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Bike Pedals

09/21/2016 12:22 PM

Just been replacing pedals on my son’s bike. He has a Haynes book which says right hand pedal has right hand thread, left hand pedal, left hand thread. That was correct (as you’d expect from Haynes). It says the threading is arranged so the action of pedalling tends to tighten the thread, and this is what I have always understood to be a requirement.

But what puzzles me is that when riding the right hand pedal crank turns clockwise looking at it, and the direction of rotation of the pedal on the crank is anticlockwise. So a right hand thread there tends to unscrew when pedalling, not tighten. And the same on the left side, of course. Any comments? Am I missing something?

Judging by how tight they were to remove, there’s not much danger in practice of them coming loose.

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#1

Re: Bike pedals

09/21/2016 12:56 PM

But the force applied is the reverse....

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#2

Re: Bike pedals

09/21/2016 1:10 PM

Wouldn't the threads make them immune from the ID 10 T syndrome?

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#3

Re: Bike pedals

09/21/2016 1:32 PM

My guess is this has been working for bicycles for about 100 years now.

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#4

Re: Bike pedals

09/21/2016 1:43 PM
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#5
In reply to #4

Re: Bike pedals

09/21/2016 2:37 PM

Thanks for that, thought there had to be an explanation. Interesting. The comments in your link about pedals falling off - I would have applied a dab of Loctite.

At one time HGV wheel nuts had left hand thread on the left side (in UK anyway). Just checked with my brother who knows more than me about it and he tells me this stopped about 25 years ago when the design was changed. I had always assumed it was because the nuts would tend to unscrew during braking, deceleration being greater than acceleration when taking off. But seems likely that's also a case of precession.

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#6
In reply to #4

Re: Bike pedals

09/21/2016 2:50 PM

Nice. Same reason a spun coin turns backward after it falls over?

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#11
In reply to #6

Re: Bike pedals

09/21/2016 7:32 PM

And a rolling stone gathers no moss!

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#7
In reply to #4

Re: Bike pedals

09/21/2016 4:21 PM

I like stuff that goes around like that, but I am easily entertained.

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#8
In reply to #4

Re: Bike pedals

09/21/2016 5:12 PM

Your first post was more accurate. When pushing on the pedal, the torque on the pedal hub tends to tighten it.

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#9
In reply to #8

Re: Bike pedals

09/21/2016 5:43 PM

The actual torque you apply to the pedal shaft is minimal. The pedal has bearings inside and spins freely.

The only force on the pedal that gets communicated to the pedal arm, I believe, is the rider's weight, periodically applied to the pedal as it turns. As the pedal arm turns, this downward force results in a twisting force between the pedal shaft and the threaded hole in the pedal arm.

I'm thinking it is mechanical precession.

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#10
In reply to #9

Re: Bike pedals

09/21/2016 7:11 PM

Yes, there are bearings in there, which is why I don't think response #4 is very good. But to reply to your comment, bearings reduce friction, but they don't reduce torque.

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#12
In reply to #10

Re: Bike pedals

09/22/2016 8:08 AM

OK, here's a picture. See if it makes sense...

You have two engaged wheels, one large and one small with the larger driving the smaller. The small wheel turns faster and in the opposite direction.

Suppose the big wheel is hollow (like a ring gear) and the small wheel is inside. Now the small wheel turns faster but in the same direction.

The big hollow wheel is the threaded hole in the pedal arm and the slightly smaller wheel is the loose threaded pedal shaft. The weight of the rider keeps the two engaged. From the right side of the bike, the pedal arm turns clockwise and the pedal shaft turn clockwise slightly faster, tightening the right handed thread.

On the left side, both the pedal arm and threaded pedal shaft turn counter clockwise tightening the left handed thread. I suppose some accident investigator long ago noticed that left pedals had a tendency to fall off.

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#15
In reply to #8

Re: Bike pedals

09/22/2016 10:28 AM

This look backward to me. If the peddle arm moves clockwise on the right side, the peddle shaft would be moving counterclockwise to the peddle arm if the peddle remains roughly parallel to the ground. This all seems backward to me. Like the right side peddles should be backwards thread. Obviously I am missing something.

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#16
In reply to #15

Re: Bike pedals

09/22/2016 11:35 AM

At first, perhaps they put both pedals on with standard (RH) threads. After the complaints came rolling in, a search for cause and effect came forth with another patent, and the correction to the issue.

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#17
In reply to #15

Re: Bike pedals

09/22/2016 11:35 AM

There is very little torque transferred from the pedal to the pedal shaft because of the bearings within the pedal. The friction of these bearings is very much less than the friction of the threaded pedal shaft inside the threaded hole of the pedal arm. Unless the pedal bearings were frozen up, there is no way you could unscrew it using the pedal.

Think of the pedal shaft as a small wheel inside a slightly larger ring (the hole in the pedal arm). They are pressed together by the rider's weight so it's like a gear train of sorts. The pedal shaft will rotate slightly faster than the slightly larger hole in the same direction.

