Physicists are still not quite sure how the bicycle is self stable..

[Fumesucker]

A riderless bicycle at speed will correct itself to stay upright until the speed drops to a rather low value, two of the most common hypotheses for why this might be, gyroscopic forces and caster, have been shown not to be necessary.

This is a PDF file so I can't quote from it but I took a jpeg of the introduction to the paper.

http://bicycle.tudelft.nl/stablebicycle/StableBicyclev34Revised.pdf

[FSogol]

1. Physicists have long denied the existence of invisible scary skeletons.

Just saying... ;)

[ret5hd]

2. i am no physicist, but i don't see how a counter-rotating wheel...

would cancel gyroscopic forces. the two counterrotating wheels are still in the same plane.

[Fumesucker]

3. The forces do indeed counter out, the net angular momentum is zero..

[RC]

8. Take the front wheel off a bicycle and hold it by the axle. Have someone spin it.

Then try to turn it. The gyroscopic effect is very pronounced. It is even felt at slow rates of rotation.
Then spin it the other way. The effect is the same.

[ret5hd]

10. i guess my question is that if i had a wheel (which we both know...

would act as you described) and attached a counter-rotating wheel to it and spun them, I would feel NO gyroscopic force?

[Fumesucker]

13. Assuming the wheels are identical and turning at the same speed in opposite directions..

Then the forces from each wheel counter the forces from the other.

Gyroscopic forces are dependent on angular momentum, the net angular momentum in that scenario is zero.

[WingDinger]

30. I dont believe it. Even the contrarotating wheel adds more gyro effect.

The bike as it leans, the contact patch goes off center. This too is a stablizing effect. Pulling the wheel to the direction of lean. Thus, stabilizing it.

[Fumesucker]

34. Whether you believe it or not it's true..

http://www.gyroscopes.org/how.asp

Sometimes precession is unwanted so two counter rotating gyros on the same axis are used.

[WingDinger]

39. While it may not be gyro pro, it still resists twisting.

[wtmusic]

40. It's completely anti-intuitive, but Fumesucker is correct.

Two counter-rotating wheels will behave the same as one stationary mass.

[WingDinger]

53. Still doesnt address the moving of the contact patch to the side it leans.

Thus, creating a righting effect.

[Tuesday Afternoon]

4. its magic

:hide:

[Angry Dragon]

5. +1000

[Fumesucker]

14. "Any sufficiently analyzed magic is indistinguishable from science".. -A Heterodyne..

[tridim]

6. Ever made a Top out of a bicycle wheel?

They are seriously stable gyroscopes.

A single rolling wheel is self-stabilizing too, not surprised that it works with two wheels.

[Fumesucker]

9. I guess you missed the part where the gyroscopic forces aren't necessary?

In my life I've found a lot of things that were intuitively obvious and turned out to be dead wrong..

To turn right on a bicycle, which way do you turn the handlebars?

http://en.wikipedia.org/wiki/Countersteering

[Kurmudgeon]

7. Science is great stuff, but they sure do change there minds a lot, back and forth many times.

[Fumesucker]

11. That's what it takes to get things correct..

:shrug:

[ret5hd]

12. Yeah! They could fix all that if they'd just write a science bible!

[frazzled]

15. Stable on roads with gouges, bumps, and potholes?

I realize this is a physics experiment, and it's interesting. But in the real world, even real bicycles with real and experienced riders don't always remain stable. Believed me, I've nursed enough fractured shoulders, collarbones, and hips, broken ribs, punctured lungs, and godawful road burn among my (crazy bicycling) family members back to health that when I see the words "bicycle" and "stable" in the same sentence I have to laugh.

[Fumesucker]

16. I've been on two wheels ever since I can remember..

And I'm sixtysomething now..

I'm aware of the phenomenon of which you speak from personal experience but that wasn't the point of my OP.

