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Discussion Starter · #1 · (Edited)
Look -- 50mph and no gears

Riding today, I'm nearing the crest of a good decent when a FedEx truck passes me. Nice. I grabbed his draft to get my speed up and pulled close to 50mph descending. BUT while in 53 x 12 on my wide-eyed decent I hit some small bumps (pretty noticeable at that speed) and my drivetrain locks up!! I look down and see my chain somehow came off the cassette and is wedged against the frame. CRAP! All was ok. I just had to coast for a while and pull over...

My rear limit looks good, so do you think I need to:
1) pull the rear stop in just a bit?
2) check the chain length to make sure I have enough tension
3) might be time to replace the cassette (~3k miles on it)
4) stop hitting bumps that fast (ie probably just a fluke)
 

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advice...

There's no way that it's a chain tension problem. Minimum tension is in the little ring and smallest cog. When you kump to the big ring, you take up 3.5 inches of chain.

I'd check the high limit screw.

A worn cassette does not cause this sort of problem and 3K is no mileage at all.

Number 4 is only a possibility if you quit pedaling; then the bump might toss the chain around.

What you experienced in not chain suck. That's a problem unique to worn chainrings,
 

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"B" screw

cmd miler said:
Riding today, I'm nearing the crest of a good decent when a FedEx truck passes me. Nice. I grabbed his draft to get my speed up and pulled close to 50mph descending. BUT while in 53 x 12 on my wide-eyed decent I hit some small bumps (pretty noticeable at that speed) and my drivetrain locks up!! I look down and see my chain somehow came off the cassette and is wedged against the frame. CRAP! All was ok. I just had to coast for a while and pull over...

My rear limit looks good, so do you think I need to:
1) pull the rear stop in just a bit?
2) check the chain length to make sure I have enough tension
3) might be time to replace the cassette (~3k miles on it)
4) stop hitting bumps that fast (ie probably just a fluke)
Consider using the "b" screw (or Campy equivalent) to move the body of the derailleur closer to the cogs. Check to be sure that the upper pulley doesn't hit the larger cogs.
 
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