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Illogical Pedal Threads!
A year ago I tried and failed to remove a pair of pedals from my 1947 Men's Roadster after having a frame bending run in with a lamp post. It wasn't until I was able to more easily remove pedals from a different bike that I discovered what I had been doing wrong.
Of course I was aware that all bikes have one pedal with reverse thread. The right crank rotates in a clockwise direction, which in turn makes the right pedal rotate counter-clockwise in relation to the crank. I reasoned therefore that the right pedal must be the one with reverse thread so that while pedalling forward the right pedal does not come loose. If the right pedal has reverse thread it will always have a tightening force on it while rotating counter-clockwise. To remove the right pedal from the crank, you would turn your wrench clockwise. Using the same logic, the left pedal must have normal thread, and to remove it one would unscrew in a counter-clockwise direction. ....or so I thought!!!
The rest of you are probably chuckling or rolling your eyes, having known all along that it's the right pedal that has normal thread and the left pedal with reverse thread. My question is, WHY???
The process which tightens the pedal axle in the crankarm during pedaling is called mechanical precession. It occurs to any round body that can turn within a round hole due to clearance between them and a radial, rotating load.
In the case of a bicycle pedal, the round object is the pedal axle while the round hole is the pedal hole in the crankarm. The radial load is the pedalling force acting through the axis of the pedal axle. In the case of a drive/right side pedal, the crankarm turns clockwise. As it does so, the radial load rotates counter-clockwise about the axis of the pedal axle.
As the load rotates counter-clockwise, the pedal axle actually rotates clockwise in the crankarm hole. Since the right hand pedal is right hand thread , it will be tightened if the radial rotating load is higher than the pedal installation torque.
The opposite happens with the left/non-drive side pedal. The crankarm turns counter-clockwise producing a clockwise rotating radial load, which in turn causes the pedal axle to turn counter-clockwise within the crankarm hole. Hence the need for a left hand thread.
This can be quite hard to grasp (and explain) but can be envisioned a bit easier using a toy based upon mechanical precession - the Spyrograph. In this analogy, the inner disc is the pedal axle while the outer ring is the crankarm hole, albeit one with lots of clearance between them. Putting the pen in a hole in the inner disc and pushing outward is the radial force while moving the pen clockwise or counter-clockwise is the rotation of the radial force simulating crankarm rotation. If you move the pen counter-clockwise, the inner disc actually rotates clockwise about its axis. It's counter-intuitive but the mechanics are sound.
Mechanical precession - I had never heard that term before today. Wow, you really can learn something new every day. Thanks T-Mar for that great explanation. I searched Youtube for videos showing the spirograph demonstration you mentioned. One video on bicycle pedals had a link to the Wikipedia page titled "Precession (mechanical)" and that page actually had a little video graphic showing it in action.
My favourite phrase from your reply is "counter-intuitive" because I didn't feel so stupid after that. The Wikipedia page also uses the same phrase. One thing is for sure: I will now always remember which way to loosen bike pedals. Cheers. -B
Remembering which way to install or remove pedals is very easy, if you've worked in a bicycle shop. You quickly learn that the most efficent way to install a pedal is to hold the pedal wrench stationary while turning the crank backwards (OK, it doesn't work on coaster brake equipped bicycles, but it's great on bicycles with freewheels or fixed gears). Remove pedals by holding the wrench stationary and spinning the crank forward (i.e driving the crank). Spinning the cranks allows you to install or remove pedals in a fraction of the time that it would take by turning the pedal wrench. Once you've done it a couple of times, you'll always remember the thread direction.
Thank-you for mentioning the animated demonstration in the Wikipedia article. That demonstrates it really well, provided you can grasp the concept that the radial force rotates in the opposite direction to the cranks. I consider myself to have quite high an aptitude in mechanics, having worked as a bicycle mechanic for several years before returning to school to study mechanical engineering. While that was almost forty years ago, I remember that it took me the better part of an evening to grasp the concept of mechanic precession. Now, let's see if I can load the animated demo for the benefit of other members.
It works! If you click on it, it moves.
Thank you EVER so much...
when very young, or younger...a friend at school showed me a small "trick".
You cut a square piece of paper, not much more than 1 inch by 1 inch, place it on top of your palm and with a finger of your other hand you start rubbing it clockwise.
The paper turns anti clock wise.
Now I know why and can finally start sleeping better.
THANK YOU !