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I've always wondered what these ridges below the teeth on sprockets are for. Can anyone enlighten me?

mysterious sprocket ridges

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They are there to help facilitate shifting. Basically, the ramps you see help when going from a smaller to larger sprocket by catching the side plates of the chain to help the chain be pulled up onto the larger cog.

Another place where you will see atypical teeth is in the front; some are shorter/different shaped than others to help shifting as well. Sometimes you see pins as well.

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    Yes, and a chain designed for indexed shifting will have pins that project out slightly, to catch the ramps on the sides of the cogs when the chain is shifted against them. – Daniel R Hicks Jan 23 '17 at 18:37
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    (These ramps and the extended pins are what make "indexed shifting" possible. Before them it was necessary to have "friction shifting", with no detents, since the cyclist needed to "overshift" slightly to cause the shift, then adjust back a little once shifted.) – Daniel R Hicks Jan 23 '17 at 18:39
  • Fantastic, thanks chaps. Years of wondering ended by a rapid answer. It seems so obvious now. – phzdw Jan 23 '17 at 20:19
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    I believe Shimano invented the idea, which they called "Hyperglide" for the rear and "Superglide" for the front. According to Sheldon Brown, the ramps allow the new cog to engage before the old cog has completely disengaged, which allows for much smoother and quieter shifts. I remember the days of friction shifters and plain cogs, when shifts took longer, you couldn't apply power while shifting (I still try not to), and chattering noise was inevitable. The new system with indexed shifting and ramped cog teeth is much nicer. – rclocher3 Jan 23 '17 at 21:50
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    @rclocher3 - Yeah, Hyperglide (and indexed shifting in general) is the single most significant advance in bike design in the past 40 years. – Daniel R Hicks Jan 24 '17 at 0:35

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