Ask a Mechanic | Choosing Lightweight Road Cranks & Fixing BB30 Crank Creaking

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 daniel.slusser@artscyclery.com. Today’s column discusses exotic crank compatability and how to silence creaks in a new BB30 crankset.

Question

I wanted a light bike so I ordered a light frame, but it came in bb30. I have decided that I want a THN M3 or an AX Lightness Morpheus crank. THM makes an adaptor for a BB30 frame that is around 15 grams. AX promises a BB30 version of their crank from time to time but it may or may not come about. What to do?

I can probably find a machine shop who will bore the bottom bracket out to PF30. It is only 2mm larger in diameter. I wonder if anyone has tried that before? From: Ian
Answer
I don’t see any advantage to boring out your frame to the PressFit30 standard as all BB30 cranks will work in both PressFit30 frames and BB30 frames. You would have a very hard time finding a machine shop that would do it too; not to mention voiding your frame warranty and risking damage to the frame if the machinist makes a mistake. Even if everything goes right, the remaining aluminum in the bottom bracket shell would be very thin as most bottom brackets are only about 2mm thick in the first place. Lastly, that aluminum insert is part of the structure of the frame and is needed to maintain the integrity of the frame.
Keep in mind when making your decision on which crank to use that adaptors are never an optimal solution. This is especially the case when you are after maximum stiffness from your cranks, as using adaptors will always be flexier compared to a dedicated BB30 crank. A true BB30 crank has a much shorter spindle than the BSA compatible 30mm spindle cranks you are considering. The laws of physics dictate that (all things being equal) the shorter the spindle, the stiffer it will be torsionally. That is the reason why I recommend going with a true BB30 crank like the SRAM Red Exogram BB30 crankset. This particular crank is optimized for the BB30 standard and has an amazing hollow carbon spider that is integrated with the hollow drive-side crank arm. I had an opportunity to see a sectioned Exogram BB30 crank at the SRAM R&D facility (meet the engineer behind it in the video below) and I don’t think you are going to find a carbon BB30 crank that will likely compete with it in terms of performance. There are of course lighter offerings out there but when stiffness and product support are taken into account, even the high-priced exotic offerings fall short. That is just my 2 cents.
I hope that helps you to make your decision. Really light road bikes are really fun!

All the little grooves left on the spindle from machining are grinding along the bottom bracket bearing races.

Question

I just bought a new Cannondale SuperSix EVO and I love it except that I am getting a really bad creak that sounds like it is coming from the bottom bracket after my third ride on the bike. Any idea what the problem might be? From: Alan
Answer
The PressFit30 bearings on your Cannondale have probably just broken in and need more preload. The crank spindle is likely walking back and forth in the bearings every time you pedal. The striations on the crank spindle left over from the machining process are grinding along on the inner races of the two bottom bracket bearings. Just add another plastic preload spacer or two between the non-drive crank arm and the bearing and you should be good to go.
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.
2014-03-05T16:12:50-08:00