Giraffe and physics

General
1

Boy, there’s something about the physics of fulcums and levers at work on the Giraffe. When I stand on the pedals, if I’m not careful the seat will shoot out from under with such force its hard (and sometimes impossible) to control.

There was some talk earlier about a possible use for the seat on stomach skill. That being, mounting a giraffe. (Climb the beast to seat on stomach, ride a bit and the transition to sit on seat).

I’m a little concerned about that transition. While holding onto the trim of my house (its cool just saying that), I’ve tried to mess with the seat. If I get it out just a smidge, there is so much force all of the sudden…kersplat!

Its tricky. Here’s me mounting. Later I found it easier to climb the post first, then back the beast into the step, then hop on.

2

Sorry, I hope this brings it:(

giraffe mounting.jpg

3

The centre of mass of giraffe and rider is pretty high up - probably somewhere in the rider’s lower abdomen. The centre of support is where the wheel touches the ground. All movement of the giraffe is caused either by gravity acting on the centre of mass, or by the tyre pushing one way or another on the ground.

If you throw a yard brush (broom) so that it spins end over end, it will rotate around its centreof mass - somewhere in the handle, a short distance from the head.

So if you threw a giraffe and rider so that the unit span end over end, it would rotate around the centre of mass (rider’s abdomen).

What this all means is that it’s very easy to move the light wheel and quite difficult to move the heavy rider. If the rider were completely rigid (say with fear;) ) then the wheel would move easily, and the rider would rotate around his/her abdomen - unless of course the rider was already ‘falling’ under the influence of gravity. So that’s why the wheel easily scoots out from under you. On a normal uni, the centre of mass is much nearer to the wheel. It’s still easier to move the wheel than the whole mass, but not as much easier as it is on a giraffe.

Riding a unicycle is like being in orbit. You keep falling under gravitational acceleration, but you never hit the ground. The difference is that in orbit, the ground cunningly curves away from you, and on a uni, the wheel should chase after you, balancing the lift against gravity. I’m surprised NASA don’t sponsor unicycle.com, really.

4

Re: Giraffe and physics

Bravo!!! … Bravo!!! Extremely well said!

Mikefule is a brilliant light in an otherwise pitch black night of
boring posts to RSU. He isn’t the only brilliant author on RSU, but
I simply had to give him this public recognition at this time.

Thank you very much for the quality posts, a good example of which is:

>The centre of mass of giraffe and rider is pretty high up - probably
>somewhere in the rider’s lower abdomen. The centre of support is where
>the wheel touches the ground. All movement of the giraffe is caused
>either by gravity acting on the centre of mass, or by the tyre pushing
>one way or another on the ground.

>If you throw a yard brush (broom) so that it spins end over end, it will
>rotate around its centre of mass - somewhere in the handle, a short
>distance from the head.

>So if you threw a giraffe and rider so that the unit span end over end,
>it would rotate around the centre of mass (rider’s abdomen).

>What this all means is that it’s very easy to move the light wheel and
>quite difficult to move the heavy rider. If the rider were completely
>rigid (say with fear;) ) then the wheel would move easily, and the rider
>would rotate around his/her abdomen - unless of course the rider was
>already ‘falling’ under the influence of gravity. So that’s why the
>wheel easily scoots out from under you. On a normal uni, the centre of
>mass is much nearer to the wheel. It’s still easier to move the wheel
>than the whole mass, but not as much easier as it is on a giraffe.

>Riding a unicycle is like being in orbit. You keep falling under
>gravitational acceleration, but you never hit the ground. The
>difference is that in orbit, the ground cunningly curves away from you,
>and on a uni, the wheel should chase after you, balancing the lift
>against gravity. I’m surprised NASA don’t sponsor unicycle.com, really.

Sincerely,

Ken Fuchs <kfuchs@winternet.com>

P.S. Let me set the record straight on an old somewhat unrelated thread:
Mikefule is definitely right about steering control being largely
transmitted through the pedals/crank arms as this is particularly true
of big wheels, especially those with axel widths greater than the Coker.

