Bottom Bracket Explained: What It Is and When It Needs Replacing

The bottom bracket is one of the most important — and most neglected — components on your bike. It sits at the lowest point of the frame, takes enormous forces, and is constantly exposed to water and dirt. Here's what you need to know.

What is a bottom bracket?

The bottom bracket (BB) is the bearing assembly that connects the cranks to the frame. It sits inside the bottom bracket shell — the tube at the bottom of the frame where the seat tube, down tube, and chainstays meet. Every time you pedal, the bottom bracket bearings allow the crank axle to spin smoothly.

Signs your bottom bracket is worn

• Creaking when pedalling: the most common symptom — a rhythmic creak in time with your pedal strokes, especially under load
• Grinding or roughness: remove the chain and spin the cranks by hand — they should spin freely and silently
• Side-to-side play: grab a crank arm and try to rock it side to side. Any movement means the bearings are worn or loose
• Clicking under load: especially when climbing or sprinting
• Stiff rotation: the cranks feel notchy or resistant when spinning

Types of bottom bracket

There are several types, and compatibility depends on your frame and crankset:

Threaded (BSA / English)

The most common type. Cups screw into the frame with standard threads. Reliable and easy to service.

• Found on most road, hybrid, and mountain bikes
• Easy to replace — cups thread in and out
• Cartridge (sealed) or loose ball bearing versions

Press-fit (BB86, BB90, PF30, BB30)

Bearings press directly into the frame without threads. Common on modern carbon frames.

• Can develop creaking if the frame tolerances aren't perfect
• Harder to install and remove — needs specialist tools
• More prone to contamination in wet conditions

Square taper

An older standard where the crank attaches to a square-shaped axle. Still found on many budget and vintage bikes.

• Simple, reliable, and cheap to replace
• Cartridge units are sealed and maintenance-free until they wear out

How long does a bottom bracket last?

• Casual riders: 3–5 years
• Regular commuters: 1–3 years
• Off-road / wet conditions: 6 months to 2 years
• Press-fit in wet conditions: can be as little as 6 months

Bottom bracket life depends heavily on conditions. Bikes ridden in rain and mud wear out bottom brackets much faster than dry-weather bikes.

Why you shouldn't ignore it

A worn bottom bracket doesn't just creak — it can damage the frame. On threaded frames, a loose or worn BB can damage the threads in the shell, making future replacements difficult. On press-fit frames, worn bearings can enlarge the bore, causing permanent creaking.

Can I replace it myself?

Threaded bottom brackets are straightforward with the right tools (BB tool and a large wrench). Press-fit removal and installation requires specialist tools and experience — it's easy to damage the frame if done incorrectly.