Measure Your Bottom Bracket for a Mid-Drive

For a mid-drive you measure two things on the bottom bracket shell: its width and its inner diameter, and you check whether there is a thread. Bafang BBS and Tongsheng TSDZ fit BSA shells 68 to 73 mm wide. Pressfit (BB86/BB92) needs an adapter (fasterbikes).

What you need

• Calipers, or in a pinch a ruler or tape measure
• Bottom bracket tool and crank puller if you want to remove the old bottom bracket (see tools)
• A torch and a rag

Take a photo of the bottom bracket area before you measure. It helps later if you need to ask a dealer.

Step by step

1. Remove crank and bottom bracket. Without the bearing out you cannot see what sits in the shell. If you only want a quick look the outside view often suffices, but the measurement is less accurate.
2. Measure shell width. The shell is the tube running across the frame that holds the bearing. Measure between the two outer edges, without cups. Common values are 68 mm and 73 mm. Much less or clearly more points to special cases, such as fat bike shells of 100 or 120 mm.
3. Check for a thread. Look into the shell. Can you see threads? Then it is a threaded bottom bracket.
4. Measure the inner diameter. Measure the opening of the shell. A rule of thumb: on BSA the inner diameter at the thread is around 34 to 35 mm. Pressfit shells are clearly wider.
5. Identify the type (see table).
6. Check space for the motor. See whether anything near the shell is in the way, such as a chainstay or a welded mount.

Which type is it?

The BSA values of 68/73 mm and the adapter statement for Pressfit come from the source named; the BB30 details are assumptions. Ask the dealer before buying whether your shell is supported.

Recognising Pressfit and BB30

With Pressfit or BB30 you see no thread turns. The cups sit in a smooth shell and are pressed in. Abbreviations are often printed on the bearing or listed in the frame specification. The frame manufacturer's website or the print on the bearing helps.

On a carbon frame with a Pressfit bottom bracket, keep your hands off. Carbon is not recommended as a conversion frame anyway (GIGA).

What else matters

• Chain line and chainstay: The motor sits wider than the old bearing. Check that chain and chainstay still have clearance.
• Front derailleur: It usually goes. Plan for a single chainring.
• Coaster brake: The freewheel inside the motor often disables it (see coaster brake).
• Frame shape: Not every frame accepts a mid-drive. Shell and tubes have to fit.

If it does not fit

If your bottom bracket does not fit, you have three ways out: use an adapter (for Pressfit), pick another motor variant, or switch to a front or rear motor. The pros and cons are in motor types compared. If you are unsure between mid-drive and hub motor, see mid-drive or hub motor.

Before ordering, measure twice, order once.

How reliable is this?

BSA 68 to 73 mm for Bafang BBS and Tongsheng TSDZ and the adapter need for Pressfit come from one source; the measuring procedure, the diameter rules of thumb and the BB30 and fat bike details are experience or assumptions that Fabio is still checking.

Does a conversion fit your bike?