How to choose the right bottom bracket

The bottom bracket is the bearing system that sits at the bottom of your frame and allows your chainset to spin. It'll eventually wear out and need replacing, but there's a bewildering number of different options to choose between.

Here's how to make the right choice.

When does the bottom bracket need replacing?

Your bike’s bottom bracket has a tough life! It has to handle all of the power you put through the pedals, and its bearings are working constantly while you're turning the cranks. Also, positioned down at the bottom of the frame, it's right in the firing line of water that's sprayed up from wet roads. 

Not surprisingly, the bottom bracket's performance will eventually start to deteriorate. It might start to creak and/or click, or the bearings could begin to feel rough when you are pedalling.

Most bottom brackets aren’t serviceable. They're designed as sealed units to be thrown away and replaced when they no longer do their job properly. For example, the Shimano Deore threaded bottom bracket has “do not disassemble” printed on it. 

If your bottom bracket starts to creak, you could try removing it from the frame and reinstalling, but if the problem is inside the bearing and it's a non-serviceable design, it's time for a new one.

Know your bottom brackets

There are lots of different types of bottom bracket out there. No, really, loads of them. Essentially, though, they can be divided into threaded and press fit varieties. 

Your bike will be designed to take a particular type. You can’t fit a threaded bottom bracket into a frame that’s designed to take a press fit bottom bracket, or vice versa.

Beyond that, there are various different types of threaded bottom bracket, and various different types of press fit. Even bottom brackets that look similar can have slightly different dimensions that mean they’ll only work with certain bikes and certain chainsets. 

Threaded bottom brackets

A threaded bottom bracket screws into a frame that is made with threads cut inside the bottom bracket shell. One common type is a cartridge bottom bracket, such as the Shimano BB-UN55

Most cartridge bottom brackets are two part: the bearings and spindle are contained in the main body which screws into your frame’s bottom bracket shell from the drive side, and an adaptor screws in from the other side.

The ends of the spindle are shaped in various different ways to work with different chainsets. The Shimano BB-UN55 bottom bracket is square taper, for instance, so it’ll work only with square taper chainsets, while the VP604 bottom bracket uses what’s called an ISIS splined design, so the chainset needs to be made with holes that match.

You also need to know the width of your bike’s bottom bracket shell – 68mm is common – and the length of the cartridge bottom bracket you’re after. That info will be written on the body of the one you’re removing or, failing that, you can easily measure with a ruler. We offer the Rockrider square taper bottom bracket in various lengths from 110mm to 131mm, for example.

Other threaded bottom brackets have external bearings and a larger diameter hollow spindle is attached to one of the cranks. The idea is to allow the manufacturers to increase stiffness and reduce weight on sports-type bikes. Of course, these bottom brackets work with different chainsets.

The Shimano Deore threaded bottom bracket, for example, is an inexpensive option that fits both 68mm and 73mm bottom bracket shells and works with common 24mm-diameter chainset spindles.

Press fit bottom brackets

Rather than screwing into place, press fit bottom brackets push into frames that are specifically designed to take them. 

Press fit systems were introduced to allow frame manufacturers more scope for design innovation, allowing them to bring down weight and increase stiffness. On the flip side, some people report more creaking with press fit designs.

There are lots of press fit standards used by different frame and component manufacturers and they’re not interchangeable. 

For example, a Shimano Ultegra Hollowtech II press fit bottom bracket will fit into an 86.5mm-wide bottom bracket shell and will work with a 24mm-diameter chainset spindle, while a SRAM DUB 92mm press fit bottom bracket is for mountain bikes with an 89.5/92mm bottom bracket shell width and a SRAM DUB 29mm spindle. You need to make sure that you replace like with like.

Replacing your bottom bracket

This will require a special tool that's specific to the type of bottom bracket system used.

The B’Twin Internal Bottom Bracket Removal Tool has 20 teeth to work with many of the most common cartridge bottom bracket designs, for instance. B'Twin's Hollowtech II Bottom Bracket Tool is for Shimano Hollowtech II external bottom brackets and others that use the same 16 external notches, such as many from SRAM.

Most threaded bottom brackets use an English thread. This means that the non-drive side loosens anticlockwise, as usual, but the drive side loosens clockwise, which is opposite to normal. Many external bottom brackets have arrows printed on them to remind you of this.

Make sure you clean your frame’s internal threads and, if the threads of your new bottom bracket don’t have dry threadlocker applied, add anti-seize compound or something like B’Twin’s Teflon Bike Grease. Then tighten your bottom bracket in place according to the manufacturer’s instructions.

If you're not an experienced mechanic and you need to replace a press fit bottom bracket from your frame, we'd suggest getting a professional – such as one of our in-store mechanics – to do the job with the right tools in order to avoid damage. It sometimes requires a lot of force!