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Author Topic: Basic dimensioning  (Read 15942 times)

Offline Dansercoer

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Basic dimensioning
« on: 21/03/2009 15:08:12 »

Dear all,

Above is a proposal for a human wheel I would like to make, the inner cylinder would be perforated, similar to newbielink:http://boards.core77.com/viewtopic.php?t=18087 [nonactive] . The difference with a hamster wheel is that it wouldn’t be stationary and is able to take ( large ) bends. The ribs seem rounded, but that’s only an optical illusion. The intention of the wheel is to give an alternative to the walking paths ( paved green space to keep your shoes clean ) in parks, a new experience with more freedom.

There will be a first estimation of the thickness of the materials (the rings shouldn't be able to amputate a hand, but shouldn't destroy too much grass either), the amount of crossbars and the diameter of the outer rings (wider than the palm of a big hand) using the Cosmos-software. The structure as drawn above would weigh ±80kg in TIG welded aluminium AW-5083 with anti-slip paint, maybe fibreboard is an option too.
But I was wondering how to define the:
- width of the cylinder a
- diameter of the two rings in the middle b

to make sure:
1) the wheel is not too heavy to make it start/stop rolling
2) it is not too difficult to make the wheel lean to one side
3) the wheel is unlikely to fall
( 4)the bends it can take are not too large )


1)

The two forces that I think decide whether the wheel is going to roll or not - rolling resistance and gravity on the person - seem to work in different directions?


2)

The wheel will lean to one side when the combined centre of mass of the wheel and the person in it is not positioned above the resting surface.
(mw.xw - mp.xp) / (mw + mp) = 0
This can be reduced to a function of a and b.


3)

The wheel will not fall when the combined centre of mass is positioned above the resting surface.
But what is the influence of inertia - as a result of the leaning movement - on the position of the combined centre of mass?

All of these could result in a graph with a and b as axes from which I could deduct their ideal value.
Parks are not perfectly flat and there might be wind, so I will have to build in safety margins.
The wheel is not intended for hilly parks with high vegetation that limits sight though.

This product designer is thanking you in advance!
« Last Edit: 29/03/2009 03:51:18 by Dansercoer »


 

Offline RD

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Re: Basic dimensioning
« Reply #1 on: 21/03/2009 17:49:19 »
I think this idea is a non-starter: it is impractical and unsafe, sorry.

If this idea is ever realised my suggestion would be take out heavy-duty public liability insurance , you're gonna need it.





I would suggest copying a hamster ball, but it's been done.
 

Offline Bored chemist

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Re: Basic dimensioning
« Reply #2 on: 21/03/2009 17:53:41 »
RD, you seem to have just proved that the bicycle won't work.

As long as the ribs on the outside of the wheel are not narrower than bike tyres I don't see a problem with this idea.
 

Offline RD

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Re: Basic dimensioning
« Reply #3 on: 21/03/2009 18:35:45 »
Impractical:-

Will sink into soft ground , (for which it is intended).
 When this occurs changing the direction the wheel is rolling in will be impossible.

Will not protect occupant from ground water & mud, (for which it was designed),
which will enter through metal mesh, (wet/muddy metal is gonna be slippery underfoot).

With dimension “a”  at apparently ~1m this 2.2m diameter aluminium wheel will easily tip over.
(assume worse case scenario when designing with any item for use  by members of public:
some users will deliberately try to tip it over).

Wheel will roll down slopes, (duh!) .
Once it has built up speed, angular momentum is going to make it difficult to stop.


Unsafe:-

The appendages of  occupants and nearby onlookers will be crushed / amputated by the sharp wheel rim.
 More injuries will occur when the wheel is accidentally or deliberately tipped over.

Fingers could get trapped in coarse metal mesh (see your pic below) and injured as wheel rotates.

« Last Edit: 21/03/2009 20:19:27 by RD »
 

Offline LeeE

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Re: Basic dimensioning
« Reply #4 on: 21/03/2009 18:40:40 »
Any wind greater than a brisk breeze could be a problem, from any direction.  Head and tail winds will make it hard to keep going or stop, respectively, and side winds will tend to turn it away from the heading you want or even start it spinning.
 

Offline RD

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Re: Basic dimensioning
« Reply #5 on: 21/03/2009 19:25:30 »
This side view is a better illustration of finger / hand hazard...




Wheely not a good idea 
« Last Edit: 21/03/2009 20:20:39 by RD »
 

Offline RD

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Re: Basic dimensioning
« Reply #6 on: 21/03/2009 23:01:29 »
I knew I'd seen this concept before ...
feature=related

Mono-cycle
feature=player_embedded

Problems when trying to stop (mono-motorbike)...
feature=player_embedded
« Last Edit: 21/03/2009 23:25:28 by RD »
 

lyner

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Re: Basic dimensioning
« Reply #7 on: 22/03/2009 20:19:10 »
The guy in the picture is using a very bright cycle lamp.

btw, have you never seen people Sorbing?
 

