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Lnewqban

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Lnewqban last won the day on April 1 2019

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  1. Having no clutch should not be too much of a problem if the throttle hardware is smooth enough from zero power up, as well as the rider's twisting input. For a tradicional combustion engine, both the delivered power and the engine braking effect come from pneumatic compression in the cylinders, which works as a shock absorber in certain way: there is a time/magnitude lapse between control input and max power and almost none for max engine-braking (as internal pressure of gases inside the cylinders grows or gets reduced with time after throttle input). To complicate things more, there is a minimum number of turns (rpm's) that an engine can achieve before stalling and turning-off; hence the need of the mechanical clutch, which also works as a magnifier of the finesse of the throttle application (opening and closing). An electric motor does not suffer any of those problems, it works based on rotating magnetic fields that are pretty solid (minimum or zero field-rotor slip), eliminating the shock absorber effect between control input and rear wheel reaction. It can also be slowed down to 1 rpm and still delivers immense amounts of torque. Not having had your practical experience, I assume that an electric motorcycle could use more finesse on the brakes and the throttle inputs, even a simultaneous combination of both in some slow maneuvers.
  2. You are correct, but only if such motorcycle is neutral steering-wise. As you know, many bikes have a natural tendency to either understeer or oversteer (if the rider releases the handlebar while the bike is leaned on a curve). Those tendencies depend mainly on geometry and profile of tires. The front contact patch of an understeering bike will "feel" less lateral force when coming out of a lean/corner as it had been forced to over-steer during the curve. In the steady conditions that you have described (while keeping zero angular input on the steering), the sliding force on each contact patch remains constant and it depends on the square value of the forward velocity of the bike and the inverse of the radius of the trajectory. As you properly have explained, any counter-steering input will instantaneously increase the value of that lateral or sliding force (especially for the front contact patch). The lean angle (and linked lateral forces) can remain constant along a curve, but real conditions of the road make it maximum only intermittently. Maximum grip or friction depends on the force that is normal or perpendicular to the surfaces in contact. The undulations of the road and the instantaneous accelerations that add to and subtract from the natural acceleration of gravity, induce a fluctuating amount of that normal force or available maximum friction or grip. Each tire has more available traction each time it rolls over a crest: that instantaneously increased normal force deforms the tire and partially compresses the springs, which push and accelerate the rider and the rest of the mass of the bike and fluids upwards. Exactly the opposite happens when the tire "falls" into a valley of the track's surface: less available traction for a fraction of a second. In other words, assuming a perfectly horizontal traverse surface of the curve (no sectional slant, slope or crown), which makes the value of the normal force that induces grip equal to the value of the weight supported by one tire, the undulating nature of that surface will make that tire and its suspension alternatively support more and less weight than normal (for a perfectly ideal flat surface). Hence, more that having a sharp value, the available grip of each tire constantly rises and falls / pulses / swings / oscillates around an average value. Similar effect (although at much higher frequencies) is produced by the vibrations coming from the rubber compound of the tire when supporting strong lateral forces (getting deformed, twisted, overheated, sheared) and when crabbing or sliding off the trajectory (while keeping grip). At microscopic level, things are changing at a very rapid rate and the surfaces are gripping each other and letting go in a very rapid sequence. Consider also that all the disturbances described above induce minimal steering inputs. Because of the trail of the front tire, while it is leaned, any close to vertical force has a perturbing effect (torque that is equivalent to a sideways force times trail distance) when the bike is in vertical position. A torque is induced into the steering, which can be over or under-steering, which significantly modifies the radius of the trajectory, which momentarily changes the magnitude of the lateral forces. Hotfoot's excellent post has perfectly explained the Physics of real life.
  3. Thanks for your answer, Roberts. If you have not done it yet, I would highly recommend you reading these old threads:
  4. Why did your coach use less time than you to complete that curve? Did you both use the same turning point? Was your entry speed comparable to his? If so, did you let your bike slowdown (before cracking the throttle open) longer than he did? What do you call hard turning?
  5. Carefully listen: https://www.youtube.com/watch?v=47ybaUjqAt0
  6. Perhaps you and I are watching different videos, or have different interpretations, or the way he explained the concept was poor, but it seems to me that the tuner was not purposely advising exactly that "brake-open throttle-turn in" timing sequence. The way I see it, that gentleman was asking the rider, whose suspension was being adjusted, not to be shy or excessively cautious about giving some gas to the engine during the first phase of a regular curve (not to coast), but rather achieving the proper weight distribution as early as possible, not over-loading the front contact patch (which needlessly over-stresses its surface), which condition when back from a spin (plus the height of the zip-tie) was the tuner's reference for finer adjusting of spring and damping. I believe that the tuner would never be able to notice any difference (regarding texture of surfaces of the tires) between the two sequences: "brake-open throttle-turn in" or "brake-turn in-open throttle", any thing that happens there just happens too fast to make any difference. In the video I watched, the tuner was able to see that the rider had done some wheelies during a lap because contradictory clues: the zip-tie moved too high (much weight on front suspension when landing back), but under-stress of the rubber surface (not enough cornering and braking forces/overall low speed). Best (non-dynamic) suspension always means best possible managing of dynamic weight transfer to pavement for most conditions, regarding forces and accelerations of braking, cornering and exiting a curve. Lacking more sophisticated sensors on the bike, the tuner's expert diagnostic fully relies on the visual clues of rubber surface (what pavement does to rubber during the periods of times the curves last) and range of suspension strokes after one or more laps, reason for which he needs the rider to keep smoothness and weight distribution as close to ideal as possible. As always, I could be wrong, though.