[caraher]

55. What they mean by stable is not that you can't crash

What they mean is something more like this: stand up a stationary bike and let go. It will fall over almost immediately.

Now set it rolling. It also falls over - eventually - but only after a much longer period of time. That is the stability they seek to explain.

[TheMadMonk]

64. "All else being equal" scenario. ie. somewhat idealised.

And if you take the right viewpoint, bikes do seem preternaturally stable. A bike in motion CAN be kept on a line with absolutely miniscule forces.

External forces, generally beyond the control of the rider, are usually what brings a cyclist low. Noticed in time, and given oportunity for control it's easy enough to ride through the hazards you mention without more than curses and dented rims.

If you're the cyclist you say you are, you know that the single biggest barrier to remaining upright is directly impedimentary forces: mud, loose surfaces, steep hills, etc.

[wtmusic]

17. It should be obvious that gyroscopic effects would be tiny

compared to what forces would be necessary to keep a bike and rider upright. Bikes stay upright because the rider can change the procession of the momentum of the bike by turning the handlebars to counteract any lateral pull of gravity. At faster speeds it's easy, but skilled cyclists can do it almost at a standstill.

http://www.youtube.com/watch?v=Q0O8ZzFuMkQ

[Fumesucker]

19. Nope, at reasonable speed a bicycle will go in a straight line just fine without a rider..

Steering input is only necessary to initiate a turn..

These motorcycles don't even have riders..

http://www.youtube.com/watch?v=jM27fujbiXs&feature=player_embedded

[wtmusic]

21. No, it won't.

Depending on the angle of the steer tube bicycles have a self-righting effect that results from the front wheel falling over in the direction a bicycle is leaning. The procession from the momentum of the bike then shifts to the opposite side, correcting the lean.

Weld the fork to the frame and a bike won't stand up for more than three seconds, even if it's going 100 mph.

[Fumesucker]

22. I ride no hands on my motorcycle on the interstate from time time..

When I'm changing out my glasses specifically. I have to force the bike from vertical by leaning my body weight and even then only gentle course corrections are possible.

[wtmusic]

24. I believe you.

The motorcycle may be able to continue for a while that way, because of the self-righting effect. But were the fork welded to the frame you'd fall over in a matter of seconds.

[Fumesucker]

25. So?

Only a moron would ride a bicycle or a motorcycle with the fork welded to the frame.

What does that have to do with this discussion?

The motorcycle or bicycle will continue in a straight line until it encounters an obstacle or loses velocity to the point it can no longer maintain stability.

[wtmusic]

26. That proves that steering is necessary

whether it happens as the result of a physical process or a rider moving the handlebars.

And a riderless bike will not "continue on a straight line" without a rider to guide it.

[Fumesucker]

28. I said "steering input" which is from a rider..

I even posted a video of riderless bikes a few post above this one, they go straight unless commanded to turn.

[wtmusic]

29. And do you believe that without any radio control input

those toys would travel in a straight line indefinitely?

[Fumesucker]

33. I don't "believe" that, I know it to be true..

That's the whole point of the OP, bicycles go in a straight line without rider input and it's not clear exactly why they do so.

The reason for this behavior is a subject of serious scientific research.

[wtmusic]

37. A thought experiment.

Take your toy motorcycle to an extremely flat surface, an airport tarmac, for example. Point it at a target and let her rip. How long is it before the motorcycle is veering away from that target - not momentarily, but progressively?

[Fumesucker]

42. Theoretically forever in a perfect world..

Even airport runways aren't perfectly flat or debris free either for that matter (you might recall the Concorde crash was caused by debris on the runway).

But with a perfect machine on a perfectly spherical world, yes it would go straight forever.

[wtmusic]

44. There you go. But perfection is not necessary for stability.

In a perfect world rockets would not need complicated gyroscopes to guide them, and I could balance my pen upright on my desk indefinitely.

