Description Of The Problem: When the rider spins the pedals backward, after a partial rotation, the chain pulls the derailleur forward, and the pedals can’t be rotated anymore. Usually, this problem doesn’t affect forward/regular pedaling, and the bike can be used as intended.
Possible Source Of The Problem
1. Contaminated Freehub or Freewheel
More often than not, the problem comes from a contaminated ratcheting mechanism inside the freehub or freewheel.
Bicycles with a coasting ability (the rear wheel can rotate independently of the drivetrain), have either a free hub and a cassette or a freewheel.
In the first case, the ratcheting mechanism is found inside the free hub.
In the second, the ratcheting mechanism is in the freewheel.
In both situations, the ratcheting mechanism operates on a similar principle.
Inside the hub or freewheel, there’s a ratcheting ring. When the rider spins the pedals forward, pawls inside the hub or freewheel “jump out” and bite against the indentations/beds on the ring.
As a result, the freewheel or cassette rotates and so does the rear hub and respectively the rear wheel.
When the rider pedals backward, the backside of the pawls brushes against the indentations on the ratcheting ring. If the mechanism is contaminated, however, it will be difficult for the freewheel or the cassette to rotate. The extra friction could cause the chain to “hang up”.
If this is the problem, the simplest solution is to lubricate the affected area.
If you have a free hub, you can spray a cleaner such as WD-40 on it.
If you have a freewheel, the easiest method is to lubricate externally by spraying the ring beside the largest cog.
In most cases, external spraying is sufficient. That said, it’s also possible to take the freewheel apart, but given the price of the unit and the number of tools and time needed for its servicing, simply replacing it makes more economical sense. There’s one exception, however.
If you have a rare freewheel that would be hard to replace, it’s better to service it because a new one could be unobtainable or too expensive.
2. Broken Hub or Freewheel
Parts of the ratcheting mechanism (usually the pawls) could break and jam the entire mechanism. In that case, however, you will experience trouble pedaling forward too. The only solution is to replace the hub or the freewheel.
The hub is more difficult to replace as the entire wheel will have to be re-laced. If the new hub is different, you may also have to purchase new spokes because the old units could be too short or too long.
It’s not incredibly difficult to re-lace a wheel, but it isn’t a simple task either. Many cyclists don’t have the time nor the needed equipment for performing the change.
3. Twisted Or Broken Derailleur and/or Derailleur Hanger
The derailleur attaches to the frame via a hanger made out of soft aluminum. The choice of material for the hanger is strategic. During a fall, the hanger will twist/bend first and won’t transmit the received stress to the frame. (Note: Older steel frames are an exception as their hangers are not separate.)
If the hanger or the derailleur are severely twisted, the chain will be prevented from moving freely. The poor alignment could prevent the rider from backpedaling.
The solutions are:
a. Restraighten the derailleur and its hanger with a hanger alignment tool.
b. Replace the hanger and/or the derailleur.
4. Severely Contaminated Cassette and/or Derailleur
If the cassette (or freewheel) and/or the derailleurs are contaminated with mud and debris, the chain may bite against the jockey wheels of the derailleur and pull the mech forward. This will prevent free rotation during backpedaling.
The solution is to clean the chain, the derailleur’s jockey wheels, and the cassette with a degreaser. Then, re-lubricate the chain.
Putting the chain on a chainring and rear cog that are too far apart laterally is known as cross-chaining.
Cross-chaining makes the pedaling process inefficient and places unnecessary stress on all drivetrain components. In extreme cases, the chain will skip or even fall off when the rider tries to backpedal.
That said, cross-chaining is rarely the cause of the discussed issue. Nonetheless, it’s still worth ruling it out if all other solutions aren’t working.
Shift into a gear combination that keeps the chain straighter and try to backpedal again. The issue may be gone.
6. A Stiff Or Damaged Chain Link
If a chain link is stiff or damaged, it can have a hard time passing through the derailleur’s jockey wheels.
During normal pedaling, the torque generated by the rider could mask the issue. When backpedaling, however, the faulty link could jam the jockey wheels and pull the derailleur.
Stand next to the bike and slowly spin the cranks while examining the chain. The problematic link will not move smoothly and may even cause the chain to skip at certain locations.
If the link itself looks intact, bending the area laterally ever so slightly should relieve the tension.
If the link is damaged, the chain will have to be replaced or shortened.
7. Broken Bottom Bracket
If forward pedaling feels normal, it’s highly unlikely that the problem is coming from the bottom bracket. Nonetheless, if all other options haven’t produced an accurate diagnosis it’s worth examining the bottom bracket too.
The simplest way to perform the examination is to remove the chain (this will isolate the bottom bracket from the rest of the drivetrain. If the pedals can rotate freely in both directions, and there aren’t weird sounds coming from the bottom bracket, the problem is probably elsewhere.
If the rotation is difficult, the bottom bracket is either too tight (older cup and cone bottom brackets have a pre-load adjustment) or damaged. Nowadays bottom brackets are not meant to be serviced and most people simply replace the unit with one of the same size when an issue occurs.