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3-speed details

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  • pj
    With planetary gears, the sum of the number of teeth on the sun gear plus the the number of teeth on the ring gear has to be evenly divisible by the number of
    Message 1 of 3 , Oct 3, 2011
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      With planetary gears, the sum of the number of teeth on the sun gear plus the the number of teeth on the ring gear has to be evenly divisible by the number of planet gears to get everything to mesh.

      Sturmey-Archer uses 20 tooth sun gears and 60 tooth ring gears in the AW/SRF3 family, and with this total of 80 teeth have four planets. This arrangement has worked well in Sturmey hubs since the K series which debuted just after WWI.

      Shimano uses 16T sun gears and 44 tooth ring gears in their three speed hubs, and with this total of 60 teeth have three planets. Three planets are theoretically better than four, because three will self-center the load. (This is one of the few things Sturmey got right with the SW3.) Using fewer teeth, theoretically the Shimano hub should be less expensive to manufacture, less smooth running and less efficient, although on a practical level I'm certain this disappears into the great wash of other variables and has no practical impact on the end user.

      SRAM (nee Fichtel&Sachs) uses 17 tooth sun gears and 47 tooth ring gears in their T3 and iMotion3, and with this total of 64 teeth uses...three planets. But...but...64 is not evenly divisible by 3!?! Upon close observation of the disassembled components of a SRAM three-speed, one will note that the three planets are not evenly spaced - they are spaced as if there where 16 planets, with 13 of them missing. Plutoed planets - quite clever! But why the odd 17 and 47 tooth gears? Gears with prime numbers of teeth preclude a pattern of wear developing in the geartrain. Crafty.

      pj
    • Rick Paulos
      Got the tooth counts handy for the other old SA hubs, the Narrow & Medium range geared hubs? That could make a nice chart (include ratios too) Rick
      Message 2 of 3 , Oct 3, 2011
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        Got the tooth counts handy for the other old SA hubs, the Narrow &
        Medium range geared hubs? That could make a nice chart (include ratios too)

        Rick




        At 08:53 AM 10/3/2011, you wrote:
        >With planetary gears, the sum of the number of teeth on the sun gear
        >plus the the number of teeth on the ring gear has to be evenly
        >divisible by the number of planet gears to get everything to mesh.
        >
        >Sturmey-Archer uses 20 tooth sun gears and 60 tooth ring gears in
        >the AW/SRF3 family, and with this total of 80 teeth have four
        >planets. This arrangement has worked well in Sturmey hubs since the
        >K series which debuted just after WWI.
      • pj
        ... More than you wanted to know:
        Message 3 of 3 , Oct 3, 2011
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          > Got the tooth counts handy for the other old SA hubs, the Narrow &
          > Medium range geared hubs? That could make a nice chart (include ratios too)

          More than you wanted to know:

          <http://www.hadland.me.uk/1theory.pdf>

          <http://john-s-allen.com/gears/hubratios.htm>
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