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Lnewqban

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Lnewqban last won the day on December 26 2017

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About Lnewqban

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  • Have you attended a California Superbike School school?
    No
  1. Experiments with Shifting Gears and Turn Radius

    https://motomatters.com/interview/2012/04/12/casey_stoner_explains_how_to_slide_a_mot.html Casey Stoner Explains How To Slide a MotoGP Bike: "It's something that only works in certain corners in this type of racing, it doesn't work in all the corners. When it does work, sometimes it can be a bit scary; you can go into the corner, and if you make a small mistake when you are sliding, the finish of it can be a catastrophe. When your heart beats really hard is when you slide when you don't really want to,"....... "There's different techniques to different corners and when they should be used, depending on grip levels, and a lot of different things. Unfortunately, most of the time these days, sliding is not the fastest way, there's only some corners where it can still work." About teaching a 5-year child how to shift gears, I recommend you this reading: https://books.google.com/books/about/Casey_Stoner_Pushing_the_Limits.html?id=npA1AgAAQBAJ
  2. Can Weight Shift Theory be debunked?

    Unless you have a fixed fulcrum to exert leverage against, you move the bike away from you (roll it a little) as you move your body off in the opposite direction. The total center of gravity (yours plus bike's) remains along the vertical line that crosses the imaginary horizontal line that connects both contact patches. If the steering is kept perfectly fixed and aligned with the contact patches, the bike does not have a reason to turn. If instead the steering is free to adjust by itself, the geometry of the front tire and angle of suspension, combined with the total weight and gyroscopic reaction (please, refer to your video and see that a left roll of the bike induces a left steering) , will slowly turn the steering towards the side upon which the bike has rolled (only if steering angles, tire's profile and pressure are neutrally set, so there are no over or under-steering tendencies). That slight counter-steering will induce a balancing slow roll towards the side upon which the rider is hanging off and the bike will commence a turn. That is the same self-balancing principle that allows a rider-less bike keep going for a while while speed is relatively high. That is a very different situation than exerting "a force (weight) at a lever point away from the center of rotation". We are starting from a out-of-balance situation. In that case, the bike will be forced to roll due to the moment created by the total center of gravity being initially far away from the line that connects both contact patches. Either or not the self-balancing capability of the steering will be strong and fast enough to compensate for that initial lack of balance depends on several factors, such as magnitude of off-set weight, weight's lever, mass of front tire and linear speed of the bike.
  3. Experiments with Shifting Gears and Turn Radius

    Absolutely! That is the whole reason for the need of selective gears: to keep the engine rotating within the range of rpm's that produces usable torque (and work) for a wider range of rpm's of the rear wheel (which translates into forward speed of the motorcycle). Except during the brief periods of coasting and engine breaking, the work of the engine pulls the motorcycle forward against the resisting forces of inertia (during acceleration) aerodynamic drag (at relatively high speeds) and (when climbing a hill) gravity. The only thing that dramatically changes the torque (and work) that the engine can deliver is the "twist of the throttle": more entering fuel and air means more powerful internal combustion, which means more internal heat and delivered torque (and work). That is true for certain range of engine's rpm and until we reach the point of full open throttle (maximum intensity of combustion and delivered torque), which is what dyno charts show. The work developed by the rear tire is always the product of its rotational speed (rpm's) times the torque it is able to deliver, which is exactly the same value as the product of its linear speed (forward speed of the bike) times the rearward force exerted over the pavement. The value of the work developed by the engine is always a little higher than the previous one, as some energy (in the form of transferred forces down the gears and chain and sprockets) is lost in the links between the crankshaft and rear tire. When the bike is moving at sustained 60 mph on a horizontal road, the position of the throttle is fixed, allowing intake of the exact amount of fuel and air that keeps two forces in balance: pushing forward force and resisting rearward force. If the bike starts climbing a hill and the throttle remains fixed (work delivered by the engine remains the same), the force resisting the rotation of the rear wheel increases due to the addition of the gravity effect. As no additional work from the engine is available, the other factor of the formula (torque X rpm) must decrease, resulting in a new state of balance at lower rpm's. The natural reaction to that is the slowing down of the rotational speed of the rear tire and forward speed of the motorcycle and reduction in rpm's of the engine. We can only allow certain amout of that reduction of the rpm's of the engine before the engine becomes real weak. If we wish keeping the same on-flat-road during the climb, we need to open the thottle up (more work delivered by the engine translates into resuming speed). If the steepness of the hill is excessive to achieve a new state of balance, even at full open throttle (no additional available work), we need to sacrifice bike speed in order to increase force on the rear contact patch via dowshifting. Returning to your original question: When the bike is moving at sustained speed on a horizontal road, the two forces are in balance: pushing forward force and resisting rearward force. If you open the throttle up some (more delivered work), the bike will accelerate due to additional torque reaching the rear tire, until reaching a new rpm X torque balance. If you open the throttle up a lot, the bike will do a wheelie due to excessive acceleration and abundant traction. If traction is not that abundant, then the additional available work must go to break the grip between the contact patch and the surface, spinning the rear wheel.
  4. Can Weight Shift Theory be debunked?

