Jump to content

Lnewqban

Members
  • Content Count

    299
  • Joined

  • Last visited

  • Days Won

    11

Everything posted by Lnewqban

  1. 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
  2. Approximately 1.96 g. That magnitude would be approximately the result of multiplying the standard acceleration due to gravity (32.2 feet/second square) by the tangent of 63 degrees (1.96). That means that, depending on specific front-rear weight distribution, each contact patch would be feeling a lateral force (trying to make it slide over) which magnitude would be a little less than the combined static weight of bike, fluids and rider. That makes that rubber compund a fantastic sticky and resilient material. The IMU (Intertial Measurement Unit) in the MotoGP bike is made up of gyroscopes and accelerometers (usually a 6-axis system) that gather information on the bike’s chassis attitude. Because the previous research we did here (above picture), we can assume that with proper hang-off of the rider, the tire-width offset is compensated in such a way that the chassis lean angle is pretty close to the theorical dynamic lean angle (line of combined CG to centers of contact patches respect to vertical). The concept that I would like to present to new riders is that the magnitude of lean angle always follows the magnitude of those lateral forces, which we create by selecting the speed of cornering for a particular curve. In other words, although we surely can improve the chassis lean angle by hanging-off, we can't directly manipulate or choose the theorical dynamic lean angle of the bike, which is a natural balancing reaction (the CG-frame-tires aligns with the new resultant leaned force of cornering) and only depends on the square of the speed and on the radius of the trajectory we choose.
  3. That is accurate, the frames (and suspensions) of motorcycles with tires of wide section will lean a few degrees more than the theorical lean angle shown in the graphic, which is the angle formed between a vertical line (the direction of gravity) and another line connecting the contact patches and the combined center of gravity of bike plus rider (please see first attached diagram of gravity and lateral forces). Hanging off reduces that difference, even eliminating it, as we discussed on these old threads: http://forums.superbikeschool.com/topic/3324-hanging-off-mathematically-quantified/?page=2 http://forums.superbikeschool.com/topic/3661-body-position-and-cog/ Note that in the case shown in that third picture-diagram of forces (Stoner's), the combined CG is relocated sideways enough to exactly compensate for the off-center relocation of the contact patch, reducing the actual lean angle of the suspensions and frame and increasing angular clearance.
  4. While riding on the mountains you should not be pushing your tires to the limit; that just means excess of danger with little reward or improvement of skills. Please, note that I mention the limit of physical pavement-rubber traction and not lean angle of the chassis. Lean angle is a natural consequence of the centripetal forces of turning and it mainly depends on the square of the speed of the bike and on the inverse of the radius of the turn: meaning in simpler terms that double the entry speed (but same radius) puts four times more lateral stress on the contact patches of the tires and that half the radius of turn (but same speed) puts double stress on those patches. The manufacturer of your sport bike tries to provide a lean clearance that matches the maximum traction capability of your tires in good pavement conditions. The resulting angle of lean must be there to compensate for those cornering lateral forces in such a way that the bike keeps its lateral balance (not crashing over towards the outside of the turn). What is happening at the same time? Both tires "feel" an increased weight of your body and the bike, as much as double (2g) around 60-degree lean; hence their sections become less pliable and have to work harder to keep grip. The suspension is now compressed by the added weight and its work to follow the surface and keep the rubber in contact with it is more difficult because the direction of the strokes is inclined while the irregularities of the track keep pushing it vertically. All the above works as described if, and only if, you have the luxury of clean dry pavement. Otherwise, your tires will have less capacity to resist the action of those lateral forces (assuming good conditions of tires, inflation and suspension, as well as good riding techniques and per-tire-weight distribution). How much less traction will you have? That is something impossible to predict on public roads, where you could suddenly find spills of oil, Diesel, anti-freezing coolant, or sand or street markings or steel manholes or animal carcasses. If you ride on the edge of available traction, you will not have available traction to use in the event of an emergency or evasive maneuver, like braking or swerving. You always choose entry speed to negotiate certain radius of a fixed corner, lean angle follows that decision adjusting itself to keep balance, and both tires and suspensions are loaded with extra forces, which should never grow beyond the limits of traction for the specific conditions of the road. Please, read some more about this: https://www.motorcyclistonline.com/riding-tips-traction-science-tires-keith-code-break https://www.motorcyclistonline.com/blogs/tread-envy-code-break https://www.motorcyclistonline.com/blogs/dragging-your-knees-code-break https://www.motorcyclistonline.com/leaning-bike-code-break http://forums.superbikeschool.com/topic/3331-have-you-ever-slid-the-front-without/
