Description of the Problem: When the rider tries to spin the pedals, they either don’t move at all or do so with great difficulty. In some cases, there could be an unpleasant grinding sound coming from the middle or the rear of the bicycle.
Possible Sources Of The Problem
- Fallen Chain
If the chain falls off and gets jammed between the chainrings, it will prevent the crank arms from spinning freely. If that’s the problem, it will be obvious upon looking at the chainrings.
To put the chain back on, push the derailleur forward to create slack and place the chain on the smallest chainring. Then, lift the rear wheel and spin the cranks by hand so that the derailleur can move the chain to the default gear.
If the chain is consistently falling off, the culprit could be:
- The chain is too long and/or has damaged links.
- The derailleur or its hanger is bent and constantly pushes the chain out of alignment.
- The front derailleur’s limit screws aren’t properly set.
- One of the chainrings is damaged.
Note: If you have a single-speed bike, the chain can still jam the chainring by wrapping around the right pedal’s axle.
- Bottom Bracket
The bottom bracket is a system of bearings and a spindle allowing the pedals/cranks to spin. The bottom bracket is located in the so-called bottom bracket shell in the middle of the bicycle.
To isolate the bottom bracket from the rest of the drivetrain, it’s necessary to remove the chain. If the chain has a quick link, use it to split the unit. If the chain doesn’t have a quick link, you will need a chain tool.
Alternatively, you can shift into the lowest gear (small chainring + big cog), push the rear derailleur forward towards the front tire and manually get the chain off the chainring and rest it on the frame. It’s recommended to use latex gloves because the chain is among the dirtiest parts of a bicycle.
You can also secure the chain to the frame with a zip tie or tape.
Once the chain is off, try to spin the crank arms. Ideally, they will spin freely and smoothly. If you can’t spin them at all, the bottom bracket is damaged and needs replacement.
If you can spin them, but you hear a creaking sound coming from the bottom bracket, a replacement is still necessary.
A bottom bracket isn’t particularly expensive to replace. If you have a cheaper bicycle, more than likely you also have a cartridge square taper bottom bracket. New units can be found for USD 10-20. It’s very important to get one that’s the right size for your bicycle.
If you have a more expensive bike, chances are that you have a bottom bracket with external bearings such as Shimano’s Hollowtech. In that case, the bottom bracket will be slightly more expensive.
If the pedals can rotate freely with the chain off, the bottom bracket is more than likely intact. That said, it’s still worth examining the bottom bracket for excessive play.
- Broken Mech
The problem may be the result of a failing derailleur, shifter, or gear cable.
For example, if the gear cable controlling the front derailleur tears completely, the derailleur will move toward the frame and in some cases, the chain will fall off. The front derailleur itself is less likely to experience damage because it sits at a fairly protected place.
The rear derailleur and its hanger, however, are exposed and can easily suffer a catastrophic failure. If the rear derailleur is twisted, it may prevent the chain from moving freely which will then stop the pedals from rotating too.
In some cases, the damage to the derailleur will be obvious while at other times the problem could be subtle (e.g., debris between the jokey wheels).
- Improperly Adjusted Cable Tension
If the tension of the gear cables isn’t adequate, the derailleurs will fail to move the chain to the needed position for adequate shifting. As a result, the chain could skip and fall between the chainrings or the rear cogs. In extreme cases, the chain will jam the rear wheel or the chainrings.
Check the cables for tears. If they’re intact, the only thing left is to re-tension them and re-index the gears as shown in the video below:
- Broken Rear Hub
The hub of the rear wheel could be problematic too. If the rear wheel can’t spin freely with the chain removed, the hub mechanism is more than likely damaged and/or extremely dirty. For example, the pawls of the ratcheting system could be broken and jam the hub.
If that’s the case, the hub, or at least the drive hub body, will have to be replaced. The procedure is not beginner-friendly as it requires a re-lacing of the rim. (There’s no other way to replace the hub.)
- Damaged Frame
If the bike has suffered a crash, the seat stays or the chainstays may have moved in a manner preventing the rear wheel from rotating freely or at all. Examine the rear triangle. The twist should be quite obvious.
Unfortunately, in that instance, the only logical solution is to replace the frame. Frame repair is often not worth the trouble unless you have an exotic one.