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What Is A Bike Bottom Bracket? Full Guide

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Many cyclists are able to carry out basic repairs on their bike, but some repairs are more of a pain than others.

Bike parts that contain bearings tend to annoy us when they wear down. You’ll hear worrying noises as you ride. Could it be your knees?

Whenever you hear creaking noises while cycling, the bike bottom bracket is always a suspect. Maybe it isn’t your joints, after all.

This article will tell you all you need to know about bottom brackets and hopefully demystify them a little so you can service or replace them.

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What Is A Bike Bottom Bracket?

The bottom bracket is a cylindrical component that holds your cranks together via their axle and lets them freely rotate using a bearing assembly. It sits in the bottom bracket shell, or BB shell (“BB” being a common abbreviation for bottom bracket).

Without a bottom bracket, you’d have no low-friction means of pedaling and delivering power to the drivetrain. Despite being a mundane bike part, it has a vital role to play. Let’s look at it a little more closely.

Bottom Bracket Parts

Keeping in mind there are various bottom bracket standards, these are the parts you might encounter in a bottom bracket:

  • Inner sleeve – central portion of BB assembly.
  • Axle or spindle – part that joins the two cranks (often fixed to the drive side crank from new on modern cranksets).
  • Adjustable cup – the removable left-hand cup that allows access to bearings for servicing (where applicable).
  • Fixed cup – right-hand BB cup, usually left in situ at all times.
  • Lockrings – keep the cups in position as you crank the pedals.
  • Bearings – a set of bearings on either side of the bottom bracket which are theoretically non-serviceable on Shimano cartridge-style BBs.

Although bottom brackets may start to seem complex looking at the list above, in reality they are often fairly simple to install or replace. Servicing them is a little more involved, but eminently doable with some YouTube tuition.

One of the biggest headaches you’ll face with bottom brackets is identifying which type you have. Because of their non-standardized nomenclature, you’ll often encounter multiple names for the same type of BB.

Video: Replacing A Shimano Hollowtech II Bottom Bracket

Main Types Of Bottom Bracket

Below, we’ll identify some of the most common types of bottom bracket.

External Threaded BSA

This BB is also known as a BSC, BC, ISO, English Threaded or Standard Threaded bottom bracket.

The Shimano Hollowtech II is a BSA-type bottom bracket with external threaded bearings. An example is the Shimano BBR60 Ultegra BSA Bottom Bracket. A nice thing about bottom brackets is that upgrades cost relatively little.

Having the bearings sit outside the bottom bracket shell (i.e., external threaded) has benefits, like extra rigidity through the increased distance between the bearings, and extra diameter in the axle for better stiffness.

These bottom brackets come in a variety of sizes, measured by the BB shell width or the width of the bottom bracket between the external bearing cups. The sizes are:

This type of bottom bracket has bearing cups with a 24 TPI pitch thread and a 1.375” (34.92mm) diameter shell. Aside from Shimano, other BB models of this type include SRAM 24/22mm GXP and Campagnolo Ultra Torque, Power Torque 25mm.

External Italian Threaded

Italian BB-threaded frames are much rarer than English threaded, and aren’t necessarily found on Italian bikes. The specifications of an Italian threaded bottom bracket include:

  • 70mm shell length
  • 36mm diameter x 24 threads per inch (TPI)

English threaded bottom brackets are incompatible with an Italian threaded bike, which has a larger-diameter shell. Even though they’re similarly designed, an Italian BB like the Dura-Ace BB-R9100 Bottom Bracket is removed by turning both cups counterclockwise.

Again, this bottom bracket type includes SRAM 24/22mm GXP and Campagnolo Ultra Torque, Power Torque 25mm.

T47 (Internal or External)

Introduced in 2015, the T47 was designed to offer the benefits of a Pressfit bottom bracket but without the creaking problems that can result from tolerance issues in frames.

The T47 comes in external or internal variants, meaning the bearings are mounted outboard or inboard, respectively.

Bottom brackets like the fully serviceable Chris King Threadfit T47 30x are akin to Pressfit PF30 BBs with threads. The “x” suffix refers to external, which would be suitable for a typical 68mm road bike BB shell with a T47 thread.

While it’s possible to tap a thread into an unthreaded Pressfit BB shell, most people that buy these will probably own a bike with a T47 threaded shell.

Video: Tapping A Thread Into A BB Shell For A T47

This bottom bracket makes sense to custom metal-frame builders in that it allows a wider area for welding and more scope for oversized tubes . The 46mm inner shell diameter is large enough to accommodate many sizes of crank axle.

The T47 is suitable for a variety of BB shell widths:

  • 68mm
  • 73mm
  • 83mm
  • 100mm

You can use a tool like the Park Tool BBT-47-16 to install or remove a T47 bottom bracket.

BB30 Press Fit Bottom Bracket

The BB30 bottom bracket is designed for 30mm diameter crank axles, though it can be used with other sizes. Unlike all the BBs mentioned so far, it fits into a non-threaded bottom bracket shell.

There are many standards of Press Fit BB, and BB30is among the more common. As their name suggests, Press Fit bottom brackets are held in place by the pressure of the bearing or bearing cup against the inside of the marginally narrower shell.

