What is a Bottom Bracket on a Bicycle & How to Maintain it

Jack Dreyer | Thursday 13th July 2023 12:00pm

A technician is working to remove the bottom bracket of a bicycle.

If you’ve made the leap to foot-powered transport and are getting stuck in with bike maintenance, you’ll likely have heard of a bicycle’s ‘bottom bracket’ – but what exactly is it, what does it do, and how can you keep it in top condition?

Let’s find out.

The bottom bracket is for pedals

While the name might indicate that it has something to do with where you sit, the bottom bracket is actually the bearing system that keeps your bike’s foot pedals locked in and turning in a regular, dependable way.

The frame on most modern bikes will usually be a welded central body that’s in a vaguely diamond shape. This is the solid bit that everything else gets attached to. This solidness helps to maintain and distribute all the stresses of the bike so that your wheels turn properly, you can steer your handlebars easily, and you can pedal effectively.

And here’s the most important thing: because the bike is entirely powered by the force of your pedalling, if there’s play in the pedals then you get a significantly reduced amount of force being transferred to the wheels, the potential to damage parts of your bike like the gears, or even to get injuries in your feet or ankles. If you’ve ever ridden a bike with front suspension, you’ll know how much more effort it takes to accelerate quickly – because the suspension cushions your forward force.

A bottom bracket helps stop this from happening through wiggle in the pedals.

How a bottom bracket works

A technician tests the play in a bicycle's bottom bracket.

The frame of the bike is usually made with a hole through it for the bottom bracket to sit in. They’re usually held in place either with a threaded connection or through a pressure-fit mechanism and they then let the pedal (or ‘crank’) spindle pass through and lock in.

In this way, the spindle is able to turn freely within the bearings while the bearings are held sturdily to the bike. Importantly, the thread is usually the opposite direction of the pedal force – so that as you’re pedalling the bike you never accidentally unthread the pedals.

How to tell which bottom bracket your bike has

This seems like it’d be an easy answer but there’s no real standardised design for bottom brackets – they’re one of the components where essentially every bike manufacturer has a different design. They all work in much the same ways, and some are compatible with certain standardised tools, but often you’ll have to consult your bike manufacturer’s guidance on what your bottom bracket is and how to get the tools to remove it.

When do you need to replace a bottom bracket?

Like with any components that have to withstand a lot of force, the ball bearings in the bottom bracket mechanism eventually wear down and don’t turn as efficiently. As they wear down, they don’t maintain a good connection between the inner and outer rings of the bearing – so they end up slipping. This is actually what makes older washing machines so loud; the broken bearings cause the drum to keep banging against the case while it’s spinning.

You usually need to remove bottom brackets if you need to replace them or, in some cases, in order to top up the grease within the spindle casing.

Need an expert?

A big issue with lots of bike maintenance is that the lack of standardisation means you often need to get specific tools for specific bikes. If looking after your bottom bracket feels daunting – we’ve partnered with bike specialists, Fettle, to deliver bike maintenance services at your local Kwik Fit centre.

Book an appointment with a Fettle technician.

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