  7. IMHO, the tuner is basically advising against precautionary coasting on a turn (50/50 weight distribution), which delays maintenance acceleration until the way out of the turn is visible and verified as safe (street visual technique). The rear suspension and tire are needlessly unloaded for too long, which is later visible in the rubber wear texture. I see no contradiction with Keith's technique of 0.1 to 0.2 G acceleration applied as soon as possible on the turn in order to achieve 60/40 weight distribution and suspension and chassis stabilization. That is impossible to do while deep trail braking up to the apex: there is always a percentage of over-loading on the front suspension and tire (and the opposite for the rear end) during the first section of the curve. During trail braking, the weight distribution remains reversed and far from the ideal 40% front/60% rear, condition that improves (tends to 50/50) as you approach the apex and gradually reduce hand pressure on the front brake lever.
  8. As usual, the answers from Hotfoot are excellent. I would like to learn from you the reason, expected benefit or reasoning behind that 40-year old habit for street riding. According to the book, once you steer for the turn, lean the bike and crack the throttle open, nothing should change until it is time to pick up the bike, accelerate and exit that curve. "Rule Number One: Once the throttle is cracked on, it is rolled on evenly, smoothly, and constantly throughout the remainder of the turn. At the point where the correct transfer of weight is achieved by the rider (10 to 20 percent rearward) by using the throttle, any big changes in that weight distribution reduce available traction. Once the bike is fully leaned into a turn, changes in tire load, either evenly (both wheels, most easily done in a crested road situation) or alternately (front to back, back to front, from throttle on/throttle off) must then either underweight or overweight the ideal load for that particular tire/bike combination." - Keith Code Discussing Physics a little further, let's see what happens (regarding forces) on the contact patch of the rear tire when extreme leaning and acceleration happen simultaneously. This article shows some schematics that help us understand how the lateral (cornering) and longitudinal (acceleration) forces are acting on the rear contact patch at 90 degrees from each other. That creates a unique resulting force that can easily grow beyond the limits of available traction (imaginary circle): https://lifeatlean.com/the-traction-zone/ Using your example of 30-degree lean and proper roll-on throttle, here is what that rear contact patch and suspension are "feeling" (regarding simultaneous forces acting in different directions): Vertical force: 60% to 70% of the total weight (bike, fluids and rider). Let's assume 600 pounds of total weight as a reference. Then the patch has 360 to 420 pounds pressing vertically down (let's use the average of 390 pounds to simplify analysis). The magnitude of that force varies as the tire rolls over crests (force increases) and valleys (force decreases) of the track or road, hence the importance of an efficient suspension that keeps the patch pressed down. If your rear tire is able of 1 G of traction, the available friction, grip or traction between rubber and pavement is 390 pounds (it equals vertical force for coefficient of friction=1) in any direction parallel to the track or road surface. We could draw an imaginary circle around the patch showing that limit of 390 pounds of available traction. If the rider forces the patch beyond that limit, the tire will slide over or skid. Lateral force for 30-degree lean angle is tan 30 x vertical force = 225 pounds pulling the tire sideways (trying to make it slide out of the curve). Rear suspension (which is working at 30 degrees from vertical) is "feeling" or supporting a total force of vertical force / cos 30 = 450 pounds (15% overloaded respect to the vertical position, while forces/shocks from irregularities of the track keep coming from a vertical direction). Rearward force is 0.1 to 0.2 G (proper roll-on acceleration rate according to the book) = 60 to 120 pounds. For those conditions, you could get away with accelerating more than recommended. For that lateral force of 225 pounds, you could apply up to 320 pounds of accelerating rearward force onto the rear contact patch before reaching the limit of the imaginary circle of traction. That means an acceleration of 0.53 G, which is very easy to achieve for a 1000 cc machine without much twist of the throttle (consider that a wheelie would happen for around 1 G or 600 pounds of rearward force). Now, as you corner harder enough to increase that lateral force (closer to 390 pounds) and lean angle (closer to 45 degrees), the amount of achievable acceleration decreases dramatically. It takes a very fine throttle control to keep the acceleration within that reduced range. Simultaneous excessive acceleration and extreme lean angle (high lateral force acting on the contact patch) can take the tire beyond its limits of traction way too easy. And that is for properly inflated warm tires on dry asphalt, just consider the outcome for wet asphalt, dust, sand or fluids, improper pressure, poor throttle control, etc. I find this article from Keith about G forces at extreme leaning very interesting: https://www.motorcyclistonline.com/leaning-bike-code-break/
