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Old 2003-10-16, 04:30 PM   #1
schroder
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World's cheapest UW -or- pedal thread thread

Hello All:

Kudos to everyone that contributes to this forum, there is a ton of information to be had.

I have been trying to make the world's cheapest ultimate wheel, namely using a throwaway 24 inch rim and tire, some plywood, screws I have on hand, etc. With this in mind I took a 9/16 pedal to the hardware store to try and find a bolt that would fit it (at least the pedal that threads the standard way) but no joy.

Can someone enlighten me on the thread size of these pedals? They are the Union ATB from unicycle.com.

Also, I have always thought that pedals on the right and left sides were threaded differently so that if the bearings freeze or become stiff, the pedal would not unscrew itself (at least not riding forward). But looking at these pedals, the right side screws in clockwise, and the left counterclockwise. It seems like a frozen bearing would then cause you to be unscrewing the pedal as you rode forward. Are there other forces at work here, or am I just looking at things the wrong way around?

Thanks all.
Phil
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Old 2003-10-16, 04:47 PM   #2
nickjb
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Go to your local bike shop (one that fixes bikes) and they will have a large box full of old cranksets with worn teeth and knackered tapers. I'm sure they will happily give you some. Make sure you get steel ones and a left and a right. Just hacksaw off the arm and weld the threaded bit to some plate.

If you can't track some down then the thread size 9/16" x 20 for MTB pedals and 1/2" x 20 for BMX. It is a pretty uncommon size (except in cycling) so the first plan is best.

The pedals are threaded so they don't unscrew during normal riding. It is opposite way that logic says it should be but it is right.
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Old 2003-10-16, 11:45 PM   #3
Mandell
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I know what you mean about it being non-obvious. I recently saw what happens when you put the wheel (and thus the pedals) on backwards though. I rode a total of about 2 blocks before the pedal started to wiggle and I had to walk home to get my tools. (That's what I get for having a machine shop work on my unicycle.) Even though it may be couter-intuitive, it really is right.
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Old 2003-10-17, 12:28 AM   #4
schroder
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Used cranks and riding backwards

Thanks for the input so far. I was thinking about trying to find some used cranks, since the price would fit in with my goal of building it cheap.

I was reading an older thread where someone was riding their UW with the pedals on the wrong sides and they came loose immediately. I am wondering, since we can ride backwards (unlike a b*ke) aren't we always in some danger of having the cranks come loose? Or would we have to ride for a mile or two (backwards) for that to happen?

Has anyone had cranks come loose while riding forwards? When I got my giraffe back from my brother a couple of months ago the pedals were both loose, and I don't think he was riding backwards. Maybe the seat got turned around, I should check.

Phil (still confused as to why)
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Old 2003-10-19, 10:38 PM   #5
Klaas Bil
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Re: World's cheapest UW -or- pedal thread thread

On Thu, 16 Oct 2003 19:28:58 -0500, schroder
<schroder.vf3xy@timelimit.unicyclist.com> wrote:

>Phil (still confused as to why)


See <http://www.sheldonbrown.com/gloss_p.html> under 'Pedal'.

Klaas Bil - Newsgroup Addict
--
Grizzly bear droppings have bells in them and smell like pepper spray. - UniBrier

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Old 2003-10-20, 03:22 PM   #6
schroder
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Precession and pedals

Thanks Klaas Bil, I found this web site also in my search for the answer.

For those who haven't looked, the answer given is that the pedals unscrew due to precession, not friction from the bearings. Precession is the effect you see when you hold one end of the axle of a bicycle tire and give it a spin. Rather than falling, the tire rotates slowly around a vertical axis.

/DIGRESSION/
Another example of precession is from WWII, when pilots discovered that they could turn and dive more quickly in one direction than the other. This was due to the angular momentum of their propeller.
/END DIGRESSION/

Precession is a cross product of a moment and angular momentum (warning, vector algebra in use!). I suppose since the pedal bends downward a bit all the time then it is not always pointing in the same direction as it goes around (from the point of view of the tire), so there could be some cross product involved.

I have come to the conclusion that the real cause has to do with the threads being in alternating compression and tension, compression and tension, constantly as we pedal. The bike page above was good to point out that the left-threaded solution was done to solve a real problem, namely that left pedals were coming loose, which is also the experience of people on the list.

Sorry for the physics lesson, just my nature.

Phil
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Old 2003-10-20, 05:25 PM   #7
ubersquish
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and for those not in the know cross product is a measure of perpendicularity, and is found by multiplying the magnitudes (or lengths) of both vectors and then multiplying by the sine of the angle where the vectors intersect
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