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Lockup when shifting to highest gear

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Lockup when shifting to highest gear

Old 07-09-13, 06:23 PM
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asmac
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Lockup when shifting to highest gear

Occasionally, esp when I'm flying downhill (and spinning the pedals) and shift to my smallest cog (already in largest chainring), the whole drivetrain suddenly locks up. That is, I can no longer pedal although, by the time I slow down and take any pressure off, everything releases and is back to normal. I have not been able to see this when stopped or duplicate it on a stand.

Any ideas as to what could cause this lockup and how to fix? It's a 9 spd XT rear derailer with a 9 spd sram chain.

Last edited by asmac; 07-09-13 at 06:28 PM.
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Old 07-09-13, 06:28 PM
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A photo of the right side of your bike taken from the back would help, but I suspect that it's related to something close to and above the cassette. On some bikes the inside bottom of the seatstay, where it joins the dropout is the problem. The chain on the outer sprocket tucks under it and runs fine, but can get snagged during a shift when it has't fully engaged the sprocket.

The same can happen with a fender screw that comes through the eye and sticks out near the cassette.

Look from the back, and see if there's anything inboard of the dropout face that may be the issue.
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Old 07-09-13, 06:28 PM
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By lockup do you mean you can't pedal? that would have to mean that the chain has caught on something. It may be that there is too little clearance between the chain and right dropout, or that bolts holding a rack or fenders on are interfering. You may be flexing things enough to cause inteference only when you are pedaling hard, or it may interfere only when shifting quickly or under power.
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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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Old 07-09-13, 08:28 PM
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Thanks for the insights. I shall spend some time on it tomorrow. One of you mentioned chain length. I built this from scratch and did my best to estimate proper chain length without really knowing what I was doing. Is there a way to adjust this with some degree of exactness? Photos will follow...
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Old 07-09-13, 09:09 PM
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I don't see how chain length would be a factor with the large chainwheel and the small cog, but info on sizing bicycle chain is easily found with a search with those or similar terms.
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Old 07-16-13, 09:01 PM
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Originally Posted by FBinNY View Post

The same can happen with a fender screw that comes through the eye and sticks out near the cassette.
I think that was it. The screw appeared to be quite far from the cassette but perhaps the chain loops up a bit before it settles on the cog when changing gears at speed. Anyhow a hacksaw has taken care of the excess bolt and the problem seems to have gone away. Thanks to all for your time.
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