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Can someone cite with sources the process by which a loose pedal pretty quickly destroys the crank arm threads necessitating a helicoil repair?

I'm interested in how the thread on the crankarm gets destroyed in these cases. Shouldn't the pedaling action tighten the pedal back onto the crank if Sheldon Brown is correct?

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    BTW I've seen a stripped thread on steel cranks (and it was the right pedal so unlikely to have been forced in the wrong way, which isn't easy on steel anyway). In this case the outer few turns were destroyed and the inner few turns did a good job of pretending to support the thread until I pedalled hard. At that point the movement Jonathon mentions became enough to take out the remaining thread. The pedal was screwed all the way home, but this demonstrates the effect of leverage on the threads.
    – Chris H
    Apr 20, 2018 at 8:21

5 Answers 5

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Once loose, the pedal spindle will describe a cone as the crank rotates. This concentrates force at certain points in the crank threads, leading to damage.

Top image shows the pedal spindle in the threaded hole in the crank. The threads are fully engaged and tightened, and forces are distributed. Note there is space between the male and female threads (somewhat exaggerated).

Bottom image show what what happens what the thread is loose. The spindle can move around in the threaded hole in response to changing pedaling forces, and forces get concentrated at the red dots.

enter image description here

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    Also to add, it should be easy to imagine the damage of the smooshed threads when the pedal eventually works out far enough to get bent out under hard pedaling power, and how opening the front of the crank wrecks it.
    – whatsisname
    Apr 20, 2018 at 22:49
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If the pedals is loose and purely rotated by pedaling, the pedal will tighten. However, the pedaling motion is not purely a twisting motion, but instead a large downward force is applied. This puts increased strain on the threads. The more the pedals moves in response to this downward force of pedaling, the more the force will be concentrated on a small section of the threads. This will cause these threads to fail, and as the pedal slowly works its way out, even more threads will fail.

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  • Why does pedalling put strain on the threads when the pedal is loose but not when it's properly tightened?
    – David Richerby
    Apr 20, 2018 at 14:24
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    @DavidRicherby As with all loose parts the force is the same but the contact surface is smaller, hence the pressure becomes large enough to deform the material. Think how you can destroy an ill-fitting wrench. Argenti's image visualizes that nicely.
    – Peter - Reinstate Monica
    Apr 20, 2018 at 15:21
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    ^ In other words, it's because the force is not distributed evenly across the surface area of the threads.
    – acobster
    Apr 20, 2018 at 15:23
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    @DavidRicherby: friction between the axle flange and the crank arm takes almost all the shear load when properly tightened.
    – whatsisname
    Apr 20, 2018 at 22:46
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The best explanation I heard is to visualise a pencil held vertically in a loosely-closed fist, such that the pencil leans at an angle.

Then move your hand in a flat circle so the pencil rolls around and along your index finger.

Notice that

  1. The pencil rotates as it moves. This is the tightening action of the pedal spindle in the crank.

  2. The pencil touches your index finger the whole way. This represents the first couple of threads in the crank and the pressures pit in place

So? If the pedal spindle has any play, the movement will slowly grind away the first thread because of the increased pressure and that it comes and goes with every revolution.

In theory its doing the same on the other side too, but we never notice that damage.

As the pedal spindle "self-tightens" it also presses the end of the pedal thread into the crank with applies a side force to the threads of the crank, also deforming and "squashing" the threads away.

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  • If visualising isn't your thing, grab a pencil or pen or drumstick or anything conveniently round and try it out. Sorry couldn't find a relevant citation, so not a good answer.
    – Criggie ♦
    Apr 20, 2018 at 10:48
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The pedalling forces don't screw the pedal in very effectively if the bearings are good. If the crank thread has taken a tiny bit of damage and is stiff (or it's just stiff anyway), pedalling won't screw the pedal in at all. You can demonstrate this without damage by putting a bike on a stand and trying to screw in the pedal by hand-pedalling, only touching the parts your foot would touch (engage a turn or two first).

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  • A few dozen or even hundred turns of the crank by hand with the wheel system unloaded is nothing. If you want to see the effect, you need to get on and actually ride a number of miles.
    – Chris Stratton
    Sep 14, 2019 at 1:56
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The right pedal has a right hand thread, the left pedal has a left hand thread. They do NOT self tighten. The self loosen. This decision to give pedal spindles this threading was done so that on a fixed gear, such as an Ordinary (Penny Farthing), you'd not break your ankles if your pedal bearings seized. They'd simply unscrew. Yea, and pigs fly.

Tighten up your pedals firmly, check them regularly and helicoils shouldn't be your crank's fate.

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  • For folks who don't believe pedals unscrew when pedaling forward youtu.be/LFbSBG7jMzY?t=127
    – Argenti Apparatus
    Apr 20, 2018 at 18:29
  • This doesn't answer the question.
    – whatsisname
    Apr 20, 2018 at 22:47
  • Did you at least read the Sheldon Brown link given by the poster? sheldonbrown.com/pedals.html . Where he talks about "precession"?
    – Craig Hicks
    Apr 20, 2018 at 23:23
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    @whatsisname The question says, “Shouldn’t the pedaling action tighten the pedal back onto the crank?” And this answers that by saying “no”.
    – David Richerby
    Apr 21, 2018 at 9:13
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    @StuartCox if the pedals on a Penny Farthing or other flat-pedal fixie seize, the seizure will simply roll your foot up off the turning edge of the pedal. Losing contact with the pedals might at an extreme lead you to crash and then break an ankle, but the damage will happen when you impact the ground - not from the pedal. You probably can't even apply enough torque by standing on a half-pedal width lever arm to break a pedal thread free in one go anyway.
    – Chris Stratton
    Sep 14, 2019 at 2:01

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