I’m struggling to index a friend's bike after a drivetrain refresh because his chain snapped (replaced chain, middle chainring and cassette). I’ve done this dozens of times on my own bikes, but this one has a specific "clunk" I can't tune out.
Specs
• Rear Derailleur: Shimano Deore M591 (Official Max: 34T)
• Cassette: Shimano Alivio 11-36T (Previous: SunRace 11-32T)
• Crankset: Deore M590 (3×9)
• Chain: Shimano HG-93 (Sized Big-Big + 2 links)
Some checks before I did the adjusting:
– I removed the shifter cable and checked if there was any corrosion, wear or resistance between inner and outer cable; no resistance and looked like new
– Shifter works like new too
– Removed the derailleur, cleaned it and checked it on damage or wear. It’s like new too
– Derailleur hanger is part of the frame and is straight as an arrow.
The Problem
My friend only rides the middle chainring and never uses the smallest rear gear/largest sprocket, so I am optimizing the indexing for that. While it shifts through the whole range, the high gears (7th > 8th > 9th) are problematic:
1. When upshifting to the smallest cogs, the chain sometimes jumps and clunks badly. It doesn’t skip the gear entirely, but it feels like the chain isn't seating into the teeth properly. Other times it shifts smoothly.
2. If I decrease cable tension to improve the upshift, it loses the ability to downshift to larger cogs.
What I've Tried
• B-Limit: Set to ~3mm gap. I suspect the 36T cassette is forcing the RD too far away from the smaller cogs, causing poor engagement.
• Chain Sizing: I used the Big-Big method. Because the link didn't match up perfectly, I had to add an extra link to reach the reference rivet, then added the standard two extra.
Could the 36T cassette be the culprit? Even though he doesn't use the largest cog, does having the B-screw set for a 36T ruin the geometry for the 11T-15T range on a derailleur rated for 34T? Or is my chain sizing slightly off causing the problems?
Photos attached showing chain tension in various combos.
Any help is appreciated!
by Particular_Spare_176