  5. I advise against that idea: friction would be the only thing preventing the bar from rotating around the fork's tube. It may be sufficient for normal riding, but it may suddenly rotate for high steering forces or head shakes or tank slappers. I would try finding the ideal position for the bars and still using a key to fix that position, perhaps drilling on the tripple or fabricating an offset kind of bridge that connects both original holes (clamp and tripple). Is the ideal position as shown in your picture?
  6. About your #2 question: Your priority for street riding should be safety, which has more to do with high alertness, with good judgement of entry speed, with understanding of traffic situations and with proficient visual skills. Your body position should be such that it serves as a good base for those things, it should be comfortable, it should keep you in total control of the machine. The extreme body positions that you see in track practices and races are not really necessary if you ride within or not much above legal speed limits. The lean angles and cornering forces on your tires should be moderate, so you will always have a safety margin or reserve to use in unexpected road hazards or traffic emergencies. You can experiment with leaning only your torso and head into the turn or even hang off your hips some, finding your most comfortable and safe body position. I would avoid dragging knees on public streets, but would know how to increase leaning angle and assertively swerve as emergency maneuvers.
  7. I believe that the only reason for steering to be less accurate is a survival reaction of one hand fighting the other. That does not mean that we should reduce or eliminate the steering torque produced by one of the hands, but that we should observe that potential SR. Your bike my have a under-steering tendency, due to geometry or tires or tire's pressure. I would experiment with lowering the front end some and/or raising the rear end, in order to reduce the trail of the steering some. That would reduce the tendency of the steering to remain on a straight trajectory.
  8. Basically, the same two processes happen simultaneously, only that in a shorter period of time than for a lazy turn. The front suspension and tire are loaded because deceleration, then that load caused by deceleration gradually yields as the load caused by the circular trajectory of quick-flick and tracing the curve rapidly increases (up to lower or similar value). You can find additional discussion about the quick-flick technique here: http://forums.superbikeschool.com/topic/4101-can-quick-turn-be-overdone/
  9. If your in-line four is carbureted and your V-tween is fuel injected, you should feel the difference. Besides the above recommendations, you could remove any current rotational slack between rear wheel and sprocket. If everything fails, I would experiment by carefully using a little bit of clutch or rear brake simultaneously at the begining of rolling the throttle on (not by the book or desirable, but better that upsetting the chassis).
  10. Excellent post, Hotfoot. ? It very well explains the "throttle should be open as soon as possible" line in the book. Prior reaching maximum lean or slidding state, the bike is always following the trajectory that the rider commands it to follow via steering and throttle. Good visual skills help me with the spatial awareness regarding where the bike is located at any time in a succession of turns and helps me decide about the proper moments to brake, accelerate and turn in.
  11. It could be that you are not following two fundamental rules of cornering: 1) Looking deep into the turn: You can only know that your trajectory is one foot off if you are looking close in front of your bike. 2) One steering for the whole turn: You may be adjusting your steering along the turn in order to achieve your goal trajectory. Think of the unintended consequences that you are creating if you are doing so, like diversion of attention, disorientation, over-stressing the front tire, etc. The way I visualize cornering trajectory: to me it is like shooting a ball into the basketball hood from a distance, you feel the cross-wind, you estimate the distance and the angle, you gut-calculate the whole flight of the ball and then you impart your best directed push hoping for the best. Sometimes you miss for little and sometimes you nail it. The hard mental, visual and calculation work in cornering happens prior the turn-in point, which is equivalent to the moment of actually pushing the ball. Let the bike "fly" describing that natural arc, free of unnecessary minute steering inputs and lean angle adjustments. Missing an apex for 12 inches may add a few feet to the corner's total trajectory, which is not a big difference for a bike that moves 88 feet per second (60 mph). Distracting your attention from proper throttle control and from reference points and from spatial location may slow your bike much more.
  12. Welcome, Don! Very true, as soon as we are not 100% mentally riding ahead of the bike, the perception (false or true) of excessive speed and lack of time and available space overwhelms our fears of not surviving the situation. "A superior pilot uses his superior judgement to avoid situations which require the use of his superior skills" - Frank Borman
