Why Does My Bottom Bracket Keep Coming Loose? (simple explanations)

Description of the problem: The drive-side cup of the bottom bracket loses its tightness over time. The bottom bracket starts making noises and feels unstable.

Possible Sources of The Problem

1. The Bottom Bracket Isn’t Tightened Sufficiently

One of the most obvious sources of this issue is an insufficiently tightened bottom bracket. This is often the case for vintage bottom brackets which require hard-to-find tools for their installation and removal. (In some cases, the user may fail to tighten the bottom bracket sufficiently due to the lack of proper tools.)

Below is a list of tools required for the tightening of different bottom bracket types:

a. Square Taper Socket

The common square taper bottom brackets require a special socket that slides into the cup and grabs a set of teeth positioned on the inner side.

In some cases, the socket has a handle attached to it. Cheaper versions come without a handle, and it’s up to the user to get one. It’s also possible to use a wrench (the socket has squared ends).

Square taper removal/installation socket
Square taper bottom bracket

b. Hollowtech Wrench

If you have an external bottom bracket such as Shimano’s Hollowtech, you will need a special cone wrench.

Hollowtech wrench

3. Retro Bottom Bracket

If you have an old-school bottom bracket such as the one below, you will need another specialized tool.

2. Damaged Threads of The Bottom Bracket Shell

If the threads of the bottom bracket shell (BBS) are damaged, the bottom bracket won’t be secure. (The BBS is the part of the frame where the bottom bracket is installed.)

To diagnose this issue, it’s necessary to remove the bottom bracket from the frame and inspect the threads of the BBS.

The threads of the bottom bracket have to be inspected too as they may have been damaged during cross-threading.

Unless the damage is excessive, the following solutions can be applied:

  • Re-tapping of the threads

There are tools such as ParkTool’s BTS-1 that can be used to re-center the threads of a bottom bracket shell. Since the tool is expensive and highly specialized, it will be cheaper to go to a bike shop or a frame builder to perform the procedure. 

  • Teflon Tape

Another option is to wrap Teflon plumber’s tape around the bottom bracket threads before installing the bottom bracket. The plumber’s tape will make it more difficult for the cups to untighten.

  • Thread locker

A few drops of thread locker compound such as Locktite Blue will also make the connection between the bottom bracket and the shell tighter.

Note: The Teflon tape and the thread locker will help only if the threads are intact or have little damage.

  • Lock ring Replacement

If there’s a lock ring, its threads should be examined too as they can easily get cross-threaded. In that scenario, the best option is to replace the lock ring.

(In the image below, the lock ring is found on the left and has notches in it so that the bottom bracket tool can grab it.)

  • Switch to a “Frame Saver” Bottom Bracket

If the threads of the bottom bracket are damaged beyond repair, one could use a threadless bottom bracket that self-tightens (a.k.a. frame saver bottom bracket).

One cup acts as a nut that tightens onto the bottom bracket body and secures the bottom bracket into the frame.

3. Internal Damage

If the bottom bracket’s bearings are in poor shape and/or the spindle is bent or broken, the bottom bracket will feel loose and unstable. Unpleasant sounds will accompany the instability. In this instance, the only option is to replace the bottom bracket.

4. Crank Arm Issues

A poor connection between the crank arms and the bottom bracket could mimic the symptoms of bottom bracket instability.

In the case of square taper bottom brackets (the most common type), the crank arms attach to the bottom bracket via bolts threaded into the spindle.

If the bolts aren’t sufficiently tight or the threads of the spindle and/or the bolts are damaged, the connection will be poor and the crank arms will wobble.

The solution is to tighten the crankarms. If that doesn’t fix the wobbling, replace the bottom bracket and the crank bolts.

FAQ: I have an external bottom bracket, and its spindle is moving from side to side. What’s happening?

The most likely issue is improper installation of the non-drive-side crank arm. (The drive-side crank arm is already attached to the spindle of the bottom bracket.)

The non-drive side crank arm mounts to the spindle via two pinch bolts. But before tightening those, it’s necessary to tighten the so-called compression plug which goes into the non-drive-side crank arm and packs the bottom bracket and the cranks together.

If the compression plug is too tight, it will restrict the smooth rotation of the crank arms. If it’s too loose, the spindle and the crank arms will wobble.

The process is described in greater detail in the video below:

Summary: What You Need To Know

The most common issues behind a bottom bracket that keeps getting loose are:

  • The bottom bracket hasn’t been tightened sufficiently, to begin with. The tool needed for the removal and installation of a bottom bracket is not universal and depends on the bottom bracket type. The two most common bottom bracket models are square taper and Hollowtech.
  • The threads of the bottom bracket shell have been damaged. If the damage is not severe, the bottom bracket shell can be re-threaded with a special tool. The user can also use a plumber’s tape or Threadlocker to strengthen the connection between the bottom bracket and the frame.
  • The bottom bracket is a retro square taper model with a lock ring that has damaged threads.
  • The bolts connecting the crank arms to the bottom bracket are not tight enough or have damaged threads.
  • The threads of the bottom bracket spindle are damaged and the crank arms cannot be secured.
  • The compression plug ensuring the tight fit of crank arms designed for external bottom brackets hasn’t been tightened sufficiently to eliminate play. (It’s also possible that the pinch bolts of the non-drive side crank arm are loose.)
  • The bearings or the spindle of the bottom bracket are damaged.

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