Читать книгу Proficient Motorcycling - David L. Hough - Страница 49

Leaning can be initiated by a number of different factors, including rider’s body English, steering the handlebars, and even road camber or a crosswind. The most powerful input is steering the handlebars, so we’ll focus on that first.

Experienced riders usually refer to a rider’s steering input as countersteering because the handlebars are steered opposite, or counter, to the intended lean. Push on the right grip to lean right; push on the left grip to lean left. That’s where some of the confusion starts because leaning and cornering is really a process of several steps, whereas countersteering is only the first very brief step in the process. The leaning/cornering process all happens within a couple of seconds, so let’s slow down the action and go through it step by step. We’ll illustrate this from the front and exaggerate the graphic a bit so you can understand what the front end is doing.

The rider initiates the lean by a brief press on the grip to steer the front wheel away from the intended direction of turn. From the saddle, it may appear that the front wheel continues in a straight line while the top of the bike leans over, but what really happens is that the front wheel steers off on a slight tangent, which causes the contact patch to track away from the turn. The bike’s mass resists lateral movement, so the tire tracking out forces the top of the bike to lean in toward the turn. For example, let’s say the rider wants to make a right turn. Pushing on the right grip steers the front wheel off more toward the left, which forces the bike to lean toward the right. The actual countersteering takes only about a half second for an aggressive lean, or one second for a leisurely lean. Let’s use the term roll in place of lean, to borrow an aviation term that’s a little more descriptive.

If you have been practicing countersteering for a while, it may seem as if you just press on the grip and maintain the same pressure. But it should be obvious that if the front wheel continues to track off on a tangent, the bike will continue to roll over until it slams into the ground. So as soon as the bike rolls toward the turn, you must ease up on that initial countersteering push to allow the front wheel to steer itself back toward center. If the bike rolls over too far, you actually add some pressure on the grip to stop the roll and stabilize the lean angle. The gyroscopic stability of the wheels helps smooth out the steering input.

Approaching a right turn, the rider momentarily countersteers the front wheel away from the turn. The front wheel tracking left forces the bike to roll right.

With the bike leaned toward the curve, the rider steers the front wheel slightly toward the curve, and the motorcycle begins to turn.

Now that the bike is leaned over and the front wheel is pointed toward the curve, the bike starts to turn. Tire traction is actually pushing against the road surface to overcome inertia and force the front end into a curving path. The front wheel is pointed slightly toward the curve, and the rider applies just enough steering input to keep the bike leaning and turning.

Of course, the bike’s forward energy tries to force it back into a straight line. We usually refer to that effect as centrifugal force. If you were to tie a connecting rod on the end of a string and swing it around your head, the outward pull on the orbiting rod would represent centrifugal force, and the string would represent the front tire. With the bike leaned over, gravity is pulling strongly on the curve side to pull the bike over; at the same time, centrifugal force is trying to roll the bike upright. Gravity and centrifugal force balance against each other. Or, more correctly, the rider balances gravity against centrifugal force by small steering corrections.

One of the interesting characteristics of gyroscopes is gyroscopic precession. What that fancy term means is that if you hold a spinning motorcycle wheel vertically by the axles and steer it toward the left, the wheel wants to lean over toward the right. Since this seems to correspond to what happens when a motorcycle is leaned into a turn, many people are fooled into believing that gyroscopic precession is the dominant force that causes a motorcycle to roll into turns. It’s a nice, simple theory but a little too simple to explain why a motorcycle behaves the way it does.

Since a motorcycle tire is in rolling contact with the road surface, tire traction enters the equation, and traction can produce much stronger forces than precession at normal road speeds. Secondarily, the steering angles are so slight during initiation of the lean that very little gyroscopic roll torque is generated. What’s more, as the motorcycle is leaned over, the front wheel is first turned away from the direction of lean and then turned back toward the lean. The net effect is that gyroscopic precession almost cancels itself out in terms of affecting roll. But the gyroscopic stability of the wheels is a necessary part of steering. If the wheels had little or no mass, it would be extremely difficult to balance the bike because of the instability.

It is primarily tire traction that forces the bike to turn away from center.

Now leaned into the curve and turning, the bike is stabilized by gravity balanced against centrifugal force.