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Old 07-09-13, 06:46 PM
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If there is nothing overtly near the dropout that would catch the chain and wedge it at high speed, check the teeth on the top pulley of your derailleur. If worn, have it replaced, or cleaned of gunk at least. Also, lube the inside of the freehub to allow it to spin freely and adjust the B-screw to better hug the small pulley a little bit more. There is a rare mode that I've observed drive trains get into if their freehubs have high friction and so they have problems of chain suck, but instead of manifesting in front chain suck, it causes the rear, in smaller cogs, where this is more separation between pulley and cog to derail off of a fouled pulley or worn pulley. If it wedges, it's good that you don't pedal hard because it's caused broken pulley teeth and cracked ceramic bushings on their centeron pulleys (expensive to replace). And you can't back pedal because of impending chain suck up front. Once you slow down, the freehub friction isn't enough to maintain the chain tension and you can back pedal a little, unwedge the chain, and pedal forward and the shift completes.

I've observed this twice with folks who upgraded to a wider range cassette on their drive trains and then started having this problem. Couldn't be sure of the cassette freehub, but don't recall it being Shimano branded and the friction and clicking were really loud. But the new chains they put on could have been a link long as well. But the B-screw setting was made to accommodate clearance for the new big cogs. And so on the small cogs, the distance between pulley and cog was greater. This isn't to say that's what you're seeing. But I'd be curious to know if you ever see this if you shift prior to your highest gear first, then wedge your drive train. In the cases I saw come through, I doubt that would ever happen. It only happens if shifting to a higher gear and the rear wheel is turning significantly faster than the speed of pedaling and therefore causing some chain suck. And I also doubt if the shift was just one or two gears. The two of them broken pulley teeth shifting from a low gear to a high gear (like many gears shifted at once).
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