Pictured is the Shimano Altus 2x crankset chainring (bike from which it comes was a 2019 model so I assume it may be one generation old, though the current chainring generation has the same peculiarity). The chainring appears to have been made by pressing sheet metal, and features a number of seemingly random (but rotationally symmetrical with 180 degree offset) divots or impressions on it. What is the purpose of these divots, or do they have no purpose other than simply being an artifact of the manufacturing process? .

  • Interesting. Those appear to bulge outward, toward the camera. Based on the currently accepted answer, though, they must be pressed inward toward the frame. Can you confirm which way they actually go?
    – FreeMan
    Feb 24, 2022 at 14:31
  • I believe the idea is that the meaningful pressing/machining for purposes of guiding the chain are on the back side, and the visible bulges are just an artifact of those
    – DerekG
    Feb 24, 2022 at 14:33

1 Answer 1


The interesting part is on the other side, when derailing: they guide the chain when going from the small chainring to the big one. 2x/3x systems very often have guides like this. Fancier ones may have more, to improve responsiveness, or they may be CNC machined (like the one on the picture below, from an XTR), but the principle remains the same.

Rear on XTR chainring

  • Interesting. So by that explanation you'd expect to have the best front-ring shifting performance at certain places in the crank's rotation?
    – DerekG
    Feb 22, 2022 at 19:25
  • 3
    @DerekG They're usually placed to follow the average cyclist's power curve as they pedal, IE weakest at top and bottom and strongest just before the feet are level.
    – MaplePanda
    Feb 22, 2022 at 19:35
  • 1
    @DerekG: Yes, it’s especially noticeable with a badly adjusted front derailleur and slow pedaling. You can actually see where the chain starts to climb up those guides.
    – Michael
    Feb 22, 2022 at 20:23
  • 1
    @Michael's right, and with a workstand both of those factors can be simulated by hand
    – Chris H
    Feb 24, 2022 at 16:51

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