    In order to communicate with the same terms, are you refering to the rolling motion of the motorcycle? Is your question limited to the reactions of the bike and steering and trajectory following a lateral weight shift of the rider? Sorry, I couldn't clearly understand your question.
  5. Experiments with Shifting Gears and Turn Radius

    Because all the gears and sprockets that link the crankshaft with the rear wheel act like a lever: the rotational speed of the rear wheel gets reduced while its applicable torque increases. For the same degree of openning of the throtle, resisting load and rpm's, the engine generates certain amount of torque or rotational force. We have to work around that more or less constant amount of torque, playing with the gears, just like it happens with a bicycle. For a greater resistive load (going uphill, for example), we have to sacrifice rotational speed of the rear wheel in order to have greater torque there; hence, we switch to lower gears. One trick for riding in the rain is to corner using a taller than normal gear, which "weakens" the available torque of the rear wheel, which creates an extra safety margin regarding any mistake with excessive throttle that could overwhelm the marginal available traction. https://www.youtube.com/watch?v=3Tc3VIDQvh0
  6. Experiments with Shifting Gears and Turn Radius

    There is more force applied onto the rear contact patch when the transmission is working in lower gears. On surfaces of poor traction (grass, dirt, etc.), the rear tire has more authority (breaks loose and pushes the bike around easier) when first or second gears are engaged.
  7. Crash Analysis

    Welcome! Perhaps the bike needs to regain the trust in the rider. Available traction can suddenly disappear under us, if the surface of the road is contaminated with Diesel, oil, sand, etc. Always be careful when the road is wet and consider that street tires may never warm up properly in those conditions, because they are cooled down by the water and spray around them. The road can be awfully slippery during a light drizzle of rain, because there is no enough water to wash away the dirt and contaminants mentioned above. Also avoid the outer half of any runabout and curve, where Diesel leaking out of trucks and sand tend to accumulate.
  8. Steer for the Rear - Ch13 of TOTWII

    Accelerating hard (enough to lift the front tire) at extreme lean is equivalent to hard braking at extreme lean: the tire will slide. The very essence of the trail braking technique is the trade off between the longitudinal force of braking and the lateral force of cornering. https://www.sportrider.com/sportbike-riding/riding-skills-series-traction-circle https://www.motorcyclistonline.com/leaning-bike-code-break Our natural gauges to feel and evaluate braking force are the degree of compression of front suspension and forward pressure of our weight on knees and hands. Our gauges for evaluating cornering force are the lean angle and the pressure of our butt on the seat.
  9. Center of gravity

