Recently I tried to unscrew the left pedal of a bicycle by turning it counter-clockwise with a spanner lengthened with a piece of pipe. The net result is the spanner broke apart and the pedal is still there.
This makes me wonder.
Is that just threaded joint or is there something else besides threading? Is the threading left-handed or right handed? Do I need anything else except what I would normally use to unscrew a threaded joint?
Like Daniel Hicks says, they are threaded opposite to each other. This ensures the act of you pedaling is constantly tightening them both. If they were both the usual right hand threads then the left pedal would eventually unscrew and fall off.
So, if you're like me and use the right hand rule to constantly assess which direction you should be turning things to make them move in the direction you want, you'll now need to switch to the "left hand rule" for the left hand pedal.
About "just threaded" vs "something else", the answer is: they are just threaded.
A possible problem, though, is rust or other chemical bonding, which might make significantly difficult to take the pedal off with raw force (a friend of mine recently broke a spanner, even applying the force in the correct direction).
As for the right vs. left rule, if you attach the spanner with the handle pointing upwards, you have to pull the handle to the back of the bike.
Pedals are threaded as said above, but over time the pedal may become stuck, especially when the pedal is made of steel and the crank is made of aluminium.
Before trying to unscrew it, you should spray some degreasant here, like WD-40. Wait for a couple of minutes and then try to unscrew. You should also use a proper pedal wrench instead of a standard wrench (easier and more force is applied). If it doesn't work, spray more degreasant, wait more, and try again. Sometimes spraying degreasant and waiting a whole day (or night) is sufficient to unscrew it, sometimes it just doesn't move, and my advice would be to just change the crank and the pedal, or else you might end by hurting yourself.
That's the recipe I used for many old bikes, and it usually works but sometimes, even if the bike isn't rusted, there's no other option than to dispose of the crank.
