How to Measure a Bottom Bracket: Height and Length Explained

This article discusses some common types of bottom brackets and describes how to measure the bottom bracket length.
By
John Watson
John Watson
Writer
John is an experienced cycling enthusiast and a great asset when it comes to writing skills. He's a Bachelor of Arts and a talented journalist. John is in charge of our blog read more
reviewed
Reviewed By
Jessica Kingston
Jessica Kingston
Expert Consultant
Jessica is our expert consultant on all things connected to biking - gear, technique, you name it. Being a pro cyclist in the past, she knows exactly how things work in and read more
Last updated: January 13, 2023
Bike The Site is reader-supported. We may earn a commission through products purchased using links on this page. Learn more about our process here

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 know what size bottom bracket you need and 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. Bottom brackets 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 bb 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 a Bottom Bracket: Height and Length Explained
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 a Bottom Bracket: Height and Length Explained
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 a Bottom Bracket: Height and Length Explained
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.

How to Measure Bottom Bracket Length and Height

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 a Bottom Bracket: Height and Length Explained
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.

How to Measure the Shell

How to Measure a Bottom Bracket: Height and Length Explained
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.

How to Measure Bottom Bracket Spindle Size

How to Measure a Bottom Bracket: Height and Length Explained
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.

Bottom Bracket Standard Size Chart

The chart below lists the majority of the bottom bracket standard measurements for current frames.

BB Standard Frame BB Shell
Inner Diameter
Common Shell Width Frame BB
Shell Threaded?
Notes
BSA Threaded
  • Standard Threaded
  • English Threaded
  • 1.37 in x 24 TPI
  • 33.6mm – 33.9mm
  • 68mm (road)
  • 73mm (mountain)
  • 83mm (Downhill)
  • 100MM (Fat Bike)
  • Yes
Most common threaded BB shell system.
  • 1.37 in x 24 TPI
  • Left hand thread on drive-side
  • Right hand thread on non-drive
TREK BB90
  • 37mm
  • 90.5mm
  • No
Trek direct-fit system. Bearings are pressed directly into frame.
TREK BB95
  • 37mm
  • 95.5
  • No
Trek direct-fit system. Bearings are pressed directly into frame.
BB86 / BB92
  • PF24 (Chris King)
  • PF41 (Hope)
  • PF86
  • PF92 (Trek)
  • BB107/PF107
  • BB121
  • PF121 (Trek)
  • BB132
  • 41mm
  • 86.5mm
  • 90mm
  • 91.5mm
  • 104.5mm
  • 107mm (DH / Mountain)
  • 121mm (Fat Bike)
  • 132mm (Fat Bike)
  • No
Designed to be used with 24mm spindle diameter cranks (Shimano HollowTech II), hence the PF24 designation. But, can also use GXP cranks in these.

Some brands call this PF41, describing the frame shell diameter instead of the crank spindle diameter.

Removable bottom bracket cups.

BB30
  • Specialized Alloy OSBB
  • 42mm
  • 68mm (road)
  • 73mm (mountain)
  • 84.5mm (Specialized MTB Alloy OSBB)
  • 83mm (Cannondale MTB)
  • 120mm (Cannondale Fat Bikes)
  • No
6806 bearings are pressed directly into the frame.

May use removable c-clips as bearing stops, or the bottom bracket shell may have molded or machined bearing stops.

Symetrical shell

BBRight (Direct Fit)
  • 42mm
  • 79mm
  • No
Cervelo R5ca Frames only. BB shell is 11mm wider on the non-drive side.

6806 bearings are pressed directly into the frame.

Specialized OSBB Alloy
  • 42mm
  • 68mm (Road)
  • 84.5mm (Mountain)
  • No
Same as BB30.

6806 bearings are pressed directly into the frame.

Cannondale BB30a
  • 42mm
  • 73mm
  • 83mm (Cannondale BB30A-83)
  • No
Asymetrical BB shell is +5mm wider on the non-drive side (BB30A). For the BB30A-83 frames, the BB shell is 5mm wider on the drive side and +10mm wider on the non-drive side.

