Get-me-home advice - stripped crank

[Bearlegged]

My understanding is that it's not to do with how force would be applied if you held a spanner on the axle and turned the cranks, but to do with the way that forces are applied to the axle and where it threads into the crank.

One way to think about it is rolling a smaller cylinder (e.g. a magic marker) around the inside of a larger cylinder (e.g. a mug). If the smaller cylinder turns anti-clockwise, it will move around the larger cylinder in a clockwise direction (and vice versa).

This counter-application of forces applies to the pedal axle. If you consider your foot pushing down on the pedal, the axle effectively pivots where it enters the crank arm, so is pushing up on the crank threads. It's by this mechanism that the forces applied by the pedal axle to the crank arm are opposite to the way to the pedalling direction. It's this counter-rotation that has the effect of stopping the pedals coming unthreaded from the crank arm.

[sean_iow]

Whist that is correct, it only applies if you push down on the pedal the whole time, if you're pushing down on the upstroke you've got bigger problems than a striped thread if you pull up on the upstroke the forces will be reversed for half the time.

The science is really for heavily loaded bushings which would rotate in the housings, i.e. steam powered water pumping beam engines. IMO it's not really applicable to pedals theses days with high quality bearings. YMMV

[Bearlegged]

But as your crankset rotates, the forces on the axle are not just up/down. You're also pushing forwards (at the start of the pedal stroke) and backwards (as you pass the 6 o'clock position).
Once you add a second pivot point into the equation, the rotational forces are reversed again - hence BB threads being RHT on the NDS, and LHT on the DS.*

*And also a contributory factor in Italian threads (among others) vanishing into obsolescence.

[ledburner]

Going back to the orignal question.... with some of your technical input.. ....
If pedaling is weird, quickly back pedal and stop...
it may avert stripping threads course, we are going to say that's what I should've done....... Tyre 2 fun...
Edit type 2 fun... Grrr