Description of the problem: The cranks feel unstable. When the rider pushes and pulls on them, they move sideways. Sometimes the motion is subtle, but it could also be very pronounced to the point where the bicycle cannot be ridden.
Diagnosis and Solutions
First, it has to be determined whether one or both cranks are affected. If only one of the cranks is moving, then the source of the problem is more than likely a loose crank arm.
If it’s the drive-side crank and the bike has a front derailleur, the shifting between the chainrings will be poor, and the chain may even drop.
If it’s the non-drive side crank, the shifting and the chain tension won’t be affected.
In both cases, the solution is to examine the attachment points of the cranks and retighten them.
The cranks attach to the bottom bracket – a system of bearings threaded into the bottom part of the frame.
The place of the frame where the bottom bracket sits is called a bottom bracket shell.
The bottom bracket type determines how the cranks attach to its spindle. For example, a square taper bottom bracket (common for older and cheaper bikes) connects to the cranks via 8mm Allen bolts that thread into the spindle.
If the bike uses an older style bottom bracket with removable ball bearings, the cranks slide onto the bottom bracket spindle and are then secured with nuts rather than bolts.
If the bike uses a modern bottom bracket with external bearings, the spindle is pressed into the drive-side crank by the manufacturer whereas the non-drive side crank has one of its ends modified so that it can be tightened to the spindle via a bolt on the crank itself.
Both Cranks Moving At the Same Time
If both cranks are moving sideways at the same time, the source of the problem is the bottom bracket.
Unfortunately, this often means that the bottom bracket is severely damaged.
The possible sources of the malfunction are:
- An unproperly “packed” bottom bracket
Bottom brackets found on retro bikes come with loose ball bearings (or ball bearing rings), two removable cups, and a lock ring.
It’s up to the mechanic to pack the bottom bracket (BB). Ideally, the BB will be tight enough not to wiggle while still moving freely. It’s a trial and error process that takes some experience.
If the lock ring is loose, the bottom bracket can come undone. In that case, merely re-packing the bottom bracket should solve the problem.
Note: It’s also possible that the threads of the spindle are damaged. In that case, the bolts (or nuts) won’t be able to retain the cranks. In that scenario, the bottom bracket, or at least its spindle have to be replaced.
- A spindle disconnected from the body
In rare situations, usually due to a hit, the bottom bracket can break and leave the spindle non-supported. A full bottom bracket replacement is in order.
- Worn or completely shot bearings
If the ball bearings are worn, they ought to be replaced. If it’s a cartridge bottom bracket, the entire unit has to go.
- Stripped Bottom Bracket Shell
One of the unfortunate scenarios are stripped threads of the bottom bracket shell due to a “savage” removal of a previous bottom bracket or improper installation of the current one.
If the threads are cross-threaded (non-material loss), one can re-chase them via a bottom bracket tap.
Bottom bracket taps are fairly expensive, but you can make your own by cutting straight grooves into the cups of an old steel bottom bracket. (video below)
However, if the shell’s threads are stripped and there’s a lot of material removed, the taps won’t be able to do their job because the diameter of the bottom bracket shell will be too large.
In that case, you can install a threadless bottom bracket.
A threadless bottom bracket comes with a removable cup which acts as a nut pulling the rest of the bottom bracket. Subsequently, the bottom bracket doesn’t need threads on the frame to remain stable.
Replacement of the Bottom Bracket
If you have a cartridge bottom bracket, and it’s badly damaged, the only logical solution is to replace it.
Replacing a bottom bracket isn’t a very complicated procedure, but it could be frustrating if you’re doing it for the first time and don’t have the necessary tools.
The procedure for removing a square taper bottom bracket is as follows:
Step 1: Remove the crank arms.
Unscrew the Allen bolts connecting the crank arms to the bottom bracket.
Once the Allen bolts are removed, thread a crank puller/extractor into one of the crank arms.
A crank puller is a simple contraption that uses a threaded road to push the cranks away from the bike.
One part of the crank puller (the black one in the image above) is threaded into the crank.
Then the mechanic holds the crank arm or the part of the crank puller that goes into the crank with a wrench and tightens the rod of the crank puller with another wrench. The rod pushes the crank arm away from the bottom bracket.
Step 2: Remove the non-drive side bottom bracket cup
The next step is to insert a bottom bracket tool into the non-drive side part of the bottom bracket and unscrew it.
Note: The non-drive side of the bottom bracket is a normal threaded cup and unscrews anti-clockwise.
Step 3: Remove the drive side cup
Unscrew the drive side cup by sliding the bottom bracket tool into it and rotating it clockwise (the drive-side cup is reverse-threaded).
The video below goes over the entire procedure in detail:
Step 4: Install the new bottom bracket
It’s the same operation in reverse:
- Apply some grease on the drive side cup’s threads and then thread the cup into the bike by rotating it anti-clockwise. Initially, use only finger strength to avoid cross-threading the cup. Once the cup is in, insert the bottom bracket tool into the cup and use a wrench to fully tighten it.
- Apply grease on the inside of the non-drive side cup. Tighten the non-drive side cup by rotating the BB tool clockwise. If you live in a moist environment, you can also put a thread-locking compound onto the threads of the non-drive side cup. Otherwise, regular grease will do.
Note: This is the procedure for a square taper bottom bracket. If you want to install another model the process is similar but requires different tools.
For in-depth video tutorials, consider watching Park Tool’s guide:
Summary: What You Need To Know
If the cranks of your bicycle are moving from side to side, consider the following troubleshooting:
- Determine whether one or both crank arms are affected. Make sure that the problem is truly coming from the cranks rather than the pedals or the chainring. If it’s the pedals or the chainring, check to see if they’ve been properly tightened.
- If only one of the cranks is giving you problems, inspect it to see if it’s tight enough. You will need an Allen key for that.
- If the cranks are properly tightened or at least appear to be, remove them and examine the threads of the spindle. If they’re damaged, the bottom bracket or at least the spindle will have to be replaced
- If both cranks are moving simultaneously, the source of the problem is the bottom bracket. It’s either loose or damaged.
- Remove the crank arms and see if the bottom bracket is fully tightened to the frame. If the bottom bracket shell’s threads are damaged, consider re-tapping them or install a thread less bottom bracket.
- If the bottom bracket is broken, consider replacing it with a similar model or an upgraded version.
- If you upgrade the bottom bracket, you will also have to buy new tools for it.
