Bearings

Bearings are the rings of small steel balls that let the moving parts of a BMX turn smoothly. They are found in the hubs, the bottom bracket, the headset and the pedals. They come in two main forms: sealed cartridge bearings, and loose ball bearings that run in cups and cones.

What it does

A bearing lets one part spin around another with very little friction while carrying load. On a BMX the loads are harsh. Landings, grinds and side impacts push hard on every bearing, and dirt and water try to get in. Good bearings keep the wheels rolling, the cranks turning and the bars steering without play or grinding.

When bearings wear, the part develops play (a knock or wobble) or roughness. Worn bearings also wear the parts around them, such as hub shells, axles and frame bearing seats.

Types and standards

Sealed cartridge bearings

A sealed cartridge bearing is a self-contained unit. The balls sit between an inner and an outer ring (the races), held apart by a cage, with grease packed inside and a seal on each side. The whole unit presses into the hub, frame or part.

Loose ball bearings

Loose ball bearings, also called cup and cone or unsealed bearings, use separate balls, often held in a retainer, that run between a cup and a cone. The rider sets the adjustment, called preload, by tightening a cone or cup. They were standard on older BMX bikes and are still found on some budget complete bikes.

Comparison

	Sealed cartridge	Loose ball
Dirt and water	Kept out by seals	Gets in more easily
Adjustment	None needed in the bearing	Needs correct preload
Servicing	Clean and regrease, or replace the unit	Strip, clean, regrease and adjust
Cost	Higher	Lower
Wear	Replace the bearing, the part is usually unharmed	Worn cups or cones can mean replacing the hub or part
Common on	Aftermarket and mid to high-end bikes	Older and budget bikes

Bushings

Some plastic pedals use a bushing, a plain sleeve with no balls. The cheapest pedals may run on bushings alone, while some better nylon pedals pair a bushing with sealed bearings. Others, such as Odyssey's Twisted Pro PC, run on loose balls. A bushing on its own is simple and cheap, but wears faster and feels less smooth than bearings.

Where bearings are used

Part	Common setup	More detail
Front hub	Two sealed bearings on most aftermarket hubs	Front hub
Rear hub	Sealed bearings in the shell and driver	Cassette hub, freecoaster hub
Bottom bracket	Sealed bearings for Mid and Spanish. Loose balls or sealed cups on American	Bottom bracket
Headset	Sealed angled bearings in an integrated head tube	Headset
Pedals	Sealed bearings, loose balls or a bushing	Pedals

Sizing and compatibility

Sealed bearings are sized by three measurements, all in millimetres:

1. Bore. The inside diameter, which must fit the axle or spindle.
2. Outside diameter. Which must fit the hub, frame or part.
3. Width. Which must suit the spacing of the part.

Bearing codes

Most cartridge bearings carry a standard code printed on the seal. The same code means the same dimensions from any maker. Some common examples:

Code	Bore	Outside diameter	Width
608	8 mm	22 mm	7 mm
6802	15 mm	24 mm	5 mm
6900	10 mm	22 mm	6 mm
6903	17 mm	30 mm	7 mm

These are examples of how the codes work, not a guide to which bearing your part uses. Check the code on the old bearing, or the maker's spec, before buying.

Letters after the code describe the seals. "2RS" usually means a rubber seal on both sides. "ZZ" usually means metal shields on both sides. Rubber seals keep water and dirt out better and suit BMX.

Headset and bottom bracket bearings often use their own sizes and angled edges, so buy them by the frame's standard. See headset and bottom bracket.

Materials

• Chrome steel. The usual ball and race material. Strong and cheap, but rusts if water gets in.
• Stainless steel. Resists rust better. Can be slightly less hard than chrome steel.
• Ceramic hybrid. Ceramic balls in steel races. Expensive, and the small gain in smoothness rarely matters on a BMX.
• Seals. Rubber contact seals keep out the most dirt. Metal shields are lower friction but less protective.

Precision ratings such as ABEC describe how tightly a bearing is made, not how tough it is. For BMX, a good seal and a solid brand matter more than a high rating.

How to choose

1. Read the code on the old bearing, or check the part maker's spec.
2. Buy sealed bearings with rubber seals for hubs, bottom brackets and headsets.
3. Pick a known bearing brand. Cheap bearings can notch, rust or seize quickly.
4. If your bike has loose ball bearings and they are worn out, consider upgrading the whole part to a sealed design.
5. Replace bearings in pairs in the same part, so both sides wear evenly.

Maintenance

• Spin parts by hand regularly. Roughness, grinding or play means the bearings need attention.
• Do not aim a pressure washer at hubs, the bottom bracket or the headset. It forces water past the seals.
• Sealed bearings can often be cleaned and regreased by lifting a seal carefully, but replacing them is cheap and reliable.
• Loose ball bearings need cleaning, fresh grease and correct preload. Too tight and they wear fast. Too loose and they knock.
• Press bearings in and out with the right tools, pushing on the race that is being fitted, so you do not damage the new bearing.
• Coastal and wet riding wear bearings faster. Check them more often if you ride near the beach or in the rain.
• See how to service BMX bearings for step-by-step instructions, and BMX maintenance basics for a routine.

Related parts

Bottom bracket, headset, front hub, cassette hub, freecoaster hub and pedals.