5

Speak for yourself! :slight_smile:

MM,

Dustin Kelm performs a trick you should probably start to work on next. I’ve seen him do the trick now several times. On his 10’er, he rides with the seat out back away from his body. Basically, he’s hanging off the front of the uni held only by his feet on the pedals and his hands gripping the saddle behind him. Its the neatest looking thing but also looks dangerously delicate in that the slightest sweaty palm would send him plummeting.

He does a great job with it and that’s why he’s Dustin and I’m not. :slight_smile:

Bruce

6

Re: Giraffe and physics

On Sun, 29 Dec 2002 10:14:52 -0600, Ken Fuchs <kfuchs@winternet.com>
wrote:

>Bravo!!! … Bravo!!! Extremely well said!
>
>Mikefule is a brilliant light in an otherwise pitch black night of
>boring posts to RSU. He isn’t the only brilliant author on RSU, but
>I simply had to give him this public recognition at this time.

I agree that Mikefule’s posts are usually eloquently-written,
well-informed, to the point, informative, and philosophical at times.
Indeed, Mikefule is one of the people in RSU of which I still spell
out every post. I wouldn’t get so much carried away, though, as to say
that everything else on RSU amounts to a pitch black night of boring
posts.

However, I think it’s kind of funny that you say this right now.
Memphis Mud’s original question, as I read it, relates to the problems
of balance and shooting-out seats when putting more weight on the
pedals, effectively creating an additional pivot point at the pedal
axis, whereas Mikefule’s reply treats the rider-and-unicycle as one
single rigid system. So in this particular case I’m not sure that MM’s
issue was fully addressed. I might be wrong of course.

>P.S. Let me set the record straight on an old somewhat unrelated thread:
>Mikefule is definitely right about steering control being largely
>transmitted through the pedals/crank arms as this is particularly true
>of big wheels, especially those with axel widths greater than the Coker.

I’m not sure if Mikefule is “definitely” right. In that old thread
that went on for too long, controversy reigned till the end. Maybe
Andrew Carter (who is not reading the forum now I think) who due to
some special construction can set his seat to spin freely, could
comment on the importance of the seat in steering?

Klaas Bil

Today (29 December) is the last day this year that is not the last day this year and also not the last day this year that is not the last day this year

7

Re: Giraffe and physics

On Sun, 29 Dec 2002 08:09:02 -0600, Memphis Mud
<Memphis.Mud.gff7n@timelimit.unicyclist.com> wrote:

>When I stand on the pedals, if I’m not careful the seat
>will shoot out from under with such force its hard (and sometimes
>impossible) to control.

On a direct-drive (regular) unicycle, if you stand on the pedals, your
weight is not on the frame, and hence the only gravitational force on
the frame is the (smallish) weight of the frame itself. Hence, only
small forces are required to keep the frame upright.

Conversely, on a giraffe, if you stand on the pedals, your (largish)
weight is still on the frame (albeit lower than if you would sit on
the seat). Therefore if the frame is not completely upright there is
an appreciable torque (force) trying to rotate the frame (go down). If
you let the seat go a little bit, that torque increases quickly which
creates the instable feeling you mentioned.

Klaas Bil

Today (29 December) is the last day this year that is not the last day this year and also not the last day this year that is not the last day this year

8

Thank you folks. Believe it or not, I understand [almost] everything you’ve written.

Its extremely sensitive up there to any faux pas. The trick is to keep my center of balance on or directly above the seat.

I think this means the shaft that holds me up there must not get more than a couple of inches off dead straight up vertical. Any more than that creates that torque I mention an it requires more strength than I can devote to controling that one aspect of my ride. (I’m still concentrating a lot).

So if I’m riding seat out front, I should be leaning over the seat. Seat out back…I guess I’d have to arch backwards to somewhat do the same thing.

