Why is the left pedal left-hand threaded?

Unicycles and Equipment
1

I’ve known for as long as I can remember that the left crank arm is reverse
threaded. I alway understood that this was to prevent the pedals from
coming loose from riding. The more I think about it the less it makes
sense.

See, when you pedal forward, the right-hand crank is travelling clockwise.
Therefore the pedal rotation is counter-clockwise. Isn’t this the same
direction you would unscrew the pedal? So it seems to me that in the event
that your pedal becomes loose (or the pedal bearings drag or seize) that
would have a tendency to unscrew both sides since the left crank travels
ccw and the left pedal turns cw which is the same direction you unscrew the
left pedal.

I started thinking about this a lot more while looking for better cranks for
my uni and wondered why we don’t just use two left cranks/pedals. Perhaps
it’s like trying to buy two left shoes… what do you do with the right?

Can someone shed some light on this issue (the left/right threads, not the
shoes).

-mg

2

Re: Why is the left pedal left-hand threaded?

In article <upM2sMZGCHA.852@cpimsnntpa03>,
Michael Grant <michael_j_grant@msn.com> wrote:
)I’ve known for as long as I can remember that the left crank arm is reverse
)threaded. I alway understood that this was to prevent the pedals from
)coming loose from riding. The more I think about it the less it makes
)sense.
)
)See, when you pedal forward, the right-hand crank is travelling clockwise.
)Therefore the pedal rotation is counter-clockwise. Isn’t this the same
)direction you would unscrew the pedal? So it seems to me that in the event
)that your pedal becomes loose (or the pedal bearings drag or seize) that
)would have a tendency to unscrew both sides since the left crank travels
)ccw and the left pedal turns cw which is the same direction you unscrew the
)left pedal.

The unscrewing action, when pedaling, comes from the far end of the
screw, so it happens the opposite way from the pedaling direction.
I’m not going to attempt an ASCII drawing, but basically, picture your
crank at 3:00, riding forward, under full power. The pedal pushes
downwards. The bottom of the pedal axle compresses against the downward
side of the hole in the crank. This tilts the axle, so on the
other side of the crank (the inner side), the TOP of the pedal axle
compresses against the upper side of the hole in the crank. If you
follow this effect around a pedal revolution, you’ll see it rotates in
the opposite direction from the pressure on the outside of the crank
hole. Hence the counterintuitive unscrewing.
-Tom

3

See the info under “Pedal” at <http://www.sheldonbrown.com/gloss_p.html>

Here is the relevant text from SheldonBrown.com

Pedal Threading
Direction
The right pedal has a normal thread, but the left pedal has a left (reverse) thread.
The reason for this is not obvious: The force from bearing friction would, in fact, tend to unscrew pedals threaded in this manner. The fact is, however, that it is not the bearing friction that makes pedals unscrew themselves, but a phenomenon called “precession”.

You can demonstrate this to yourself by performing a simple experiment. Hold a pencil loosely in one fist, and move the end of it in a circle. You will see that the pencil, as it rubs against the inside of your fist, rotates in the opposite direction.

Ignorant people outside the bike industry sometimes make the astonishing discovery that the way it has been done for 100 years is “wrong.” “Look at these fools, they go to the trouble of using a left thread on one pedal, then the bozos go and put the left thread on the wrong side! Shows that bicycle designers have no idea what they are doing…”

Another popular theory of armchair engineers is that the threads are done this way so that, if the pedal bearing locks up, the pedal will unscrew itself instead of breaking the rider’s ankle.

The left threaded left pedal was not the result of armchair theorizing, it was a solution to a real problem: people’s left pedals kept unscrewing! I have read that this was invented by the Wright brothers, but I am not sure of this.

4

im of the bearing friction side of things,combine this with vibration too.

5

I think Gilby addressed this question in a previous thread. If the bearing were a bushing, the pedal action itself would tend to unscrew the pedal. But they are ball bearings and the balls are what transfer torque to the pedal axle by friction. The frictional forces from a ball bearing tend to screw the pedal in. Draw a picture of the outer race (pedal), the balls, and the inner race (pedal axle). If you turn the outer race clockwise the balls will all turn clockwise. On the other side of each ball a counterclockwise torque is being exerted against the axle. This is opposite to the direction the pedal (outer race) is being turned.

6

Re: Why is the left pedal left-hand threaded?

Thanks for the info. I should have thought to check SheldonBrown.com before
posting.