  13. You are welcome, Jaybird What you have been analyzing and trying to understand is very complex dynamics, reason for which most riders don't even bother learning the "why" of these things. The books that explain the whole interconnection of steering, wheels, masses, forces, etc. in a motorcycle are very dense to read and difficult to comprehend. I believe that there is value in understanding the basics of the Physics behind riding a motorcycle in a proficient way. It is difficult to explain those principles to inexperienced riders without going too deep into the subject and causing confusion. Most mentoring/teaching is limited to "do this to achieve that and go practice it". The experienced rider has the advantage of having tested what works and what does not, of having felt those forces and the reactions of the machines during enough time to make sense of those principles. If serious about this, by persistent observation during thousand of miles, an educated rider becomes more aware and more sensitive about the dynamics of riding and develops a finer input of all the controls and sense of balance. The Physics then becomes less abstract and more in harmony with our senses and minds. In order to function as a motorcycle rather than as a bag of potatoes, all the forces and moments acting over a motorcycle in different directions must be in balance. If our control inputs or road conditions break that balance, a brief transition period follows, during which the machine does its magic to self-adjust to a new state of balance. If that state is not physically achievable, a fall will follow. Counter-steering is a clear example of that: the rider intentionally steers the bike out of balance (out of its rectilinear path), inducing many reactive forces, movements and moments for a very brief period of time, forcing the machine into a new state of balance (onto a curvilinear path). If the machine continues on in one of the two states of balance, the rider is doing nothing or too little to modify those, like it happens in the No BS bike demonstration. If the machine is upset by incorrect control inputs from the rider, like closing the throttle during a big rear tire slide, the machine can go from stable cornering balance to unstable transition to out of balance (highside fall) really quick. The speed of the motorcycle is very influential about the steering, gyroscopic reactive forces, rolling and balance, reason for which counter-steering is so powerful in a superbike at high speeds, but almost negligible for a trial bike at walking speeds. http://www.dynamotion.it/eng/dinamoto/8_on-line_papers/effetto giroscopico/Effettigiroscopici_eng.html
  14. Talking about chairs, it has occurred to me that we can discuss the actions of monkeys (passengers) in sidecars races. By moving around for each corner, they do what you describe about your folding chair: they relocate the total or combined center of gravity as far from the motorcycle or as close to the rear tire as possible. Rather than trying to make the motorcycle and sidecar roll, they compensate the natural rollover tendency during fast cornering as much as possible. That rollover tendency is induced by the combination of centrifugal effect and height of the center of gravity respect to the road. A regular sidecar could be comparable to the situation that you have pictured above: a motorcycle with a dramatic asymmetrical weight to its side. Would the bike yield to the induced roll? Let's say that thanks to the third wheel, that weight does not roll the bike over and instead keeps it vertical. If we weld the steering to the frame keeping the steering bar perpendicular to the bike and then make the bike and sidecar gain speed on a straight trajectory, the contraption will describe a straight line. As the bike happily cruises along, if we suddenly remove the sidecar wheel, even with the stability induced by the two remaining main gyroscopes of the contraption, that asymmetrical mass or weight will be able to roll the bike until the sidecar axis hits the ground (the lateral balance will be lost). The bike, even while leaned over, will try to keep going along the straight line (assuming no dragging forces from that dragging axis) because the steering has not changed. Riding with a Motorcycle Sidecar: http://www.steves-workshop.co.uk/vehicles/bmw/sidecar/riding/sidecarriding.html Yes, a substantial weight with some lateral leverage is able to roll a motorcycle in movement or tip the stationary chair of your example over. Nevertheless, without the complicity of the steering capability, the bike will not turn, even if leaned over. The following video shows that the steering capability of a motorcycle, with or without a sidecar, has a powerful influence regarding directing it onto either a straight or a circular trajectory in a precise and controlled manner ....... and what it seems more important: combined with speed and rider's skill, it is able to lift that asymmetrical weight and keep it balanced at will, even on a left turn, in which the centrifugal effect tries to take the chair down. The maneuver is known as "flying the chair". https://www.youtube.com/watch?v=k6ZSSPY32Jk
  15. 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
  16. 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 an 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.
  17. 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.
  18. 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.
  19. 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
  20. 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.
  21. 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.
  22. 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.
  23. This shows the experimental process and the result for a 2013 BMW R1200GS: http://www.me.unm.edu/~starr/moto/cm.pdf
  24. 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.
  25. 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?
×
×
  • Create New...