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@Thomas_M
Upon cleaning my bike and re-lubing the chain, I noticed something odd with the Ultegra RD-6800:
When in 53t font and lowers 4 gears on the rear (both on 25t and 28t cassettes), the chain jumps off the lower pulley wheel. Pedalling backwards will put the chain back on, but pedalling forward will throw it off again.
The indexing is fine and the shifts happen properly. (Apparently it has been doing this for some time without me noticing as the inside of the cage shows marks from chain rubbing.)
Any thoughts on the cause?
Thanks