# B-Tension Mystery Solved - At Least for Me



## mikiek (Aug 18, 2014)

I have had this problem since the first time I took the RD off the bike and put it back on. There was another recent thread with the same problem. We're talking Shimano 6800 RD (maybe others) - the B-tension screw does nothing. You can screw it all the in and all the way out and the pulley doesn't move at all.

I came across a very old post regarding this topic and someone was saying that they actually take the derailleur off, turn the B screw and remount the RD and see how close/far the pulley is to the cog. He described this as an iterative process.

If you take a look at the piece that the screw screws into, after the screw hole it makes a 90 degree turn, flattens out and fits between the hanger and the RD mount (hope I described that well enough). Now when you adjust the B screw this piece is supposed to move up or down the screw.

My problem is that I always crank down on that 5mm derailer bolt to make it super tight. We don't want the derailleur falling off right? Well when the RD is screwed in that tight there is no way in hell the B screw piece is going to move! What happened to me was the screw actually chewed into the hanger tab. The screw moved, not the other piece.

My solution was to loosen up the RD bolt 1 - 1 1/2 turns. Not so loose that the RD flops around but loose enough so the B screw piece can move. Do that and the adjustment works as advertised.:thumbsup: Be sure to tighten up the RD bolt when you're done.

I checked the Shimano specs to see how tight that mounting bolt is suppose to be - 8-10 nm. I know I was hand tightening well beyond that. Sadly, I did torque the bolt correctly but the B screw piece still did not move.

Hope this helps someone.....


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## MMsRepBike (Apr 1, 2014)

When screwing in a b-tension screw I always unweight the derailleur. Meaning I spin it clockwise until the b-tension screw lifts off of the hanger. It makes it a tight squeeze to get a screwdriver in there but it still fits. This way you're never tightening the screw when it's contacting the hanger. Should prevent you having to loosen or tighten the main fixing bolt for the derailleur.


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## rcb78 (Jun 15, 2008)

The b tension screw does work, it's just subtle, and it won't be visible if you're just turning the screw and nothing else. Some people will back pedal while making the adjustment, I prefer making 1/4 or 1/2 turns and then run it through the gears real quick. The derailleur will sit in a new position after you shift it through a few gears. It's really more of a dynamic setting rather than a static one. And as said above, twist the derailleur up to relieve the spring tension while turning the screw, it just makes it easier.


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## mikiek (Aug 18, 2014)

MMsRepBike said:


> When screwing in a b-tension screw I always unweight the derailleur. Meaning I spin it clockwise until the b-tension screw lifts off of the hanger. It makes it a tight squeeze to get a screwdriver in there but it still fits. This way you're never tightening the screw when it's contacting the hanger. Should prevent you having to loosen or tighten the main fixing bolt for the derailleur.


I tried that a few times but it didn't help in my situation.

The odd thing is before I took the RD off the first time, I was adjusting the B screw and it was moving the pulley without having to loosen the RD. Just like it was supposed to. But now, even when I tighten the RD bolt to spec the B screw does nothing.


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## AndreyT (Dec 1, 2011)

mikiek said:


> I checked the Shimano specs to see how tight that mounting bolt is suppose to be - 8-10 nm. I know I was hand tightening well beyond that. Sadly, I did torque the bolt correctly but the B screw piece still did not move.


In a derailleur like 6800 the B-screw acts between two parts that are supposed to be able to move relative to each other regardless of how tight the RD mounting screw is. I.e. you are supposed to be able to adjust the B-screw without loosening the mounting bolt. If you RD does not move under B-screw action, then something is really wrong with it.


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## mikiek (Aug 18, 2014)

AndreyT said:


> In a derailleur like 6800 the B-screw acts between two parts that are supposed to be able to move relative to each other regardless of how tight the RD mounting screw is. I.e. you are supposed to be able to adjust the B-screw without loosening the mounting bolt. If you RD does not move under B-screw action, then something is really wrong with it.


The RD is moving (rotating) just fine. It was the B tension piece (wish I knew the real name for it) that was not rotating when adjusting the B screw.

I'll give rcb78's method a shot. But it is real easy & quick when I loosen the RD bolt. You can actually see the pulley moving towards/away from the cog as you turn the screw.


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## mikiek (Aug 18, 2014)

MMsRepBike said:


> When screwing in a b-tension screw I always unweight the derailleur. Meaning I spin it clockwise until the b-tension screw lifts off of the hanger. It makes it a tight squeeze to get a screwdriver in there but it still fits. This way you're never tightening the screw when it's contacting the hanger. Should prevent you having to loosen or tighten the main fixing bolt for the derailleur.


Well, the ole man is slow but he does catch on eventually. I wasn't rotating the RD far enough to lift up the B bracket. The bracket finally did raise off the hanger tab and I was able to adjust.

Thanks for that.


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## MMsRepBike (Apr 1, 2014)

MMsRepBike said:


> When screwing in a b-tension screw I always unweight the derailleur. Meaning I spin it clockwise until the b-tension screw lifts off of the hanger. It makes it a tight squeeze to get a screwdriver in there but it still fits. This way you're never tightening the screw when it's contacting the hanger. Should prevent you having to loosen or tighten the main fixing bolt for the derailleur.


Here is a visual representation of exactly what I'm saying here.


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