In article <
[email protected]>,
[email protected]
(Prometheus) wrote:
> Leave the Engineer to decipher it all!
Are you sure?
> The high-speed stability of a bike comes from its traction. At high rotational speeds, the
> majority of a tire's tractive force (actually a frictional force) is involved in merely keeping
> the tire rotating. If you were to try to impart any lateral force(as is done by a turning of the
> handlebars), its effect would be greatly reduced. The same is true in your car. High speed
> cornering requires a much larger turning radius than at low speeds, because you have less
> avaliable(unused) traction. So, yes, a bike is more stable at high speeds, but for the same reason
> a car has less maneuverability at high speeds.
This is, to paraphrase Pauli, not even wrong.
The high-speed stability of a bicycle--assuming the bicycle's geometry is stable--is largely a
matter of its momentum, not its traction. It resists changes in direction in proportion to its mass
and speed.
At high speeds, attempting to corner requires a greater force because you're trying to resist a
greater amount of inertia.
A useful example might be to consider a frictionless object like a spaceship (close enough). If it
is moving, you can get it to change direction by applying a force. The faster it is moving, or the
faster you want it to change direction, or the heavier it is, the more force required to change its
direction, and the less time you get to apply it (F=ma, the force here must be enough to counter the
object's m and get the desired a, which in this case is a handy way of describing how quickly you
want the object to change direction, since acceleration describes rates of changes in vector, not
just speed).
On a bicycle you have to use tire traction to impart these forces. That is, you can't let the tire
traction be overcome by the direction-changing forces you are essentially driving through the tire.
Exceed the tire's ability to resist this force, and you slide. Since the tire's coefficient of
friction doesn't change appreciably at speed, and you don't change your mass appreciably at speed
(assuming a cyclist, and not that spaceship I had earlier), the maximum amount of F you can push
into the system at any moment is constant. The higher acceleration required to counter the greater
inertia at higher velocities essentially means it takes more distance to make the same change of
direction (because you can't make it any faster than you can at slow speeds), and more distance
equals bigger turning circles.
In other words, the faster you go, the more room you need to change direction, unless you can absorb
a greater amount of force at any instant, which you can't on a bicycle.
Ryan Cousineau, BA (English), SFU.
--
Ryan Cousineau,
[email protected] http://www.sfu.ca/~rcousine President, Fabrizio Mazzoleni Fan Club
.