Offline RD

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Re: Basic dimensioning
« Reply #8 on: 22/03/2009 21:34:36 »
btw, have you never seen people Sorbing?

Like Liza it's with a "zee" ... http://www.zorb.com/main.html

They throw a bucket of water* in the Zorb with the passengers: it must be like being in a washing machine.

[* Maybe this preemptive soaking is to make washing the vomit out easier]
« Last Edit: 22/03/2009 21:50:21 by RD »
 

Offline Bored chemist

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Re: Basic dimensioning
« Reply #9 on: 22/03/2009 21:59:06 »
btw, have you never seen people Sorbing?

Like Liza it's with a "zee" ... http://www.zorb.com/main.html

They throw a bucket of water* in the Zorb with the passengers: it must be like being in a washing machine.

[* Maybe this preemptive soaking is to make washing the vomit out easier]
I suspect it's because some of the adverts feature young women in T-shirts.
However, since the videos show that essentially the same ide works I can't see why it's impossible. Daft perhaps, but perfectly possible.
 

Offline Dansercoer

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Re: Basic dimensioning
« Reply #10 on: 23/03/2009 03:19:07 »
When the outer rings are sufficiently wider than the palm of a big hand - which will be the case -  it will be very hard to grab them the way you would grab a line. The wheel is able to change direction because it behaves like a chopped cone, not like a bicycle where you have to force a wheel to change direction. This same bicycle analogy proves that it won’t sink that much into the ground like bored chemist said; the wheel is heavier and the rings are thinner, but the contact surface is bigger because of the diameter and the fact that most of the time there is more than one ring touching the ground.
Of course the tracks are deeper than footprints, but I have the impression that they are not as harmful to the lawn as a shortcut worn away by pedestrians, just think about lawn aerators. Small rocks shouldn’t be a problem either, the diameter and width of the wheel are too big to feel much of an impact, large rocks can be seen.

I got some good advice though:
I’ll keep the perforations in the metal mesh smaller than the smallest finger without making it non-transparent.

I’ll put the project aside for a little while to think about the worst case scenario, maybe I’m getting too attached to it as well.  ;)
You’re right, people could deliberately tip over the wheel, there might be a storm (putting the wheel on its side helps) and the angular momentum makes that the braking distance is relatively long (it won’t be anything like the monowheel crash though, this wheel is much wider).
 

Offline RD

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Re: Basic dimensioning
« Reply #11 on: 23/03/2009 09:23:17 »
Just noticed yet another problem: muddy water on the wheel will rain down upon the occupant...



feature=related

If you made the wheel double skinned: an inner and outer mesh separated say 30cm, the wheel would be less likely to sink into soft ground and at 30cm impossible to grip, (the outer mesh could be more open than the inner one to maximise visibility). However producing a curved outer mesh would be difficult to manufacture and increase the weight and particularly the angular momentum of the wheel, making it very difficult to start and stop. 

Even ignoring safety issues, this human hamster-wheel idea is a non-starter, sorry.
« Last Edit: 23/03/2009 09:32:31 by RD »
 

Offline RD

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Re: Basic dimensioning
« Reply #12 on: 23/03/2009 10:10:23 »
Even with a 20cm rim it is still possible to run over ones own forearm / hand / digits ...
 

Offline Dansercoer

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Re: Basic dimensioning
« Reply #13 on: 23/03/2009 18:08:01 »
It’s like putting a hand between the spokes of a bicycle, but yes it can happen, so I’ll add it to the worst case scenario list.
I don’t agree with the mud though, nothing dripped off when I did a little simulation with a thick knife aka ring.
Dewed grass won’t touch the mesh either as it is too short, which is different in the video.
 

Offline RD

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Re: Basic dimensioning
« Reply #14 on: 23/03/2009 20:22:26 »
I don’t agree with the mud though, nothing dripped off when I did a little simulation with a thick knife aka ring.

The purpose of the wheel is to prevent feet getting wet, now you're saying the wheel itself will not become wet (?)
The amount of contact between the wheel and the ground & grass will be greater than footprints, (wheel is rolling not stepping),
 i.e. the wheel will gather more water than shoes would, some of which will rain down on the occupant.


Quote
Dewed grass won’t touch the mesh either as it is too short,



Here's an idea: try going for a walk on park land after rain, wearing ice skates, to see how deep you sink in.
« Last Edit: 23/03/2009 21:32:22 by RD »
 

Offline Dansercoer

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Re: Basic dimensioning
« Reply #15 on: 23/03/2009 23:35:47 »
Let’s say there’s one footstep of 200cm2 every 75 cm.
200cm2 / 75 cm = 2.7 cm (width for two tracks)
This is a lot more than we need.
No need to make the calculation for the ice skates.

