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Bicycle Repair/crank arm gets loose while riding


As I pedal my bike the left crank arm begins to get loose. I hand tighten the nut as far as I can and turn the bike upside down so I can push the pedals by hand and the crank remains tight. But the next time I ride my bike the crank arm becomes loose again. Do you have any idea why this happens? Thanks in advance.

  If the bike has been ridden for a significant amount of time with the crank arm(s)/bolt(s) loose,  there is a high probability that the crank arm(s) may be ovalized and need to be replaced.
  What has happened is that the precision tapered press fit of the relatively softer aluminum crank arm has been widened or wallowed-out by the loose interface with the hardened steel  crank spindle. The normal press fit interface is quite strong, but still not strong enough to resist the torsional (twisting) forces that occur during normal riding. The result is that all bikes which use a square-taper press fit crank must have the torque on the fixing bolts/nuts checked periodically, and especially within the new bike "break-in" period.  In effect, the fixing hardware may appear loose, but in fact, the crank arms have crept further inwards on the spindle, thus moving inboard of the original crank fixing hardware position.
  It is very important that a torque wrench of some variety be used when tightening crankarm fixing hardware, because the desired torque value (usually around 24-27 Foot/Pounds)  is much more than "hand tight" and measures the desired "placement" or positioning of the crank arm on the tapered spindle.  

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James Novak


I can answer most inquiries with respect to bicycle assembly, repair, tuning and maintenance. I am less than inclined to speak on such questions as "What kind of bike should I buy?", &/or "What is this bike worth?", etc. Any answers to these types of questions may correlate poorly to an acceptable "fact/sarcasm" ratio.


7 seasons of "Professional" bicycle service/repair, etc. 4 seasons as Instructor, Professional Bicycle Mechanic training college. n-1 seasons of riding, cracking, crushing, mutilating and breaking bikes of any & all varieties, and thus requiring intense and at-hand skills development, etc.

Certified Professional Bicycle Mechanic; Winterborne Bicycle Institute/Conestoga College

Past/Present Clients
The Hub Bicycle Shop, Hespeler Winterborne Bicycle Institute

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