On Tue, 17 Jun 2003 17:46:03 -0400, Kirk Gordon <
[email protected]> wrote:
> The wheels on a bicycle ARE gyroscopes, and stabilize the bike
> EXACTLY the way that any gyro will stabilize anything it's attached to.
Indeed. However, the effect is negligible. If it weren't, a bike, once up to speed, would coast
happily without a rider, rather than falling over almost immediately. It doesn't.
> 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.
Why don't you calculate the moment of inertia for a typical bike wheel plus its rotational velocity.
See how big the effect really is.
> 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
Just like driving a car. Whoop-de-doo.
> 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;
You haven't actually tried this much, have you? I can balance my bike perfectly well when moving at
about 20 centimeters per second, aka under 1 km/h, as well as at the highest speeds I've ever been
able to reach safely.
>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.
Christ. At high speeds, with hands off, you STILL balance by moving your weight around, but just
like steering corrections *they are very small and subtle*.
>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
The same goes for bicycle wheels. Do the math.
Jasper