-mg

“john_childs” <john_childs.6mp2a@timelimit.unicyclist.com> wrote in message
news:john_childs.6mp2a@timelimit.unicyclist.com…
>
> See the info under “Pedal” at
> <http://www.sheldonbrown.com/gloss_p.html>
>
> Here is the relevant text from SheldonBrown.com
>
> Pedal Threading
> Direction
> The right pedal has a normal thread, but the left pedal has a left
> (reverse) thread.
> The reason for this is not obvious: The force from bearing friction
> would, in fact, tend to unscrew pedals threaded in this manner. The fact
> is, however, that it is not the bearing friction that makes pedals
> unscrew themselves, but a phenomenon called “precession”.
>
> You can demonstrate this to yourself by performing a simple experiment.
> Hold a pencil loosely in one fist, and move the end of it in a circle.
> You will see that the pencil, as it rubs against the inside of your
> fist, rotates in the opposite direction.
>
> Ignorant people outside the bike industry sometimes make the astonishing
> discovery that the way it has been done for 100 years is “wrong.” “Look
> at these fools, they go to the trouble of using a left thread on one
> pedal, then the bozos go and put the left thread on the wrong side!
> Shows that bicycle designers have no idea what they are doing…”
>
> Another popular theory of armchair engineers is that the threads are
> done this way so that, if the pedal bearing locks up, the pedal will
> unscrew itself instead of breaking the rider’s ankle.
>
> The left threaded left pedal was not the result of armchair theorizing,
> it was a solution to a real problem: people’s left pedals kept
> unscrewing! I have read that this was invented by the Wright brothers,
> but I am not sure of this.
>
>
> –
> john_childs
>
> john_childs (at) hotmail (dot) com
> ------------------------------------------------------------------------
> john_childs’s Profile: http://www.unicyclist.com/profile/449
> View this thread: http://www.unicyclist.com/thread/18867
>

7

RE: Why is the left pedal left-hand threaded?

> threaded. I alway understood that this was to prevent the pedals from
> coming loose from riding. The more I think about it the less it makes
> sense.

I know what you mean. But the way to solve this one, since the threading
obviously works, is to work from the solution backward. Why does this work?

If you’re not sure it works, just turn your seat around and ride the
unicycle backwards for a few days (or hours). That’s all the convincing
you’ll need.

> See, when you pedal forward, the right-hand crank is travelling clockwise.
> Therefore the pedal rotation is counter-clockwise. Isn’t this the same
> direction you would unscrew the pedal?

I think the force comes from precession of pressure on the pedal bearings.
In other words, as the pedal goes around the pedal stroke, the forces acting
on it go around in a circle also, from one side of the spindle to the other.
Over time, this rotation causes loosening of the threads. Don’t think of the
pedal body, think about the spindle part, which is what is threaded to the
crank.

I don’t know if that’s accurate, or well-explained. But I don’t question it
any more, because I know it’s true.

8

RE: Why is the left pedal left-hand threaded?

>> threaded. I alway understood that this was to prevent the pedals from
>> coming loose from riding. The more I think about it the less it makes
>> sense.
>

I asked this same question a year or two ago. It seemed to me that the pedals
were threaded in a way that would make them UNscrew as they were turned. I
don’t remember exactly how it was explained to me or by whom, but it made
sense, and worked out something like this: Make a small circle with your thumb
and forefinger, just open enough to loosely grasp the middle of a pencil. Your
hand is the end of the crank-arm, and the pencil is the shaft of the pedal. Now
move the end of the pencil in a circle to simulate the force applied when you
pedal. Notice that the shaft of the pencil itself tends to rotate in the
direction opposite that in which you are “pedaling” its end. It’s this force -
I don’t know what it’s called - that the pedal-threads are designed to cope
with, not the force of the pedal turning on its shaft.

  • Joe
9

It works. You don’t understand why, even though it’s been explained to you. It still works. You just don’t understand it, but then again, you never really worried about it until someone pointed out to you that maybe you don’t understand it. Now it’s starting to bother you, because you think that maybe you should understand it. You’ve read the explanations, but you don’t understand them, so, you start making up your own “theories”. Somehow, they don’t help, and now you’re riddled with doubt…

What if the sky is falling?

What if those durned pedals are threaded all wrong, and it’s only our blind faith that’s kept them from falling out onto the street…?

Look! That bicycle’s pedals have just flown into the gutter!

Way back when, I had a 1969 Plymouth Fury… The lugnuts on the driver’s side (in America, so the LEFT side) were left-hand threaded… You can go ask the Chrysler Corp. why, (Why? Why?) but I’m guessing they took a little wisdom from the one-human powered vehicles that lent so much of their technology to the “other” horseless carriage…

10

my left pedal seems to defy physics and undo itself anyway, or is it the pedal grabs,idling, one footing and riding backwards.

why have pedals that screw on anyway?
it just dosen’t seem to make sense to screw on somthing that gets turned around.(and on unicycles in both derictions

wouldn’t we be better off with splined pedals?
or even just pedals that had longer threaded sections that protruded from the inside of the crank so you could put a retaining bolt on.

bike pedal technology just wasn’t designed for unicycles.

perhaps we should convince a couple of bike manufacturers that they would be able to make an absolute fortune by introducing splined pedals, and convincing people to upgrade(they probably could too)

sure i’d have to upgrade my cranks, but i’d be glad not to have to carry a spanner with me everywhere.

