Quoted message said:Pb? Walther said:Not at all. Gyroscopic forces give bikes increased stability at higher speeds. When you are going
really slowly, it is rather easy to fall over, at high speeds you have quite a bit of stability.
Oh? Could you please explain how that occurs. You may have noticed that roller-blades are more
stable at higher speeds exactly like ice skates. These are not Segways and do not use gyroscopic
forces for stability. With increased speed stability improves from more rapid response to steering
corrections.
The wheels on a bicycle ARE gyroscopes, and stabilize the bike
EXACTLY the way that any gyro will stabilize anything it's attached to.
A Segway is inherently stable from side to side because it has two wheels on the same horizontal
axle, with the center of mass (including the rider) located between the wheels. It therefore
uses a gyro with a vertical axle, which resists front-to-back tipping. A bicycle is inherently
stable in the fore and aft directions, because that's where the wheels are, and because the
center of mass is also between the wheels. A bicycle needs to be protected from side-to-side
tipping; and that protection (when the bike is moving fast enough) comes from the horizontal
axles of the gyro-wheels. And, as with all gyroscopes, the faster the wheels rotate, the more
stable they become. The Segway is different only in the fact that it has an ADDITIONAL gyroscope
which is always moving at high speeds. A bicycle's gyros roll on the ground, so gyroscopic
stability is directly proportional to speed of travel. A slow moving bicycle becomes unstable in
exactly the way a Segway would if it's internal gyro were to slow down.
The extent to which a gyroscope will produce stability depends on it's speed of rotation, and
also on its "moment of inertia". Moment of inertia is what physicists call the combination of a
spinning object's mass and the distance of its mass from the center of rotation. Basically, if
two different size wheels had exactly the same mass, and were both rotating at the same RPM, the
larger wheel would be a more effective gyro. Or, if the two wheels were the same diameter, and
were moving at same RPM, the heavier wheel would be the better gyro. This means that very light
bike wheels need to spin faster than heavy wheels, in order to create the same gyroscopic
stability.
More rapid response to steering at high speeds is not an improvement in stability. It's a mixed
blessing with two kinds of effects. There is, of course, some minimum speed at which a steered
vehicle doesn't operate very well. On boats, which need water flowing across their rudders, this
is called "steerageway." On a car, obviously, you can turn the steering wheel all you want, and
nothing will happen if the car isn't rolling. Keeping a bicycle upright when it's speed is too
small to create gyroscopic stability requires that the front wheel be turned in the direction
the bike wants to fall. If the wheel is rolling, this causes the wheels to move under the center
of mass, and to recover (momentarily) verical balance. If the wheel is rolling very slowly, of
course, then it will be slower to get back under the rider. This means that either the change in
wheel direction must be more dramatic, or that the rider exercise more care to balance on his
own, so that his/her center of mass doesn't get very far from the line between points where the
wheels touch the ground.
High speed, however, makes every little change in steering wheel direction, from any cause, have
a more dramatic effect on the vehicle's direction of travel. To steer well and stably at high
speed requires smaller, more subtle steering corrections than at low speed. It will also require
more rapid and accurate response from the driver/rider to correct for changes in steering wheel
direction that are cause by bumps, gravel, slopes in road surfaces, etc.
When steering action is considered at both extremes of the speed range, it becomes clear that
very fast isn't any better than very slow; and that the best speed (for steering purposes
only) occurs at some moderate speed that's appropriate for an individual rider's sense of
balance, reaction time, and skill with small adjustments on the handlebar. Very high speeds,
without gyroscopic stabilization, are as much an enemy to effortless, carefree steering, as
very low speeds.
What makes a bike more stable AND easier to steer at high speeds is the fact that the
gyroscopic action of the wheels provides increased resistance to tipping, so steering
corrections for that purpose become smaller and less frequent. At the same time, the
fast-spinning front wheel/gyro resists motion of any kind other than parallel to its axle, and
therefore doesn't get bumped off course by road hazzards as easily as it would at lower speeds.
It also requires the rider to exert more force on the handlebar before the wheel will change
direction, which has the nice and convenient effect of dampening and minimizing the extent to
which the front wheel CAN be turned, except when the rider makes a conscious effort to do so.
To test this, of course, it's only necessary to sit up for a moment, and take your hands off
the handlebar. Riding hands-free is tough to do at low speeds because there's nothing to keep
the front wheel from wandering anywhere it wants to. The rider must continually shift his/her
weight left and right, keeping the center of mass over the wandering wheels at all times. At
higher speeds, however, the front wheel's gyroscopic action prevents it from wandering around,
and keeps it rolling in a nice straight line. In this case, the rider must NOT shift weight, or
the center of mass would move away from the center of the wheel line, and the front wheel would
resist changing direction to correct.
With regard to ice skates and roller blades: I'm not aware that these are more stable at high
speeds. In fact, that seems very unlikely. If there is any truth to this idea, then it has
nothing to do with a real increase in the stability of the skates or blades, but probably with
the total momentum of the skater, and the increased inertial tendency to overcome the effects of
small obstacles like pebles on a path, or divots in an ice surface. (The wheels on rollerblades
are gyroscopes, too; but their diameters and masses are so small that they'd need to move at
insane speeds before the effect could matter.) There is, like a bicycle, a quickened ability at
high speeds for the skates to move back under the center of a skater's body when correction for
balance is needed; but the problem of corrections needing to be more precise and subtle also
applies. The result is the same mixed blessing described for bikes, above.
What DOES make these devices more stable, in actual practice, is lengthening the blades. With
one skate on each side (left and right) of his/her center of mass, a skater can easily control
lateral stability with leg muscles. Front to back stability, however, is achieved by creating a
longer footprint. That's also why skis are so long.
KG
