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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schroder - Occaisional Unicycler
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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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nickjb - one wheel short
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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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Mandell - Keen Beginner
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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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schroder - Occaisional Unicycler
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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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schroder - Occaisional Unicycler
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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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schroder - Occaisional Unicycler
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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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ubersquish
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