Welcome to our Ask a Mechanic column where our expert mechanic Daniel Slusser answers your bike maintenance questions. If you have a question for Daniel, please post it on our Facebook Wall or e-mail Daniel directly at firstname.lastname@example.org. Today’s column offers some tips on Shimano derailleur tuning in response to a couple of our instructional videos on the Art’s Cyclery YouTube Channel.
Question: I have a Shimano Di2 system. When riding in the 7-8-9 gear (i.e large chainring in the front) the chain chafes against the cage of the front derailleur due to the angle of the chain. At the extremes, i.e lowest and highest gear it is fine. What is wrong with my setup? From: YouTube
Answer: It sounds like your derailleur might be positioned too high. Make sure that the bottom edge of the outside of the derailleur cage is 1-3mm above the large chainring when the derailleur is shifted into the small chainring. Rotational alignment may be off as well. Make the outer cage parallel with the big ring. The last thing to check is your limit screws because these can change the way the front derailleur will automatically trim.
Question: When my rear derailleur is perfectly aligned with the largest cog. The lowest part of the derailleur hits the spokes. When I adjust the limit screw so the derailleur won’t hit the spokes, the gears grind and wanting to go down to a higher gear. What do I do here? From: YouTube
Answer: Your derailleur hanger is bent inward. When properly aligned the two derailleur pulleys should be sitting vertically on top of each other and parallel with the rear cogs. Use a Park Tool DAG-2 to realign the hanger. Your local shop will have one in their service department and can help you with this if you want to avoid purchasing this costly shop tool. Once your hanger is straight, you will need to retune your rear derailleur starting over at step one because alignment changes everything.