Don’t get mad if your derailleur is slapping the largest cassette cog. It’s fixable.

From Rookie’s Keyboard,

Hello, friends

It’s time for my weekend post (although I am not sure if the entire post will be ready by the end of the weekend). 

Today, I am going to talk about a somewhat weird but popular issue that cyclists face – contact between the top pulley of the derailleur and the largest cog of the cassette.

A bicycle courier that I met the other day had this issue and gave me the idea for the post. I know that you haven’t asked for such details, but I like to add context to almost everything I write. 

The possible culprits are:

  • The Mythical B-screw

The go-to solution is to re-adjust the B-screw. The B-screw is almost a hidden feature (if you’re a rookie). It’s essentially a screw that pushes away the cage of the derailleur and therefore dictates the distance between the top pulley and the cassette. 

Tightening the B-screw lengthens the distance between the big cog and the pulley.

If the B-screw is fully tightened, and yet the pulley is still touching the biggest cog, the problem lies elsewhere.

To put the B-screw in the correct setting, you will need a long Philips screwdriver. A gap of about 5-6mm between the large cog and the top pulley is the standard.

  • The Cassette and Derailleur Aren’t Meant For Each Other

Derailleurs have a maximum large cog capacity based on their length. Derailleurs with short cages have a lower capacity than models with medium and long cages. 

So, if your cassette has a 1st gear that’s simply too big for the derailleur, the top pulley could act weirdly. 

For example, if the largest sprocket has 42 teeth, but your rear derailleur has a cassette size limit of 32 teeth, then the combination will fail. 

The options are:

  1. Switch to a derailleur with a higher capacity
  2. Use a derailleur hanger extender a.k.a. “goat” link (image above)
  3. Switch to a cassette with a smaller first gear/large cog.
  • A Needlessly Long Chain

If the chain is excessively long, it will fail to stretch the spring of the derailleur. In return, the derailleur would compress, and the top pulley would go up and potentially touch the large cog.

The solution is to remove a link and in rare cases two. You can learn how to size a chain in this article.

  • A Poorly Assembled Rear Derailleur

There is a slight chance that the derailleur has been improperly assembled after a cleaning process.

Search for an image of the same model online and compare it to yours.

If the cage has been reversed, you will have to re-assemble the derailleur. It’s not that hard. Just schedule a quiet time of the day to do it. Use a card box to store the parts to avoid losing any.

  • Improperly Routed Cable Housing

If the cable housing near the rear derailleur has a large loop or another weird shape, it may be stopping the shifter from pulling enough gear cable for a full shift.

As a result, the rear derailleur fails to assume a proper position away from the large cog.

To fix this, you will have to detach the gear cable from the derailleur and re-arrange the housing. 

If you don’t have housing and cable-cutting pliers, this is a good opportunity to get a pair. BTW those can easily cut spokes too. 

  • The Derailleur Hanger Is Damaged Or The Wrong One

The derailleur hanger (DR) is the link between the derailleur and the frame. The DR is made of soft aluminum on purpose. During a crash, the hanger should bend or break without transmitting torque onto the frame. 

However, due to its softness, the hanger often gets out of alignment even without a fall. If the hanger isn’t properly oriented, it may cause rubbing issues.

In that case, you will have to replace the hanger or use a derailleur hanger alignment tool to re-straighten it.

derailleur hanger alignment tool

If the bike has been bought second-hand, it’s also possible that the previous owner had installed a derailleur hanger that isn’t even designed for the used derailleur or frame.

  • The Derailleur Hanger’s Bolt Is Insufficiently Tight or Damaged 

If the bolt attaching the derailleur to the hanger isn’t tightened sufficiently, the derailleur could move and touch the largest cog. It’s also possible that the bolt or the threads of the frame and/or the hanger are damaged. 

In that case, it will be necessary to replace the hanger or re-tap it. If the issue comes from the frame’s threads, re-tapping them will be the only option. 

  • The Rear Wheel Is Off-center
a basic spoke wrench

In a perfect world, the distance between the rim and each seat stay of the frame is the same. If it isn’t, then the wheel is known to be off-center.

If the wheel is off-center to the right, the cassette will get too close to the derailleur and may get slapped by the top pulley during shifting.

The solution is to re-dish/re-center the wheel. The process isn’t for beginners and requires a truing stand, a spoke wrench, and a dishing gauge. 

  • The Derailleur’s Tension Spring Is Too Old/Damaged

Derailleurs have a return spring tensioning the chain. When the spring is damaged or worn, the derailleur loses strength, and the chain pulls it forward too much.

The derailleur may get in an awkward position resulting in rubbing against the cassette.

If that’s the case, the logical solution is to replace the entire derailleur if it’s cheap or to search for a new spring if the derailleur is high-end. 

I know that I’ve listed a lot of possibilities…to the point where some of you may be confused. That’s because I wanted the post to be maximally detailed.

But the three most common reasons are:

  • A poorly set B-tension screw
  • An excessively long chain
  • The cassette is too large for the derailleur

All three are fixable for free..or almost free.

Well, I hope you find this post useful. If you want to read about more topics that help rookie cyclists excel in their pursuit of a better riding experience, consider coming back to my site again.

Until next time,

– Rookie






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