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.
Question: I’ve been watching your Youtube videos to try and adjust my Ultegra Di2 gears. Regardless of any adjustments I make, the chain will not shift down to the smallest rear cog. This is the case whether it’s on the large or small front ring. All other movements are smooth and there’s no noise or scraping. Any advice would be appreciated. From: Ian
Answer: I have a few ideas on what could be wrong. First off I would check your derailleur hanger alignment. If the hanger is bent inward or twisted, then the derailleur can’t get to the smallest cog and it will probably need to be replaced. Also check to make sure that the hanger anchor bolts are tight.
Next, make sure that your high limit screw is adjusted properly to let the derailleur go to that outboard extreme position. Lastly, check your index adjustment to make sure that you haven’t adjusted it so far inboard that you are missing your smallest cog. Check this by adjusting the index as far outboard as it will go, and then see if it still won’t go into gear.
If none of these solutions work, don’t worry, it’s not your fault. Your derailleur hanger might be too thin. It’s rare, but it happens. Put a 3/8″ washer between the derailleur bolt and the hanger to space the derailleur further outboard on the bike. This will get it to move over the extra amount that you need to push the chain to the small cog. Keep in mind that if you go this route you will need to start your derailleur tuning over from the start with the derailleur in its new position. This last trick is almost never needed for Di2 setups, but can be very helpful for mechanical rear derailleurs where the spring isn’t strong enough to execute a shift at the outboard extreme of its range.
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.