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precession of the front wheel does not cause a motorcycle to counter steer. Motorcycles counter steer because they fall over when you apply steering input. That is it. When you turn one way the bike falls the other way because of lateral acceleration, period. The reason that it falls is because it is laterally unstable(there is nothing mechanically holding it up). The fact that bikes fall over when you steer them is the basis for understanding why motorcycles behave the way that they do. Motorcycles counter steer at all speeds. Some people believe that the gyroscopic precession of the front wheel takes effect once a certain speed is attained. There are only two ways to hold a motorcycle up, mechanically (your leg or kick stand) or via lateral acceleration. At low speeds and steep lean angles lateral acceleration may not be sufficient and steering to full lock may not generate enough lateral acceleration to hold a machine up. At this point the machine will fall to the ground unless you hold it up. Counter steering will work as long as it can produce sufficient lateral acceleration to overcome gravity. As long as you are not trying extreme lean angles you can counter steer almost to a complete stop. If you are not counter steering then you are holding it up mechanically. At low speeds the bike does not run out of gyro precession, it runs out of lateral acceleration. What causes some of the confusion is that at low speed and modest lean angles the front wheel steers in the direction of the turn a noticeable amount. What proponents of this theory are missing is that the front wheel points into the direction of travel at all speeds. If this is not true than it makes no sense to even speak of steering at all. |
few variations from traditional theory. The key difference being that a bike by definition cannot be laterally stable. Bikes have been around for over 100 years and yet there are still misconceptions about how they work. On this page I am going to try to explain why motorcycles do some of the specific things that they do. |
highside Because they are laterally unstable. When the rear tire looses traction in a turn it will cause the bike to get sideways in relation to the path of travel due to inertia (the mass tries to continue in a straight line). While the rear has lost traction the front tire continues to hold the line. This is why the bike does not immediately low side and why the chassis becomes more upright as it goes farther sideways. When the rear tire regains traction while the motorcycle is sideways relative to the path of travel the bike rotates (changes lean angle) very rapidly. This is true whether the bike is traveling in a straight line or leaned over in a turn. It is also true regardless of how far the rear steps out whether it is 2 ft. or an 1/8 in. There will be a lean angle change that corresponds. Slides cause oscillations because the lean angle change that occurred due to the tire regaining traction while misaligned with the path of travel forces a steering change due to the precession of the front wheel. Remember, the resulting lean angle change that occurs from the tire biting while misaligned with the path of travel is due to the center of gravity's lack of stability. The degree to which a bike is sideways and leaned over when traction is regained will dictate whether an oscillation or a highside will be the result. A highside occurs when an oscillation exceeds a bikes ability to recover from the last sequence. A violent tank- slapper that rotates a bike to the point of crashing is physically the same as a highside. |
the front wheel slightly crossed up in order to change lean angle. This clip demonstrates Rossi's genius as a rider. He gets the bike to change lean angle and avoids wheel wobble by touching the wheel down only briefly. Notice how rapidly the bike changes lean angle and how quickly it slows down but continues to change lean angle as he carries the wheelie. Before the front wheel can get back off the ground precession straightens out the front wheel which slows the rate of change. As he carries the wheelie it is evident that he is still applying pressure at the bars. Even though he is carrying a wheelie the bike is still changing lean angle and the resulting precession must be resisted by the rider. Clip #3 In this clip the rider on the Proton attempts to do the same thing that Rossi did in the previous clip. However, he did not achieve all of the lean angle that he needed by the time that he landed the wheelie and was forced to steer aggressively the moment it landed. He also changes lean angle with his body while carrying the wheelie. He is able to do this because while the front wheel is off the ground it cannot change lateral acceleration. The lean angle change that occurs while he is carrying the wheelie cannot be from precession of the front wheel because the change began after he turned the wheel to the left. I am in no way critiquing these riders, only observing. Clip #4 Notice how the front wheel changes direction in perfect synchronization with lean angle change. It is not a coincidence. The precession is the reaction to the lean angle change. The two must happen simultaneously. In this particularly violent example notice how the bike is wagging. The bike is nearly highsiding with every wag. When the tire bites while misaligned with the path of travel it forces a rapid lean angle change. The front wheel responds (due to precession) by turning into the lean change. Again, that is why if you cancel out the gyroscopic force of the front wheel, wheel wobble is impossible |
precession from the front wheel (which is a reaction to the act of changing lean angle) provides the energy for wheel wobble. Clip #1 Capirossi is in transition from left to right. His steering input changes the lean angle which causes precession to steer the front wheel to the right which causes the bike to stop changing lean angle. This momentarily allows him to steer back to the left which starts the cycle over until he releases pressure at the bars. Part of what started the wobble in this clip is that the rider aggressively changed steering input while the rear was sliding. That caused the rear tire to suddenly gain traction and quickly rotate the bike to the right. Of course that caused the front wheel to generate precession. The rider's aggressive steering input moves the contact patch forward when the bike is leaned over which effectively reduces trail. Short trail reduces steering effort at the expense of twitchy handling because it allows the rider to initiate a rapid change in lean angle. But a rapid change in lean angle only makes the torque generated by the front wheel that much greater. The mass of the bike changes lean angle with very little resistance. Because the rider must fight the torque produced by the precession of the front wheel it forces him to exert substantial energy to overcome an unnecessary force and in this instance caused a wobble. |