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Oval Cog

  1. Jul 7, 2005 #1
    this is just hearsay from another engineer
    but i guess there is a guy in the Tour de France right now that is using an oval front cog for more downstroke power, and for an easier upstroke.

    has anyone here heard of this?
    i can't seem to find anything more on it

    also would there be problems with the chain tension?
  2. jcsd
  3. Jul 7, 2005 #2
    A number of years ago Shimano introduced their Biopace chain ring, which was oval in shape. The idea behind the oval chainring is to increase the effective gear ratio on the down stroke and make it lower at the bottom stroke. The shape of the chain ring wasn't quite as extreme as the one being used in the Tour though.

    Chain tension shouldn't be a problem because the derailleur is designed to take up slack on the chain.
  4. Jul 7, 2005 #3


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    I remember those oval Shimano rings, they never really caught on for whatever reason.

    The theory behind them is that when you're pedalling, there's a 'dead spot' (when your feet are at 12 and 6 o'clock) where you can't push on the pedals with as much force as, say, 3 and 9 o'clock.

    An oval chain ring reduces the gear ratio during this 'dead spot', by effectively reducing the diameter of the chain ring, effectively increasing torque. When you're past the dead spot, at the part of the stroke where you can apply maximum power, the chainring diameter effectively increases. This can also reduce wind-up in full suspension bikes.
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