Bike Forums

Bike Forums (https://www.bikeforums.net/forum.php)
-   Bicycle Mechanics (https://www.bikeforums.net/forumdisplay.php?f=10)
-   -   B Screw question (https://www.bikeforums.net/showthread.php?t=1256191)

sknhgy 08-02-22 04:48 PM

B Screw question
 
All the B Screw adjustment on my Shimano derailler is taken up and the pully JUST BARELY clears the largest cog. It might even be rubbing a little.

Since all the adjustment is taken up --- Should I remove a few links from the chain in order to get a little more clearance?

Thanks in advance.

Bill Kapaun 08-02-22 05:01 PM

How many extra links can you SAFELY remove?

https://cimg8.ibsrv.net/gimg/bikefor...f5abe736d1.jpg
This is a quickie method to determine how much extra chain you have. No sense getting your hands dirty if you don't need to.
Put it in BIG:BIG Cog/chainring.
You want 1-2" excess. More than that can exacerbate your problem to some degree, depending on "how much"
Does the RDER have adequate chain wrap & cog size capacity.
IOW, what's different then before?

Polaris OBark 08-02-22 05:02 PM

I don't think shortening the chain would help. Buy a longer bolt (I think they are M4) at the hardware store -- strongest steel you can find, as you don't want it to start to bend.

70sSanO 08-02-22 05:24 PM

Sometimes if a chain is too long, shortening it might help.

A longer bolt might help. The problem with longer bolts is keeping them on the hanger tab.

Some RD’s just have weak b springs. I found that with a couple of Shimano RD’s rated for 34t and 32t had an issue. On one bike I used a locknut on the end of the b screw to gain enough contact with the hanger tab. Not pretty, but it works.

John

andrewclaus 08-02-22 05:47 PM

Another hack is to turn the B screw around and have the head sit on the hanger.

irtazasheikj 08-03-22 12:59 AM

Buy a long chain, if you can. Otherwise, removing chains would not help.

hokiefyd 08-03-22 04:52 AM

With most traditional style derailers, where the upper pulley is on a different axis than the top of the cage (and swings up vertically as the cage rotates rearward), shortening the chain can absolutely buy you some room here.

As Bill Kapaun suggested, put the drivetrain in the big-big configuration to see if you can shorten it some. If you determine that you can indeed remove one inner and outer link pair, then put the drivetrain back in the low gear combination and use a small piece of wire or two small screwdrivers as pictured above to mock up what your derailleur cage and upper pulley will do with that one link pair removed. It may rotate that cage just forward enough to sneak that upper pulley down out of the largest sprocket.

A longer chain will not help in this case -- it will generally make the problem worse. A longer chain allows the cage to pull further rearward, which will cause the upper pulley to ride even higher.

And all of the above, again, assumes a traditional style derailer. This does not apply to some of the Shimano Shadows or SRAMs where the upper pulley is mounted coincident with the upper cage pivot. In that case, the upper pulley does NOT move vertically as the cage swings, and chain length should not have an effect on upper pulley position.

sknhgy 08-03-22 06:02 AM

Thanks. I'm gonna try a longer bolt and see if that works. This is a fairly old bike. At one time or another I might have replaced the chain without comparing it to the one I was replacing.
Or I might have changed cassette/cog size and not adjusted chain length accordingly. But I'm just guessing here.

cxwrench 08-03-22 09:31 AM


Originally Posted by irtazasheikj (Post 22596813)
Buy a long chain, if you can. Otherwise, removing chains would not help.

What are you trying to say here?

easyupbug 08-03-22 09:47 PM

You can try as some have said:
reversing the B screw but that makes adjustment messy
add a nut to the end of the screw for more surface area
get a longer screw
do check you hanger and derailleur incase they are bent
shorten the chain don't buy a longer chain!

Leisesturm 08-03-22 09:55 PM

What about those Wolf Tooth things that let the RD ride lower to clear a big cog?

sknhgy 08-04-22 06:46 AM

I ended up taking a few links out of the chain. Don't know if its my imagination, but would that make it so my shift levers require more effort to push?

The longer screw didn't work. Hard to explain but it didn't line up.

rccardr 08-04-22 07:07 AM

If the bike has horizontal rear dropouts you can unscrew or remove the setscrews and pull the wheel back as far as it will go.
Will require adjusting rear brake shoes.
Usually gives you that extra little bit of clearance between the upper pulley and largest cog when in small/big.

andrewclaus 08-04-22 08:19 AM


Originally Posted by sknhgy (Post 22598142)
I ended up taking a few links out of the chain. Don't know if its my imagination, but would that make it so my shift levers require more effort to push?

The longer screw didn't work. Hard to explain but it didn't line up.

In post #4 you got some warning about a longer screw not hitting the hanger tab. That was the reason for the suggestion to turn the screw around.

If you made the chain too short, it could be very hard to shift into the large-large combination at least. It shouldn't make a noticeable change in small-small. Be sure you didn't put any crimps in the cable or housing.

bikeme 08-05-22 04:02 PM

Almost touching is way too close. I was misled by Shimano instructions of having it very close for years. I've found that 5mm between the largest cog's and pulley's teeth is great. Yields plenty of chain wrap on big cog and quieter drivetrain. Per the attached video, also check the b-tension with the chain in big ring and highest cog so there's no belly in the chain (provided chain is proper length). Kyle is does a great job of explaining what b-tension actually does and how to adjust it! FYI, he's a mtb guy but info applies to road bikes too. EDIT: additional thought--if this is a mtb with an after-market extra big gear, you might need a longer (like 25mm) b-tension screw that the RD originally came with.


