Description of the Problem: When pedaling backward, the chain goes from a large cog to a smaller one even though the rider isn’t touching the shifter.
Possible Sources Of The Problem
One of the most frequent sources of this issue is cross-chaining.
Cross-chaining is the result of extreme gear combinations (e.g., the largest cog combined with the smallest chainring).
In those gears, the chain looks diagonal to the bike when looked at the from above.
When the rider backpedals, a crossed chain will try to “straighten itself out”. This is possible because the chain tension is lost and the front derailleur cannot effectively guide the chain and prevent unwanted movement.
As a result, the chain can move down or come off the cassette. This problem is very common with 1x drivetrains because the setup is inherently prone to extreme cross-chaining.
If you have a 2x or 3 drivetrain, you can prevent cross-chaining, but a 1x setup does not give that option. In that case, the best bet is to simply avoid backpedaling when the chain is crossed.
- Contaminated Freehub Body or Freewheel
The back-pedaling motion is possible thanks to the ratcheting mechanism of the freehub body (if the bike uses a cassette) or the freewheel.
If the mechanism of the freehub body or the freewheel is contaminated, it may fail to keep up with the chain when back-pedaling. As a result, the chain can get stuck and/or come off.
To understand why one has to become familiar with the ratcheting mechanism of a free hub.
The figure above shows the basic structure of a rear hub.
When the rider spins the pedals normally, the pawls of the freehub body open and bite against the teeth of the ratchet. Then, the entire hub starts rolling in a clockwise direction.
During backpedaling, the freehub body is rotating anti-clockwise, and the pawls are sliding against the teeth of the ratchet (fig.2)
If that mechanism is contaminated and consequently not operating as efficiently as needed, the user can experience issues with the chain moving in an unwanted manner.
The same applies to freewheels. The only difference is that in the freewheel case the ratcheting mechanism is in the freewheel rather than the hub.
The simplest solution is to spray the hub/freewheel with a degreaser.
Of course, the ideal approach is to disassemble the hub entirely, clean it and re-grease it. However, this is a procedure that takes a lot of time and requires additional tools that not everybody has.
If you have a freewheel, you can also replace it as they tend to be cheaper.
- Contaminated Derailleur
If the derailleur has accumulated a lot of dirt, its jockey wheels may get stuck when pedaling backward. As a result, the derailleur will block the chain from moving and create unnatural behavior.
The problem does not manifest when pedaling forward because the power transfer to the chain and consequently the chain tension are greater.
Cleaning the derailleur with a brush and a degreaser while paying special attention to the jockey wheels should alleviate the issue.
- Improperly Adjusted B-Screw
The b-tension screw is used to adjust the distance between the top pulley of the rear derailleur and the cassette or freewheel.
When the b-screw is screwed in, it pushes against a lip on the derailleur hanger. In consequence, the derailleur “decompresses” and “descends”.
If the B-screw isn’t adequately set, and the derailleur is too close to the cassette, problems including a moving chain while back-pedaling can arise.
Usually, the B-screw needs to provide about 5mm of space for proper operation of the derailleur. You can read more about that topic here.
- Insufficient chain tension
Low chain tension increases the chances of unwanted chain movement too. In some cases, it may be necessary to replace the chain if it’s worn or remove a couple of links to increase the tension. The procedure requires a chain tool.
FAQ: Is this issue dangerous?
In general, no because bikes aren’t designed to be pedaled backward anyway. Of course, there are situations when the rider has to backpedal slightly (e.g., to avoid hitting a rock with the pedal or crank), but multiple revolutions are usually only done when the bike is barely moving (e.g., waiting at a stop sign).
If the drivetrain is otherwise in good condition, avoiding back-pedaling will remedy the problem in most cases.
Summary: What You Need To Know
- The number one reason for a dropped chain during back-pedaling is cross-chaining. The problem is very common for 1x drivetrains. The solution is to avoid back-pedaling when riding in a gear combination that causes cross-chaining. Still, an inspection of the drivetrain is in order to rule out unexpected technical issues.
Other sources of the problem could be:
- Contaminated freehub or freewheel with a ratcheting mechanism that doesn’t rotate as fast as needed
- Improperly adjusted B-screw
- Insufficient chain tension