Bottom Brackets

What is a bottom bracket and what is its main purpose?

The bottom bracket (BB) consists of the bearings that sit inside your frame's bottom bracket shell. It allows your crank to rotate friction-free. Since BMX riding exerts enormous forces on this area during landings and grinds, bottom brackets must be manufactured to be particularly robust and precise.

How is a bottom bracket constructed?

Sealed Bearings

The actual bearings that sit tightly in the frame (or are screwed in). They are sealed against dirt.

Tube Spacer

A tube that sits between the two bearings inside the frame. It prevents the bearings from being crushed laterally when tightening the crank bolts.

Cone Spacers

Dust covers placed on the outside of the bearings. They protect against dust and ensure the correct distance to the crank arm.

Washers

Thin shims to finely adjust the chainline (chain alignment).

What types of bottom brackets are there?

The type of bottom bracket is determined by the frame.

Mid BB

The absolute standard in modern freestyle BMX. The bearings are pressed directly into the frame (41mm diameter). Large, robust, and durable.

Spanish BB

Also press-fit, but the bearings are slightly smaller (37mm diameter) and lighter than the Mid BB. Popular for clean looks.

Euro BB

These bearings are screwed into the frame (threaded). Used almost exclusively in BMX racing today.

US BB (American)

The old standard with very large bearing cups (51mm) that are pressed in.

China BB

A specific standard with a smaller outer diameter (31.1mm) primarily used in Flatland frames to save weight and allow for compact designs.

What materials are used in bottom brackets?

Hardened Steel

The balls and races of the sealed bearings are made of extremely hard steel to withstand high loads.

Aluminum / Plastic

Cone spacers are often made of CNC-machined aluminum for a premium look or durable plastic (composite), which creates less friction during grinds.

Which bottom bracket is right for my needs?

To find the right bottom bracket, you need to know two measurements: Which type does your frame require? (Outer diameter) and what axle diameter does your crank have? Tip: If you switch from 19mm to 22mm (or vice versa), you must replace the bottom bracket, even if the frame remains the same.

How do I install a bottom bracket?

1. Clean the bottom bracket shell in the frame and apply some grease to the bearing seats.
2. Press the first bearing in straight (ideally with a press tool, or carefully with a block of wood and a hammer).
3. Important: Insert the tube spacer! Do not forget this, otherwise the bearings will break.
4. Press the second bearing into the other side. Make sure the tube spacer is centered.
5. Place the cone spacers on the bearings before sliding the crank axle through.

How do I maintain and care for a bottom bracket?

• Avoid direct high-pressure water spray on the bearing seals.
• When removing the crank, wipe old dirt off the cone spacers and bearings.
• A bottom bracket is a wear part. As soon as it runs rough or wobbles, it should be replaced.

How do I identify and fix common problems?

• Creaking when pedaling: Often the bearing is not broken, but grease is missing between the bearing and frame or axle and bearing. Remove, clean, regrease.
• Crank turns with difficulty: The tube spacer was likely forgotten or is too short, causing lateral stress on the bearings when tightened.
• Play (wobble) in the crank: The bearings might be worn out or spacers are missing between the bottom bracket and crank arm.

What do the specifications mean for bottom brackets?