Go Back  Bike Forums > Bike Forums > Bicycle Mechanics
Reload this Page >

RD B-Screw Question

Notices
Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

RD B-Screw Question

Old 03-31-23, 08:23 AM
  #1  
mara777
Newbie
Thread Starter
 
Join Date: Mar 2023
Posts: 29
Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 26 Post(s)
Liked 5 Times in 3 Posts
RD B-Screw Question

I may finally take my bike to the LBS to ask my questions and see if anything needs fixing/adjusting (problem is there are none locally and I've been stressing myself out for weeks trying to decide where to go).

I am confused about the RD B-screw. I've read it's purpose is the keep the guide pulley from touching the cog. Thankfully, my guide pulley doesn't touch the cog, but the B-screw doesn't appear to be limiting anything. I'm checking this while on the smallest chainring/largest cog as online guides indicate. I have the B-screw as far out as possible, screwing it in would just push the limiter even further away from the little piece it's suppose to limit (sorry I don't know the name for this?). I can manually push the RD so the pulley is in the cogs (because there is nothing limiting it), as shown in the second image.

Am I missing something? Is something wrong with the way my RD is set up? I am so confused 😐


The reason I'm even wondering about this is I feel a lot of vibration in my pedals while pedaling at high speeds on the large chainring/smallest two cogs. This is something I want to ask the LBS about. I do not believe it is related to the pedals or BB. I think it is something to do with the RD because if I maintain the same speed but move one cog larger, I don't feel the same vibration.


mara777 is offline  
Old 03-31-23, 08:51 AM
  #2  
Barry2 
LR÷P=HR
 
Join Date: Sep 2019
Location: SF Bay Area
Posts: 1,840

Bikes: Holdsworth 1979 Special, C-dale 1993 MT3000 Tandem & 1996 F700CAD3, Cervelo 2022 R5 & 2018 R3, JustGo Runt, Ridley Oval, Kickr Bike 8-)

Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 717 Post(s)
Liked 941 Times in 547 Posts

Barry
Barry2 is offline  
Likes For Barry2:
Old 03-31-23, 08:51 AM
  #3  
Andrew R Stewart 
Senior Member
 
Andrew R Stewart's Avatar
 
Join Date: Feb 2012
Location: Rochester, NY
Posts: 17,324

Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 3883 Post(s)
Liked 3,102 Times in 1,896 Posts
After setting chain length by whatever method you chose the B screw is done but in the low (large cog) rear and inner front. This is the cog combo that will suffer from pulley knock if the B spring can't swing the parallelogram/body down/rearward enough, although all but this combo will likely work well.

My first thought was that the B pivot is not freely moving. Removing the rear wheel and pulling on the der body should confirm that the B spring and pivot are smoothly working. My next thought is that of chain length. I wonder if the knocking you hear in the hi/smallest couple of cogs could be the cage over wrapping and the chain is rubbing on the cage or itself.

While not those last two small cogs, I see a fair amount of wear on the rest of the cog teeth. I would not be surprised if there's not more that will need to be repaired or replaced... Andy
__________________
AndrewRStewart
Andrew R Stewart is offline  
Likes For Andrew R Stewart:
Old 03-31-23, 11:53 AM
  #4  
FBinNY 
Senior Member
 
Join Date: Apr 2009
Location: New Rochelle, NY
Posts: 37,155

Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter

Mentioned: 132 Post(s)
Tagged: 1 Thread(s)
Quoted: 5013 Post(s)
Liked 1,131 Times in 659 Posts
I suspect that there MAY be a mismatch between the RD and hanger. And/or that the RD has a counter spring which controls it's angle rather than relying on the chain to pull it forward until the B-screw engages the stop tab. See the image below showing the stop tab at the bottom, and compare to yours. In any case, actually working trumps all theoretic discussions of how it's "supposed" to work, so don't fret over it.
__________________
FB
Chain-L site

An ounce of diagnosis is worth a pound of cure.

“Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin

“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN

WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FBinNY is offline  
Likes For FBinNY:
Old 03-31-23, 01:15 PM
  #5  
Crankycrank
Senior Member
 
Join Date: Jan 2009
Location: San Diego, CA
Posts: 3,304
Mentioned: 10 Post(s)
Tagged: 0 Thread(s)
Quoted: 734 Post(s)
Liked 801 Times in 582 Posts
In your photo, to my eyes, it looks like the pully is either touching or too close to the cassette cog. You would need to screw in the B-screw until there is a little more room between them. B-Tension adjustment is always a little foggy to see the change while adjusting. Make an adjustment and run the bike through the gears while pedaling and check again.
Crankycrank is offline  
Old 03-31-23, 03:21 PM
  #6  
maddog34
Senior Member
 
maddog34's Avatar
 
Join Date: Jul 2015
Location: NW Oregon
Posts: 2,077

Bikes: !982 Trek 930R Custom, Diamondback ascent with SERIOUS updates, Fuji Team Pro CF and a '09 Comencal Meta 5.5

Mentioned: 13 Post(s)
Tagged: 1 Thread(s)
Quoted: 896 Post(s)
Liked 272 Times in 212 Posts
ummm, Why does the Rear Axle look like it's NOT FULLY SEATED in the Frame? Let's get the Basics sorted out before trying to adjust things........................

