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 explains Shimano Di2 wiring options and answers another bottom bracket compatibility question.
Question: I’m looking at getting the new Dura Ace 9070 Di2 group. One of the main reasons I am interested in it is because I want to easily install clip-on aero bars for some TT and Tri races I plan on doing this year. What all do I need to make this work? From: Isaac
Answer: You have plenty of options, Isaac. Rather than try to explain them all in writing, I’ve scanned a page in the Shimano STI Dealer Manual that illustrates the various options and how to wire it up. Just choose the option that works best for you and only use the parts called out in the diagram for your setup. All of the Di2 wiring components are available by clicking on the highlighted link. The remaining Dura Ace 9070 and 9071 components that you will need are also available by clicking on the links above. If you want two buttons per aero extension, use the optional R671 Di2 Shifter Set.
Question: I bought a new carbon mountain bike frame with a pressfit bottom bracket that is 89.5mm wide with an internal diameter of 41mm. I have several unused square taper and ISIS bottom brackets and cranksets that I could use with it. Are there any adapters available that would allow me to use a threaded square taper bottom bracket in my new frame?
If no adapter is available, my next question would be, what are my choices as to the type of BB and matching crankset types that can be used? I want to stay away from Shimano because of some bad experiences in the past. What other options do I have? What about Sram GXP? Can I use a track or double (road) crankset with that? Is that a problem? From: Timmi
Answer: Your new frame has the BB92 mountain bottom bracket standard that was developed by Shimano. There are no adaptors available that will allow you to use a square taper or ISIS bottom bracket with BB92. The good news is that Sram GXP mountain cranks will work with this bottom bracket standard. You just need to use the right bottom bracket bearing set. Here is a link to the BB92 GXP Pressfit bottom bracket bearings you need.
Because BB92 is a mountain specific bottom bracket standard you won’t be able to use road or track cranks, even if they are GXP. The equivalent road version of your frame’s bottom bracket standard is BB86, and it is narrower. The spindle on the road cranks won’t be long enough to work with a BB92 frame. Even if they did fit, you might run into problems with the chainring not clearing the drive side chainstay.
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.