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

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

Just saying... ;)

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

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.

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

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.

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.

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

Thus, creating a righting effect.

:hide:

They are seriously stable gyroscopes.

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

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

:shrug:

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.

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.

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.

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.

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

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

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.

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.

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.

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.

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.

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

those toys would travel in a straight line indefinitely?

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.

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?

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.

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.

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.

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

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.

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.

That's what this entire argument has been about.

Unless it is somehow disturbed by external forces?

Because you have specifically stated that would not happen.

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.

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.

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

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.

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

The physics are well-established.

Raked triple trees were famously deadly for this..

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?

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.

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.

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.

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

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

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..

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.

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

;)

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

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

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?

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.