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My b-tension screw does absolutely nothing…

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  • March 21, 2018 at 2:52 pm #28273
    AfterPeak

    So I have built a bike with ultegra rear derailleur and have a 11-25 cassette. Never touched the b-tension screw from new just installed the derailleur. Anyway I thought I would have a play with it to increase the crispness of my shifting and noticed it does nothing if screwed all the way in or out. Is that normal/possible?

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  • March 22, 2018 at 8:42 am #915215
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    AfterPeak

    Acm wrote:

    Acm wrote:
    Fairly sure Shimano derailleurs’ upper knuckles are sprung, so the B screw increases the tension on a spring rather than it being a direct adjustment. The position of the derailleur is therefore determined by the balance between the spring on the upper knuckle and the spring for the jockey cage. My guess is the jockey cage spring is a fair bit stiffer so the B screw doesn’t have much effect. SRAM derailleurs differ in that the the B screw acts against a stop, so adjusting it has a direct effect on the derailleur’s position. From experience, the B screw adjustment isn’t really important with Shimano, whereas it can have a pretty big impact with SRAM

     

    Nice answer. Yes the spring that holds the reraillaur to the hanger is pretty powerful. I have made the mistake of trying to service one once. Took me forever to retwist it back into position and slip the securing pin/washer thing back on (especially after packing it with grease).

    March 22, 2018 at 8:33 am #915213
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    AfterPeak
    Grahamd wrote:
    Zooming in on the photo, there appears some swarf as though the threads could have been stripped. 

     

     

    Thats just threadlocker. It comes pre added to the screw when it arrived. I did read that you should pull the cage away when you screw it in to stop pressure on the hanger but that just makes the job more fiddly

    March 21, 2018 at 11:46 pm #915211
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    Grahamd

    Zooming in on the photo,

    Zooming in on the photo, there appears some swarf as though the threads could have been stripped. 

     

    March 21, 2018 at 11:12 pm #915209
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    ktache

    I have been adjusting my b

    I have been adjusting my b-screw with my JIS 2 screwdriver,  it fits beautifully and doesn’t put stess on the grooves in the head.  Proper tool, proper job.

    March 21, 2018 at 10:07 pm #915207
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    Acm

    Fairly sure Shimano
    Fairly sure Shimano derailleurs’ upper knuckles are sprung, so the B screw increases the tension on a spring rather than it being a direct adjustment. The position of the derailleur is therefore determined by the balance between the spring on the upper knuckle and the spring for the jockey cage. My guess is the jockey cage spring is a fair bit stiffer so the B screw doesn’t have much effect.

    SRAM derailleurs differ in that the the B screw acts against a stop, so adjusting it has a direct effect on the derailleur’s position. From experience, the B screw adjustment isn’t really important with Shimano, whereas it can have a pretty big impact with SRAM

    March 21, 2018 at 7:16 pm #915205
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    AfterPeak

    Difficult to get a good image
    Difficult to get a good image but…

    March 21, 2018 at 7:10 pm #915203
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    AfterPeak

    Yeah no seizing or anything
    Yeah no seizing or anything like that. It all moves freely (its brand new too) and I can see it pressing against the flat part on the hanger as it should. I will add a pic

    March 21, 2018 at 6:26 pm #915201
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    StraelGuy

    Nope, like SpikeBike’s, I can

    Nope, like SpikeBike’s, I can see the screw is pushing against the drop-out. I might remove the chain and have another play.

    March 21, 2018 at 6:14 pm #915199
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    Jimthebikeguy.com

    2 things. Your cassette is
    2 things. Your cassette is fairly narrow anyway so you dont need much b screw, as the guide pulley will be miles from the 25 sprocket. And, look at how the mech is attached and check the obvious; when you wind the b screw in, is it actually driving against the flat outcrop on the end of the hanger? Or is it in fresh air?

    March 21, 2018 at 5:50 pm #915197
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    StraelGuy

    Both mine were new

    Both mine were new derailleurs. Not worried because they both both shift perfectly.

    March 21, 2018 at 5:44 pm #915195
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    The Gavalier

    Is the derailleur pivot bolt

    Is the derailleur pivot bolt seized? Check that the mech pivots freely when you move it. 

    March 21, 2018 at 4:16 pm #915193
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    AfterPeak
    SteveAustin wrote:
    It mightnot do anything if under tension, ie the cable is attached and tightened, and occasionally the chain in place can cause it to look like it isnt doing too much. so disconnect cable/chain, and then try it.

    i built a bike with a dura ace mech and that never looked like it was moving much tbh. never affected the shifting if it was in or out

    Interesting. I could certainly check that. If it doesnt change gap under tension (i.e. when actually in use for shifting) then I suspect it doesnt do anything in or out.

    March 21, 2018 at 4:13 pm #915191
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    SteveAustin

    It mightnot do anything if

    It mightnot do anything if under tension, ie the cable is attached and tightened, and occasionally the chain in place can cause it to look like it isnt doing too much. so disconnect cable/chain, and then try it.

    i built a bike with a dura ace mech and that never looked like it was moving much tbh. never affected the shifting if it was in or out

    March 21, 2018 at 3:54 pm #915189
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    AfterPeak

    Its ultegra 6800 50-34, 11-25

    Its ultegra 6800 50-34, 11-25

    The screw is working fine as I have see it screwing through against the hanger but the derailleur gap between jockey wheel and cog does not alter (with as you say small front and large rear cog).

     

    I am wondering if with a 11-25 the b-screw is not needed as there is no much slack in the system?

    March 21, 2018 at 3:32 pm #915187
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    VeloUSA

    A little more info would be

    A little more info would be helpful as I have never encountered a new Shimano RD b-screw not functioning properly. I’m not saying it can’t happen just that I haven’t seen one fail. So, with the chain on the smallest ring (front) and largest cog (rear) are you saying when you tighten (increases gap) or loosen (decreases gap) the b-screw you can’t visually see the gap change between the cog and upper jockey wheel?

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