How to Measure Bottom Bracket: Helpful Guide

This article discusses some common types of bottom brackets and describes how to measure the bottom bracket length.
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Last updatedLast updated: March 25, 2022
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It’s no secret to anyone with experience riding a bike that soft-pedaling makes the activity less strenuous. As such, how to facilitate softer pedaling is a question that should be on every rider’s mind, and the answer is a better bottom bracket, and how to measure bottom bracket size is critical to that.

This answer, however, brings up another question; what is a bottom bracket?

The bottom bracket (BB) of your bike is a hollow, cylindrical component that links the pedals’ crank arms to the bike frame. It is basically a metal axle that possesses two bearings. Each end of the bottom bracket ends in a crank into which the force exerted on pedals is transmitted.

Read on to figure out how to improve your bottom bracket without damaging your bike.

Types of Bottom Bracket

The first thing to note about bottom brackets is they aren’t all the same. They are manufactured in different sizes and types. You would need to figure out the exact type that is suitable for your bike and this is determined to some extent by the frame type.

According to Wikipedia Trusted Source Bottom bracket - Wikipedia The bottom bracket on a bicycle connects the crankset (chainset) to the bicycle and allows the crankset to rotate freely. en.wikipedia.org , the term “bracket” comes from the early history of bicycles where tube fittings were used to hold frame tubes together in lugged steel frames, which also held the spindle and bearings. As implied, the size has to be right as well, hence the necessity of bottom bracket measurements.

Size aside, though, the most common of these bottom bracket types are the press-fit bottom bracket, external bottom bracket, and cartridge bottom bracket.

Press Fit bottom bracket

How to Measure Bottom Bracket: Helpful Guide

This type of bottom bracket does not feature internal threading. They usually have a larger bearing than the bore of the frame.

The bearings are mostly industrial bearings that can be gotten cheaply. A press-fit (or interference fit) is created when these bearings are either forcefully pressed into the bottom bracket housing (as in a BB30) or inserted horizontally in a plastic shell in the housing (e.g. Pressfit BB92). The latter variant is now widespread on mountain bikes, as the frame construction tolerances can be somewhat more lenient.

Press-fit bottom brackets are quick to install, but they are notorious for wearing out quickly if the bearings are changed too often. If you’re in the market for one, the Shimano New BB71/BB72 Press Fit Bottom Bracket XT/Ultegra Variable offers improved durability and at a great price too!

External bottom bracket

How to Measure Bottom Bracket: Helpful Guide

External bottom brackets allow you to have both large bearings and a large, hollow bottom bracket spindle.

These configurations are easy to identify because you can see the aluminum cups containing the bearings located on the outside of the frame. Most have a 34.7mm x 24 English thread (BSA type), meaning the right cup is screwed counterclockwise and the left cup clockwise (aka reverse threading). The alternative Italian threading means both sides are screwed in the same direction.

The major benefit of this type, especially the modern external ones, is weight reduction. External bottom brackets allow you to have a large, hollow bottom bracket spindle. This is just as durable as a small, solid spindle but requires less steel, consequently reducing weight. Steel or high-quality steel alloys are often choice materials Trusted Source How to buy a bicycle | Sport | The Guardian Bikes are better value than ever, but it’s worth taking time to get the right one. www.theguardian.com for making brackets due to their high tensile strength and excellent shock-absorbent properties.

Though they have their perks, external bottom brackets aren’t great in terms of durability. Most people would often go for a cartridge square-taper type due to their low prices and averagely longer lifespan.

An extra tip – ceramic bearings are better known for better performance and durability than steel ones.

Cartridge type bottom bracket

How to Measure Bottom Bracket: Helpful Guide

This type of BB is commonly found on cheaper bikes.

This is a much older design consisting of two main components – the spindle and the sealed bearings. The short spindle, characteristic of this design, is fused to the sealed internal bearing to form a single unit.

Cartridge types are easier to work with as changing them doesn’t require any extra work, you just loosen and take out the old one and put a new one in, just like this Origin8 Torqlite Square Taper Bottom Bracket.

The appeal of the cartridge types is that they are inexpensive, although they are easily damaged if they aren’t torqued down properly.

Tools you’ll need to measure bottom bracket

You now know what type of bottom bracket you are using. Then comes the actual measuring. Before you take any readings, you need to make sure you have the appropriate tools nearby. You would need:

1. Metric Rulers or Calipers

This device often has two pairs of “jaws” on the main scale and is used to measure inside or outside dimensions.

2. Spindle

The spindle connects the two crank arms and is the axis around which the pedal rotates.

You would also need a wrench or screwdriver and a crank extractor to remove the bike’s crankshaft.

Measuring bottom bracket

Before we get started, it should be noted that a great BB alone does not guarantee that you’d be pedaling with ease. A stuck pedal or a rusty bike chain can also hamper pedaling. Check out this article on what to do when you have a stuck pedal or this one if you are dealing with a rusty bike chain.

Having clarified that, to measure your bike’s bottom bracket, it first has to be accessed by removing the crankshaft and pedal assembly. This axis allows the crankset to operate with reduced friction.

Preparing

How to Measure Bottom Bracket: Helpful Guide

It is crucial to remove the crankshaft not to damage the bike.

As previously mentioned, the bottom bracket is hidden beneath the crankshaft and pedal assembly. Both of those need to be taken care of before the bottom bracket measurements can be taken.

To remove the crankshaft;

  • Turn your bike upside down, or use the bike stand. Check how the pedal is connected to the crank (whether with a nut or screw) on the crankshaft. Then using a wrench or screwdriver that matches the connecting screw or nut, unscrew the piece, and keep it safe.
  • Behind the first nut/screw is another that secures the crankshaft. Slide the crank extractor into the crankshaft opening and tighten that nut too.
  • With the crank extractor in place, turn the bushing till it fastened properly.
  • Once the pedal feels loose, slide the crankshaft off the bottom bracket.

After completing these steps, you can now figure out how to measure the bottom bracket height.

Measuring the shell

How to Measure Bottom Bracket: Helpful Guide

You should see a horizontal, cylindrical casing – that is, the shell.

If you hadn’t done so earlier, turn your bike upside down.

With your metric caliper, measure the length of the cylindrical casing from one end to the other. Take care not also to include the length of the spindle for this stage. You can also use a ruler for this step, but calipers are generally easier.

The length of the shells is usually in millimeters, and most bracket shells are generally 68, 70, or 73 millimeters long.

Measuring the spindle

How to Measure Bottom Bracket: Helpful Guide

The common length ranges between 113-122 millimeters.

On how to measure bottom bracket spindle length, repeat the same process used for measuring the shell. This time though, align the caliper jaws on either spindle on both sides and take your readings.

If your bike’s crank is connected with nuts instead, don’t measure the connected parts of the spindle.

Final thoughts

By now, you should be familiar with the process of measuring your bottom bracket length and the bottom bracket spindle length.

To recap, we discussed the common types of bottom brackets, starting with the press-fit bottom bracket, then the external bottom bracket, and the cartridge type.

You also know that you need to dismantle the crankshaft to measure the crankshaft before reaching the bottom bracket spindle. An important reminder – when measuring the bottom bracket length make sure not to include the spindle pins in the measurement.

If you found this guide on how to measure bottom brackets helpful, you can also check out our other bike-themed guides like; how to raise handlebars on a road bike.

References

1.
Bottom bracket - Wikipedia
The bottom bracket on a bicycle connects the crankset (chainset) to the bicycle and allows the crankset to rotate freely.
2.
How to buy a bicycle | Sport | The Guardian
Bikes are better value than ever, but it’s worth taking time to get the right one.
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