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Chain falling off cassette (not what you are thinking)

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Chain falling off cassette (not what you are thinking)

Old 07-03-21, 08:03 PM
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ronopolis
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Chain falling off cassette (not what you are thinking)

Shimano Ultegra drivetrain. This isn't the "normal" chain falling off of the cassette issue. I can't shift the chain off of the cassette, the shifting is all spot on. But...

I'm in my biggest gear (chain is on the smallest cog in the cassette). I'm going down a hill, eventually get to 35mph or so and pedaling isn't really helping. So I'm pedaling hard and then I suddenly stop pedaling. The rear cassette has momentum, so it continues to rotate a tiny bit more, which causes the chain to move forward and down to the chain stay. The chain almost touches (or maybe slightly touches) the top of the chain stay. Then the cassette rotates back a few degrees and the chain snaps back (or really, up) into its normal position. And at this point the chain falls off the cassette, to the outside (away from the hub). The chain is off the cassette, and the cassette is now free spinning. I can shift the chain back on to the cassette but as you can guess, this is a huge pain. It doesn't happen every time, but often. More than once I've started to pedal and "Oh Crap my chain is off."

I've never had a bike do this. What is going on? Again, I can't shift the chain off of the cassette, and the shifting is spot on. I sized the chain using the normal "wrap the chain around the biggest rear/biggest front and add one link" measurement. But is my chain one link too big?

I live in Colorado, so this is really annoying, I never have a flat ride.
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Old 07-03-21, 08:14 PM
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What model Ultegra RD?

Sounds like your B screw adjustment is too slack, and/or your chain is too long.
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Old 07-03-21, 08:26 PM
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Originally Posted by Kabuto View Post
What model Ultegra RD?

Sounds like your B screw adjustment is too slack, and/or your chain is too long.
Ultegra RD-R8000 GS R Derailleur. B screw adjust... hadn't thought about that one. I think it was correct when I built the bike, but... maybe not anymore?
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Old 07-03-21, 09:53 PM
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Originally Posted by Kabuto View Post
What model Ultegra RD?

Sounds like your B screw adjustment is too slack, and/or your chain is too long.
Highly, extremely, almost definitely not this. If the chain was too long you'd only see it in the small ring/small cog or 2. Wouldn't matter a bit anywhere else. Same with b screw adjustment. What kind of wheels do you have? Sounds like a Mavic freehub that needs cleaning/lubing.
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Old 07-03-21, 10:18 PM
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Looks like you're laying down the watts like Cavendish!
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Old 07-03-21, 10:37 PM
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Originally Posted by cxwrench View Post
Highly, extremely, almost definitely not this. If the chain was too long you'd only see it in the small ring/small cog or 2. Wouldn't matter a bit anywhere else. Same with b screw adjustment. What kind of wheels do you have? Sounds like a Mavic freehub that needs cleaning/lubing.
Mavic Ksyrium UST CL Disc. Yikes. Can you explain a bit more how the hub is making this happen? Is the freehub clean/lube relatively straightforward? I built this bike from scratch, I know how to do a lot of basic stuff, but never got into hub maintenance.
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Old 07-03-21, 10:45 PM
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Originally Posted by cxwrench View Post
Highly, extremely, almost definitely not this. If the chain was too long you'd only see it in the small ring/small cog or 2. Wouldn't matter a bit anywhere else. Same with b screw adjustment. What kind of wheels do you have? Sounds like a Mavic freehub that needs cleaning/lubing.
This.

Sounds like drag in the freehub. Should be videos to help you clean/repair this.
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Old 07-03-21, 10:49 PM
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OK, I'll start figuring out the freehub drag!
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Old 07-04-21, 01:19 AM
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It sounds like you have to be careful when really hammering it. My chain/cassette is messed up too but I just got used to it and adjusted my pedal stroke accordingly
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Old 07-04-21, 05:03 AM
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Originally Posted by cxwrench View Post
Sounds like a Mavic freehub that needs cleaning/lubing.
That was my first thought. Inboard bearing a plastic bushing, what were they thinking.

You can verify this is the issue by the fact the cranks turn when you wheel the bike in a tall gear.

Last edited by Kimmo; 07-04-21 at 05:07 AM.
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Old 07-04-21, 05:53 AM
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Almost certainly going to be a draggy freehub. I get a similar thing happening on my direct drive turbo trainer if I sprint hard and then suddenly stop pedalling. Fortunately it doesn't happen on the road as the freehub on my wheel has much less drag.
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Old 07-04-21, 09:14 AM
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Originally Posted by LarrySellerz View Post
It sounds like you have to be careful when really hammering it. My chain/cassette is messed up too but I just got used to it and adjusted my pedal stroke accordingly
That's just stupid. If it's not working correctly you fix it, not 'adjust your pedal stroke'.
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