So if the shaft is loose, the pedalling action will tighten it instead of unscrewing the pedal, which might result in injuries to the rider that I would rather not think about.

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#29
In reply to #15

Re: Bike pedals

09/24/2016 8:55 AM

That was exactly my thoughts when I started the thread, but others have explained the paradox, if that's not too grand a word.

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#21
In reply to #8

Re: Bike pedals

09/22/2016 4:29 PM

That doesn't make sense. How can torque on the pedal, tighten the pedal to the pedal crank ?

The pedal is not mounted on a fixed shaft. The pedal floats on its own shaft because it's supported by bearings on both ends, so it's rotation is independent of the pedal crank.

This is why there are "flats", on the pedal shaft adjacent to the threads where a wrench can be afixed to tighten the pedal shaft onto the pedal crank arm.

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#22
In reply to #21

Re: Bike pedals

09/22/2016 4:39 PM

This was stated clearly in post #4:

post #4

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#25
In reply to #22

Re: Bike pedals

09/22/2016 4:58 PM

Oh, I didn't think we were talking about " real " bike pedals. Sure, the mechanical recession is used to indicate the " slop " between the ball or roller bearing and the bearing race.

But try this: lightly screw a pedal onto a crank arm, as you ride the bicycle forward, the pedal will screw into the crank arm and it may appear that it has tightened into the crank arm, now if you were to pedal backwards with the same intensity as when you pedaled forward, then the pedal will unscrew, and that is why a juxtaposition is placed onto the pedal shaft so torque could be applied to secure the pedal with more torque than could be applied by simply turning the pedal by hand,foot or the riders weight.

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#30
In reply to #21

Re: Bike pedals

09/24/2016 8:58 AM

How else could you tighten it if there weren't flats on the shaft?

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#13

Re: Bike Pedals

09/22/2016 8:48 AM

Given that pedals are intended to be operated by the foot, shouldn't that read "left foot thread" and "right foot thread"?

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#14
In reply to #13

Re: Bike Pedals

09/22/2016 9:06 AM

ba-dump, ching!

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#18

Re: Bike Pedals

09/22/2016 12:15 PM

If you want to repurpose the old pedals...

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#19

Re: Bike Pedals

09/22/2016 12:39 PM

Here's the bottom line. (play on words there)

If it wasn't REALLY necessary, bicycle manufacturers would not be using reverse threads on one side of their bikes.

It costs more, and is confusing.

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#20

Re: Bike Pedals

09/22/2016 1:12 PM

Yes - and when you have to remove the bottom bracket (bearing set for the spindle, and on the older "square" or "JIS" bottom brackets, the spindle as well) that the crank arms attach to, for the same reason, they are left and right, but reversed. The right side (crank ring side) has a left thread, while the pedal there is right hand thread and the opposite a right hand thread with a left hand threaded pedal. Ponder that for a while. Makes neophyte bike mechanics life tough the first time or two they try to work on this portion of a bike. After a few hundred, it becomes natural to turn the wrench or tool the correct way.

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#23

Re: Bike Pedals

09/22/2016 4:42 PM

Judging by how tight they were to remove, there’s not much danger in practice of them coming loose.

No, but if you get in a hurry after a rebuild and forget to tighten them....well, strangely enough, they will work looser, not tighter, and a new crank arm is definitely in your future budget. I killed a $200 Shimano 5600 three weeks ago this way. By the time you feel the pedal wobbling, it is way too late to tighten them.

Now maybe we need that explained as well. They work loose if not tight - but stay tight if tightened. Kind of strange, isn't it?

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#27
In reply to #23

Re: Bike Pedals

09/24/2016 8:24 AM

You're not serious in your final question are you? If you forget to tighten any bolted fitting it's likely to work looser, by sod's law if nothing else.

Presumably your bike has RH thread on right, LH on left? If so it seems the self-tightening effect described in this thread didn't help.

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#24

Re: Bike Pedals

09/22/2016 4:45 PM

If this thread were a horse, it would have died 3 days ago.

Just tighten the damn pedals and ride the bike!

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#26
In reply to #24

Re: Bike Pedals

09/22/2016 4:59 PM

ROFLMAO and clenching legs together. Heck it's time to go pee.

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#28
In reply to #24

Re: Bike Pedals

09/24/2016 8:30 AM

Have you checked out the thread on dark matter in General recently?

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#31
In reply to #28

Re: Bike Pedals

09/24/2016 3:20 PM

either one, the horse stinketh methinks.

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#32
In reply to #28

Re: Bike Pedals

09/24/2016 4:57 PM

Yes, but it doesn't "matter".

I've always wondered why they didn't put a hex fitting in the end of the shaft, instead of flats by the thread end.

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#33
In reply to #32

Re: Bike Pedals

09/24/2016 5:05 PM

That's a " Murphy's law " thing, open end wrenches were invented before hex wrenches were.

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