Point being - there's nothing, absent the self-righting phenomenon I mentioned earlier, that makes a riderless bicycle different from an unsupported pen standing upright on a desk.

[Fumesucker]

45. How long will an unsupported pen stand up on your desk in the real world?

Because a riderless bicycle or motorcycle will stand up in the real world until it either slows down below self stability speed or encounters an obstacle of some sort.

[wtmusic]

46. Exactly as long as a bike with a fork welded to the frame.

(if it was a tiny bike with the same mass). But these are details.

[Fumesucker]

47. In the real world bikes don't have the forks welded to the frame.

You're trying to make this about something that has nothing to do with real bicycles or motorcycles, what you are describing is an awkward two wheel cart.

[wtmusic]

48. That's the self-righting mechanism at work.

In high school I had a physics teacher who brought in a toy bicycle with the head tube angled inward instead of outward.

Give it a nice gentle push and instantly it flopped over (the wheel "fell" away from the direction of the turn, making the balance even worse). Great demonstration.

[Fumesucker]

49. But you said a bicycle doesn't have a self-righting attribute..

That's what this entire argument has been about.

[wtmusic]

50. #21. nt

[Fumesucker]

51. So then you agree that a bicycle or motorcycle will continue in a straight line..

Unless it is somehow disturbed by external forces?

Because you have specifically stated that would not happen.

[wtmusic]

52. That's one part of the equation

but it would also have to be perfectly balanced to begin with. And it would have to be on a perfectly level surface, both of which are impossible to achieve in practical terms.

Just like balancing a pen - nothing special. Well understood, for centuries.

[wtmusic]

32. Steering input can also result from weight shifts

which is how bicycles (and unicycles) are ridden without hands.

On your motorcycle, whether you're aware of it or not, you're probably leaning slightly to correct your direction when your hands are off the bars. It doesn't take much with weight and speed (aka, momentum) on your side.

[Fumesucker]

36. Yes, the bike goes straight unless I shift my weight..

It actually takes considerable weight shift just to get a gentle course correction, I believe I mentioned that already in one post.

[AndyTiedye]

35. Self-Righting Due to the Angle of the Fork

The fork is at an angle for a reason. That is what makes it stable.

It will be more or less stable, and less or more easy to maneuver, depending on the angle of the fork.

Welding the fork to the frame would defeat the self-stabilizing property.


Those physicists need to talk to someone who builds bicycles.

[wtmusic]

38. Bingo.

This whole paper is goofy - the references are from the 19th century.

The physics are well-established.

[Fumesucker]

43. It's quite possible to drastically alter the stability of a bike without changing the head angle..

Raked triple trees were famously deadly for this..

[leveymg]

18. If it's all angular momentum, is a bike with a heavier wheel more stable?

It would seem that in order to change direction quickly, and for the rider's shift in balance to have the most immediate effect you want a smaller diameter, lighter wheel, like on a road racing bike, while directional stability demands a big, honkin' Harley wheel to counteract and keep vertical the typical fat, drunken big honkin' Harley rider. ;-) :beer:

So the imparted stability is actually in both planes - vertical as well as directional? Yes?

[Fumesucker]

20. A bike will try to stay vertical, you really have to force it from the vertical..

That's why it's possible to ride no hands, I do it from time to time on my motorcycle when I'm changing from regular to sunglass or the other way, it takes both hands to do and I have a cruise control, I can make minor course corrections by shifting my weight but it really takes handlebar input to maneuver at all briskly.

[leveymg]

23. Gotya. But, what about the effect of the weight and diameter of the wheel?

[Fumesucker]

27. I imagine it has some effect but steering geometry is more important in my experience..

I've changed front forks on a bicycle before and had it make a major difference in the stability of the bike, one set of forks moved the tire contact patch forward in relation to where the steering axis intersects the ground, the bike was considerably less stable with that one change, downright twitchy in fact.