    This shows the experimental process and the result for a 2013 BMW R1200GS: http://www.me.unm.edu/~starr/moto/cm.pdf
  10. Steer for the Rear - Ch13 of TOTWII

    Regarding the bicycle coasting question: The forces acting over the contact patches in the case of coasting are only the portion of the total weight that each tire carries (vertical force vector pointing down) plus the lateral force trying to deviate the bike from a linear trajectory (horizontal force vector pointing towards the center of the circular trajectory described by the bike). Fortunately, the magnitude of that lateral force for a particular tire depends on the portion of the total weight that that tire carries. The other two factors on which that horizontal force depends are the radius of that circle (the smaller the radius, the stronger the force) and the square of the speed (the faster the bike, the stronger the lateral force, squarely. The possible reasons for the slides of those rear tires in the video are many. As Hotfoot pointed above, the rear tire could be worn, improperly inflated, too hot or too cold or under excessive braking or accelerating forces. Even when both tires were made of the same material (possible same brand and pattern) and being rolling over similar surface and trajectory, the available traction of the front tires in each case were higher than the one for the rear tires. Disclaimer: I am not expert; only an enthusiast of Physics, Math and motorcycling. Without being scientifically rigorous, this is what I have learned from books and from riding many miles in all weather conditions: 1) Understanding what is happening in the relatively small areas of both surfaces where rubber and asphalt meet (contact patches of both tires) is extremely important, as much for avoiding crashes as for high performance riding. The proficient rider develops a fine sensitivity about this; he/she can almost feel what is going on there, based on visual evaluation, comparative experience and feed-back from suspension and steering. 2) Traction or grip is the capability of both materials in contact (rubber and asphalt) to persist on staying together, not dramatically and uncontrollably sliding respect to each other, whether or not lateral forces (excerted in any direction that is parallel to the surface) are applied onto that contact patch. Physics tells us that such capability only depends on two things: nature of materials in contact (molecular structure and superficial roughness) and force that is exerted in a direction that is perpendicular to the contact surface (also known as normal force). After many laboratory tests, a "coefficient of friction" is obtained for each combination of materials, which is just the rate between the magnitude of the lateral force that is able to induce an slide and the normal force. That coeficient is a fixed number, which means that both forces are proportional and that in order to achieve more traction, you must increase the perpendicular or normal force. Example: double normal force in magnitude gives you double traction or friction, being the contact patch able to resist double lateral force in magnitude. 3) Considering only the available traction of each contact patch of our machines: While rolling at high speed over a less than flat and perfect asphalt surface, our tires deviate some degree from that pure Physics concept of traction. Static perpendicular or normal force: Static weight on each contact patch is the available perpendicular or normal force . That is only true for a horizontal surface; if there is camber of the track or road, that normal force becomes smaller than the weight. If the bike is going up or down a sloped road, there is more static weight on the lower tire and less on the one located higher. Dynamic perpendicular or normal force: Then, there is a dynamic load, which is fluctuating (greater and smaller than the magnitude of the weight in static conditions). When a tire rolls over a crest of the road's surface, its contact patch "feels" a higher weight or load (the suspension compresses and the available traction improves), the opposite happens when the tire rolls over a valley. When we accelerate or decelerate (brakes or engine brake), we dramatically change the distribution of the total weight of bike, fluids and rider between both contact patches and suspension (from 50/50 up to 0/100). Notice that it is much more difficult for the overloaded suspension to follow the crest and valleys of the track's surface, also that the overloaded tire suffers more dramatic changes in tire profile and contact patch area. Coeficient of friction: Between a rolling tire and asphalt, this is not as constant as the lab experiments demonstrate. We are not dealing here with a loaded chunk of flat rubber that resists slidding. In the case of a rolling tire, any contact patch is rapidly disappearing to be followed by the next contact patch a few inches ahead (reason for the side-walking of the tires while cornering hard, when combined with lateral deformation of each contact patch, the next one lands off track). There is the molecular attraction of the two materials, but there is also the number of macro-imperfections of the asphalt and how deep the rubber "flows" into those; hence, bigger area and higher pressure of contact improves traction. Then, we have the dependence of the rubber on temperature: too cold and does not deforms well enough to dig into the surface, too hot and the integrity and resilience of the rubber degrades and slippery oils start to come out. There is also the time factor: being a flexible solid behaving as a very viscous fluid, rubber needs time to "flow" down and sideways and to get twisted: too quick the load application and the rubber reacts as a hard rock (imagine the water resistance on your body diving from 30 foot trampoline versus walking into a pool); hence the advice about smooth control inputs. Finally, foreigner material that eventually infiltrates between both surfaces, such as sand, water, Diesel, dramatically reduce the coefficient of friction (bassically sliding friction becomes rolling friction, as those molecules and small particles act like little balls). 4) Finally, considering only the horizontal forces that the contact patches "feel" as reaction forces back from the asphalt (those trying to slide or detach both surfaces in contact, inducing a skid or a slide) on each contact patch of our machines. There are three fundamental types of these horizontal forces: A) Longitudinal forces (horizontal force vectors pointing directly forward (accelerating) or rearward (decelerating). B) Lateral force (horizontal force vector pointing towards the geometric center of the circular trajectory). C) A combination of A and B above (horizontal resulting vector pointing at certain angle between the trajectory and radius lines), which happens when we simultaneously corner/accelerate or corner/brake. The magnitude of those forces are limited by the traction that is available at each moment, which can be represented as an imaginary circle around each contact patch. Each time that one of these forces grows beyond that limiting circle (available traction), the inexorable laws of Physics punish us with a skid or a slide (excessive entry speed, for example). Exactly the same result happens each time that the circle of available traction suddenly shrinks and its diameter becomes smaller than the the magnitude of any lateral force for that tire (cornering and crossing over a patch of sand, for example). I find "A twist of the wrist 2" book to be very accurate about the Physics of motorcycling. It is not scientifically rigorous, but it seems to me that the principles that it explains were based on accurate and objective observations. I have extensively experimented with its tips, techniques and advise, and all work regarding performance and safety. Once again, excuse me for the long post.
  11. Steer for the Rear - Ch13 of TOTWII

