In the realm of modern human-pedaled and electric-assist tricycles (e.g. for carrying kids, passengers, cargo, etc.) the single wheel at the back configuration is called a "tadpole" and it's common and considered stable. Caveat: "stable" is relative. Example: Bicycles are unstable at rest but stable at speed, compared to any tricycle which is stable at rest but unstable at speed, particularly when turning. But plenty of fun if you're willing to lean. See also: professional motorcycle sidecar racing. :-)
I was wondering about having a trike with both back wheels motorised (or a wheelchair?). Weight at the front does make it easier to fall off though I feel.
Having the front be single wheeled leads to the a lot of instability because turns change your center of gravity. I just bought a recumbent bike and I had to search extensively to find a nice two-wheeler because the 2 front wheel 3-wheelers are quite popular, just like the motorcycle equivalent. Most bikes and motorcycles with three wheels have the same steering linkage you'd find in a car.
At least bicycles are stable. This onewheel thing cannot work without a computer balancing it (+ the rider). If it fails the rider falls on their face. It would be the equivalent of a bike fork breaking.
A bicycle with compliant training wheels, half way between a
bicycle and a tricycle, is uncontrollable
ANDY RUINA, Cornell University,
Mechanical Engineering
We have built and tested a vehicle that can balance and
steer like a bicycle, a tricycle, or anything in between. A bricycle is essentially a
bicycle with springy training wheels. The sti?ness of the training wheel suspension
can be varied from in?nite, when the bricycle is a tricycle, to zero, when it is a bicycle.
One might expect a smooth transition from tricycle to bicycle as the sti?ness is
varied, in terms of handling, balance and feel. But the situation is more complicated.
Rather, the controllability of a bicycle depends on gravity. Without gravity, lean
and direction cannot be controlled independently. Springy training wheels e?ectively
reduce or even negate gravity. Indeed, experiments with the bricycle show problems
when the total e?ective gravity is about zero. People can then still balance easily
but can no longer turn the brike. The theory and experiment show a qualitative
di?erence between bicycles and tricycles. A di?erence that cannot be met halfway.
Tricycle layout (2 wheels in back, 1 in front) cars have always cornered poorly compared to the reverse. I honestly don't know why this is, but it would be a concern for me with this vehicle.
Long before bicycles, people observed that rolling wheels (e.g. coins) are stable above a certain speed, falling over when they slow down.
(This is actually true of bicycles, for a similar reason: like a single wheel, they steer in the direction of a fall, and that counteracts the fall, because if you steer left, you fall right and vice versa.)
The possibility that a two-wheeled vehicle could be stable when moving should have manifested itself to some of the brightest minds in European science as far back as at least the Renaissance.
Bicycles are entirely stable without a rider as long as they’re moving. The human is only really necessary to provide motive power. There are a ton of videos showing this.
It is totally unlike the intentional instability of a modern jet fighter.
Scooter I can kind of see the argument for. But bike not so much. Assuming we’re talking peddle bikes here, then they’re naturally self stabilising, and capable of far heavier breaking than a One Wheel is.
A One Wheel requires quite a lot of active balance from the user, a bike requires some balance, but hard cornering on a bike, or slamming the breaks on hard don’t generally result in unplanned dismounts, even with amateur bike riders. A One Wheel on the other hand, even experienced riders will still end up doing unplanned dismounts when performing rapid unexpected manoeuvres (such when avoiding another vehicle or pothole).
It’s only anecdata, but I’ve personally never had a serious unplanned dismount on a bike (after thousands of hours), and includes while mountain biking. I’ve have had a couple of nasty unplanned dismounts on a One Wheel just riding around a park. I suspect broader stats (assuming anyone’s even collected them) would probably find that my experiences of each mode of transport is pretty representative.
If that failed to be stable, all that would prove is that particular design was unstable, not that there is no design with those features that is stable. This is a problem of finding a general rule, not just a particular design that is stable or unstable.