9

Thinking about it a bit more…

Take two unicycles: a normal one with the seat 3 feet off the ground and a giraffe with the seat 5 feet off the ground.

Lean each of them at 30 degrees from the vertical, or 60 degrees from the horizonal.

The seat of the giraffe has ‘fallen’ further than the seat of the normal unicycle.

(I think it is (1 - sin 60) X the height of the uni, but I’m very rusty on maths with sines and cosines.)

As the centre of mass of the uni/rider combination is somewhere just above the seat, it follows that the centre of mass of the giraffe/rider has fallen further than the centre of mass of the normal uni/rider.

Therefore, there is more work to be done to restore the centre of mass to the balance position.

The answer is not to lean the giraffe to as big an angle as the normal uni. However, when learning, particularly mounting and setting off, the rider may have ‘learned responses’ about the relative positions of seat and pedals which lead him/her to lean the ‘expected’ amount - which is now too far!

Incidentally, it doesn’t matter how much weight is on the seat and how much is on the pedals. If the rider’s position is secure so that his/her body mass is held in the right position relative to the seat, then the effect will be exactly the same whether his/her weight is distributed 50/50 between pedals and seat, 60/40, 40/60, or any other split.

That’s for ‘dead balance’. For control, the rider will exert more leverage on the cranks if his/her weight is more on the pedals than the seat.

10

Re: Giraffe and physics

“Mikefule” <Mikefule.ggt7m@timelimit.unicyclist.com> wrote in message
news:Mikefule.ggt7m@timelimit.unicyclist.com…

> The seat of the giraffe has ‘fallen’ further than the seat of the normal
> unicycle.
>
> (I think it is (1 - sin 60) X the height of the uni, but I’m very rusty
> on maths with sines and cosines.)
>
No detectable corrosion there at all Mike. …and for the purpose of the
rest of this post, your 30% tilt results in a fall of about a 7th of the
height of the seat. Or let us say a 13% fall. :wink:

Now, let us apply this with rigorous scientific method, and somewhat less
precise mathematics (due mainly to the inability of most human bodies to
retain the same angle as the unicycle during falling) And we will apply it
to my own somewhat dodgy riding abilities. In particular to my poor attempts
to ride smoothly and in a straight line.

I am now able to describe my unicycling progress in a totally new way. As a
total newbie I used to fall off. This should be known as a 100% fall, a
perfect fall if you like. (again ignoring said imperfections of the human
body)

Now with my vast experience I now have the ability to ride a couple of
hundred metres in a straight line without a perfect fall. However that 200
metres is probably best described as a linked series of imperfect falls,
both forward and backward (I ignore sideways for the purposes of this
discussion). I continue to ride by applying energy to accelerate the wheel
in such a way as to correct those falls, and by thus not allowing them to
become perfect. Many have described unicycling as learning to balance,
which could also be described as learning to make your falls less perfect
(or smaller). As I get better my falls get less and less perfect, until at
some future time, the ever hopeful observer may not even be able to detect
them, but I will still be progressing by a series of much smaller falls.
So a good rider, making smooth progress, is merely controlling his falls at
a far greater rate than I.
Or alternatively: I am far better at falling than he is.
I think my falls are now only 5% perfect falls, ( or 5% UPDs?) , and I am
striving for total imperfection.

Naomi :wink:

This post may well appear as total nonsense to some, but it is not quite
that good…

Honest: I have a perfect hourglass figure…head full of sand and bottom
getting heavier by the minute.

11

Less lofty observation (because that’s all I’m capable of):

The direction of Gravity could be drawn as a line from the sky, straight down to the center of the earth.

Perfect balace of a Giraffe can be described as holding the uni where the Seat, shaft, and axle are along this imaginary line (wheel on ground, seat in sky for now).

