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.
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 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splined (Octalink) interface and there is also a standard called ISIS, though these latter two are rarer, having been superseded by other designs.
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.
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.
Press-fit bottom brackets that don’t require a threaded shell on the bike frame are becoming more popular. They allow designers to make composite frames that don’t require tough inserts for the bearings to sit in. There are a number of standards here too. Shimano’s Press-Fit 92 uses the same spacing as their external, threaded Hollowtech II system and is cross-compatible with those cranks using the right 24mm spindle.
There is also BB30, which uses even larger diameter bearings for a stronger, stiffer and lighter axle, but this requires a dedicated crank or the use of a converter such as the Wheels Manufacturing adaptor.
The bearings are highly important, as their quality dictates how much energy is lost as friction and how long they’ll last. If they are within your budget fully sealed cartridge bearings are preferable. They are non-adjustable, fit and forget units that don’t need any maintenance but have to be thrown away when they are worn out.
More expensive units use bearings that have much higher tolerances, resulting in less friction and longer life. Mountain bike bottom brackets often have a larger amount of sealing for the increased demands of off-road use, which increases friction as a side effect. More lightly sealed, lower friction ‘Road’ versions are also common.
It is now possible to buy bottom brackets that have the bearings and races made from a super hard and strong ceramic material rather than the usual steel. The longer life is a big positive, but many manufacturers also claim reduced friction, which makes them attractive to serious time triallists and other riders for whom even slight gains in performance count.
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