The issue is that no one has found a way to characterize all possible designs (within certain constraints, such as two wheels each attached to a rigid frame, the frames joined by a hinge) which are self-stable. They know some conditions that are necessary, such as at least one factor linking lean to steering and the design needing a steering force applied to turn in a steady turn. They have not yet characterized what conditions are sufficient for a stable design.
What you want, to say that you fully undertsand how a bicycle works, is a set of conditions which are both necessary and sufficient for a bicycle to be self-stable. If you build a bicycle which meets those conditions, then it will be self-stable (at some speed; certain designs may be self-stable over a wider range of speeds while some may only be self-stable at a narrow range of speeds); if you build a bicycle which does not meet those conditions, it will not be self-stable at any speed.
We've gotten closer over the last century; initially it was believed that gyroscopic force was necessary, but that was disproved. Later it was believed that trail was necessary (or either gyroscopic force or trail was necessary; I haven't read the older paper), but that has now been disproved. We now know a couple of necessary conditions (listed above), but they are somewhat weak necessary conditions, and we don't yet have (as far as I know) a set of sufficient conditions (conditions which, if they hold true, will guarantee that the bicycle will be stable, regardless of other changes to the design), beyond a few known designs which are demonstrably stable.
In fact, if you follow from the paper in Science to the "Supplementary Online Materials" (which is actually the full-length paper; what's published in Science is really an extended abstract), you will see that they prove that "no combination of positive gyroscopic action, positive trail, or positive steer axis tilt are either necessary or sufficient for self-stability over at least a small range of speeds." They construct models of bicycles that lack each of these things but are stable, and have all of these things but are unstable.
this leads me to wonder - has there been a bike design where the rear wheel is allowed to freely "steer" as well as the front wheel? sorta like a crabwalking bike able to side pedal when needed.
Modern automobiles have incorporated rear wheels that turn together with the front wheel (I believe in opposite directions during a turn) in order to decrease the turning circle and improve maneuverability - I'd kill for a bike that has an electro-servo controlled rear wheel that turns to help the biker corner, etc.
I looked up the point you are making. The data backs you up, 3 wheelers suffer from inherent instability on turns. http://www.youtube.com/watch?v=QQh56geU0X8 "When the single wheel is in the front , the vehicle is inherently unstable in a braking turn, as the combined tipping forces at the center of gravity from turning and braking can rapidly extend beyond the triangle formed by the contact patches of the wheels. This type, if not tipped, also has a greater tendency to spin out ("swap ends") when handled roughly." What a pity. I was quite excited about the prospects.
This. However I wonder what it would be like to ride a bike on two rolling-roads so that the front and back wheels moved in opposite directions. Or, indeed, a trike/quad with large coaxial wheel separation and contra-rotating rolling-roads.
Not really, no. Motorcycles are inherently stable due to a combination of gyroscopic stability, wheel geometry and rake geometry.
They are so stable in fact, that at high speeds one needs to use a technique called counter-steering in order to induce a gyroscopic and geometric reaction that sends the motorcycle into the required position. This technique is also used by some bicycle riders, to a smaller extent and also partly subconsciously.
Motorcycles are very stable. In comparison, the steering and suspension dynamics of four wheeled vehicles have to be very finely tuned in order to avoid feedback effects, instabilities, and this tuning has to be balanced with phenomenons such as oversteer and understeer. Overconstraining is also the cost for many issues, as is the consumer demand for bad form factors that lead to loss of control, rolls, and sometimes even death-wobbles.
Tricycles, such as this one, at least aren't overconstrained, but they are a lot less stable as they have neither the inherent reactive stability of bicycles nor the balanced resistance to torque while maintaining stability. This is further compounded by demand for non-optimal geometries, such as the one in the article, that lead to even less stability. This obviously doesn't apply to vehicles such as the Yamaha Nikken.
Own and ride a Brompton, which has 16" wheels. It's perhaps marginally less stable than a full-size bike, but I'm willing to believe that it's mostly down to steering geometry (smaller wheels limit how far behind the intersection of the steering axis and the ground you can put the contact patch) not gyroscopic effects.
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