The Bottom Bracket and Your eBike Conversion - Guide to Proper Fit and Removal Issues

The Bottom Bracket and Your eBike Conversion - Guide to Proper Fit and Removal Issues

by Nathan Bogner on Mar 15, 2024

Ready to convert but still aren't sure if the motor will fit? Let's talk about one of the most important components of your conversion: the bottom bracket. This often-overlooked part plays a crucial role in ensuring a seamless eBike conversion experience for two reasons.

  • First, you need to have the right fit.
  • Second, removing the BB is the hardest part of the entire process, requiring special tools.

What is a Bottom Bracket (BB)?

Think of your bottom bracket (BB) as the heart of your bike's pedaling system. It's the part of the frame where your cranks (the arms your pedals attach to) connect to the chainring, which sends your pedal power to the rear wheel. That assembly fits into the BB shell of the frame. The shell houses the bearings that allow your pedals to spin smoothly. This whole system is called your crankset. The chain turns the rear sprockets (also known as the rear gears or cassette) and makes you go.

Bottom Bracket Tools

  1. Let's figure out what we're working with. Park Tool has a fantastic guide for identifying your bottom bracket type:
  2. If you've identified your BB, you'll know the specs of the tool to buy. Park Tools has this guide to help you find your tool. 
  3. The more common BB tools can be found on Amazon or at your local bike shop.


For Motor Installations:

The one special BB tool you need comes with the conversion kit. Problem solved!

For BB Removals:

Each standard and different brands have special BB tools to remove their brackets. If you've identified your BB, you'll know the specs of the tool to buy. Park Tools has them all but can be kind of expensive. The more common BB tools can be found on Amazon or at your local bike shop.

Need Help?

Since you will only need this tool for this project, see if your local bike shop will remove the BB for you. They might charge a small fee or even do it for free.

Understanding Mid-drive Motor Mounting

The motor housing has a spindle that slips into the BB shell, replacing your original crankset, and held in place by a locknut and bracket. You attach the cranks and pedals to the motor spindle. The electric motor itself sits in the housing just below the bottom bracket.

Where the spindle extends past the shell determines the position of the front chainring and its alignment with the center of the rear cassette. For a smoother conversion, check these dimensions before purchasing your motor.

Mounting Dimension Requirements

The base models of mid-drive motors from Tongsheng and Bafang have the same mounting requirements:

  • 35mm inner shell diameter
  • 68mm-75mm shell width
  • 12mm clearance at the bottom of the shell

You can use calipers or a tape measure to figure these out.  Not to worry if you don't have metric tools, we have this handy BB Fit Guide to help you out.


My BB is wider than 75mm. What can I do?

There are wider BBs out there. Fat tire bikes sometimes have a wider BB because of the wider frame. If your BB shell width is between 75mm and 120mm, both brands have models with wider spindles. We can get these as special orders, so factor in the extra time it will take for them to arrive.

My BB Shell Inner Diameter is Larger – What Now?

Larger shell diameters are used for one-piece cranks, also called “Ashtabula” or “American Fit”. This presents two problems. You’ll need an adapter to reduce the shell's diameter, and this adds width to the shell, potentially causing clearance issues. We have asymmetrical BB adapters to solve this problem!

Does Threaded or Press Fit (Unthreaded) BB matter?

Both work! The spindle slips through the BB shell and is held on by a large locknut, so any threads inside the shell don’t matter.

My BB Shell is 35mm but the spindle won't slide through. Now what?

  • Check the Cables: [Image: Close-up of cables near BB shell, then another photo with cables rerouted] See if your brake or shifter cables are interfering. You might be able to reroute the cables to go over the bottom brackets on one or both sides.
  • Screw Inside the Shell: A plastic cable bracket is attached with a screw that might extend inside the shell. Backing that screw out a turn or two usually solves this.
  • Welds Inside the BB Shell: These should be ground down at the factory, but there can be a leftover bump. Use a Dremel to carefully grind down that last millimeter of material.
  • The Motor Casing is Catching: The motor shell is two parts screwed together. Unscrew the housing, slide the spindle through, and replace the housing. (Note: There's a warranty sticker, so consult with us before attempting this.)

Can I Convert My Carbon Frame eBike? (Important Consideration)

Yes, but... If you have a clearance issue that might require grinding the inside of the BB shell, absolutely do NOT do this. Pass on this conversion.

What Is Chain Alignment And Why Does It Matter?

Chain alignment is how the front chainring aligns with the gears in the rear. Ideally, you want the chainring to align with the center of the cassette for smooth shifting and efficient power transfer.

It’s okay if your alignment is off by a few millimeters. If it's off by quite a bit, your chain will be at a significant angle in higher or lower gears. This causes extra stress on the chain and can cause it to hop off the front chainring.

For example, if you have a 10-speed cassette, you would want the chainring to align right between 5th and 6th gear.  Slight deviations outside of these gears are fine. If, however, it aligns closer to the 3rd or 4th gear, or 7th or 8th, you'll likely have shifting problems in the extreme gears.

We have spacers to fine-tune the chainring position. If it is too close to the frame (rear chainstays), then you might not be able to convert this bike. In some cases where alignment is close but you still experience chain drops, adding a chain guide will keep things running smoothly.

My bottom bracket is stuck and won't come off. Now What?

Don't panic! A seized bottom bracket isn't super common, but it happens, especially with older bikes.  Here are things to try:

  • Penetrating Oil: Spray on some WD-40 or Liquid Wrench (get these at an auto parts store).
  • Breaker Bar: This gives you extra leverage to break that stubborn nut loose. Remove tires, seat, and accessories like mirrors so you don’t damage them. And put a blanket down on the floor so you don’t scratch the paint.
  • DIY Solution: If all else fails, check out this video from RJ The Bike Guy: [link to RJ's video]. He shows a tool you can build using hardware store parts that works wonders.

Conversion Not Working? Don't Panic, It's Reversible

Removing the bottom bracket is standard for bike upgrades. This non-destructive process (except for the weld grinding situation*) means you aren't altering the parts or frame. 

Take pics during disassembly – they'll make reinstallation a breeze. If you cannot finish, we have a 30-day return policy:

*If you did the grinding method and that didn’t work, as long as you didn’t grind down any threads, you can still reinstall your BB.

Let’s Get That Bottom Bracket Off and Your New Motor On!

Don't let the bottom bracket intimidate you! With a little understanding and the right support, you can easily conquer your e-bike conversion.

CTA: Let's get your e-bike dream rolling! Contact us to discuss your conversion goals.

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