Also, aluminum doesn’t absorb water or mud the way most shoes do, just take your ice skates and make a rotating movement in a puddle. I said the mesh won’t get wet (unless it rains on it), of course the supporting rings will be wet where they touch the ground.
« Last Edit: 23/03/2009 23:42:27 by Dansercoer »
 

Offline Dansercoer

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Re: Basic dimensioning
« Reply #16 on: 24/03/2009 01:27:10 »
I see RD's response is gone, but anyway, here’s my reply;
Let’s say your (rectangular?) feet sink 1cm.
The width of the smallest ring should be minimum 13cm - the length of a large hand palm - for the digits not to get amputated.
So for the wheel to sink until the mesh you will need a supporting length of 1200 cm2 / (13 x 1,2cm) = 77cm
The wheel would only have to sink 6cm into the ground to get this length as the outer diameter of the small rings is 220cm + 2 x 13cm = 246cm
-> This leaves 7cm until the mesh.
I deliberately took 13cm rather than the bigger central rings, and 1,2cm rather that the fat 2,7cm, both of these are in “your” favour.
Top soil is normally softer too.

Having said this; I still didn’t get any further with my initial questions (in red), could somebody point me in the right direction?
If not, a recommendation for easy to use simulation software might help too.
I’m not ignoring the tips, but making those calculations won’t hurt.
« Last Edit: 24/03/2009 01:29:03 by Dansercoer »
 

Offline Bored chemist

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Re: Basic dimensioning
« Reply #17 on: 24/03/2009 20:05:36 »
People can walk on their hands. The hand is therefore perfectly able to stand the weight of the body. As long as the wheel rim is wide enough to spread the load across most of your hand then, while it might hurt a bit and may even bruise, there wouldn't be any lasting damage. OK, to allow for the weight of the wheel itself you should be careful to err on the side of caution but I still think it's possible.

The talk of ice skates is just silly. Nobody was sugesting an "edge" that thin.

I don't see how anyone can say "Even ignoring safety issues, this human hamster-wheel idea is a non-starter, sorry." when there are videos on YT of essentially the same thing.
 

lyner

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Re: Basic dimensioning
« Reply #18 on: 25/03/2009 00:00:15 »
Your hands don't need to be outside the periphery of the wheel - they can be on a handle inside, protected.
But, far from protecting you from the wet, the wheel will rotate and lift water from the ground to above your head, dumping it all over you - plus other, not such pleasant things. There is no way it could be going fast enough to shed stuff away from the passenger.
 

Offline Raghavendra

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Re: Basic dimensioning
« Reply #19 on: 26/03/2009 09:13:17 »
 I should re-open my old books and study again...
 

Offline Karsten

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Re: Basic dimensioning
« Reply #20 on: 26/03/2009 20:18:39 »
 What is the purpose of this contraption? Why not walk?
 

Offline Dansercoer

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Basic dimensioning
« Reply #21 on: 29/03/2009 03:54:09 »
Somebody told me my rolling resistance should be orientated in the opposite direction acting from the bottom of the wheel in order not to contribute to the rotation of the wheel. I find this very confusing;
newbielink:http://webphysics.davidson.edu/faculty/dmb/PY430/Friction/rolling.html [nonactive]
-> The first diagram agrees with the above because the wheel is not rolling yet.
But down the bottom of the page there’s “distribution of the normal forces creates a net torque negating the rotational contribution of the friction” ?
newbielink:http://cnx.org/content/m14385/latest/ [nonactive]
-> Here the formulas are completely different (no rolling resistance coefficient) and there’s no deformation.
 

Offline Raghavendra

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Basic dimensioning
« Reply #22 on: 29/03/2009 10:17:48 »
Resistance for what?
 

lyner

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Basic dimensioning
« Reply #23 on: 29/03/2009 13:34:10 »
Somebody told me my rolling resistance should be orientated in the opposite direction acting from the bottom of the wheel in order not to contribute to the rotation of the wheel. I find this very confusing;
http://webphysics.davidson.edu/faculty/dmb/PY430/Friction/rolling.html
-> The first diagram agrees with the above because the wheel is not rolling yet.
But down the bottom of the page there’s “distribution of the normal forces creates a net torque negating the rotational contribution of the friction” ?
http://cnx.org/content/m14385/latest/
-> Here the formulas are completely different (no rolling resistance coefficient) and there’s no deformation.


I think the confusion may arise because of Newton's Third Law and the need to choose the appropriate "action" or "reaction" force.
Also, for a vehicle to be driven forward, there is the necessity  of having a frame or chassis to allow for a torque to be applied. (It seems to be implied rather than explicit in the paper).  This is usually achieved by using another contact with the ground (the other axle) or, in the case of a unicycle - a forward tilt of the rider on the seat.
 

Offline Dansercoer

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Basic dimensioning
« Reply #24 on: 30/03/2009 03:35:27 »
I think the confusion may arise because of Newton's Third Law and the need to choose the appropriate "action" or "reaction" force.

How is the rolling resistance force orientated according to you, and what would be its origin?
In Physicsforums somebody told me the following: “I drew the rolling resistance force vector vertically upward, at the bottom of the wheel, at a horizontal distance b in front of the wheel centre ground contact point. Rolling resistance coefficient b would generally be much less than r.

I hope I can find this rolling resistance coefficient in the first place...
 

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Basic dimensioning
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