    All excellent answers, gentlemen! Based on those, and talking in forces and traction terms only, why are the rear tires sliding in this video, while the front tires are not?
  12. Steer for the Rear - Ch13 of TOTWII

    Please, excuse the math and the physics; it was just an attempt to show and quantify the limits that we are dealing with in this discussion. Allow me to insist on some basic concepts, which are very important for you and for any rider to visualize and understand in a clear and solid way. You should know and apply these things without thinking too much about them while riding a fast motorcycle in a proficient way. The limit to "roll it harder, sufficiently enough to lift the front" is the available traction of the rear tire in those cornering conditions. If you can do it without inducing a slide of the rear tire, it is only because your tires have not been loaded enough (with cornering speed) in the previous stages of cornering: your entry speed was lower than it could have been. In other words, you will be trying to compensate a slow entry with a fast exit. Why is that? I believe that this is the point that unfortunately I have failed to explain properly. Hence, I will try the questioning approach, hoping to make you see some limits: 1) Would a unicycle corner better that a bicycle? Why? 2) Do you believe that a motorcycle doing a wheelie becomes a motorized unicycle? Please, explain. 3) What limits the maximum speed at which a sport motorcycle can negotiate a turn of 200-foot radius? How does rain affects that? 4) What forces are acting over the contact patches of a bicycle that is cornering and simultaneously coasting? 5) Regarding turning a motorcycle along a corner, should the lean angle be considered a consequence or a cause? Please, explain. 6) What causes a wheelie (for a vertical attitude of the bike)? How can it be controlled? Why? 7) What forces are loading each contact patch during a wheelie?
  13. Steering Video No Bs Bike