[leveymg]

31. Right, that's caster angle.

The tiny front wheel they put on choppers is to compensate for all the self-centering force of the extended forks that imparts a lot of caster. A full-size front wheel on a chopper would be very difficult to turn.

[Fumesucker]

41. Nah, it's for looks..

Bobbers are similar to choppers but they have the big front wheel.. I've never heard any complaints about them being hard to turn.

[leveymg]

54. That Bob doesn't have nearly as much rake as a classic chopper

Compare the Bob with these early '70s vintage bikes:



And, take a look at the new wave choppers. They have a low aspect wide radial at the rear with a larger diameter, but still relatively narrow and very lightweight wheel at the front.

http://4.bp.blogspot.com/_NJeah5qWqTA/TKgMWJ5GnnI/AAAAAAAAAGA/TmqqVCkQN3w/s1600/chopper+2.jpg

[Fumesucker]

56. Someone thinks this works well enough to put it into limited production..

Kind of the ultimate bobber if you will.

http://www.leonhardtweb.de/en/gunbus/design.htm



FWIW I used to have a Honda 750 chopped a lot like the foreground one in your pic, I wised up eventually and realized it wasn't the kind of bike I really enjoyed riding, my favorite ever ride was a hot rodded RD350, a smoking, cackling, shrieking handful of acceleration and razor sharp corner carving and my second favorite was an FZR400 which just omitted the smoking part and substituted a rumpa-rump four stroke idle for the two stroke cackling.

I wasn't ever really a chopper kind of biker..

[leveymg]

57. Notice he's not actually riding that monster. Throw a saddle and handlebars on a P-47.

Can you imagine trying to pick that thing up if it got dumped? Maybe if you're a 7-foot tall, 370 pound NFL lineman. Maybe.

[Fumesucker]

61. I've seen regular guys pick up a dressed Gold Wing by themselves..

And people ride Boss Hoss Chevy V8 bikes that aren't so far off weight wise..

[leveymg]

62. How many chrome trees had to die to make that Bad Boy Toy?

;)

Personally, I found this one was more the right size for cutting apexes on the Pacific Coast Highway between Big Sur and Santa Cruz:

[lumberjack_jeff]

63. The issue is trail. Put the forks way out in front of the steering head and you can use a big wheel.

It's certainly possible geometrically to have a lot of rake angle, a big front tire and a modest trail (or caster)

[Igel]

58. Don't know much physics.

Just read about the slope on train wheels, though: The bottoms are angled. When they go around a turn the mass pushes the train slightly higher, producing a smaller wheel radius on the inside curve and a larger wheel radius on the outside curve. Nicely, the outside wheel has to go a greater distance on the outside curve than the inside wheel on the inside curve.

Perhaps it works sort of the same way with bikes.

When you steer a bike you can hear the rubber grating on the road even at speeds that don't allow the bike as a whole to skid. The side of the tire closer to the ground, the inside radius, moves a lesser distance than the part of the tire on the outside radius. Yet the center of gravity is on the inside radius side, providing slightly greater pressure and friction at that part of the tire. The inside curve radius will win when it comes to determining velocity. But the part of the tire on the outside curve will also produce friction with the pavement and provide a torque that will swing the wheel toward the outside of the curve.

In other words, if the wheel turns right, even slightly, the wheel-road contact will produce a force to steer the wheel left. The force should be fairly small, esp. if the deviation from a straight path is slight, so when you're steering you wouldn't notice it.

Possible, O ye physicists?

[bananas]

59. They're not sure how bees fly, either. nt

[trotsky]

65. Old myth.

http://www.straightdope.com/columns/read/1076/is-it-aerodynamically-impossible-for-bumblebees-to-fly

[lumberjack_jeff]

60. Whaa?

A bicycle with a positive caster angle? Anyone who's ever pushed a piece of wheeled furniture around knows that having the contact patch in front of the steering axis is inherently unstable - the wheel immediately steers to bring the contact patch behind.