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Or, in other words, What does the B-Tension screw do? If you’ve tried to adjust your own derailleur, chances are good that you’ve wondered the same things. But, since that odd ball screw tends to confuse a lot of people (more than one reader has asked essentially the same question), we thought it would be worth a closer look…

AASQ #14: What does the B-Tension screw on a derailleur do?
B-Tension screw location on new Dura Ace rear derailleur

To make sure we weren’t missing anything, we reached out to one of the most knowledgeable people we know on the subject – Shimano’s Mountain Bike Product Manager, Nick Murdick. Simply put, the B-Tension screw is there to control the gap between guide pulley (the upper pulley on the derailleur) and the bottom of the cogs. Since this gap will always be most easily seen and tested by the relationship between the derailleur and the largest cog on the cassette, it’s recommended that you set your adjustment here. It’s also recommended that if you have a front derailleur, it should be shifted into the smallest chainring.

OneUp Components RAD Cage replacement Shimano derailleur cage to offset pulley for oversized rear cogs
You want this gap to be as small as possible.

OneUp Components RAD Cage replacement Shimano derailleur cage to offset pulley for oversized rear cogs OneUp Components RAD Cage replacement Shimano derailleur cage to offset pulley for oversized rear cogs

This used to be a fairly straight forward adjustment. Just get the guide pulley as close to the bottom of the cog as possible without it making noise. But as cassette ratios get wider, the recommended gap has ballooned from 1-2mm to 5, 6, 7, or even 8mm depending on the set up. The larger the tooth count difference between the top two cogs, the more space you’ll need.

Since you have to set the B-Tension adjustment to clear the largest cog, this is where cassette adapters (i.e. putting a larger-than-stock cog on the back of your cassette) tend to have a detrimental effect on shifting. Nick points out that Shimano has very strict engineering requirements when it comes to their products, so it’s not surprising to find a little wiggle room in what actually works.

So what does happen when the B-Tension adjustment is too far away from the cog? Essentially, it just slows down your shifting. It will still work, but as the derailleur pushes on the chain from the inside, the shift takes place when the chain picks up the tooth on the next cog. When the derailleur is farther away, this will slow things down. It will still shift, just not as crisply. The flip side is that if it’s too close, the derailleur can try to push the chain into the side of the larger cog, but too low for it to pick up the chain, and things can get jammed up.

In the end, you still want the B-Tension adjusted so that the upper guide pulley is as close to the cogs as possible, without making noise or creating physical interference. How close that actually is depends greatly on the cassette and derailleur you’re using, but in general – the closer it is, the better your shifting will be.

And if you really want to nerd out on derailleurs and chain theory, Nick suggests The Dancing Chain by Frank Berto.

Got a question of your own?  Click here to use the AASQ form, or find the link under the Contact menu header up top anytime a question pops into your mind!

14 COMMENTS

  1. Shimano tech docs states: Adjusting the B-tension adjust bolt
    Mount the chain on the smallest chainring and the largest
    sprocket, and turn the crank arm backward.
    Turn the B-tension adjust bolt to adjust the guide pulley to
    be as close to the sprocket as possible but not so close that it
    touches. Next, set the chain to the smallest sprocket and
    repeat the above to make sure that the pulley does not
    touch the sprocket.

  2. … What is not true about this? Yes the exact required B tension gap is sometimes slightly different, especially for Eagle, but for most sram drivetrains the same basic theory of get it as close as possible without making noise, aka forcing the chain into the side of the next largest cog, is 100% true.

    • I’ve found that the red b-tension guide for eagle rear derailleurs is useless. I believed in SRAM for the first few eagle derailleurs I set up and the shifting was garbage until I adjusted the b-tension in.

      • Keep in mind that on full suspension frames, gap should be checked at the sag point. Bikes typically exhibit chain growth when suspension is compressed, moving the upper pulley down some.

    • TS- for older Shimano (anything with a spring loaded b-pivot) the adjustment was to run the upper pulley as close to the largest cog while not making noise while backpedaling. If you were to do the same for SRAM, you would, in fact, have the chain run into the next larger cog when shifting to another easier gear. Yes, both are about eliminating a noise, but they are two totally different noises.

  3. SRAM eTap 11-25 works best for me at 8-10mm and the same for Red 22 Hydro 11-28. Tech docs claim 6-8mm but that close and shifting down to the 13t and smaller are labored, that’s just my observation.

    • Good question. Especially given Campy’s adjustment works opposite Shimano’s and SRAM’s, and even more especially given Eagle’s B-tension adjustment is pretty much contrary to the way B-tension has been set always and forever. And it only works when set up that way.

      This “article” is a useless contribution to bicycle technical literature.

  4. How about everyone starts with a good dose of RTFM. If that doesn’t get you to a well running setup, try small adjustments until you reach the best functioning. It’s not hard. It’s only a single screw.

    • This guy gets it.
      Shimano: si.shimano.com
      SRAM: sram.com/service
      The manual will get you there most of the time and the knowledge you pick up even thumbing through it will get you the rest of the way.

  5. It is not about “stupid questions” It is about the lack of interesting subjects to cover, more than enough said for a single screw, just read the manual as already mentioned and give us something more interesting to read please

    • @Karel, updated to a less weird, less randomly Japanese link since Berto appears to no longe manage that URL. Thanks for the heads up.

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