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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?

31 comments

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StraelGuy [1402 posts] 3 months ago
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To be honest, I've noticed exactly the same thing on the last two 105 groupsets I've fitted to my bikes sad.

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VeloUSA [250 posts] 3 months ago
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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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SpikeBike [89 posts] 3 months ago
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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?

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SteveAustin [124 posts] 3 months ago
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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

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SpikeBike [89 posts] 3 months ago
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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.

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The Gavalier [91 posts] 3 months ago
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Is the derailleur pivot bolt seized? Check that the mech pivots freely when you move it. 

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StraelGuy [1402 posts] 3 months ago
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Both mine were new derailleurs. Not worried because they both both shift perfectly.

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jterrier [188 posts] 3 months ago
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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?

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StraelGuy [1402 posts] 3 months ago
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Nope, like SpikeBike's, I can see the screw is pushing against the drop-out. I might remove the chain and have another play.

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SpikeBike [89 posts] 3 months ago
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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

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SpikeBike [89 posts] 3 months ago
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Difficult to get a good image but...

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Acm [43 posts] 3 months ago
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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

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ktache [784 posts] 3 months ago
4 likes

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.

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Grahamd [951 posts] 3 months ago
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Zooming in on the photo, there appears some swarf as though the threads could have been stripped. 

 

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SpikeBike [89 posts] 3 months ago
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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

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SpikeBike [89 posts] 3 months ago
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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).

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kil0ran [852 posts] 3 months ago
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SpikeBike wrote:
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

I learnt this lesson the (not too) expensive way when my B-tension screw bored a hole in my hanger - clearly sharper/harder than the hanger. Since then have been extra cautious particularly as one of my steel frames doesn't have a replaceable hanger.

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SpikeBike [89 posts] 3 months ago
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kil0ran wrote:
SpikeBike wrote:
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

I learnt this lesson the (not too) expensive way when my B-tension screw bored a hole in my hanger - clearly sharper/harder than the hanger. Since then have been extra cautious particularly as one of my steel frames doesn't have a replaceable hanger.

 

kil0ran out of curiosity what happens if you cant get a replacement hanger? Is it worth buying a spare hanger when you buy a new bike? 

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kil0ran [852 posts] 3 months ago
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I usually request a spare hanger. Having said that my LBS has a hanger encyclopedia so as long as it isn't completely trashed they can find one that's a reasonable match, and then fettle accordingly

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SpikeBike [89 posts] 2 months ago
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kil0ran wrote:

I usually request a spare hanger. Having said that my LBS has a hanger encyclopedia so as long as it isn't completely trashed they can find one that's a reasonable match, and then fettle accordingly

I think I am going to request one now. I always thought LBS could make you one.

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DaSy [819 posts] 2 months ago
2 likes

I always used to go to GearMechHanger.com -Gear Mech Hangers for any replacement hangers. It was something I used to require pretty regularly and for some pretty esoteric frames; they always came up trumps.

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DaveE128 [996 posts] 2 months ago
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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

Uh... I think you may have misunderstood what the B-tension adjustment does. Unless it's a shadow mech, if you don't have a chain on you will not see the effect. Whether the cable is attached or not is irrelevant.

The B-tension setting changes the angle that the main mech body sits at, and therefore the vertical distance between the upper jockey wheel (guide pulley) and the cassette sprocket. It does this because the angle is set by the balance between cage tension and b-tension. If the cage spring tension is higher, the cage moves up towards the cassette. If the b-tension is higher the cage moves down away from the cassette.

The actual reason you may not be seeing the difference is friction in one or both pivots.  To find the equilibrium between the two springs, you might need to induce some movement by shifting across the whole cassette and back.

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SpikeBike [89 posts] 2 months ago
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DaveE128 wrote:
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

Uh... I think you may have misunderstood what the B-tension adjustment does. Unless it's a shadow mech, if you don't have a chain on you will not see the effect. Whether the cable is attached or not is irrelevant.

The B-tension setting changes the angle that the main mech body sits at, and therefore the vertical distance between the upper jockey wheel (guide pulley) and the cassette sprocket. It does this because the angle is set by the balance between cage tension and b-tension. If the cage spring tension is higher, the cage moves up towards the cassette. If the b-tension is higher the cage moves down away from the cassette.

The actual reason you may not be seeing the difference is friction in one or both pivots.  To find the equilibrium between the two springs, you might need to induce some movement by shifting across the whole cassette and back.

So adjust up and down cassette and repeat? Still see nothing

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matthewn5 [1190 posts] 2 months ago
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Give the derailleur a wiggle after adjusting it. It does move (or at least, my Ultegra rear mech used to).

If you really want to sharpen up the shifts, replace the upper pulley with one of those alloy ceramic bearing jobs you see on Ebay. Shimano upper pulleys have sideways movement built in to dull down the shifting -- as the cage moves across, the pulley doesn't completely move until the right part of the cassette comes around. Putting a pulley that's rigid laterally makes a big difference. Whether you'll like it or not is another matter! But certainly worth a try.

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Judge dreadful [292 posts] 2 months ago
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Make sure the derailleur hanger is butted up against the screw, when it’s wound as far out as possible, before you start. I’ve had lots of bikes where the hanger has been fitted badly, in relation to the derailleur, rendering the b screw adjustment limited, or useless.

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stenmeister [352 posts] 2 months ago
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ktache wrote:

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.

 

Top recommendation Sir, I'm getting one of these.laugh

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Grahamd [951 posts] 2 months ago
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stenmeister wrote:
ktache wrote:

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.

 

Top recommendation Sir, I'm getting one of these.laugh

Wholeheartedly agree, embarrassing that it took me years to order one. Got mine from Amazon, about £7 , so no nonsense with import duty / tax.

 

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Peowpeowpeowlasers [604 posts] 2 months ago
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Pedal backwards.  You'll see the change in position.  Postion the screw so the chain stays at least 5mm away from all the cogs.

The screw on a Shimano derailleur simply adjusts the tension on a very large, fat powerful spring wrapped around the bolt that secures your derailleur to your hangar.  You can't see this spring unless you remove the surclip from behind the bolt and then push the bolt out.

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SpikeBike [89 posts] 2 months ago
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Peowpeowpeowlasers wrote:

Pedal backwards.  You'll see the change in position.  Postion the screw so the chain stays at least 5mm away from all the cogs.

The screw on a Shimano derailleur simply adjusts the tension on a very large, fat powerful spring wrapped around the bolt that secures your derailleur to your hangar.  You can't see this spring unless you remove the surclip from behind the bolt and then push the bolt out.

 

Cheers I will give it a go. FYI never try and service that spring. Pain in the backside to get that surclip back in and hold the spring wound. throw in the bin and buy a new derailler  1

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SpikeBike [89 posts] 2 months ago
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OK I have finally got the bike on my work stand to have a good play with the screw.

With the advice above (pedal backwards + shift up and down the cassette between adjustments) I can now see a small movement. I certainly can't get the gap less than 5mm but it's close to that so I am happy.

Thanks everyone. I am going with mystery solved.

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