You've explained where left arms go: into the trash.
That path through the BB is very hard on inadequately tightened left arms. Many (usu. inexpensive) new bikes will shed their left arms in a month of riding if they are not tightened by the dealer. Caloi cruisers in particular I remember seemed merely hand tight out of the box, requiring numerous turns before even considering torque. One large maker seems to be getting theirs tighter this year than in the past.
Mssr. Brandt has asserted, I think, that a properly tightened arm can crawl up the spindle which then allows the bolt to come loose, which can then allow the crank to work loose if the bolts are not checked...and that well-installed metal crank caps can prevent all this. This all seems right, though again if they are never tightened well to begin with I think bolts and arms just fall off together.
Jeff Slotkin, usually at The Bicycle Shoppe, Charleston, SC
On Jan 3, 2010, at 12:00 AM, Jon M. Crate wrote:
> Two Left Arms
>
> It is easier for me to understand where the mates to my sox go than to
> understand where the left crank arms go.
> I can see the load runs (in reverse) from the chain to the chainring, then for
> the right arm directly up the arm to the pedal and for the left side it goes
***
> through the bottom bracket to the arm and them pedal.***