When The Rear Derailleur Of Your Bike Is “Slapping” The Cassette…

Description of the problem: A part of the rear derailleur, usually the top pulley, is touching the largest cog of the cassette.

Possible Sources Of The Malfunction

Note: Don’t examine the area with the bike upside down. In that position, gravity is pulling the derailleur and the chain, and it’s easy even for an otherwise fine and adjusted derailleur to touch the largest cog.

The issue described above can have many sources such as:

1. Poorly Adjusted B-Screw

Most people are familiar with the limit screws on a rear derailleur, but many haven’t even heard of the so-called B-screw.

The B-screw on a rear derailleur is used to regulate the distance between the top pulley and the largest cog of the cassette.

Tightening the B-screw increases the space between the big cog and the pulley.

If the B-screw is all the way in, and yet the pulley is still in contact with the biggest cog, the culprit is elsewhere.

Note: To adjust the B-screw, you will need a long Philips screwdriver. A gap of about 5-6mm between the cog and the pulley is considered the norm.

2. Low Capacity

If the rear derailleur doesn’t have the capacity to reach all the cogs on the cassette, rubbing may occur. There are three ways to solve this problem

  • Switch to a derailleur with a higher capacity
  • Install a derailleur hanger extension a.k.a. “goat” link
  • Switch to a cassette with a smaller first gear

FAQ: How can I know if my derailleur can cover the entire cassette?

1. Check the number of teeth on the largest sprocket/cog.

2. Search for your derailleur online to find its total capacity and maximum cassette size.

If your largest sprocket has 42 teeth, but your rear derailleur has a cassette size limit of 34 teeth, then the combo will not be successful.

The total capacity of the derailleur, on the other hand, determines whether the model could support the entire drivetrain (the combination of chainring and cogs).

The formula for calculating total derailleur capacity is:

Total Capacity = (Big Chainring – Small Chainring) + (Largest Sprocket – Smallest Sprocket)

If you have a 42t large chainring, a 28t small chainring, a 34t large sprocket and an 11t small sprocket), you will need a derailleur with a total capacity of at least: (42-28)+(34-11)=14+23=37.

3. Excessively Long Chain

If the chain is too long, it won’t stretch the spring of the derailleur sufficiently. As a result, the derailleur would compress, and the top pulley will elevate itself.

Removing a link or two from the chain could fix the malfunction. The procedure will require a chain tool.

For more info on how to size your chain, consider reading this article.

The chain should be short enough to stretch the derailleur.

4. An Improperly Assembled Rear Derailleur

If the derailleur’s cage is rubbing against the cogs, the derailleur may be improperly assembled.

It’s highly unlikely that this is the problem if the derailleur is new, but when the unit is disassembled for servicing, it’s possible to put it back together in the wrong order.

To see if this is the issue, search for an image of the same model online and compare it to yours.

If the cage has been reversed, you will have to disassemble the derailleur and fix the orientation.

5. Poorly Routed Cable Housing

If the cable housing near the rear derailleur has a weird shape (e.g., an abnormally large loop…etc.), it may be preventing the shifter from pulling the amount of cable required for a full shift.

In consequence, the rear derailleur doesn’t move sufficiently to assume a proper position further away from the large cog.

If this is the issue, you will have to disconnect the gear cable and “tidy” the housing. If the housing is too long, you will have to cut it. To do that procedure, you will need special cutting pliers designed for cable housing.

6. Faulty Derailleur Hanger

The derailleur hanger (the piece connecting the derailleur to the frame) is made out of soft aluminum. This is a deliberate choice meant to turn the hanger into a strategic point of failure.

During a crash, the hanger bends or breaks and thus prevents or at least minimizes the damage to the derailleur and frame.

Unfortunately, the softness of derailleur hangers makes them a frequent source of problems.

If the derailleur hanger is deformed, it may cause rubbing issues.

To fix the problem, you will have to replace the hanger or use a derailleur hanger alignment tool to straighten it.

Moreover, your bike may be equipped with a short hanger preventing the derailleur from reaching the largest gear.

The derailleur itself may have the capacity to work with the cassette, but it fails to do so because the hanger is of insufficient length. If that’s the case, a derailleur hanger extender should help.

You may also have a derailleur hanger that’s not designed for your bike. If your bicycle is new, this is super unlikely. But if the bike’s been serviced, the hanger may have been replaced with a different one.

To know if this is the case, examine the area. If the derailleur hanger is the original one or an exact copy, it will fit nicely into the frame. There won’t be unusual gaps.

Check the surface of the hanger too. If it’s filed rather than smooth, another hanger may been “re-shaped” to fit your bike.

If that’s the case, search for an original hanger.

7. The Rear Wheel Isn’t Centered

The distance between the rim and each chainstay of the frame should be the same. If it isn’t, then the wheel is off-center.

If the wheel is off-center to the right, the cassette will get unnaturally close to the rear derailleur. During downshifting, the derailleur may hit the big cogs.

If this is the problem, the wheel will have to be re-dished. The process requires a truing stand, a spoke wrench, and a dishing gauge. If you don’t have the skills and equipment to perform the procedure, take the wheel to a bike mechanic.

8. The Derailleur Isn’t Tightened To The Hanger

If the bolt connecting the derailleur to the hanger isn’t fully tightened, the derailleur would move and may touch the largest cog, especially if you’re riding on uneven terrain.

9. A Worn Tensioner Spring

Derailleurs have a spring that keeps the entire chain under tension. When the spring breaks or wears down, the derailleur becomes “weak”, and the chain pulls it forward too much.

In some cases, the derailleur assumes an awkward position causing rubbing against the cassette.

If you have a bad derailleur spring, the solution is to replace it or buy a new derailleur.

What Are The Most Likely Causes?

The situation may involve any of the culprits mentioned above.

Having said that, the most likely sources of the malfunction are:

1. Low capacity derailleur that cannot run across the entire cassette.

2. An excessively long chain

FAQ: Can’t I just replace the B-screw with a longer one?

Technically, you can. It’s not unheard of. You could also reverse the B-screw (thread it from the inside) to increase its effect.

Having said that, this isn’t a good permanent solution as the angles of the derailleur will be negatively affected.

Also, a drivetrain made of the right components wouldn’t necessitate such a measure.

FAQ: Why is my rear derailleur hitting the spokes?

This question requires an article to be fully answered.

In a nutshell, the reasons are:

1. Bent Derailleur

During a fall, the cage of the derailleur can bend inward towards the spokes.

The derailleur can also be harmed during transportation.

Unless the damage is severe, the cage can be straightened.

2. Bent Derailleur Hanger

If the derailleur hanger is bent inward, it will bring the entire derailleur closer to the spokes.

If this is the case, you will have to replace the hanger or realign it.

3. Off-center Wheel

If the wheel isn’t centered, the spokes themselves may be too close to the derailleur.

Note: If the derailleur is touching the spokes, the bike isn’t safe to ride. The derailleur may get into the spokes while you’re cycling. The result will be a trashed derailleur and rear wheel.

Leave a Reply