Bottom Bracket

A bottom bracket is the set of bearings in the frame's shell that lets the crank spindle spin. Each frame is built for one bottom bracket standard. The standard has to match the frame and the cranks.

What it does

The bottom bracket sits in the round tube at the base of the frame, called the shell. The crank spindle passes through it and spins on the bearings. This keeps the cranks turning smoothly while the frame takes your weight and the force of landing.

BMX bottom brackets get more stress than road bike ones. They take hard landings, side loads from grinds, and dirt and water. Poor bearings or loose cups can make the cranks wobble or creak.

Types and standards

There are four main BMX standards. They differ in how the bearings fit the shell and in which spindle size they carry.

Standard	How the bearings fit	Notes
American	Cups press into a large smooth shell (about 51 mm inside).	The original BMX standard, found on older and entry level frames. Takes one-piece cranks, or 19mm spindles with adaptor cups.
Spanish	Bearings press into a smooth 37 mm shell.	Popular for years. Used with 19mm and 22mm spindles.
Euro	Threaded cups screw into a threaded shell (about 34.8 mm, 24 tpi).	Less common on BMX.
Mid	Bearings press into a smooth shell of about 41 mm.	Common on newer frames. Available for 19mm and 22mm spindles.

Spanish and Mid bearings press straight into a smooth shell. This keeps the parts count low and suits modern frames. American BBs use pressed-in cups in a larger shell, and Euro BBs use threaded cups that screw into the frame. Shell dimensions are close between some standards, so never rely on looks alone.

Bearing sizes

The bearing's inner diameter has to match the crank spindle.

Spindle	Bearing inner diameter
19mm	19mm
22mm	22mm

The outer diameter and width depend on the standard and shell. Do not guess these. Buy a kit made for your frame standard and your spindle size. A set often includes bearings, spacers and a retaining ring or cups.

How to tell which one your frame has

1. Check the frame spec. The maker's website or the original build sheet will list the standard. This is the easiest route.
2. Look at the shell. If you can see threads on the shell, it is a Euro BB. If the shell is smooth, it is American, Spanish or Mid.
3. Measure the shell inside diameter. Remove the bottom bracket and use calipers. Compare your measurement with the maker's spec, because sizes between standards can be close.
4. Check the cranks. The spindle diameter (19mm or 22mm) tells you what bearing inner size you need.
5. If you are unsure, take the frame and cranks to a bike shop. They can match a bottom bracket kit for you.

Sizing and compatibility

• Spindle diameter: 19mm or 22mm, matching your cranks.
• Shell standard: matched to the frame. This decides how the bearings fit.
• Spacers: used to set the chainline and spacing between the bearings and the crank arms.
• Chainline: the sprocket should line up with the rear driver. Adjust with spacers if needed. See sprocket and chain.
• Tool needs: pressed-in bearings need a press or a bearing tool. American cups need a cup press, and threaded Euro cups need a cup tool.

How to choose

1. Identify your frame's standard first.
2. Pick the spindle size of your cranks.
3. Buy sealed bearings from a known brand.
4. Choose a kit that includes all the spacers you need.
5. Check the length of the spindle against the bearing spacing.

If you are buying a frame and cranks together, check they use the same standard. Mixed up standards are a common building mistake. See how to build a BMX bike from parts.

Maintenance

• Spin the cranks with the chain off. They should turn smoothly and quietly.
• Check for side-to-side play by pushing a crank arm. Any play means the bearings or cups need attention.
• Keep bolts and cups tight, and grease thread-in cups.
• Replace the bearings if they feel rough or gritty.
• Avoid pressure washing the bottom bracket area.
• See BMX maintenance basics for more.

Related parts

Cranks, sprocket, chain and frame geometry.