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A Guide to How to Measure Headset Bearings

How to Measure Headset Bearings

The sealed headset bearings are the most important part of a bike’s steering system. The function of these bearings is to keep the handlebars and fork straight while transferring the torque from the front wheel to the frame, which in turn transfers it to the rear wheel. If your seals are worn down or defective, then water and dirt can get into them and cause rusting, corrosion, stiffness or even failure. This guide will help you measure sealed headset bearings on all types of bikes!

What you need to Measure 

If your bike has a branded headset, it may have an easily identifiable model number that makes sourcing the right replacement bearing easy. If not, then you’ll need to either get in touch with its manufacturer or identify what bearings are needed based on their specs. And if they’re in good condition and all markings can be found outside of them (such as numbers), simply inspect these pieces for this information so no further work is required!

However, the model numbers are small and more than likely become permanently hidden by corrosion or being run loose. Sealed and cartridge bearings are found in many modern bottom brackets and hubs, with three key measurements; external diameter, internal diameter, and height. 

Modern sealed headset bearings are typically quite different to those found in your hubs and bottom brackets, which is because the bearings are angular. This allows for off-axis loads that would normally cause them to skip or seize up completely with a ball bearing design.

A headset bearing has those same three measurements as above, but then you need to know the angle of the inner and outer races. It is imperative that these angles be correct for a smooth ride; if not, your bike will shake when you pedal or brake hard! The bicycle industry has commonly stuck with 36º and 45º here – though there are various combinations- so it’s important that they’re measured accurately in order to keep everything running smoothly. Brands such as FSA have long offered to measure tools for this task, but with some knowledge about using vernier callipers (a simple business card can also do!), anyone should be able to measure their own bearings without any difficulty at all.

Measuring Inside, Outside and the Height 

A vernier calliper is a highly useful tool for measuring decimal points of a millimetre, such as when you need to measure the width or thickness of something. Unfortunately, there’s not always an accurate ruler around so it’s easy and quick with this handy instrument!

With the vernier calliper and the worn bearing in hand, it’s important to make note of the three major dimensions of the bearing; 

  • The Internal Diameter – Measure the widest portion of the inside of the bearing while keeping the tool square to the flat edge of the inner bearing race.
  • The External Diameter – By measuring the widest portion of the outside of the bearing while keeping the tool square to the flat edge of the outer bearing race. 

After measuring the external and internal diameter, you will need to measure the height of the bearing by simply clamping the vernier over the bearing. 

How to Measure Bearing Angles 

You can measure the angle of your bearing by using a rectangular object like a business card. If it fits perfectly against both sides, then you have 0º bearings and if there’s any gap in between them, that means they are not at 45º. 

The trick for measuring the outside angle (most of which are 45º) requires you to have a spare bearing with a known 45º angle. Here you simply place the old bearing against the new and try to form an upright right triangle from them. If they’re both 45° bearings then your square corner is complete, but if one or both bearings don’t make it at exactly 90 degrees, that’s when things get interesting!

Matching the Size and Common Headset Bearing Types 

The trick for measuring the outside angle (most of which are 45º) requires you to have a spare bearing with a known 45º angle. Here you simply place the old bearing against the new and try to form a 90º angle from them. If they’re both 45° bearings then you’ll have an even corner – if one or two is 36°, however, your angles may not be square!

For Further Information on Headset Bearings, Contact the Team

If you need help measuring your headset bearings, call Aire Velo Bearings. No matter what type of bike you have, we can find the right parts for it! We know that sometimes buying a new set of bearings is not in everyone’s budget and so our experts are always happy to offer their expertise on how to measure them without any cost involved. And if all else fails with figuring out which size will work best for you or your bike, give us a call today