SRAM makes this plastic tool for adjusting the B-gap on their Eagle derailleurs. My question is, if I want to measure this distance without this tool (say with a caliper instead), how should I go about it? Between which two points should I make the measurement and what's the value I should aim for?

  • 1
    Standard B-screw adjustment is to get a gap of 5-6 mm between the tips of the teeth of the largest sprocket and upper idler, measured with a 5 or 6mm hex wrench. The Eagle tool seems to leave a bigger gap. Perhaps someone with a tool can measure the tooth-tip gap. Jan 23, 2019 at 14:36
  • 1
    The 'progressive-degressive' method might help. Turn to the maximum gap. then slowly and carefully reduce by half or quarter turns until it gets sub-optimal. This bit I forgot: then go back that last bit. Never had problems with any bike that way. (I won't take liability for this.)
    – Carel
    Jan 23, 2019 at 18:42

2 Answers 2


This measurement is given by both Shimano and SRAM as the shortest distance between the circles formed by the tips of the teeth of the largest cog and those of the guide pulley.

The spec for Eagle is 15mm.

A simple way of actually doing it is to hold up a 15mm wrench, caliper, track nut, etc to it.

Give it a little fine tuning once the chain is on and you're setting up the shifting. You'll be able to tell how small changes (1/4 turn-ish) have some bearing on how smoothly and quietly it all works.

  • 1
    Wow - 15mm is quite a large gap. I've run some shimano ones at about 0mm difference!
    – Criggie
    Jan 25, 2019 at 8:18
  • I can only tell it makes a difference on whether the chain engages correctly with the largest cog or if its teeth stay aligned more with the rollers of the links. I suspect I have other problems like a bent hanger because I can't get it to shift right both on both ends of the cassette at the same time.
    – Artefacto
    Jan 25, 2019 at 13:29
  • 1
    Yeah, bent hanger sounds likely. Jan 25, 2019 at 17:06
  • I would add the chain should be on the second largest cog, not on the largest one.
    – Artefacto
    May 14 at 10:51

the gap is 15mm with the suspension at 30% sag

  • Welcome to Stackexchange - please browse the tour and see how the site is organised, and thank you for your contribution - feel free to try some other questions too.
    – Criggie
    Jun 10, 2020 at 0:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.