Originally Posted by
pdlamb
I think you're missing my point. As subsequent posters have noted, a properly installed and torqued square taper crank does not loosen. Properly stated, the original question would be, "Why are improperly installed left side cranks more likely to loosen?" (Leaving for follow-up, more likely than what? Properly installed right side cranks??)
In fact, properly stated, the question would be "Why are left crank arms more likely to loosen than right crank arms?" It wasn't until very near the end of the period of ubiquity of square-taper cranks that torque wrenches came into more or less common use in bike stores, so "proper" versus "improper" installation would have been impossible to quantify in most shops. Regardless, that left crank arms have been observed to loosen more frequently than right crank arms comes down to the fact that the right crank doesn't impose a twisting force on the spindle whereas the left crank does.
Incidentally, a tip for those of you who still have square-taper cranksets to service: to set the torque for the crank bolts with a minimum of effort, apply a torque wrench to one crank bolt and an ordinary socket wrench to the other. When you hear the torque wrench click, you know that both crank bolts are torqued to the proper spec. (I've shown this technique to a number of mechanics over the years; some immediately get how this works; others have to think about it for a while.)