3alarmer 08-06-22 12:33 PM

...
...if it's a "fairly old bike" there are some limitations on the largest cog many of the older rear derailleurs will accept.
Sometimes, you can push that another dog or two, but it's generally a bad idea. They still work then, but not well.

You can look up a lot of the capacity numbers for older derailleurs on VeloBase: link

Bill Kapaun 08-06-22 01:16 PM


Originally Posted by sknhgy (Post 22598142)
I ended up taking a few links out of the chain. Don't know if its my imagination, but would that make it so my shift levers require more effort to push?

The longer screw didn't work. Hard to explain but it didn't line up.

Do you have the proper amount of slack as shown in POST #2?

79pmooney 08-06-22 01:49 PM

Question - is this a place where a Wolf Tooth hanger extender can be used? I thought that was what they were for. (But what do I know. I'm still at 28 teeth and downsizing chainrings.)

cxwrench 08-06-22 03:02 PM


Originally Posted by 79pmooney (Post 22600807)
Question - is this a place where a Wolf Tooth hanger extender can be used? I thought that was what they were for. (But what do I know. I'm still at 28 teeth and downsizing chainrings.)

Wolf Tooth hanger extenders do only 1 of the 2 things a derailleur needs to do and they don't even do that well. I cannot stand those things. They move the derailleur down, that's all. That will allow a larger cog than the derailleur is spec'd to work with...BUT...they make the upper pulley further from the cogs and shifting suffers. The other thing they can't do is magically make the pulley cage longer so it won't wrap the extra chain you'll need for the bigger cogs/longer cage. People blindly suggest them but don't have the basic knowledge to know why they don't work. It even says that right on the WT site.
d

Iride01 08-06-22 04:38 PM

Shimano says they should be as close as possible without touching. You only said they might be rubbing. Did you ever find out? Here is what Shimano said for my rear DR on a past bike...


Adjusting the B-tension adjust bolt

Mount the chain on the smallest chainring and the largest sprocket, and turn the crank arm backward. Turn the B-tension adjust bolt to adjust the guide pulley to be as close to the sprocket as possible but not so close that it touches. Next, set the chain to the smallest sprocket and repeat the above to make sure that the pulley does not touch the sprocket.
page 14 https://si.shimano.com/en/pdfs/dm/RD...003-09-ENG.pdf

sknhgy 08-06-22 05:09 PM


Originally Posted by Bill Kapaun (Post 22600768)
Do you have the proper amount of slack as shown in POST #2?

Actually, I wasn't clear what to look for using that test.
I took the bike out for a couple hours today. The levers did seem a bit stiffer than normal. But it shifted good.

Bill Kapaun 08-06-22 05:28 PM


Originally Posted by sknhgy (Post 22601001)
Actually, I wasn't clear what to look for using that test.
I took the bike out for a couple hours today. The levers did seem a bit stiffer than normal. But it shifted good.

What was difficult to understand about "You want 1-2" excess..."?

cxwrench 08-07-22 07:50 PM

You're kidding, right? Bill explained it WITH a photo. Put the bike in big/big. Use the screwdriver/whatever to see if you can pull any extra slack.

sknhgy 08-08-22 09:41 AM


Originally Posted by Bill Kapaun (Post 22601014)
What was difficult to understand about "You want 1-2" excess..."?


Originally Posted by cxwrench (Post 22602394)
You're kidding, right? Bill explained it WITH a photo. Put the bike in big/big. Use the screwdriver/whatever to see if you can pull any extra slack.

You guys aren't explaining yourselves well. All you are saying is to pinch the chain. Then you get snippy when a person doesn't understand what to be looking for. Here's a couple other threads where the same thing went down:

https://www.bikeforums.net/bicycle-m...ut-pedals.html
https://www.bikeforums.net/bicycle-m...-new-gear.html

But it's OK. There's always Youtube to help us.

cxwrench 08-08-22 09:51 AM


Originally Posted by sknhgy (Post 22602950)
You guys aren't explaining yourselves well. All you are saying is to pinch the chain. Then you get snippy when a person doesn't understand what to be looking for. Here's a couple other threads where the same thing went down:

https://www.bikeforums.net/bicycle-m...ut-pedals.html
https://www.bikeforums.net/bicycle-m...-new-gear.html

But it's OK. There's always Youtube to help us.

I can't imagine it being explained any better. If you don't understand what is being done in Bill's photo I don't know what to say. One more time...shift the bike into big/big (carefully in case there isn't enough chain) then put the screwdrivers in the chain as shown in the photo. If there's enough slack to do that you're good to go. I just twist the chain w/ my fingers but if you don't want to get dirty use the 2 small screwdrivers. If you need a video to make that more easily understood then watch a video. At that point it should be crystal clear.


All times are GMT -6. The time now is 06:20 AM.


Copyright © 2022 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.