When I stand up off the seat on my regular Uni, my 200 lbs force is being exerted on the axle at the center of the wheel. There is no front/back force being applied to the seat. (There is left/right force because of my awkward pedal technique). But if left/right force could be held equally, then the seat could swivel front and back with almost no effort. (i.e. hold the tire upright and drop the seat, it effortlessly drops forward or backward)

When I stand up off the seat on the Giraffe, my 200 lbs force is being exerted on the shaft. As soon as this point of force moves in front of the original imaginary gravity line, my 200 lbs force gets its own new imaginary gravity line. This new line does not go down the shaft and through the axle point. The forwad force created on the seat is formidible. And the further in front of that original gravity line the more powerful.

I’m resolved to this phenomenon. Its part of Giraffe-dom. Deal with it or crash.

Another question is: As the Giraffe gets taller, isn’t this force lessened because ones feet (point of force) goes up the shaft? Therefore as the seat comes out front, it doesn’t move as far in front of that “original gravity line”. On a 30 foot Giraffe, a man holding the seat out front would only move the new gravity line an inch or so forward. And the shaft angle hardly changes.

I think therefore that this phenomenon is the worst on the 5 foot giraffe.

12

I think Klaas is exactly right. It has less to do with center of mass and more to do with weight being on the frame. The unicycle pivots about the axle (fulcrum) When you stand up on a normal uni, the mass is applied at the fulcrom and so there is no moment arm, so it takes no force to hold up the uni. On a giraffe the mass is applied away from the fulcrum. so the moment is
mhsin (theta). Where h is the bottom (er middle) bracket distance from the axle, m is your mass and theta is the angle from vertical. This moment is manefested as the seat trying to jump from your hand.
-gauss

13

Re: Giraffe and physics

On Mon, 30 Dec 2002 10:50:26 -0600, Memphis Mud
<Memphis.Mud.ghhac@timelimit.unicyclist.com> wrote:

>Another question is: As the Giraffe gets taller, isn’t this force
>lessened because ones feet (point of force) goes up the shaft? Therefore
>as the seat comes out front, it doesn’t move as far in front of that
>“original gravity line”. On a 30 foot Giraffe, a man holding the seat
>out front would only move the new gravity line an inch or so forward.
>And the shaft angle hardly changes.
>
>I think therefore that this phenomenon is the worst on the 5 foot
>giraffe.

Good point, you might be right. Giraffe designs exist in which the
pedal axle is cut in two halves and located below the top of the tyre.
It could technically be just a couple of millimeters above the axle of
the wheel, in which case the effect in question would be very small.
On a 30 foot giraffe I doubt if anyone would want to experiment with
seat out though!

Hey, now I think of it, the pedal axle could also be below the wheel
axle. It would give the unicycle a self-uprighting effect if you would
stand on the pedals! (Use only short cranks, though.)

Klaas Bil

Today (30 December) is the last day this year that is not the last day this year.
0

14

Re: Giraffe and physics

On Mon, 30 Dec 2002 02:10:10 -0600, Mikefule
<Mikefule.ggt7m@timelimit.unicyclist.com> wrote:

>(I think it is (1 - sin 60) X the height of the uni, but I’m very rusty
>on maths with sines and cosines.)
Your goniometry knowledge is still shiny.

(rest of correct story snipped)

>Incidentally, it doesn’t matter how much weight is on the seat and how
>much is on the pedals. If the rider’s position is secure so that
>his/her body mass is held in the right position relative to the seat,
>then the effect will be exactly the same whether his/her weight is
>distributed 50/50 between pedals and seat, 60/40, 40/60, or any other
>split.
With all due respect, but this is where you miss the point. More
exactly, it’s your assumption that “the rider’s position is secure so
that his/her body mass is held in the right position relative to the
seat”. This assumption is in conflict with MM’s very problem: “the
seat will shoot out from under [me]”. I think the reason is that when
the rider stands on the pedals of the giraffe, he will tend to stay
upright while the uni falls. Sitting on the seat does secure the
rider’s position.

Klaas Bil

Today (30 December) is the last day this year that is not the last day this year.
0