Description of the problem: The b-tension screw of the rear derailleur is too short to adequately do its job. In consequence, the top pulley of the derailleur is touching the largest cog of the cassette or freewheel.
Definitions
Derailleur hanger – The piece connecting the rear derailleur to the frame. Derailleur hangers are made of soft aluminum. They are a strategic point of failure.
During a fall, the derailleur hanger is supposed to break or bend and thus minimize the stress on the derailleur and the chainstay (frame).
B-Tension 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”.
When Is a B-Tension Screw Too Short?
If the lip of the derailleur hanger is too far away from the b-tension screw, the length of the b-screw may be insufficient to provide the required space between the top pulley and the largest cog.
Two factors determine whether you will experience this problem:
1. The design of the derailleur hanger (the position of the lip)
2. The length of the b-screw
Since derailleur hangers are specific to the frame, it’s technically possible for a b-screw to be long enough for one bike but short for another.
In that case, you may experience problems such as rubbing of the rear derailleur against the cassette.
What To Do If The B-Tension Screw Isn’t Long Enough?
The common fixes for a short b-tension screw are:
1. Reversing The B-Screw
When you use a b-screw the normal way, the head of the bolt remains on the other side and keeps the effective length of the b-screw a bit shorter.
A popular way of “lengthening” the b-screw is to reverse it.
To do this, you will have to remove the rear derailleur, unscrew the b-screw and thread it from the inside.
This trick provides a couple of extra millimeters because the head of the screw acts as an extension.
This solution has the following downsides:
- Instability
When you reverse the screw and extend it maximally, it becomes unstable because only a small part of it is threaded through the hole. Also, most screws are tapered towards the end.
- Difficult adjustment
Reversing the screw makes adjustment uncomfortable because the head of the screw becomes hard to reach.
2. A Nut On The Contact End
Another option is to put a nut on the end of the b-screw touching the derailleur hanger. The nut will increase the surface area and better support the position of the derailleur.
If you choose to rely on this method, it’s recommended to put threadlocker on the screw to prevent the nut from untightening.
Another option is to use a fiber nut. (The plastic part of the nut acts as threadlocker.)
3. Get a Longer Screw
You could get a new screw from a hardware store. The size of most b-screws is M4 metric.
It’s recommended to get an Allen head screw because they’re harder to strip. Also, all bike multi-tools have Allen keys on them.
When picking up a new screw, take the old one with you to compare sizes and threads just in case.
Tip: Consider filing the contact end of the screw. Otherwise, the screw will scratch or dig into the derailleur hanger.
The B-screw Doesn’t Do Anything. Why?
If playing with the b-screw has no effect on the derailleur’s position even when the length is sufficient, the screw itself may not be the problem.
The other possible culprits are:
- A Faulty Derailleur Hanger
The derailleur hanger is another part of the tandem. If it’s bent or installed improperly, the b-screw may fail to reach the lip. The aftermath would be a non-functioning b-screw.
If the hanger is bent, you will have to replace it or straighten it via a derailleur alignment tool.
- A Derailleur Incompatible with The Cassette
If the maximum cassette size supported by the rear derailleur is exceeded, then no amount of b-screw tweaking will fix the problem.
To improve the shifting performance, you would have to install a derailleur hanger extender (doesn’t always work) or switch to a derailleur that has the capacity to support the large cog on your cassette.
- A Bent Derailleur
If the derailleur is bent, the b-screw may fail to come in contact with the hanger. If that’s the problem, you will have to straighten the derailleur if possible or replace it.
- The Derailleur Isn’t Fully Tightened
If the derailleur isn’t fully tightened to the hanger, the b-screw and the lip of the hanger may not come in contact.
- “One of Those Derailleurs”
Some entry-level derailleurs offer poor adjustability, and the b-screw doesn’t have the expected effect.
- Excessively Long Chain
If the chain is too long, it will fail to keep the derailleur under tension. As a result, the derailleur would compress and remain in a higher position. Removing a few links from the chain via a chain tool will resolve the issue.
What’s The Best B-Screw Setting?
In general, it’s best to keep the top pulley as close as possible to the cassette to increase chain wrap and ensure crisp shifting but not close enough to cause chain rubbing.
The classic steps for adjusting the b-screw are:
1. Shift to the largest cog and smallest chainring.
(If the pulley clears the largest cog, it will clear them all.)
2. Unscrew the b-screw by a few turns. Don’t pull it all the way out.
3. Tighten the screw in small increments. After every turn spin the pedals backward. If you spin them forward, it may look like the derailleur isn’t moving.
4. Repeat the procedure until the upper pulley clears the largest cog.
The main purpose of the screw is to get the top pulley of the derailleur away from the largest cog and prevent rubbing.
As soon as the necessary clearance is provided, the b-screw is set.
What Signs Indicate That The B-Screw Needs Adjustment?
1. The top pulley is touching the largest cog.
2. The chain is making noise even though the limit screws of the derailleur are adjusted properly.
Ultimately, however, the setting of the b-screw isn’t as critical as some make it to be.
If the derailleur has the capacity to shift across the entire cassette, and the limit screws are adjusted properly, chances are that the b-screw is not going to create trouble.
Truth be told, most people don’t even know that such a screw even exists.
FAQ: My B-screw doesn’t fully contact the hanger. Is this normal?
Sometimes the b-screw comes at an angle preventing it from contacting the lip of the derailleur hanger with more than one edge.
While this set-up, isn’t optimal, it doesn’t necessitate immediate intervention if the system is working as expected.
If there’s a problem, you could inspect the hanger and derailleur for damages.
You could also tread a nut onto the screw to increase the contact area.
FAQ: Does the B-screw need frequent adjustment?
Unless there’s a clear problem, you won’t have to adjust the b-screw unless you’re switching to a much larger rear cog (e.g., 34-40t).
Another situation when adjustments could be necessary would be a complete disassembly of the derailleur for servicing purposes.
If you take your derailleur apart completely, you will have to re-set the position of the b-screw.
FAQ: Is it possible for a derailleur not to have a B-screw?
It’s rare but not impossible. Some early Shimano models (e.g., Shimano Altus c20) do not have a dedicated b-screw.
Also, some post-2001 Campagnolo derailleurs do not use a b-screw while others have the screw positioned on the lower pivot.
If you have a somewhat modern derailleur (produced in the past 10 years or so), it’s unlikely for it not to have a b-screw or an equivalent adjustment.