  9. Gianco, why do you think that pushing on external handlebar while cornering is wrong? Some bikes are naturally under-steering, yours may have that tendency for the tires that it is wearing. That means that the front tire will try to under-steer by itself when leaned. By keeping pressure on the external handle, you are compensating for that tendency and keeping everything in balance. You know exactly how much pressure to keep by feeling the bike balanced while cornering (not falling into the turn or out of it). Whenever you are "too slow for that moment" or at the ideal cornering speed, the bike is leaning exactly what it needs to lean to keep lateral balance of forces for that particular speed/radius-of-turn combination. As soon as you released the external pressure that was necessary to compensate for the under-steering tendency of the bike, a small counter-steering happened by itself (the internal handle-grip moved forward some), which leaned the bike excessively for that speed and you immediately felt the bike was falling into the turn (the lateral balance of forces had been ruined). Remember, we never directly select the lean angle, we only choose speed and radius of turn; then, the bike leans as far as it needs in order to find the lean angle that balances all lateral forces. By hanging-off we reduce the lean angle of the chassis, but the dynamic lean angle of balance (of the combined center of mass) remains the same for same speed and trajectory of the curve/radius of line.
  10. You are correct, Gianco. It is about covering the course as quickly as possible, using street tires only. The riders need to go fast between cones (or pylons), braking-in and accelerating-out very hard as well; that is why they install bigger than normal rear sprockets. Around the cones the situation gets reversed and they need to go slow to rotate (change directions) as quickly as possible. They don't discuss cornering mph, but degrees of rotation per second. Because centrifugal effect depends on the square of velocity and on inverse of radius, at very low velocities the radius becomes much more important in that equation. If the rider reduces the radius to a minimum without quickly slowing down (as much as needed), the centrifugal effect will flip the bike out of the turn (bassically an out of control counter-steering). At full lock (no chance to steer), the balance is achieved by braking (more lean angle and tighter circle) or accelerating (less lean angle and wider circle). Of course, moving the upperbody in or out also changes lean angle and radius of turn (low speed = small gyroscopic effect), but main balance is carefully achieved by controlling a very accurate slow speed during a critical section of the circular trajectory. Either or not parts of the chassis drag over the pavement is a consequence of all the above: proficient Gymkhana riders don't need or purposely look for maximum lean angle. For more information about these techniques, please see: http://amgrass.com/forum/index.php https://www.youtube.com/watch?v=8ZFdxEWpefI
  11. The book that you have mentioned has the answer to your original question: "What makes the bike turn the same as it was leaned more without hanging off? It is explained in Chapter 3: Less lean angle requires more effective steering angle in order to keep the same radius of turn (please, see figure 3.18 of page 3-13): "Increasing lean angle tends to increase the effective steering angle." It is a simple geometrical problem, there is no need to complicate it with camber thrust, slip angles, etc., because the magnitudes of the forces of cornering and the dynamic lean angle remain the same, either or not you hang-off. The chassis reduces its lean angle when the rider hangs-off while cornering, which changes the relative geometry among the three planes: the ones containing the rear tire, the steered front tire and the curve (track surface).  You may want to do the following experiment: Fill up a wide recipient with water (the surface of the water will work like the plane of the curve). Make a central 10-degree bend in a small rectangular piece of cardboard (one side will work like the plane containing the rear tire and the other side like the plane of the front tire). Keeping the bent edge and both sides vertical, deep the piece of cardboard into the water. Looking from above, turn the cardboard just like a bike would lean over to turn and note how the angle formed between both lines that intersect the surface of the water and each side of the cardboard gets bigger as the lean angle increases. That angle is the effective (or kinetic) steering angle, which would force the bike to turn tighter (reduced radius of turn) if the rider would not compensate for this phenomena by steering a little less. If that experiment still does not convince you, we could use the following well stablished formula: Radius of turn = [Wheelbase x Cosine of chassis lean angle] / [Steer angle x Cosine of caster angle] As wheelbase gets a little bit smaller and caster angle remains constant, when the rider hangs off while cornering, the cosine of the chassis lean angle increases (example: cos 45=0.707 and cos 40=0.766). That change would increase the radius of turn some, making the bike run wide respect to the desired trajectory. In order to avoid that from happening, the rider must compensate by increasing the steer angle a little. Another geometrical way to analize that: Imagine a perfectly vertical line running underground by the center of the circular trajectory of the motorcycle. Disregarding slip and camber thrust, the extended axis of both wheels must intersect with that vertical line. As those wheels are leaned more, the point of intersection moves deeper into the ground, which reduces the angle formed between the extended axis of both wheels. Hence, the steering angle must be reduced some in order for the bike to keep tracing the same circular trajectory.  