Our Guide to Buying: Bottom Brackets

Where is the Bottom Bracket on a bicycle?

Your bottom bracket is housed in the bottom bracket shell of your bike frame, it connects your chainset to the frame and allows the cranks to turn smoothly. When selecting a replacement bike bottom bracket you need to make sure that you get the correct model. It’s not an easy task as there are a number of different bicycle bottom bracket standards in addition to different quality models. This guide will help you get the right one to fit your bike and also your pocket.

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What are the main types of bottom bracket?

There are two main types of bicycle bottom bracket, namely those that thread into your frame and those that are pressed in. We will go into each type of bottom bracket in more detail, to help you discover the best choice for your bike.

Threaded Bottom Brackets

Threaded bottom brackets come in a number of different versions. Cheaper and older designs use bearings that sit inside the frame with a captive (fixed) axle that the crank arms are tightened onto. There are a multitude of standards for the captive axle. The most common and cheapest is a square interface (square taper), but Shimano offer a multi splined (Octalink) interface and there is also a standard called ISIS, though these latter two are rarer, having been superseded by other designs.

What do modern threaded bottom bracket designs use?

Most modern threaded designs now use a three-piece design that has a pair of larger diameter bearings that sit outboard of the bottom bracket shell, which an axle that is permanently fixed to one of the crank arms threads through.

This design has a large advantage in that the bearings sit outside the bottom bracket shell and can be of a much larger diameter, making them much more hard wearing. The axle can also be made thinner and lighter. These designs include Shimano Hollowtech II and SRAM GXP, although these two systems aren’t compatible.

What bottom bracket do I have

To find out the size of bottom bracket needed, measure the inside of the bottom bracket shell in your frame, it will be 73mm, 70mm or 68mm. Some older frames may have Italian threaded bottom brackets, instead of the more modern English.

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