And, from what i'm seeing in your photos, you're worried about something that isn't a problem.. the Stops you imaged are to prevent the der. from unwrapping it's knuckle spring and look to be just fine.

Invert the bike, Loosen the QR lever, PUSH the Wheel down into the Frame, retighten the QR. 1/3 to 1/2 lever swing to the fully seated position is a good tension on a QR... adding extra tightness will only ruin the axle bearings.

Last edited by maddog34; 03-31-23 at 09:53 PM.
maddog34 is offline  
Likes For maddog34:
Old 03-31-23, 10:28 PM
  #7  
Kontact
Senior Member
 
Kontact's Avatar
 
Join Date: Apr 2011
Posts: 5,176
Mentioned: 38 Post(s)
Tagged: 0 Thread(s)
Quoted: 3217 Post(s)
Liked 668 Times in 445 Posts
Originally Posted by mara777 View Post
I may finally take my bike to the LBS to ask my questions and see if anything needs fixing/adjusting (problem is there are none locally and I've been stressing myself out for weeks trying to decide where to go).

I am confused about the RD B-screw. I've read it's purpose is the keep the guide pulley from touching the cog. Thankfully, my guide pulley doesn't touch the cog, but the B-screw doesn't appear to be limiting anything. I'm checking this while on the smallest chainring/largest cog as online guides indicate. I have the B-screw as far out as possible, screwing it in would just push the limiter even further away from the little piece it's suppose to limit (sorry I don't know the name for this?). I can manually push the RD so the pulley is in the cogs (because there is nothing limiting it), as shown in the second image.

Am I missing something? Is something wrong with the way my RD is set up? I am so confused 😐


The reason I'm even wondering about this is I feel a lot of vibration in my pedals while pedaling at high speeds on the large chainring/smallest two cogs. This is something I want to ask the LBS about. I do not believe it is related to the pedals or BB. I think it is something to do with the RD because if I maintain the same speed but move one cog larger, I don't feel the same vibration.
What you are missing is that the B screw is not a limit at all. On the older design Shimano derailleurs like this one (and copies), the B screw changes the internal spring tension of the rear pivot where it mount to the bike. When you screw it in, the spring force makes the derailleur attempt to swing down and back. This is in opposition to the spring in the pulley arm that is attempting to pull the derailleur forward and up by tugging on the chain.

Shimano has moved away from this design, but the idea was to get the derailleur to self adjust to shifts on the crankset. If your cassette is not particularly large in the low cog, the derailleur may not need any added B tension to keep it away. But if you want to know what it sounds like when the derailleur is too close, pedal the bike in the workstand and push the derailleur forward until the jockey pulley rumbles against the cogs.

Originally Posted by FBinNY View Post
I suspect that there MAY be a mismatch between the RD and hanger. And/or that the RD has a counter spring which controls it's angle rather than relying on the chain to pull it forward until the B-screw engages the stop tab. See the image below showing the stop tab at the bottom, and compare to yours. In any case, actually working trumps all theoretic discussions of how it's "supposed" to work, so don't fret over it.
There is no such thing as a "mismatch" between a modern Campy style hooked hanger and a modern hanger mounted derailleur. That's a normal hanger with a normal derailleur.

Originally Posted by maddog34 View Post
ummm, Why does the Rear Axle look like it's NOT FULLY SEATED in the Frame? Let's get the Basics sorted out before trying to adjust things........................

And, from what i'm seeing in your photos, you're worried about something that isn't a problem.. the Stops you imaged are to prevent the der. from unwrapping it's knuckle spring and look to be just fine.

Invert the bike, Loosen the QR lever, PUSH the Wheel down into the Frame, retighten the QR. 1/3 to 1/2 lever swing to the fully seated position is a good tension on a QR... adding extra tightness will only ruin the axle bearings.
The axle looks perfectly normal in this replaceable hanger dropout.



This whole thread is about a problem that isn't a problem - how all Shimano, Campy, Sachs, Mavic, Microshift double spring derailleurs work and have worked since the introduction of the 1971 Shimano Crane derailleur and its "Servo Pantograph" dual spring design. Surely I am not the only person on this thread that knows what B tension is?

@mara777, if you have a function problem or a noise, let's talk about that. But your B screw sounds like it is working as designed.
Kontact is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell or Share My Personal Information -

Copyright © 2023 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.