Threaded #8: A guide to sizing cartridge bearings

A bearing is to a bicycle what a smear of peanut butter is to a P&J sandwich – you’d just be in a jam without it. Previously, I covered all the fancy and specialist tools for smoothly removing worn and crusty cartridge bearings from a bicycle, and in the next edition of Threaded I’ll finally get to the punchline with how to install those spinning rings of steel.

In this edition, I’ll be covering the all-important aspect of knowing what bearings you need, how to measure them, and common pitfalls to avoid. Consider this a tips-heavy and tool-lite version of Threaded. Yep, a world first!

This is part two of the Threaded series of replacing Cartridge bearings. If you haven’t already, check out part one here. A quick word of warning, many of the topics covered here are typically the domain of professional mechanics. If you’re yet to master derailleur tuning and wheel truing feels like witchcraft, please consider employing a professional to replace cartridge bearings. Regardless of that, I hope you find this article interesting.

As covered previously, cartridge bearings (aka, sealed bearings or deep-groove ball bearings) have become the most common type of bearing on the modern bicycle aimed at enthusiast or performance usage. These bearings are designed to be a self-contained unit and are made up of an inner race, an outer race, the ball bearings, and in most cases, a retainer for the ball bearings and protective seals.

This article focuses solely on cartridge bearings. However, other bearing types are also commonly found. Many bikes sold today still feature cup-and-cone loose ball bearings, nylon bushings, or similar alternatives to cartridge bearings. These alternatives are often simpler to service but can introduce unique intricacies. Park Tool is a good resource for beginner guides related to other common bearing types, such as cup-and-cone hub bearings. 

OK, back to cartridge bearings. No standards exist for what size or type of bearing a bicycle component must use. It’s common for a single rear hub to have multiple different bearing sizes within it, or a mountain bike’s full suspension linkage to use a different size at every pivot location. And then, just as a gotcha, occasionally you’ll find custom-sized bearings that may be doubled in width, have a different inner diameter, or an extended inner race. Needless to say, it’s a topic that deserves a dedicated article (like this one!).

Adding to the stress, there are instances where the bearing codes printed don’t correlate with the actual size, or more commonly, those codes are no longer legible due to corrosion and abrasion. And it’s here that I hope the lessons of this article are most helpful. 

In case you didn’t know, the Wide World Web has become a fantastic resource for accessing information. More than ever before, manufacturers are uploading dealer manuals, product diagrams, or schematics of their products for you to locate required product numbers needed for service. The best examples don’t just give you the manufacturer’s part number for an otherwise standard bearing but rather the required bearing dimensions or bearing type. 

However, there are also times when a manufacturer has decided to keep such information exclusively for its dealer network (hate this, #righttorepair!) or perhaps considers a bearing-based component to be non-serviceable (commonly seen with freehub bodies, pedals, and some headset cups). In these cases, it’s normal for a mechanic to dissemble a component to determine what replacement cartridge bearing is required. 

In my experience, crawling through forum threads or YouTube for answers on bearing sizes can be a huge time suck with questionable results. Rather, I always try to go to the source. My standard method is to Google for “Brand x Model x Schematics” (or “dealer manual”, or “exploded view”). For extra efficiency, you can also add “.pdf” to your Google search to prioritise results for official PDF documents; sometimes, these will be from the brand or at other times, you may locate bootleg copies intended for dealers only.