. Adaptation of bottom bracket and shell

(1) BSA shell and bottom bracket 

Square taper bottom bracket, splined bottom bracket and external bearing bottom bracket are suitable for British threaded bottom bracket shells. When matching with the square taper or splined bottom bracket whose width is 68mm, the minimum width of the shell should exceed 108 mm. If you want to install an external bearing bottom bracket, usually two 2.5mm gaskets are required on the right side of the bottom bracket and one gasket is required on the left side. However, when the shell width is 73mm, only one 2.5mm gasket is required on the right side. 

(2) BB86/90/92 shell and bottom bracket 

These types of bottom brackets support all integrated cranksets that can be used on British threaded bottom brackets. Press-fit BB91, GXP bottom brackets, and Campagnolo’s BB90 bottom brackets are suitable but they cannot support BB30 bottom

(3) BB30 shell and bottom bracket 

BB30 shell is suitable for a variety of bottom brackets and has good compatibility. It can also be converted into an ordinary British threaded bottom bracket shell by using the adapter mechanism.
Thanks for watching. 

Don’t hesitate to leave a message regarding the bottom bracket shell and bottom bracket if you have anything to share. We are looking forward to your idea and story.




[1] ‘Cycling Bible’ Editorial Board. Cycling Bible [M]. 1st Edition. Beijing: Machinery Industry Press, 2013.

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