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 post concerns chainline problems with Shimano Di2 systems. Both questions were posted as YouTube comments on our How to adjust Shimano Di2 Rear Derailleurs Video.
Question: My problem is when I’m shifted into the top 2 cogs (either one) my chain changes automatically shifts to the small ring. When I shift back to the big ring and apply torque to the chain (and I can hear it sort of grinding), it drops back to the small ring. I am not touching anything for this to happen. What is going wrong? From: Rob
Answer: Your front derailleur needs to be retuned. When the chain is shifted into the two largest cogs in the rear, and also in the big ring up front, Di2 front derailleurs will automatically adjust the trim to avoid rubbing on the front derailleur. This is pushing the chain off of the big ring. You need to change your front derailleur to shift further outboard to avoid this problem. If this doesn’t work, you have a chainline problem and need to have your crank set up further inboard. I’m not sure which crankset you have on your bike, but running a Shimano crankset from the same series as your shifters will of course help as well.
Check out the video below to learn how to dial in your Di2 front derailleur:
Question: If I use the small ring with small cogs (34×11, 12, 13, 14) it seems that the chain touches the 50t big ring? Is it normal? I am using Bora ultra wheels and an FSA SL-K compact crankset. Do you think I can solve this by using an 11-speed chain? From: Antonio
Answer: Your crank is the problem. Spacing between the chainrings on Shimano cranks is a little different in order to avoid this issue. One thing you can do improve the problem with your FSA crank is to move the chainline outboard by moving any spacers you are using from the left side of the crank to the right side. This will reduce the rubbing when you are in the small ring and small cogs. If you aren’t using any spacers for your application then you are out of luck and will need to purchase a Shimano crankset.
Daniel Slusser is a professional bicycle mechanic with over ten years of experience. He holds a bachelor’s degree in business administration from HSU and a master’s degree in history from Cal Poly. When he is not riding, wrenching, or writing he enjoys spending time with his wife and two children.