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I ride an early 90s Scott Unitrack CST with the original Shimano Deore drivetrain it came with. Same as this one. When pedaling, the right pedal has a peculiar feel like a sudden inward jerk at a certain position, roughly when the right crank is parallel to the down tube. This is more noticeable when pedaling lightly. Looking down when pedaling, the right crank appears to stretch or play inward and outward. There is no detectable play when forced with hands.

I'm given to think a busted ball bearing in the crankset is responsible, but I would like to get your opinion about the cause and urgency of the issue before I take it to the shop.

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Sounds like a loose crank arm to me. If so, it's urgent (or perhaps past urgent) that you get it tended to. – Daniel R Hicks Jul 3 '14 at 14:53
Specifically, the more you ride it the worse the damage will be, and very quickly you get to the point of having to replace the cranks. You may already be at that point. – Móż Jul 3 '14 at 22:59
Thanks for the replies. I will take it to the shop ASAP. – nucularbum Jul 4 '14 at 6:55

If you can't feel it with your hands, maybe you should look at the pedal. One of my pedals does similar thing. When a crank was getting loose on one of my bikes it was felt with hands too, and when coasting. Also, it may be bent chain ring or broken tooth on a chain ring.

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