Where Left Hand Crank Arms Go

Excellent theory, Jim. Much better than the spindle bolt or nut being right hand threaded on the non-drive side whereas the pedal on that side is left hand to keep it tight.

Tell me, is there a different gremlin for each bike malady or does the critter you show take care of them all??!!

Best,
John "wheelson"

One thing I’ve noticed with a few cottered cranks is the non drive side arms have seen some blowtorch action in the past to get a stubborn cotter pin out. I’m guessing a few got thrown in the bin and another arm was used from something else.

I don't understand why there would be a twisting force on either crank bolt in service if the crank arm is seated on the taper.

hi-i think maybe if we all look for the missing socks from our laundry we might find the missing cranks...[just thinking out loud]; personally i'm still looking for the girl on the lambert bike..........

Dracco is correct in that there is a circular force on the crank bolt through the action of pedaling. This is through "precession" and because the crank bolts are right-hand threaded on both sides, tending to tighten the right arm bolt and loosening the left arm bolt. This is why the left pedal is left-threaded and the right pedal is right-threaded. There is technically no motion between the pedal spindle and the crank arm but it is there through I believe some flex of the threaded joint. Pedals seldom work loose, though, because of the opposing threading.
With the crank arm, there is no opposing threading for the left side. Now the force is applied from the pedal through a lever arm the length of the crank arm. This would tend to cause this joint, the tapered one, to flex at a greater force than that of the pedal. Of course, the crank arms are not always tightened correctly, I see it all the time at the shop, even on new bikes (especially low end bikes). So, as the joint flexes it allows the bolt precession, the right side tends to tighten or at least stay tight, while the left tends to loosen. The tapered joint does flex, if you notice the dark markings on the spindle tapers, this is fretting or a actual rubbing or movement between the two surfaces. This is most pronounced when the crank and the spindle are both steel.
Over my bike shop years, I've replaced many left arms, seldom right arms. I've also seen cases of crank arms falling off on the trails I usually ride. Always the left arm held in one hand while the hapless rider-now-pedestrian pushes the bike along. One case where the crank arm caps, held in disdain by classic bike racers, actually performs the function of keeping the bolt somewhat attached to the bike.
DISCLAIMER: This is Wheelson Conjecture. I am not a mechanical or materials engineer. My engineering degrees are in electrical and electronics. I did spend about half of my career in an engineering mechanics lab for a major tooling systems manufacturer doing destructive testing and failure analysis, working with people that actually did understand this stuff!
Best,
John "wheelson"

It's not easy to visualize with a square taper as opposed to a threaded joint like the pedals.

Brian, your conclusions are correct in quality of materials and precision of fit as being major culprits. The variations of taper, ISO, JIS, old Campagnolo, and variations within those types due to manufacturing tolerances are big contributors. Perhaps the biggest one, in my opinion, is inadequate tightening of the crank bolt or nut. Lack of a torque wrench or even a torque spec when you do, as well as fear of breaking the bolt or nut off, and lastly, a relaxing of the torque setting may all contribute. With every bike we get for a tune up at our shop, I'd say 90 percent have inadequately tightened crank bolts.

Remember how the auto tire dealers recommend a re-torqueing of alloy wheel lug nuts after a week or so after the installation?

And then there's the controversy among the bike mechanics and enthusiasts: do you grease the tapers when you install a crank or is the oil from your fingers more appropriate or should the tapers be de-greased?? This subject regularly comes up on most all the bike forums, and even those of us who have worked in bike shops for years and have attended factory service schools cannot seem to agree.
Best,
John "wheelson"

OK, Wheelson, you have me spooked. I just got back from a 34 miler on a bike where I just replaced the BB and tightened the crank nuts "tight enough." All I could think about was whether my left crank might come off unexpectedly.

What is the recommended torque for the nuts on a 45 year old spindle that came on a Japanese bike with Sugino cranks? I'll use a torque wrench and won't blame you if I snap something. Anything would be better than being preoccupied on every ride from now on!

Brian

I read an article about using loctite on the left crank arm bolt.
Had a nice Claud Butler and the left crank arm would always work loose. The previous owner must’ve used it loose and damaged the tapered axel as I swapped the arm and it solved nothing. In the end I used loctite on the threads and sold the damn thing on.

I don't know about tin shims for cranks but I do use old guitar picks for brake toe-in shims.

So, we are all guitar players besides vintage bike riders then?

Cool, never tried one of those, I’ll stick with my Fenders