Headset bearings form an integral part of a bicycle’s steering system. They are usually found in pairs. A single bearing at the top of the head tube and another bearing at the lower end of the head tube. Headset bearings operate at very low speed and their primary function is to support axial load (ie the weight of the bike and rider) as opposed to radial load which is the turning of the handlebars. They are commonly angular contact bearings with some chamfers used for locating geometric positions. Occasionally, some large deep groove ball bearings are used. Do not hesitate to contact us if you need assistance to determine the correct replacement. Measuring Headset bearings are notoriously problematic because they do not follow conventional ISO or JIS standards for numbering. It is always advisable to measure the physical bearing if at all possible. Use of the bearing number to size the bearing is an option, but it is not recommended. Headset bearings are often imperial derived (inch) sizes. The use of a vernier caliper or digital caliper is highly recommended. A micrometer would be advantageous. The figure below shows all of the measurements required to correctly identify a headset bearing. Inner Diameter (this does not need to be extremely accurate) Outer Diameter Height (width) Internal (Inner) Chamfer Angle in degrees External (Outer) Chamfer Angle in degrees
