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 question deals with rear derailleurs that don’t want to shift when the chain is on the big chainring and big cog simultaneously.
Question: I’ve watched your videos about how to adjust Shimano front and rear derailleurs many times and really really like them. The videos have helped immensely with me knowing how to adjust my derailleurs and they are done really well.
I just adjusted my rear derailleur (Ultegra) following the videos advice and it shifts well as long as I’m in the little ring in the front. As soon as I shift to the large ring in the front, the rear derailleur won’t shift from the largest cog down, unless I adjust the barrel adjuster so much that it won’t shift right once I shift back down to the small ring. Do you know how I can fix this so there’s balance between both the big and small rings in the front? From: Rob
Answer: I suspect that your chain is too short. With the derailleur pulled tight when you are shifted into the big ring and big cog, it is difficult for the derailleur to pull the chain over to the next smallest cog. Make your new chain as long as possible, but still functional in the small ring and smallest cog combo. This will get you the perfect chain length at the other end of the spectrum so long as you aren’t using a super wide range cassette like an 11-30 or 11-32.
The video below will help you to get the right chain length.
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.