RIPPERTON said:
My neighbor and I both have Shimano Biopace gear on our roadies but recently noticed the high spots on the chainrings were timed to engage the chain when the pedal was at 12 o'clock slowing the crank down during the dead spot which is the opposite of what you want. ....
Well, , , -it's the opposite of what
you want. There's at least a couple different goals here, and you can't attain both at the same time.
On an upright bike and with the pedals vertical--if you put the high spots horizontal, then the rider can move through the dead spots faster.
On an upright bike and with the pedals vertical--if you put the high spots at roughly the one/seven o'clock positions, then you're shifting the torque load off the upper part of the pedal stroke, and pushing it into the lower part. The reason you'd do this is if you got knee pain early in the pedal stroke, but not in the lower-half of the pedal stroke (like me).
You're kinda limited with a typical chainring's 72-degree increments however. Q-rings are the best right now for playing these games, even though they don't have the same shape as the BioPace did.
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I put up a page
here showing the different shapes of some non-round chainrings compared, with their long axis oriented the same way and resized to the same maximum diameter. This diagram
doesn't show them oriented as they are properly installed however; I meant to add another diagram showing that but haven't gotten around to it yet.
...And I only traced the best photos I could find online, and the shape depends on what size chainring you choose to use as an example (the smaller ones were usually "less-round" than the larger rings of the same brand). Additionally, there were at least two "official" BioPace versions (I and II, the second was milder than the first) and at least one more than that: someone from online with a small collection of BioPace rings claims there's at least three different shapes for a given size, and possibly four.
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