6806 bearings are pressed directly into the frame.

PF30
  • PressFit-30
  • 46mm
  • 61mm (Specialized OSBB/FACT)
  • 68mm (Road)
  • 73mm (Mountain)
  • 83mm (Downhill)
  • 100MM (Fat Bike)
  • No
Removable bottom bracket cups.
BBRight (Press Fit)
  • 46mm
  • 79mm
  • No
Cervelo Frames except R5ca. Also used by 3T.

Removable BB cups.

BB shell is 11mm wider on the non-drive side.

Specialized OSBB Carbon (Road)
  • 46mm
  • 61mm (Road)
  • No
Specialized carbon road frames with 61mm wide shells for use with Specialized FACT cranks.

Removable BB cups.

Cannondale PF30a
  • 46mm
  • 73mm
  • 83mm (Cannondale PF30A-83)
  • No
Asymetrical BB shell is +5mm wider on the non-drive side (PF30A). For the PF30A-83 frames, the BB shell is 5mm wider on the drive side and +10mm wider on the non-drive side.

Removable BB cups.

Cannondale Asymmetric Integration (Ai)
  • 46mm
  • 83mm
  •  No
Cannondale Asymmetric Integration (Ai) found on Scalpel-Si and F-Si System frames. BB shell is 83mm wide, but symetrical

Removable BB cups.

T47
  • Threadfit 30i
  • M47 x 1 Threaded
  • approx. 46mm ID
  • 68mm
  • 73mm
  • 83mm
  • 100mm
  • Yes
Frame’s bottom bracket shell is threaded with M47 x 1 threads.
  • Left hand threads on the drive-side
  • Right hand threads on the non-drive.
386EVO*
  • 46mm
  • 86mm
  • No
* Essentially a PF30 with 86.5mm wide shell.
392EVO*
  • 46mm
  • 92mm
  • No
* Essentially a PF30 with 92mm wide shell.
Colnago ThreadFit82.5
  • 41mm
  • 86.5mm
  • Yes*
* Colnago standard with a threaded BB shell. Aluminum cups are threaded into the frame that result in the BB86 standard (41mm ID x 86.5mm width).
Look BB95
  • 65mm
  • 90mm
  • No
Unique to Look frames. Requires bearings and hardware from Look
Wilier BB94
  • BB93
  • 37mm
  • 93mm
  • No
Unique to Wilier. Bearings press directly into the frame. Similar to Trek BB90
Truvativ ISIS Overdrive
  • M48 x 1.5 Threaded
  • 68mm – 100mm
  • Yes
Little used threaded BB shell standard. Usually found on Cyclops indoor bikes. *NOT COMPATIBLE WITH T47*
Italian
  • 36mm X 24tpi
  • 70mm
  • Yes

Crankset Standard Sizes

After you’ve identified the bb size on your mountain or road bicycle, identify the key crankset dimensions in order to select the correct bottom bracket for your frame.

Crankset Spindle Diameter Bearings Notes
Shimano HollowTech II
  • 24mm
  • 24×37 (Drive)
  • 24×37 (Non-Drive)
Current Shimnao crank specification. Spindle length varies depending on type of crank (road, Mountain, Triple, etc.).

Two-piece system. Crank spindle is pressed into the drive side crank arm.

GXP
  • 24mm/22mm
  • 24×37 (Drive)
  • 22×37 (Non-Drive)
SRAM / Truvative crank system. Spindle diameter is 24mm on the drive side, and 22mm on the non-drive side where the splines are. The crank slides completely through a 24×37 drive side bearing and stops against a 22×37 non drive side bearing.

Two-piece system. Crank spindle is pressed into the drive side crank arm.

Uses wave washer on drive side of crankset to take up any play in crank.