11

Oooh! Nice one! Good idea! Let’s bring the technology up-to-date! How about pedals with retaining nuts on the outside ? Oops! There’s still that durned thread problem…

OK. Splines.

Or… cotter pins?

Any other ideas on how to make pedals uni-directional?

12

RE: Why is the left pedal left-hand threaded?

Just another confirmation that the 100 year rule applies. In other words,
the left pedal has not been left-hand threaded for 100 years by mistake.

I changed the crank arms today on my Steve Howard prototype MUni. I put the
original 170s back on, after using 150s for the Downieville Downhill.
Somehow I wasn’t paying attention, and put the cranks on the wrong side. I
didn’t notice this until LESS THAN FIVE MINUTES of riding had transpired.
And those pedals were on tight. I had not taken them off the cranks, and
used different pedals for Downieville. All I really did was ride down and
then up one little hill (the uphill race course from the first three MUni
Weekends). Both pedals were noticeably loose, and I had to return to the car
for the pedal wrench Glad I noticed this before heading up the trail,
because I don’t have anything to tighten pedals with in my normal riding
pack. I turned the seat around.

Due to the unique design of the Steve Howard frame, I’ll be able to turn the
wheel around the right way without having to go through too many steps. Just
take out the six screws holding the fork legs to the crown, turn it around,
and screw them back in.

Just goes to show, not matter how many times you’ve done it, you still might
get things wrong when you put even the simplest machines together…

Stay on top,
John Foss, the Uni-Cyclone
jfoss@unicycling.com

“If we are what we eat, then I’m easy, fast, and cheap!”

13

RE: Why is the left pedal left-hand threaded?

> my left pedal seems to defy physics and undo itself
> anyway, or is it the pedal grabs,idling, one footing
> and riding backwards.

I think it’s mostly the pedal grabs. I never had a problem with loose pedals
from riding backwards, even when I was learning. And idling, technically, is
50/50 so it should leave them where they were.

Somebody else mentioned something about tightening pedals because he does
trials every day. I’ve never heard of loose pedals being a problem for
unicyclists, other than people with wheels on backward or two of the same
kind of crank. So the new skills of pedal grabbing might be the main
problem.

For those kinds of activities (grabbing, grinding, etc.), it seems a
different approach to the whole drivetrain and bearing area of the unicycle
would be in order. But I’ll save that for another thread, another time.

> why have pedals that screw on anyway?

Because they’re cheap to manufacture, and mostly this hasn’t been a problem.

> perhaps we should convince a couple of bike manufacturers
> that they would be able to make an absolute fortune by
> introducing splined pedals, and convincing people to
> upgrade(they probably could too)

That would be a hard sell in both cases. I don’t think you could make a
splined pedal for less than several times the price of a mass-produced
traditional one. Your only market would be people who have a similar
problem.

Do many people have problems with chronic pedal-looseness? Not counting
people with the wheel on backwards, etc… :slight_smile:

JF

14

Re: Why is the left pedal left-hand threaded?

>Oooh! Nice one! Good idea! Let’s bring the technology up-to-date!
>How about pedals with retaining nuts on the -outside- ? Oops! There’s
>still that durned thread problem…

Just reverse the threads on the locking nut and use a smaller diameter
end so there’s no contact with the threads in the crank arm. Thus for
the right pedal, it is screwed into a right hand (clockwise) thread and
held in place with a smaller diameter left hand (counter clockwise)
locking nut.

But, why bother? Just, torque the pedals with the pedal wrench
(spanner) extremely tight, to the point where it feels like it might
break. Shortly after learning to ride backwards a long long time ago,
my pedals would unscrew because I would ride backwards where ever I
went. I simply torqued the pedals on a tightly as possible and they
never unscrewed again when riding exclusively backwards. (I did
consider simply rotating the seat 180 degrees, but that would have been
cheating.) However, extreme tightening of the pedal could be
over-stressing the crank arm threads, so I remove pedals only for
airplane trips and sometimes not even for that.

Sincerely,

Ken Fuchs <kfuchs@winternet.com>

15

The idling and riding backwards. Idling isn’t quite 50-50 because the forces that your foot places on the pedal are not balanced towards and away from the wheel. The pedal wants to move away from the wheel more than it wants to move towards it, because your foot exerts more pressure away-from-the-wheel. The precession effect, then, which as Sheldon Brown says, is the culprit here, is more effective in the away-from-the-wheel direction.

