How to Measure a Bottom Bracket for a Mid-Drive eBike Conversion

Not so long ago the only bottom bracket decisions you needed to make were spindle length, shell width, and English or Italian threading. It can seem overwhelming when facing the multitude of “standards” on bike brands, especially when each one claims to be the best option.

Electric bike motors sit in one of three places, between the pedals in the bottom bracket, the front hub, or the rear hub. So, what is the bottom bracket? How do you measure it? Continue reading to find out more.

Measuring a bottom bracket is easy. Get a set of calipers or an accurate ruler (preferably in mm). With the bike flip upside-down measure the frame at the bike crank. Be sure to measure the frame (usually painted) and not the bottom bracket lock nuts.

The distance is usually:

• 68mm (most common)
• 100mm (Fat tire bike)
• 120mm (Fat tire bike)

Heck it’s probably easier to watch my video below.

What is a Bottom Bracket?

The bottom bracket on a bike is a component that connects the bike’s crankset to the frame. This allows it to rotate independently of the frame. These brackets are normally threaded, but can also be pressed, into the frame’s bottom bracket shell. This is a sleeve that connects the tubes to the bike frame’s front. Without this bracket, your bike transmission cannot rotate.

This portion of a bike requires maintenance at times. Some can be rebuilt or adjusted, while others are designed to be replaced instead of repaired. When the bottom bracket fails, the bearings will make unpleasant noises and can develop play. (Source)

What Are the Common Types of Bottom Brackets?

If your bike is 10 years old or newer, odds are it has one of three standard bottom brackets: press fit, threaded, or thread thru. The best way to know what kind of bottom bracket your bike has is to refer to the owner’s manual or the manufacturer’s website.

A press fit bottom bracket is also called an unthreaded bottom bracket. This type uses a smooth shell to slide into the frame. A cup slides into the bracket shell and is pressed together. Press fit is most common in carbon frame bikes.

Threaded bottom brackets rely on threads to tighten them. This category can be further divided into cartridge style, adjustable cup and cone, and external bearing systems. Most modern bikes use an external bearing system. This system requires the bottom bracket shell and bottom bracket cup to thread together.

Thread thru is like the press fit in that the bottom brackets combine to form a smooth, non-threaded shell with threaded cups. Instead of threading to the frame, thread thru bottom brackets thread into themselves. This connects the right and left cup together inside the smooth bottom bracket shell.

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What are Standard Measures for Bottom Brackets?

When choosing a bottom bracket for a mid-drive motor powered eBike, there are six sizes available, but three are the most sold. Sizes come in 68, 73, 92, 100, 110, and 120 millimeters. 68mm, 100mm, and 120mm are the most sold sizes. These three can fit all other sizes.

For example, the Bafang BBS02 68mm model can fit both 68mm and 73mm shells. Make sure you do not buy a bottom bracket until you are fully sure of the shell type and diameter.

What Are Spacers for Bottom Brackets?

Spacers for the bottom brackets are used to adjust the position of the spindle and add extra clearance. This allows you to fine tune the chain line. If your inner chain ring is too close to the chainstay, you may add spacers to move the crank outward.

Spacers are a great way to perfect the chain line on your bike. They can also help when replacing symmetrical bottom brackets from an older non-symmetrical type of bracket. (Source)

Learn more about spacers here: https://bafangusadirect.com/products/bafang-bbshd-bottom-bracket-spacers

Why Do Some Bikes Have Larger Bottom Brackets?

With the popularity of fat tire bikes, wider bottom brackets have become more common. The larger tires require the chain stays to be wider apart forcing the crank arms to be wider and in turn a wider bottom braket.

The proper bottom bracket is a necessity on all bikes, whether it is a mountain bike or a fat bike. Fat bikes need their own option. The shell width is larger than other bike types, typically 121mm to 131mm. The skinnier the tire the smaller the shell width, typically.

Carbon bikes usually have wider shells, too, so it pays to double check the measurements on these before you replace the bottom bracket.

RIDE e MTB Tip: Converting a carbon fiber frame bike to electric is a bit more difficult. The mid-drive motor needs to be torqued down really tight onto the bottom bracket shell. I use the words, you almost can’t over tighten it. This will most likely cause damage to a carbon frame.

How Long Do Bottom Brackets Last?

Most bottom brackets are built to last. Some say they get up to 10,000 miles from their bottom brackets. This all depends upon the type of riding you are doing. If your bracket is constantly exposed to moisture, or your ride often in mud or dirt, it may need to be replaced more often.

Bottom brackets are designed to last for several thousands of miles. Most cyclists need to replace theirs every one to two years, depending on the miles traveled and conditions. If your bike has an older bottom bracket, you may be able to grease this as part of your bike’s regular maintenance. Newer bottom brackets are designed to be replaced instead of fixed. (Source)

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How to Measure the Bottom Bracket

You want to measure the inside of the bottom bracket shell for your frame. This can be done with calipers to ensure proper measurement. It will likely be between 68mm and 73mm. The owner’s manual of your bike or the manufacturers website may also have this information. It is best, however, to measure it yourself so you can be doubly sure you are purchasing the proper bottom bracket.

If you are unsure about measuring the bottom bracket yourself, you can take it to a bike shop and ask them to measure it for you. This will help give you some peace of mind that you are looking for the proper size.

Finishing Up

The bottom bracket is a critical component of a bike that connects the chain rings and crank arms to the frame. This lets the crank set rotate freely and with reduced friction. This makes pedaling easier. There are different sizes and types of bottom brackets for bikes.

Ensure when purchasing an eBike conversion kit for your bike that you get a bottom bracket that properly fits your bike frame. You can measure the bottom bracket using calipers or a traditional measuring tool. You can also reference your bike’s user’s manual or check the manufacturer’s website. This will ensure you get the proper bracket for your bike!

RIDE e MTB Pro Tip: With eBikes growing in popularity learning all the terms and definitions can be a bit overwhelming. Let me help with this comprehensive glossary. 👉 eBike Glossary

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