Proficient Motorcycling. David L. Hough
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Название: Proficient Motorcycling
Автор: David L. Hough
Издательство: Ingram
Жанр: Сделай Сам
isbn: 9781935484677
isbn:
Mass Shift
While you are straddling your bike, you might also notice that when you turn the handlebars, the steering head also moves sideways (laterally). If you turn the bars to the left, the steering head (and the whole front of the bike) shifts laterally to the left of the contact patch. That means that when the front wheel is turned away from center, the weight on the front end is shifted toward one side. For instance, with the front wheel pointed toward the right, you’ll feel gravity pulling the bike over toward the right. In other words, steering from side to side can actually help balance the bike even if the wheels are not rolling. Of course, as the motorcycle begins to roll ahead, it reacts differently than it does when standing still. With the bike rolling down the street, everything moves around in a complex dance.
Because of trail, the steering head moves sideways when the front end is steered.
Contact Patch Location
If you were to ride your machine through a puddle of white paint, you’d see a painted stripe all the way around the tread, maybe one or two inches wide. But even though we could see that this contact stripe is a big ring around the tire tread, it’s a lot easier to discuss front-end geometry if we agree to think of it as the small contact patch (CP) where the tire touches the road at any particular moment. Be aware that the location of the CP can shift forward and back as well as sideways.
Trail can decrease or increase as the front wheel rolls over a bump.
Consider what happens when the wheel rolls over a bump. As the tire first contacts the bump, the CP instantaneously shifts forward and then follows the bump backward until the tire rolls onto level ground again. If the bump is steep enough (a curb, for instance), the CP can momentarily jump ahead of the steering axis. That’s why a steep bump or dip yanks the handlebars around (and why riding no hands over bumps and grooves isn’t a clever idea).
Now, lean the bike over on the sidestand, with the front wheel pointed straight ahead. Get down on your hands and knees, and look back toward the front tire. Observe that the CP is no longer in line with the bike centerline. When you lean the bike over into a curve, the CP shifts laterally toward the direction of lean. That causes tire drag to steer the front wheel more toward the curve.
Tire Cross Section (Profile)
With a narrow, round-profile tire, the CP doesn’t move far to the side. But with a wide, low-profile tire, the CP shifts farther sideways than with a round-profile tire, for the same lean angle. And the farther out the CP, the greater its off-center drag. The CPs of both front and rear tires shift laterally as the bike leans over, so the sizes and profiles of front and rear tires are interrelated. That’s one reason changing tires to different profiles, or changing just one tire to a different size or profile, can change how the bike handles, for better or worse. It’s very fashionable these days for cruisers to wear extremely wide rear tires, but handling usually suffers when function takes a backseat to style.
When a wheel is leaned into a curve, the contact patch on the tire shifts toward the edge of the tread. CP shift is greater with a wide, low-profile tire than with a narrower tire.
Self-Balancing
With carefully selected rake, trail, and tire profiles, a machine can have good self-balancing dynamics, whether moving upright and straight ahead or leaned over into a curve and turning, and whether at fast or slow speeds. The point I don’t want you to miss is that the front-end geometry is designed to balance itself. If the bike leans over to the right, the CP shifts farther right, steering the front wheel more toward the right. As the wheel tracks away from center, that rolls the bike back toward vertical. When the machine returns to vertical, the gravity, steering head position, and CP all balance again.
When the motorcycle begins to fall over, the location of the front tire contact ring moves slightly, increasing drag on that side of the tire and countersteering the front wheel back under the center of mass.
As the front wheel steers itself back under the mass, the bike is rolled upright and balanced again but on a slightly different path.
If you watch a motorcycle cruising down the superslab (freeway), you’d swear it follows an absolutely perfect straight line. But if you could measure accurately, you’d discover that it rolls ever so slightly from one side to the other as it balances itself, sort of like a clock pendulum. This self-balancing act is more obvious at slower speeds because the front tire requires greater steering input at slower speeds than at higher speeds to get the same effect.
If you were to ride your bike slowly through a puddle of white paint and then go back and look at the tire tracks, you would observe that the front tire sometimes tracks to the left and sometimes to the right. In other words, the front tire rolls along in a snakelike track as the bike continuously rebalances itself.
Gyroscopic and Inertial Stability
Two big contributors to straight-ahead stability are the inertial effect of the motorcycle /rider mass and the gyroscopic forces generated by the spinning wheels. Perhaps the best way to think of inertia is that objects “want” to keep on doing whatever they are doing. Kick a brick sitting on the ground, and you’ll discover that it doesn’t want to move. Throw the brick, and it’s obvious it wants to keep moving, at the same speed and in the same direction. The popular unscientific term for property of matter is momentum. The correct name for this is inertia, but if we start to add vectors and forces, we’d need to start calling it kinetic energy. To avoid a war over definitions, I’ll just call it forward energy.
Even if the front wheel tracks off on a tangent, forward energy attempts to pull the mass of bike and rider back into a straight line again.
A motorcycle, once up to speed, wants to keep rolling along, straight ahead, at the same speed. Forward energy contributes to straight-ahead stability by pulling the motorcycle’s mass back toward center and by providing a resistance against which the tires can react. For example, if the motorcycle starts to drift away from center, forward energy attempts to pull it back into a straight line again.
The wheels of a motorcycle also contribute to stability but in a different way. Spinning wheels generate gyroscopic energy that resists changes in position. A spinning gyroscope wants to stay spinning at the same angle. The gyroscopic effect of the wheels helps keep the motorcycle from making any sudden changes in direction.
There are four main forces acting on a motorcycle to СКАЧАТЬ