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I have a Planet X EC-130 carbon road bike with Ultegra group set. I have the stated problem above; it mainly happens while in the smallest chainring and does it at all speeds. It sometimes goes 20 miles with no issues then will do it 6-12 times in quick succession.

I have had a friend ride the bike while I observe and the rear derailleur is being snagged somehow and springs towards the front of the bike before returning to its normal position, it doesn’t cause any issues just a very annoying loud metallic sound. Rear cassette and chainrings are fine, derailleurs have been thoroughly cleaned, new chain has been fitted and bike is kept and maintained to a high standard.

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    Sounds like something called “chain suck”. Search around for that.
    – Paul H
    Jan 24 at 16:40
  • Sorry to seem pedantic and can you find a better translator? 'Rear derailleur catches and springs towards front of bike' is (just) understandable but 'when in predominately in the smallest front chainring…' relies on guesswork. Can you find a better translator for 'when in predominately in the smallest front chainring…'? Jan 24 at 21:50
  • Can you reproduce it in a work stand? Or hang the bike on a beam or string or something from the saddle, like this: dero.com/wp-content/uploads/images/products/saddle-buddy/… Pedal with your hands, shift and see if you can see any strange behavior. The bike should shift smoothly into all gears, the chain should be properly aligned with the sprocket and the rear derailleur and its hanger shouldn’t be twisted or bent.
    – Michael
    Jan 25 at 8:03
  • @RobbieGoodwin First quote: the derailleur cage rotates forwards (CCW viewed from drive side), meaning a sudden decrease in chain slack. Second quote: the problem primarily occurs in the small chainring (the bike likely has a 2x).
    – MaplePanda
    Jan 26 at 2:09
  • Thanks for that… Jan 26 at 22:56

1 Answer 1

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Sounds like the chain is staying attached to the chainring while pedalling.
This is called Chain Suck

This pulls the low-tension lower run of chain forward, so the rear derailleur's cage is pulled forward against spring tension.

The spring pressure builds so the derailleur pulls the chain off the binding point and snaps back to its normal position.

Causes:

  • Damaged/bent teeth in the chain ring that can grip the chain. If this, it will always happen in the same spot in your pedal stroke.
  • Damaged chain that manages to grip the chainring as it passes. If this is the cause, then the issue will appear about every-to-every-second pedal stroke. Unlikely since you have a new chain.

You mention this only happens in the small chainring- if its happening in small/small combinations, I wonder if the chain is snagging on the inside of the big chainring?

If you can replicate this in a workstand it will be much easier to find the cause. Do be mindful of fingers and hair when working really close to the chain; it can be quite unforgiving if it gets a taste of you.

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