Bicycle Wheel: Newsletter Challenge (02/28/06)

Spin the wheel & rest the axle in the crook of your finger, I'd guess. The gyroscopic action of the spinning wheel should counter its natural tendency to fall away from you.............until it slows down & you get your finger caught in the spokes, that is ;-)

I think you are right.

Yes, if you spin it fast enough and do it without introducing wobble, in theory, it would work for a short time.

However, those wheels don't have a lot of mass and the rotational friction with the axle would soon slow it down and then you have your finger caught in the spokes! A dumb bet, for sure.

I responded earlier as an "anonymous coward" -forgot to log on! Anyhow, I would simply pass a finger through the spokes and rest the middle of the axle on that finger. (Asuming that you have a long enough and strong enough finger!)

If you spin the wheel it will try too keep its position. This is because of the conservation of angular momentum. Gyroscopes also work on this principle. This is also the reason why we keep going with a bicycle without falling or why a coin doesn't flip over while rolling. That's all conservation of angular momentum. So it is possible to lift it with your finger

I have wondered if it would be possible to not only maintain angular position but actually induce a lift effect by forceably changing the angle. Instead of allowing the gyroscopic action to turn you in one direction you can turn in the other direction and cause the end of the shaft that passes through the wheel to rise. I used to play around with this with a shaft and fly wheel. I never had the time or funds to pursue investigating this phenomenon further. Does anyone know of any research along these lines? Can enough lift be produced to overcome the weight of the mechanical apparatus?

Find the time & the funds! If you succeed, you will have discovered anti-gravity !!!

Because of the symtrical design of the wheel and the fact that the axle is at the center, a force in the axial direction would act as a point load at the center of the axle. By holding the wheel out on your finger tip at arms length and spinning in a circle you should be able to angle the wheel in a bit and find the point of equilibrium where the angular acceleration will hold the wheel against your finger tip

Can't be a88ed to register so I'm an AC. Not very practical though. I've noticed that when I did old the axle on one finger and spun the wheel roud, the wheel did in fact remain spinning in a verical plane but.... it started to rotate in a horizontal plane, causing me to have to move round in a circle to keep the wheel cleany on my finger. Explain that!

I don't remember the details on why, but the gyroscopic effect defers any attempt to tilt the axis by 90 degrees. The axle (and wheel assembly) is trying to fall over (down) because of gravity. The gyroscopic effect defers this motion to a horizontal plane.

The change of rotation in a vertical plane to a horizontal plane can be explained by simple vector dynamics using the standard "right-hand rule" to give the resultant rotational vector. The initial direction of the rotational vector of the vertically rotating wheel is horizontal and through the axle of the wheel. When the wheel is supported on the axle with one finger, a moment is set up about the center of gravity of the wheel. This moment is represented by a second horizontal vector with direction normal to the first vector and through the rim of the wheel. When these two vectors are crossed (ie. a moment is applied to the axle of the rotating wheel), the resultant vector is in the vertical direction according to the right-hand rule (ie. direction of the axle moves from horizontal (or vertical spinning) to vertical (or horizontal spinning)).

It's called precession. There's a torque trying to take the wheel away from vertical (which would happen quickly if it weren't rotating). This is a horizontal vector = wt of wheel x distance from finger to shaft centre. The spinning wheel has angular momentum also in horizontal plane but at right angles to the the above torque. These two vectors combine to give an angular velocity, precession, at right angles to both i.e. about a vertical axis, as you observed. Putting in some figures, 50mm to wheel centre, 600mm dia. and 500 rpm (corresponding to road speed ~ 35mph) precession velocity comes to ~ 1rpm, which sounds about right. Mass of wheel doesn't matter as it cancels. Precession is also seen in toy gyroscopes where a ball on the frame rests in a cup on a small tower.

Your friend is right. It can't be done

"Your friend is right - it can't be done"
Well..., that all depends on what is NOT said:

It CAN be done, IF ... you would be allowed to rotate (slowly) around your own axis. It CANNOT be done if you are to stay completely immobile.

I used to do this several times a day as a part of my work in a tech&science museum (NEMO Amsterdam F.Y.I., I can recommend!). It's basically a trick: by turning the wheel fast enough, and by trying to cover up my own rotation, I was able to give the impression of me standing still. But if I would really stand still, the wheel would eventually fall (slowly and half-sideways of course, because of the gyroscopic forces)

The same phenomenon can be felt more clearly if you were to hold the same spinning wheel with one hand on one side of the axle, and then try to tilt the wheel towards your body... you will feel what happens. The wheel will force itself to rotate partly sideways instead of straight towards you and you will not be able to force your will onto the wheel ... these are the forces at work, my friend ! MWOUAH HAH AH AH AAAHHHHHH !!!

Seeing&feeling this work in live action makes it a lot easier to understand, but in fact I think this must be one of the most difficult phenomena to explain scientifically!! Every engineer can mumble something about gyroscopics and vectors, but who can actually explain it in laymens terms, in detail, scientifically??? Let yourself be known and fame will be your reward.

Signed,
Bart

[Anonymous Coward] == [Lazy Bastard] == [Typical Engineer]

"You know, I'll bet I can hold that wheel up off the ground vertically - normal upright orientation - with just one finger on the axle. No tools, no ropes, just one finger." He says, "You're on." Who wins the bet?

Of course you can do it by spinning the wheel and using the gyroscopic effect, but here is another, although cheesier, method:

Take the wheel over to the nearest stable, flat vertical surface, preferably one with a very high co-efficient of friction, like a concrete or brick wall. Hold the wheel upright with one hand and push the axle straight into the wall, while supporting the other end of the axle with your finger. Unless you are a 90 lb. weakling, you should be able to both support the axle on one end and push it against the wall with enough force to create enough friction to overcome half the weight of the wheel (your finger is supporting the other half).

See, you ARE using only one finger. Hey, no one said anything about walls, only tools and ropes!

As a matter of fact, we did this in the course of my first university physics course (1966). It entailed the use of the aforementioned bicycle wheel and an industrial grade lazy susan. The subject stood on the lazy susan and suspended the spinning bicycle wheel on an extended finger tip. The force of gravity acting on the spinning wheel resulted in a horizontal moment of force which caused the student to rotate (precess, as my learned collegue has said) about the vertical axis. As the question before us specifies that no tools be used, are we allowed the use of the lazy susan? If not, the person holding the bicycle wheel must continually turn to accommodate the precessional motion of the axle. You win the bet, but look rather foolish in the process. PS: If the lazy susan is allowed, get help mounting the turntable. The student fell trying to get on the one in class.

Oh...and of course you wouldn't look too foolish spinning around on a lasy-susan in the middle of the driveway.

Note that looking foolish was not predicated solely on turning oneself around. Presumably either way would look somewhat odd.

If scientists, engineers and other pioneers worried about looking odd, we'll still be living in caves!

Or use an office chair as we did in A-level physics. No H&S issues there

I see no reason you must put your finger on the outer part of the axle. He could easily put his finger between the spokes and at the center of gravity on the axle and simply let it hang. I hope I didn't miss something, as this would be an embarassing 1st posting.

You only missed solving the problem the hard way!

(Well, I guess that part between the spokes is the "hub", so maybe the challenge was well-defined, but it certainly could have been more clearly stated. I wouldn't want to try this one as a bar bet without making that clear.)

Suck the air out to flatten the bottles. Then put the cap back on to keep them flat. You do this when you drink the last of the soda - it saves time.

HUH! Hey Horace! We're talking about spinning wheels here...