My wife has got hold of a 2022 Trek Powerfly FS 4 Equipped Gen 2, which has been upgraded to a 12 Speed rear cassette; I have now replaced the shifter and rear mech to a 12 Speed one and put a new chain ring on it, but when the lowest 3 gears the chain appears to be rubbing on the mech cage, could t...
@stumcuk - a few things to check here, in rough order of likelihood:
1. B-screw (most likely culprit)
If the original cassette was 11-speed with a smaller largest sprocket, and the new 12-speed cassette has a larger one (say, going from a 46t to a 50t or 52t), the B-screw may not be wound out far enough. This pushes the top jockey wheel too close to the large cogs, and the cage fouls under load. Wind the B-screw out (clockwise when viewed from the drive side) until there's a clear gap - Shimano spec is typically 8mm between the top jockey and the largest sprocket tooth, SRAM varies by derailleur.
2. Bent hanger Yes, absolutely possible, and worth checking even if the bike looks fine - hangers bend subtly and it's not always visible to the eye. A derailleur alignment gauge is the only reliable test. Given the symptoms are in the lowest gears specifically (where lateral load is highest), a bent hanger is a credible cause.
3. Cable tension Low-end cable tension issues can cause the derailleur to sit slightly inboard in the large sprocket range, which can rub the cage. Make sure indexing is set with the barrel adjuster correctly - low-limit screw is a separate check, but that's more about the derailleur not reaching the largest sprocket at all.
Quick triage: • What cassette did it come with, and what have you fitted?
• Which derailleur have you swapped to?
That'll narrow it down - but I'd start with the B-screw before pulling the hanger off.