Many brands of bike use a BB30 shell in some of their range, including Boardman, Cannondale and Specialized. Cannondale developed BB30 as an open-source standard, but it also uses proprietary standards like BB30A and BB30-83 Ai.

SRAM is a major manufacturer of BB30 bottom brackets, as evidenced by the SRAM Dub BB30 Bottom Bracket for 68mm road bike BB shells. An MTB version is also available with a 73mm width.

The BB30 has a 42mm inner diameter, hence it’s also known as the PF42.

PF30 Press Fit Bottom Bracket

Similar to the BB30 BB is the PF30 Press Fit Bottom Bracket, except it has a larger 46mm inner diameter. For that reason, it’s also called the PF46.

PF30 bottom brackets and BB30 bottom brackets are not interchangeable.

A PF30 BB like the FSA PressFit 30 has sealed bearings that press into cups, and the cups press into the bottom bracket shell. In a BB30 bottom bracket, the 6806 bearings press directly into the BB shell of the frame, which requires tighter tolerances.

BB86/92 Bottom Brackets

The BB86/92 standard of bottom brackets has multiple names, including PF24, PF41, PF86, PF92, BB107/PF107, BB121, PF121 and BB132.

The names and numbers are derived from the various bottom bracket widths this standard accommodates as well as the inner shell diameter (41mm) and axle width it was designed for (24mm).

Canyon is one of the bike manufacturers that uses this standard extensively. Note that PF92 (BB92) is a mountain bike bottom standard and PF86 (BB86) is for road bikes, but a  product such as the Race Face EXI BB92 Bottom Bracket fits either shell width.

What Bottom Bracket Fits My Bike? How To Measure Up

It can be confusing trying to identify your bottom bracket type if you wait until there’s a problem to do so. It’s worth noting this information when or before you buy the bike.

One reason you might want to follow the advice above before buying a bike is that certain bottom brackets on certain bikes have a reputation for creaking.

There are a few ways you can tell which bottom bracket fits your bike. If your bike has an external threaded cartridge-style bottom bracket, a model number or standard is likely to be clearly visible. Then all you’ve got to do is Google it and buy similar.

Not all bikes are going to make it easy to ID the bottom bracket, so you might have to get a caliper out and measure it.

Measuring Up

The first thing you’ll measure is the bottom bracket shell from the underside of the bike. How wide is it? Common measurements are 73mm for mountain bikes and 68mm for road bikes, but there are variations.

“Old-fashioned” square taper bottom brackets will have an axle/spindle running through them, which you also have to measure end-to-end. Obviously, you’ll need to remove the cranks first to do this, which will involve taking out a fixing bolt.

Video: How To Measure A Bottom Bracket

Tool-Fitting Clues

Another key way to identify your bottom bracket is to look at tool-fitting clues, which requires you to first remove the cranks. See if the BB has notches, splines, pin holes or wrench flats, which will help you identify the BB with a few minutes of online research.

A bottom bracket with none of these identifiers will be a Pressfit BB.

Note: some “thread-thru” or “thread-together’ bottom brackets that fit into a Pressfit non-threaded shell look like conventional threaded bottom brackets. You may have to remove the bottom bracket or consult your local bike shop to safely ID these.

Bike Bottom Brackets FAQs

Here are some frequently asked questions about bottom brackets:

Do All Cranks Fit All Bottom Brackets?

No. Cranks with 24mm axles will theoretically fit into many frames, but they must still be matched to the correct bottom bracket.

What Causes A Bottom Bracket To Creak?

One common cause in Pressfit bottom brackets is loose tolerances in some bikes frames where the bore diameter is too big in the BB shell and allows movement. Worn or dry bearings are another cause.

How Often Should You Grease A Bottom Bracket?

That depends on how often you ride your bike and in what conditions. Some bottom brackets, like Shimano Hollowtech II cartridge-style BBs, are not officially designed for servicing but can last for thousands of miles before replacement.

How Can I Tell If My Bottom Bracket Needs Servicing?

You’ll notice a rough feeling and grinding noise when pedaling, and/or you may detect play in the crank arms. Many modern BBs are sealed units that aren’t intended for servicing, but you can replace them with serviceable alternatives.

Why Are There So Many Bottom Bracket Standards?

From the beginning of the 21st century, manufacturers have strived to create bottom brackets with bigger, more durable bearings and thicker, stiffer crank spindles. Many bottom bracket forms have been created as part of this evolution. That being said, the non-standardized naming convention for BBs exaggerates their number.

Conclusion

If you start hearing an irritating abrasive or creaking noise from your bike, it could be your bottom bracket. It’s best to rule out other, simpler causes first, like seat posts, pedals or quick-release skewers. But don’t be afraid to tackle the BB if necessary.

This article set out to give you a greater understanding of bottom brackets or at least help you to identify the one on your bike.

Please feel free to add comments of your own or share with family and friends.

Glenn H
Glenn H
When I’m not contributing articles to Bike Push, I can often be found cycling on the rural roads around me. If I can help you benefit from bicycling in some small way, I’ll consider it a win.

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