A leaned motorcycle will always have an effective steering angle that is smaller than the one for a 4-wheel vehicle describing the same curve.  The exercise of Motorcycle Gymkhana is a different solution to a problem that is different: make the tightest quick turn around a cone. The maximum speed at maximum lean angle will make you slower in this particular case, try that experiment as well. Since speed must be much smaller than during normal Superbike track cornering, the smallest radius of turn of the rear tire is the key to turn the bike 180 degrees as quickly as possible. For the same reason explained above, the Gymkhana rider wants the chassis to be as leaned as possible during the slowest section of the tight turn. At full stop lock of the steering, the radius of turn (and the circular trajectory of both tires) will be smaller as the chassis lean angle increases: there is a greater effective steering angle. Lock the steering of a bicycle at a pronounced angle and push it while at different sustained lean angles for each completed circle and you will see that the smallest circle corresponds with the biggest lean angle. For the above formula and description of angles, please see "Steering angle" here: https://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics
  12. Your language is good enough for us to communicate about dynamic of motorcycles, my English is not much better. According to Newton, everything that has some speed wants to move on a straight line by itself and must be forced to turn. The forces of steering (wheels pointing in different directions) and friction between tires and pavement are the only things that force a car, truck or a motorcycle to turn, not the lean of the bike. A motorcycle can be leaned and still move on a straight trajectory if both tires are kept perfectly aligned forward. We only lean the bike to create a balance of forces between gravity and centrifugal effect and that balance is kept during the turn regardless of how much the rider hangs off. The more you lean a bike, the less misalignment both tires must have to keep the same circular trajectory and the front contact patch moves away from the rear one, which means less steering is needed (although the difference may not be noticeable). One of the reasons is that the distance at which the axis lines of both tires intersect each other must increase as the bike is leaned in order to keep the same horizontal radius of the curve. Please, take a look at these schematics and text in Italian: http://www.dynamotion.it/eng/dinamoto/8_on-line_papers/Pneumatici/Pneumatici_ita.htm
  13. The main advantage I see is pre-loading the rear sprocket, chain and rear suspension while the chassis is still pitching nose down due to deceleration. The rear suspension remains more or less extended during the transition, rather than returning to normal after prior getting extended again under power. The top leg of the chain is slacking while braking and the transition to power always has a shaking effect, plus some dead rotation of the sprocket (if some play exists between the rear sprocket and rubber connectors to the wheel). That transition used to be less abrupt for carburated bikes than it is for the ones equipped with fuel injection (there is a time lapse after control input). For street riding, it is safe to use the rear brake for that purpose, rather than simultaneously manipulating the throttle and front brake controls in a fine manner.
  14. It could be off some degree. As far as I understand it, the lean angle shown in the display of a MotoGP is calculated based on the rate of leaning over speed from a vertical, using radial acceleration data supplied by the IMU. Because the inertial reference of the bike changes when cornering, there is no way for the blind (it has no horizon visual reference) IMU to directly "feel" and measure lean angle. Any blindfolded passenger of that bike would be lost about angle as well, his/her only clue about the intensity of cornering and resulting lean would be the sensation of increased body weight. The only accurate way to calculate the lateral acceleration and resulting force is by accurately knowing both speed and radius of the trajectory. I understand that chassis lean angle is the most evident clue that we normally have to guess how much stress we are putting on the contact patches, but the above graphic of lateral g-acceleration versus angle of lean shows us that strees-angle relation is not linear. What amazed me about this remarkable article written by Keith ten years ago was the fact he exposed (opening my eyes to this phenomena) about the needed finesse required when approaching maximum lean angle due to the rapid increase on dynamic forces, loads and stresses: "The barriers then are both physical sensation and visual orientation, and I believe there is a make-or-break point. That point is 45 degrees of lean. At 45 degrees, the forces are a bit out of the ordinary. Along with the normal 1g down, we now also have a 1g lateral load. As a result, the bike and our bodies experience an increase in weight. That's not native to us, and acts as both a distraction and a barrier. Once we become comfortable with 45 degrees and attempt to go beyond that, the process begins to reverse. Immediately we have more lateral load than vertical load, and things begin to heat up. Riders apparently have difficulty organizing this. Suddenly, we are thrust into a sideways world where the forces escalate rapidly. While it takes 45 degrees to achieve 1g lateral, it takes only 15 degrees more to experience nearly double that (depending on rider position and tire size)." https://www.motorcyclistonline.com/leaning-bike-code-break
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