B-Tension Problem Shimano Altus Derailleur
I have a problem with adjusting the B-tension on my Shimano Altus rear derailleur.
The problem is when adjusting to the smallest sprocket, there isn't enough spring tension to bring the guide pulley close enough to the sprocket, even with the adjustment screw turned all of the way out.
I'm wondering if there is a pre-load spring adjustment on the derailleur?
There is nothing mentioned in the user manual.
I know that the chain isn't too long, as it is the factory original and only has 2,000 Km on it.
To make sure, I measured it with a steel ruler and also the Park Tools chain wear tool and it is fine.
With this condition it makes using high gear almost impossible, as the chain jumps in this gear.
On a Sram derailer, the "B" screw is to adj the distance between the jockey wheel and the largest cog. On my Sram X7 the distance is listed at 6mm. To my knowledge it isnt a tension adj it is a measure adj to insure proper clearance between the jockey wheel and the largest cog. The minimum listed adjustment will insure quick and proper shifts.
The chain jumping in the smallest cog is not a b-tension screw problem. Are you trying to use a new chain with an old cassette?
Did you check chain length when in big ring/big cog?
Shift the chain into the largest rear sprocket and then the smallest front chainring. If the upper RD pulley is so close to the largest rear sprocket that it makes noise turn in the B-tension screw to pull the upper pulley away from the cassette eliminating the noise. Sometimes you cannot get the pulley close enough to the smallest cassette sprocket depending on your RD hanger, RD, and cassette combo. For the most part this is OK as long as the RD is not running on the largest rear sprocket in the smallest front chainring.
Thanks for the replies, Guys:
joejack951: Yes, B-tension does affect chain jumping on the smallest cog. I proved this by temporarily putting on a cable tie to bring the guide pulley to the correct position ... chain jumping stopped immediately.
AndrewP: Yes chain length is correct - in fact it is still the factory chain, as stated in my original post.
rydabent : Yes this adjustment is to change the guide pulley's position. It is achieved by slightly rotating the dérailleur, which either tensions, or de-tensions the spring.
Ingleside : Yes, I have performed all of the factory recommended adjustments.
The problem is that the spring controlling the rotation of the guide pulley has no tension at all when you try to adjust the guide pulley to the correct position in low gear.
Re-stating my original question: Is there a way to pre-load the spring to give the needed range of adjustment?
You proved that putting a cable tie to bring the guide pulley close to the small cog stopped the jumping. You didn't prove anything about the B-tension screw and it's effects. I'll repeat, if your chain is jumping on the small cog, it's not a B-tension problem. You likely need to properly set your high stop screw and maybe have your derailler hanger aligned.
Originally Posted by ramjet1953
You didnt answer my question. The original chain length may have been wrong. If the derailler idlers are not at the limit of their movement in the big/big combination, the mechanism may not have enough movement to take up the chain slack on the smallest cog. However I think the problem is more likely adjustment of the high limit screw and/or bent hanger.
Originally Posted by ramjet1953
Just thinking out loud, but I wonder where the chain would go if you completely released the shift cable.
Wind the derailer cage 360 degrees countercllockwise and then install the chain.
Thanks for all of your comments and suggestions!
The problem is now solved.
AndrewP : You were right!
The factory chain was far too long.
I checked the length as per Shimano's recommendations and found it was 4 links too long.
I guess it is like the famous court scene in the TV series 'The Odd Couple', where Feleix is before a white-board and says, "When you assume you make an ass out of you and me (ass-u-me).
Anyhow, with the chain off I re-adjusted the high and low end stops and then re-installed the chain.
I then adjusted the B-tension.
Shifting is now almost silent and is as smooth as silk.
Much better than when I first bought the bike.
I guess the factory must had set-up things in a bodgy way originally.
Grand Bols : Thank you very much for answering my original question. It is appreciated.
Again thanks to all of you for your help.