Bike Issues: A Jumpy Rear Derailleur When The Chain Is “Loaded”

Description of the Problem:

Option 1: When the rider is pedaling hard, there’s a moment when the chain makes an abrupt jump/skip. The jump happens unexpectedly and hurts the rider’s control and concentration. Without foot retention, the rider could lose control over the cranks.

Option 2: In certain gears, the chain doesn’t quite get on the teeth of the used cog. The result is inefficient pedaling accompanied by a skipping sound.

Possible Sources Of The Problem

1. Worn Cassette Cogs

With time, the teeth on the cogs wear down and become pointier/sharper. The new shape makes it easier for the chain to slip forward, especially when the rider is generating lots of torque. Hence why the jump/skip usually happens when starting and climbing.

Since the mid-cogs are the most used ones, they often report this problem first.

The only solution is to replace the cassette and the chain. The chainrings may need a replacement too.

If that’s not possible at the moment, the problem can be “quarantined” by avoiding the cog with the most wear. If the shifter has a speed indicator, it will be helpful to mark the faulty gear with a marker and avoid it.

Ultimately, however, cassettes, chains, and chainrings are consumables and have to be replaced periodically.

2. Chain Issues

The chain can also be the culprit. If the chain is worn and therefore stretched beyond the healthy limit, it can start slipping.

However, a new chain can also create this issue when it’s coupled with a worn cassette. A new chain and an old cassette don’t operate well together because the spacing of the chain is too narrow for the worn cassette.

Another chain issue could be a stiff link. If the chain has been shortened, the connection point could become abnormally stiff. Unless a technical error has been made during the procedure, flexing the chain at the problematic location ever so slightly should remedy the issue.

3. Bent Derailleur Hanger

The derailleur hanger is the unit connecting the derailleur to the frame. Derailleur hangers are made of soft aluminum and are designed to bend during a fall to avoid transmitting stress to the chainstays.

This engineering protects the frame but makes the hanger very susceptible to misalignment. Sometimes the user would damage the hanger without even realizing it when locking the bike or transporting it.

If the hanger isn’t perfectly straight, it brings the derailleur out of position and creates shifting issues.

One of them would be incomplete shifting. When the rider makes a shift, the derailleur does not move the chain sufficiently for it fully grab the desired cog. The result is skipping.

There are two ways to proceed.

a. Replace the derailleur hanger

b. Re-align the derailleur hanger with an alignment tool

The first approach is cheaper initially because a basic hanger doesn’t cost as much as a quality alignment tool. But if you’re working on many bikes, a derailleur hanger alignment tool will save you money in the future by reducing the need to always buy new hangers.

It’s also important to note that old steel bikes come with hangers that are part of the frame. Those hangers are not meant to be replaced unless absolutely necessary. The replacement process is complex because the hanger first has to be machined and then someone has to braze or weld it to the frame. It’s much simpler to simply re-align the existing one. In that case, the tool will be helpful once again.

4. Bent Derailleur Cage

A bent derailleur cage could create symptoms identical to that of a bent derailleur hanger.

To fix the issue, it will be necessary to re-straighten the cage or replace it. If the bike doesn’t have a high number of gears, the cage can be re-aligned by eye with a high degree of success. If the gears are over 9, however, the process will be a bit more difficult because more accuracy is required for proper shifting.

With every additional gear the number of cogs on the cassette increases, but the hub’s spacing is kept the same to ensure compatibility with multiple cassettes. Consequently, the distance between the gears gets smaller, and the derailleur needs to be more precise to offer high-level performance.

Note: After re-alignment of the hanger or the derailleur’s cage, it will be necessary to reset the limit screws on the derailleur and re-index the gears due to the new effective position of the derailleur.

5. Improperly Adjusted Gears

If the gears haven’t been indexed properly, it’s possible to have insufficient or too much cable tension during certain shifts. If this is the case, the derailleur won’t be able to shift accurately. The solution is to re-index the gears. The process is not extremely complicated but requires some experience and guidance. Consult the video below.

FAQ: What are the dangers of a skipping/jumping derailleur/chain?

In the first situation (Option 1), the rider can unexpectedly lose control.

Imagine the following. You’re pedaling up a hard hill. You shift to a lower gear and when you press on the cranks, the chain makes a weird sound and the drive side crank suddenly sinks. In a situation like that, it’s easy to lose balance.

If there’s no foot retention, the rider may lose control over the cranks completely (slipped foot) and get hit in the shin by the crank arm or a pedal. In some cases, the rider may even fall on the handlebars due to the sudden loss of balance.

The second situation (Option 2) is less dangerous because it doesn’t involve a sudden drop. However, when the chain is continuously skipping, most of the effort generated by the rider will be lost. Thus, you will be pressing the pedals hard or spinning them with great RPM without generating lots of speed.

Summary: What You Need To Know

  • If the teeth on certain cogs are worn, they will fail to retain the chain when the rider is generating a lot of torque. As a result, the chain will make sudden jumps. To remedy the problem, it’s recommended to replace the cassette and the chain.
  • If the chain is continuously skipping or in other words failing to fully climb onto the desired cog, the sources of the problem are the rear derailleur’s alignment or improperly indexed gears. In the first case, the culprit is either a bent derailleur hanger or a bent derailleur cage. In the second, the adjustment of the gears isn’t correct.

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