Pedal grabs might tend to loosen the threads or shock the connection but would not unscrew the pedals unless circular motions are involved. One-footing puts more pressure on the pedals (thus enhancing the precession effect) but, unless you are one-foot idling or one-footing backwards, would tend to screw the pedals on instead of off.

The greater the pressure on the pedals, the more effective precession is at unscrewing the pedals. So riders who are heavier, put less weight on the seat, or are riding high-force terrain with the wheel on backwards would have more problems with pedal unscrewing. Were there a backwards uphill uni race, it probably would be a good strategy for a racer to reverse the wheel during competition.

I had the same problem when I started learning to ride backwards. A solution that has worked well for me is to use blue loc-tite on the pedal threads and tighten the pedals quite hard.

The precession effect has to do with the threads “walking” within each other. The bearing type or bearing friction in general don’t have anything to do with it. Even if the pedal bearings were frictionless, and the pedal’s axle did not bend, precession and pedal unscrewing would still occur. The circular force about the pedal axis that does the unscrewing arises from the fact that the true pedal axis is wobbling (describing a cone) about the ideal pedal axis. The wobble, called precession, comes from the foot’s pressure on the pedal throughout the cycle and is allowed partially by thread slop and partially by the elasticity of the material out of which the threads are made (it also arises from the pedal’s axle bending, but that is not relevant to this discussion). The unscrewing effect takes place between the pedal and crank threads, outside the pedal body. This may be why loc-tite, which helps fill the thread gap, and harder tightening, which reduces the thread gap, have been so effective.

16

it is probably the grabs, because i tend to grab mostly with my left side and thats the pedal thats always loosening up.

still i’d like to fantasise about splined pedals.

no more spanner in my pocket to fall on.

my reasoning behind convincing the bike manufacturers was that because mtb is such a big market, it could support a new type of pedal that was overspec’d, they’ll always be people who buy realy good downhill mtbs and never take them off road. in my town theres hundreds of kids riding about with splined cranks on bikes that have never seen dirt. and it realy irks me, that i don’t have a splined hub yet. o.k. i’m jealous.

and it’s only a new pedal axle and crank that you’ll need

if i ever end up with a realy stupidly huge amount of cash to spend on a uni i’ll get profile to make me a set,
i’ve heard that they’ll make you anything as long as you pay them enough cold hard cash.

mind you though its only trials and muni riders that need splined uni hubs and 145 crank arms and they sell loads of units.

i know its a far fetched idea that anyone would ever bring such a cool setup to market, but let me indulge myself.

mind you, if my splined hub and cranks arrive and the next day someone starts making splined pedals i’ll be realy irate.

17

Re: Why is the left pedal left-hand threaded?

n a message dated 6/24/02 4:31:59 PM, evilewan.6rqjn@timelimit.unicyclist.com
writes:

<< my reasoning behind convincing the bike manufacturers was that because
mtb is such a big market, it could support a new type of pedal that was
overspec’d, they’ll always be people who buy realy good downhill mtbs
and never take them off road. in my town theres hundreds of kids riding
about with splined cranks on bikes that have never seen dirt. and it
realy irks me, that i don’t have a splined hub yet. o.k. i’m jealous. >>

mountain bikes don’t use platform pedals, they use clipless pedals. You
would have to go trough the BMX industry. Thats where our pedals come from.

__
Trevor Andersen

18

Re: Why is the left pedal left-hand threaded?

>mountain bikes don’t use platform pedals, they use clipless pedals. You
>would have to go trough the BMX industry. Thats where our pedals come from.
>

DH mountain bikers use platform pedals, so do freeriders and trials riders, and
all BC Mtbers, (the cool ones). :wink:

-Dylan

19

Suddenly the writings of a Mr Terry Pratchett spring to mind… don’t question why the world exists, just in case you come to the conclusion that it shouldn’t and the world decides to agree with you…

Phil, just me

20

Thanks guys. I love to stand corrected when I learn something. My apologies to the industry. Why didn’t my Physics professor mention this ‘precession’. I’m sure I would have taken note. I assume this process is aggravated by cheap threads (mentioned earlier in this thread) and I can envision the forces being reversed at the other end of the axle. I haven’t actually had pedal problems in years. But, my original comment came from experience many years ago when jumping bicycles into lakes was a common pastime. The dried out bearings overcame the precession, and unscrewed the pedals every time I forgot to oil them (while re-greasing every other #@?$ bearing on the bike). It was a high maintenance pastime. So, as I mentioned earlier (especially for MUni riders playing in the mud) if you are having problems, red Lock-tite and oil should help. And I know it’s basic (or I thought so yesterday), but if all of this confusion has you totally confused, just make sure you have a clockwise thread on the right, counterclockwise on the left. I’m sure there are a few unicycles out there that have the seat accidentally reversed.