    I was corroborating your statement, which I found to be correct. I agrre with your last post as well. Sorry about the confusion.
  14. Steer for the Rear - Ch13 of TOTWII

    1. The bikes want to follow a straight line, if they can, if not forced otherwise (remember the Newton's law we described priorly?). A sphere rolling on a flat surface can't make a curvilinear trajectory. A disc (like a coin or a tire) can, if rolling over the same flat surface off a vertical position (leaned). For keeping that leaned attitude, someone or something must have initiated the leaned rolling. The Newton's law manifests itself in the case of the rolling leaned disc, however, there is a force making the disc deviate from the straight trajectory, turn instead over a circle: a portion of its own weight constantly pulls the disc towards the center of that disc. Same force is what keeps the motorcycle leaned and turning in that out of-the-curve wheelie. If that centripetal force (portion of the weight) equals the centrifugal effect of the circular movement, the bike as well as the disc will keep describing the same circle over and over again. Yes, your wheeling bike will keep turning even after the end of the curve (you must put your front tire down and counter-steer to decisively straighten the trajectory up). If that centrifugal effect becomes bigger that the centripetal force (portion of the weight), then the bike tends to open the circle (to constantly increase its diameter). That is what makes a leaned bike that is excessively accelerated (greedy application of throttle) run wide and subsequently reduce the lean angle. That is also the reason that the leaned and turning bike in that wheelie will tend to return to a straight trajectory, if enough acceleration and time is provided. 2. You could accelerate harder than 0.2 g, let's say 0.9 g and achieve 10/90 weigth distribution. Consider that your rear contact patch will be loaded with harder longitudinal forces (exactly 90% the magnitude of the combined weight of your bike and your body, because F = mass x acceleration). If the rubber of your good racing tires is able to achieve traction of 1.1 g, then, your rear tire can only deal with around 0.3 g of cornering lateral force (around 20-degree lean). Then, you will be forced to slow down to prevent a slide (like riding on wet-dirty pavement). There it goes your excess of acceleration due to the need to moderate your speed. Even if you make the turn in those conditions, you will only have a fraction of the grip that you could have in the front contact patch (only 10% of the weight on there, remember?) for countersteering out of the turn. 3. Let's do some math: Sustained acceleration of around 1.0 g (32 ft per second or 22 mph of additional speed per each second in the curve) will give you a nice sustained wheelie. If the limit of available traction allows a maximum speed of 80 mph in that curve, that will be your leaving or exiting speed. If it takes 3 seconds to complete the curve in the described conditions of strong acceleration, your maximum entering speed must be 80 mph - (22 mph x 3 seconds) = 14 mph. Your average speed through the curve would be (80 + 18) / 2 = 49 mph. Another rider following the recommended 40/60 weigth distribution (acceleration of 0.1 g) would increase the speed at a rate of 2.2 mph per each second, which means same exit speed of 80 mph and entering speed of 80 - (2.2 x 2 seconds) = 75 mph, being his average speed of 77 mph. Note that he used 2 seconds while you used 3 seconds (gross approximation only). Hope those numbers help you see the dramatic difference. There is more than maximum available traction involved in this picture of moderate or recommended weight distribution: steering, suspension, tire performance, stability, as mentioned in my previous post. Practical considerations: You want to keep your steering tire loaded with a decent amount of weight or available traction. To force a speeding heavy bike in and out of a lean angle requires muscular effort and a solid fulcrum, which is a gripping front contact patch. That loaded front tire can save you in an emergency swerving and in a slide of the rear tire. Imaging going through a quick chicane with only 10% of the total weight on the front tire? Please, excuse my long post.
  15. Steer for the Rear - Ch13 of TOTWII

    I believe that the answer to your question can be found in the last section of that chapter: Stable suspension. Perhaps re-reading Chapter 3 could help you see the whole picture more clearly.
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