SRAM DUB
  • 29mm
  • 29×42
SRAM introduced the DUB cranks in 2018. 29mm diameter spindles. Meant to be used across all BB standards.
BB30
  • 30mm
  • 6806 (Drive)
  • 6806 (Non-Drive)
30mm spindle diameter cranks. Used in both BB30 and PF30 bottom bracket shells.
PF30
  • 30mm
  • 6806 (Drive)
  • 6806 (Non-Drive)
30mm spindle diameter cranks. Used in both BB30 and PF30 bottom bracket shells.
Rotor 3D
  • 24mm
  • 24×37 (Drive)
  • 24×37 (Non-Drive)
Splined interface much like Shimano HollowTech II.
Rotor 3D24
  • 24mm
  • 24×37 (Drive)
  • 24×37 (Non-Drive)
Splined interface much like Shimano HollowTech II.
Rotor 3D30
  • 30mm
  • 6806 (Drive)
  • 6806 (Non-Drive)
Rotor 3D+
  • 30mm
  • 6806 (Drive)
  • 6806 (Non-Drive)
Rotor 3DF
  • 30mm
  • 6806 (Drive)
  • 6806 (Non-Drive)
RaceFace EXI
  • 24mm
  • 24×37 (Drive)
  • 24×37 (Non-Drive)
Splined interface much like Shimano HollowTech II.
RaceFace Cinch / SL
  • 30mm
  • 6806 (Drive)
  • 6806 (Non-Drive)
RaceFace Cinch is a three-piece system. Different length spindles can be used depending on the frame/bottom bracket configuration
Specialized S-Works FACT
  • 30mm
  • 6806 (Drive)
  • 6806 (Non-Drive)
Cannondale Hollowgram SL / SI / SISL2
  • 30mm
  • 6806 (Drive)
  • 6806 (Non-Drive)
Cannondale crank for use in BB30A or PF30A frames. 3-piece crankset with separate, replaceable spindle. Can be used in BB30, PF30, BBRight, 386EVO frames with the appropriate length spindle.
FSA BB386EVO
  • 30mm
  • 6806 (Drive)
  • 6806 (Non-Drive)
Designed to work in frames shell width from 68mm to 86.5mm.
FSA BB392EVO
  • 30mm
  • 6806 (Drive)
  • 6806 (Non-Drive)
Designed to work in frames shell width from 68mm to 92mm.
FSA MegaEXO
  • 24mm
  • 24×37 (Drive)
  • 24×37 (Non-Drive)
Splined interface much like Shimano HollowTech II.

NOTE: FSA MegaEXO crank spindle diameters can be as large as 24.07mm. These cranks will not work in normal 24×37 bearings. Please see our 24.07 x 37 bearing for use with these cranks.

Praxis M30 THRU BB
  • 30mm
  • 6806 (Drive)
  • 6806 (Non-Drive)
Crank spindle diameter is 30mm from end to end.
Praxis M30 BB
  • 28mm / 30mm
  • 6806 (Drive)
  • Proprietary Non-Drive Bearing
Crank spindles are 30mm and then have a step-down to 28mm to lock on the non-drive side bearing.
Cane Creek eeWings
  • 30mm
  • 6806 (Drive)
  • 6806 (Non-Drive)
Road version designed to work in frames shell width from 68mm to 86.5mm.

Mountain version designed to work in frames shell width from 73mm to 92mm.

Campagnolo Ultra Torque
  • 25mm
  • 6805N (Drive)
  • 6805N (Non-Drive)
2-piece crankset using a hirth joint in the middle of the spindle. Bearings are pressed onto the spindle next to each crank arm instead of pressed into the bottom bracket cups.
Campagnolo Power Torque
  • 25mm
  • 6804 (Drive)
  • 6804 (Non-Drive)
Spindle is permanently attached to the drive-side crank arm. Drive side bearing is pressed on the spindle up against the crank arm. Non-drive side bearing is pressed into the non-drive side bottom bracket cup.
Campagnolo Over Torque
  • 30mm
  • 6806 (Drive)
  • 6806 (Non-Drive)
Similar to other 30mm spindle cranks. Bearings are pressed into the bottom bracket cups.

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.
Leave a comment

Your email address will not be published. Required fields are marked *

X