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I was trying to adjust the "H" and "L" screws on my rear derailleur to as to adjust the gears better to a Wahoo Kickr I just set up. However I accidentally gave a turn to another, completely different screw. I tried to set it back as it was, but I don't know what that screw does, and what I should look for if I somehow knocked it out of adjustment. And whether not having that screw correctly set could in some way be important -- that is, cause damage, or mess with something in a bad way. As said, I think it's exactly as it was, pretty much -- I was being cautious turning the H and L screws in any case, a quarter turn at a time or so.

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    Once the H and L screws are adjusted, they "usually" do not need to be adjusted from wheel to wheel to smart trainer. I say usually because if tolerances are not kept by manufacturers, that would be a reason for adjustment. The other would be if the H/L screws moved since they were last adjusted. The B screw adjustment you were curious about could may need an adjustment when switching to a cassette with a different range, particularly the largest cog.
    – Ted Hohl
    Mar 23, 2023 at 18:44
  • @TedHohl Another reason for readjusting the rear derailer after a wheel change is if the wheels' axles use different width locknuts. The axle with a thinner locknut on the drive side will place the cassette further outwards, increasing the rear chainline. I'm not sure if there is a standard for hub/axle manufacturers to follow regarding rear chainline.
    – Robert
    Mar 23, 2023 at 21:08
  • @Robert good point. That is why I qualified my comment with "usually." There are exceptions to usually, like the one you mentioned. That would be a good example of a manufacturer not adhering to tolerances and not being in line with whatever standards the industry is following. And the industry doesn't follow them sometimes as well.
    – Ted Hohl
    Mar 23, 2023 at 22:51
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    Just a thumbs-up for @TedHohl's comment above. When switching between my wheel with 11-32 cassette and trainer with 11-25 cassette, I don't need to adjust the H or L limit screws but need to give the B-screw three turns anti-clockwise when moving to the trainer and 3 turns clockwise when moving back to the wheel, otherwise shifting on the trainer can be quite hesitant. If your cassettes are the same you shouldn't need to adjust anything once set up correctly.
    – BigglesZX
    Mar 24, 2023 at 17:34
  • I've had that screw just fall off due to road vibrations and being unscrewed too far. After adjusting do check that it does not move when pushed sideways. Do replace the blue stuff in 2 years or 3.
    – Vorac
    Mar 31, 2023 at 3:24

2 Answers 2

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It is called a B-screw. You can find more in the Dealer's manual for your derailleur, but basically it allows to change the angle around the pivot (related to tensioning) and the distance from the cogs.

This video shows how it is used for adjustment:

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  • Thanks! I will watch that. For now, If I adjusted it more or less back where it started, I should be fine? That is, how "picky" is the adjustment?
    – Cerulean
    Mar 23, 2023 at 15:45
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    @Cerulean It is not very picky. As long as you are not hitting the cogs or it is not way too far, it will work fine. Mar 23, 2023 at 15:51
  • @Cerulean I always set that in the middle of the adjustment range, and never had a problem. If there are problems then screw or unscrew according to the video tutorial above. I am always skeptical with any adjustments the bike comes with by default. On a bike I bought new some time ago this B screw was so unscrewed, almost falling out. The shifting worked fine, but it's nice to not lose the B screw.
    – Robert
    Mar 23, 2023 at 21:26
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According to the manual the B-tension adjust bolt allows you to change the derailleur so that the top pully does not interfere with the sprocket.
You want it close but not interfering.
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