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: My left crank arm fell off when I was riding the other day. This is the second time it has happened. What is causing this? How do I fix it? From: John
Answer: Left crank arm detachment is a sinister problem but an ounce of prevention can save you from a pounding headache. Just periodically torque the crank bolt, as they tend to come loose on the left side. This is because the bolt has a right-hand thread and the motion of the left crank arm tends to loosen it when riding. For some reason left-side bottom brackets were never made with left-handed threads even though left pedals have left-handed threads to prevent a similar problem from developing at the pedal.
The bad news is that once your crank falls off it is so damaged that it will never stay reliably attached from that point on no matter how much you torque the bolt (as displayed in the picture above). So replace the arm and remember to keep that bolt tight and you won’t have any more problems.
Question: I recently recabled my Tarmac SL4 frame with internal cable routing. As I was working, the derailleur cables in the downtube got tangled somehow and it didn’t shift very well. I have fixed it now but was wondering if you have any ideas on how to avoid this problem in the future? From: Steve
Answer: Oh what a tangled web resides when first we place the cable inside. I believe I can disentangle you from this problem however. There is a relatively easy fix you can employ. When you replace the cables you should install them one at a time and completely finish the installation before you do the next one. That means getting all the housing installed and anchoring the cable. This helps to prevent tangles when feeding in the next cable although this is not a sure fire fix.
When you go to do the second cable, after you get it fed through the frame you can check your work by sliding an old piece of housing over the cable you just installed. If you experience resistance while you are feeding the housing up the frame or if you feel the housing twisting in your hand then you know you have a tangle. To fix it just pull the cable back out and try it again, repeating the process until you get it right.
If some of you readers are wondering if your cables are tangled from a bungled install there is an easy way to check it. When you are shifting one derailleur place your hand on the exposed cable of the other derailleur. If you feel the exposed cable move then you know that the two are twisted as one is affecting the other when they should be completely independent.
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.