Rear derailleur catches and springs towards front of bike when in predominately in the smallest front chainring

Question

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.

Answer 1

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.