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Thread: A serious thread about pedal...thread

  1. #1
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    A serious thread about pedal...thread

    This week i installed new pedals on my bike. The pedals are indicated left and right, because their thread is different. I assume that's done in order to make sure you don't unscrew them accidentally due to the rotary motion of pedalling.

    However, from this reasoning it appears that they made the thread go the wrong way:

    To screw a pedal onto the bike, you must make a CW motion for the right pedal, a CCW motion for the left pedal. But if the pedal would not freely rotate over its own axle (like when it's dirty), fixing its orientation with your foot would mean you unscrew them while riding your bike. Because in that case, the right pedal makes a CCW motion relative to the erm "vertical pedal crankshaft like bar", and the left one a CW relative motion.

    How come the thread is chosen like this when this reasoning shows that it would make dirty pedals prone to unscrewing themselves?

    And directly linked to that, how come then pedals don't unscrew themselves in reality?

    Is it just a strong enough bond and is the thread indeed chosen "wrong" or is my reasoning incomplete or wrong, and is the thread direction chosen correctly?
    With sufficient thrust, water towers fly just fine.

  2. #2
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    Quote Originally Posted by Nicolas View Post
    This week i installed new pedals on my bike. The pedals are indicated left and right, because their thread is different. I assume that's done in order to make sure you don't unscrew them accidentally due to the rotary motion of pedalling.

    However, from this reasoning it appears that they made the thread go the wrong way:

    To screw a pedal onto the bike, you must make a CW motion for the right pedal, a CCW motion for the left pedal. But if the pedal would not freely rotate over its own axle (like when it's dirty), fixing its orientation with your foot would mean you unscrew them while riding your bike. Because in that case, the right pedal makes a CCW motion relative to the erm "vertical pedal crankshaft like bar", and the left one a CW relative motion.

    How come the thread is chosen like this when this reasoning shows that it would make dirty pedals prone to unscrewing themselves?

    And directly linked to that, how come then pedals don't unscrew themselves in reality?

    Is it just a strong enough bond and is the thread indeed chosen "wrong" or is my reasoning incomplete or wrong, and is the thread direction chosen correctly?
    Yes, it seems counterintuitive and no, I don't know why the threading was chosen. Of course the pedal does not freely rotate at the joint of the crank and pedal. In practice, I've never heard of a pedal rotating off of a bike (i bike afair bit) and since most/all pedals have sealed bearings there should never be a dirt issue.

    The only problem I've had was I clipped my pedal on the ground and bent the shaft in the pedal. Within two weeks the pedal sheared off completely when I was biking to work.

  3. #3
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    A Google search turns up a lot of website pointing out that left pedals are left-hand threaded, and stating that this is done so that regular pedal motion will tighten rather than loosen.

    But they don't specifically explain how that works, and Nicolas' assertion that it's backwards seems to me, too, to be correct.
    Sometimes you win, sometimes you learn

  4. #4
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    The pedal goes in from the outside of the bike, toward the centerline. From this position, the pedal on the left side as you ride it would need to be tightened with a CCW rotation as seen from the side of the bike. As you pedal, the rotation of the axle in the pedal will continue to be counter clockwise. Quick question: How many that have it going the other way are looking at it from the centerline of the bike, outwards? This will make the rotation CW, but since that's not how the pedal is installed, it's backwards.

    Ask me how tight I got the oil drain plug on my VW learning this little bit of trivia... Hint, I had a 4 foot pipe on the wrnch for leverage.
    I'm Not Evil.
    An evil person would do the things that pop into my head.

  5. #5
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    Here you go, Nicolas.

    The left-hand pedal has a left-hand thread, to prevent it becoming loose from an effect called precession. Although the pedal turns clockwise in its bearing relative to the crankarm, the force from the rider's foot presses the bearing against the crankarm thread at a point which rolls around clockwise, thus slowly pulling the outside of the bearing anticlockwise because of friction.
    I'm not quite sure if I'm visualizing this correctly, but at least it's an explanation.
    Sometimes you win, sometimes you learn

  6. #6
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    I'll have to read a bit more into it and think a bit more about it before I can visualize it, but thanks!

    I'm amazed I actually made myself clear in my post

    The only problem I've had was I clipped my pedal on the ground and bent the shaft in the pedal. Within two weeks the pedal sheared off completely when I was biking to work.
    Except for a difference in time span, I had the exact same problem! I clipped it onto a sidewalk, and within a few months the pedal sheared off completely when I was biking to work. The pedal shaft showed a clear fatigue break surface, due to the oscillating rotations the bent pedal made.
    With sufficient thrust, water towers fly just fine.

  7. #7
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    OK I can visualize it now! I visualize the braking force applied by the pedal onto the pedal shaft as explained in your link, and indeed the friction slowly forces it in the right direction.
    With sufficient thrust, water towers fly just fine.

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