Ask a Mechanic: Di2 Compatibility and Building Wheels

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

Question: I have a Shimano Dura-Ace Di2 setup on my Specialized Tarmac. The only non-Shimano part is the SRAM Red BB30 crank. I was out riding and after executing a front shift my rear derailleur pulled up and over the dropout and was torn off the hangar. The replacement derailleur cost me a pretty penny so I don’t want to repeat this problem. Any ideas on what went wrong? What can I do to prevent this problem in the future?   From: Ed

Answer: My grandpa always warned me the robots would turn on us and we wouldn’t see it coming. However, I have a plan to pacify them by feeding them high quality chains and chainrings.

I believe your Red crank is to blame for the malfunction. A call to Shimano confirmed this is a common problem. They also informed me that non-Shimano chains have been a culprit in these situations as well. If you were using all Dura-Ace components the repair would have been covered under warranty.

Shimano’s advice to keep components in the family also applies to their mechanical groups. This is important to all of us, especially riders that purchased a new bike within the last two years. During this period I have noticed that product managers are often specing bikes with KMC chains rather than SRAM or Shimano. I suspect this is a way for manufacturers to save a couple of bucks on each bike. Back in the old days this was accomplished through the use of low-end bottom brackets but that option doesn’t exist given the latest crank and bottom bracket standards. What this means to you is that when it comes time to replace your original chain, just be sure to use the appropriate Shimano, SRAM, or Campy chain for your application.

Keep the robots happy lest they become our metal masters!

Question: I am learning how to build wheels and have put together a few wheels successfully but I am struggling to get them round. My wheels also seem to lose spoke tension quickly. Do you have any suggestions on how to fix these issues?   From: Ritchey

Answer: Wheel building is not the dark art that many make it out to be. Building good wheels just takes practice. Of course having a good teacher makes a big difference too! Wheels that de-tension do so because they are tensioned unevenly and/or are not tensioned highly enough. All of the spokes work together to keep a wheel together and like a chain, they are often as strong as their weakest link. Completing the tensioning process in the right order will help you to get and keep the tension even throughout the build process.

The key to getting wheels round is to first look for flat spots in the wheel. The sections that are flat have too much spoke tension pulling on them. Always work using even numbers of spokes so you don’t pull the rim to one side or the other. Use 2,4, or 6 spokes that span the affected area and give each of them a quarter turn clockwise to relieve the tension. Next, do the opposite for high spots by tightening the spokes in these areas. Once you get the wheel round, then get it true. For more on truing wheels, click here. After you have the wheel round and true with moderate spoke tension, add tension to each of the spokes by turning no more than a quarter turn each. It will likely take anywhere from 2-6 laps around the wheel to accomplish this.

Once the desired tension is achieved (too high is generally better than too low) then you need to stress relieve the spokes. This is the process of seating the spokes into the hub properly while straightening out the bow just above the j-bend. First, press this area down with your thumbs to get it straight (you can do this earlier in the build too). Second, squeeze the parallel spokes on each side by gripping two at a time in each hand, on each side, and squeezing them together. Keep the squeezing motion parallel with the rim. Don’t worry about pulling the wheel out of true. Doing each side simultaneously will largely prevent this from happening. Work your way around the wheel and then check your tension.

At this point the tension is likely to have dropped. So, add a quarter turn at a time until the original tension is restored and repeat the stress relieving process. Once this is done, do a final truing and you should be good to go.

When you do become a proficient wheel builder, don’t give me or anyone else the credit, just act as though you have mastered a voodoo process that only those in tune with the vibrations of the cosmos can fully comprehend. At least that is what most wheel builders do, so you will be in good company!

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 University. When he is not riding, wrenching, or writing he enjoys spending time with his wife and two children.