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Description of the problem: During pedaling the rider can feel the cranks move up and down. Pedaling feels unstable and inefficient. Sometimes there’s noise coming from the bottom bracket that augments when the rider increases intensity.
Possible Sources of The Problem
Insufficiently Tightened Cranks
Untightened cranks rather than the bottom bracket could be the culprit. Currently, there are two main types of cranks based on their attachment mechanism to the bottom bracket – square taper cranks and external bottom bracket cranks.
Square taper cranks are secured to the bottom bracket via bolts threaded into the axle.
If the bottom bracket has external cups, then the drive-side crank is a part of the axle. The non-drive side crank is secured to the axle via a compression bolt (this bolt packs the entire bottom bracket) and two pinch bolts.
If the compression bolt or the two pinch bolts are loose, so will the cranks and the axle.
Insufficiently Tight Bottom Bracket
Another source of the problem could be an insufficiently tightened bottom bracket. If the bottom bracket isn’t secured into the bottom bracket shell to the right torque settings, it will keep coming loose and wobble.
To tighten the bottom bracket, you will have to remove the cranks and use the appropriate tool for your bottom bracket model.
If you have a square taper bottom bracket (the most common), you will need the tool below:
The tool slides into the cutout groove of the bottom bracket as shown in the video below:
If you have an external bottom bracket such as Shimano Hollowtech, you will need a special cone wrench to tighten the cups.
An Improperly Adjusted Retro Bottom Bracket
If you have an older bicycle, chances are that it has an old-school square taper bottom bracket. Modern square taper bottom brackets are sealed and not meant to be serviced. When they get bad, the user replaces them.
The retro square tapers, however, can be serviced for a long time, usually until the bearing races get uneven to the point where grinding occurs.
If you have one of those bottom brackets, it’s possible that its cup & cone setting is inadequate and allows too much play. Ideally, the bottom bracket will be tight so that there’s no play but also free enough for the cranks to spin smoothly.
To learn how to service a similar bottom bracket, watch the video below:
Damaged Bottom Bracket Threads
If the threads of the bottom bracket shelf are damaged, then the bottom bracket won’t be secure and will wobble.
If the damage to the threads is fairly small, they can be restored with a special tool which is a bit expensive. Hence why it’s recommended to go to a bike shop to perform the procedure.
If the threads are beyond repair, one can use the frame with a “frame saver” bottom bracket.
In the case of “frame saver” bottom brackets, one cup acts as a nut that tightens onto the bottom bracket body and secures the bottom bracket into the frame. As a result, the bottom bracket does not have to rely on the threads of the bottom bracket shell.
Damaged Bearings
Another source of the problem could be highly worn or damaged bearings. If that’s the problem, the only solution is to replace the bottom bracket entirely. The only exception would be the retro square taper bottom brackets mentioned above. They are serviceable and allow the user to replace the bearings.
Broken Axle
It’s rare, but the axle of the bottom bracket could be bent or even broken causing the cranks to rotate unevenly.
Problematic Pedals
If the pedals are not in optimal condition, they may be creating the feeling of instability and wobbling.
The usual problems are:
- Worn or missing ball bearings
Sometimes the pedal bearings will experience significant damage over time to the point where individual units of the ball bearing system fall out. Consequently, the pedal axle stops receiving support and the pedal starts wobbling. The solution is to disassemble the pedal and replace the bearings.
If that’s not done on time, the bearing races will be destroyed and the entire pedal will have to be replaced.
- Untightened pedals
It’s not uncommon for a pedal to get loose and starts wobbling. Check the pedal bolts to see if they have been fully tightened. You will need a pedal wrench (15mm cone wrench). Some pedals can be tightened with an Allen key too.
Summary: What You Need To Know
The most likely sources of a wobbling bottom bracket are:
- Insufficiently tightened crank arms
- Insufficiently tightened bottom bracket
- Damaged threads of the bottom bracket shell
- An improperly adjusted cup and cone bottom bracket (retro models)
- Broken bottom bracket bearings
- Damaged pedal bearings
