Jump to content


  • Content count

  • Joined

  • Last visited

  • Days Won


Lnewqban last won the day on December 26 2017

Lnewqban had the most liked content!

Community Reputation

18 Good

About Lnewqban

  • Rank
    Cornering Master

Profile Information

  • Gender

Previous Fields

  • Have you attended a California Superbike School school?
  1. 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.
  2. 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
  3. 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.
  4. 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?
  5. 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?
  6. 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.
  7. 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.
  8. 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.
  9. Steer for the Rear - Ch13 of TOTWII

    We care the most about inducing the 40/60 weight distribution via throttle control when we need maximum performance from the tires and the suspension, when cornering on asphalt as fast as possible. If, while cornering like that, we put more weight on one tire, we compress that suspension and load that tire beyond the optimum state or conditions. The suspension becomes harder, the contact patch becomes a little bigger and the profile of the tire less pliable. Following the irregularities of the pavement is more difficult for the tire. The rubber becomes less elastic and it changes its shape more slowly. Once the weight carried by that tire while cornering hard reaches a crtical point, the available traction that the over-loaded tire can offer rapidly decreases. During a leaned wheelie, all the weight of the bike and the rider is on the rear tire and on the rear suspension. That tire would not be able to develop the traction demanded by the lateral forces of extreme cornering, which normally surpass the value of that weight. The wheelie always happens during the way out of the corner and at a lean angle that is much smaller than the max lean angle required by that turn. If the rider tries to wheelie the bike at that max lean angle, when the lateral forces of cornering on the contact patch are close to the max, the tire would slide. The tire would not slide only if the rear contact patch has been unloaded enough from lateral forces in a way that its performance can be reduced by the the extra weight.
  10. Steering Video No Bs Bike

    Using arm's force to countersteer and make any bike turn, we cancel the self-correcting property of the steering geometry. Motorcycles don't really need a rider to avoid cornering:
  11. Steering Video No Bs Bike

    First Newton's law of motion: In an inertial frame of reference, an object remains at rest or continues to move at a constant speed along a straight line path indefinitely, unless acted upon by a force.

    You are welcome The difficult part of your question is the 50% throttle: something hard to get precisely accurate. Full or partial throttle only means that the power delivery of your engine would be maximum or partial, leading to similar results than in the OP's case. The power plant of any bike delivers the necessary torque (rotational force) at the necessary rate (rpm's or torsional speed) to compensate for the forces that resist the acceleration and movement (internal friction, hills and aerodynamic drag mainly). At partial throttle, you are taming the engine to deliver exactly what you need to achieve certain final speed or rate of climbing or acceleration (a specific amount of resistive forces). At full throttle, you are full feeding the engine to generate maximum torque-rpm's combination (HP), which will be naturally resisted by certain amount of resistive forces (aerodynamic drag force grows with the square of the speed), which will result on maximum acceleration and speed on a level road. Electric motors have a delivery of power that is more or less linear with the "throttle opening". They will burn themselves trying to give you exactly what you ask. That is the reason for which electric bikes need few or no gear box to select gears. An internal combustion engine is a pneumatic machine that very much depends on "breathing" and over-the-piston pressure. That breathing is determined by intake, valving and exhaust and rpm's. That pressure is determined by the expansion of the gases due to the heat of the combustion. That makes them have a delivery of power (HP) and torque that is a curve rather than a line. For partial throttle, the amount of mix (air plus fuel) is artificially restricted (carb(s) or FI alike) via increasing intake resistance (butterfly damper(s)). For full throttle, the amount of mix is allowed to be has high as possible. After certain point along the range of rpm's, the breathing or mix intake gets compromised due to turbulences, needed time to expel the exhaust gases and valve floating (lack of time to fully close) and torque followed by HP begin to decrease. A good selection of the transmission steps and sprockets (the equivalent to manipulating the diameters of the wheels in the OP) tries to match max speed with the point of rpm's on the curve where the engine is stronger. That selection is always a compromise for tracks of different configurations (max acceleration out of curves (max torque) versus max speed on straight sections (max HP)), in order to complete the circuit as quickly as possible. If you wrongly select that 22-inch wheel for a track of few fast turns only and long straights, that other bike with a 27-inch will have an advantage over yours, reaching max speeds that are higher than yours. At full open throttle, the engine of your bike will reach its max HP about the same rpm's than the other engine, but it will not reach the max speed of the other bike with the bigger wheel. What happens is that your engine will deliver higher rearwards force onto the contact patch of the smaller rear wheel than needed to counteract the resistive forces generated by that speed (excess of rear wheel torque), but will restrict its own breathing or choke itself as soon as it tries to turn that smaller wheel faster to keep up with the other bike, resulting in less torque to fight the resistive forces (returning to the balance point). That is what happens when you extend a gear (second gear, for example) beyond the proper point of switching, the engine keeps screaming, but the bike does not move faster. When you switch to the next gear is the equivalent to replacing your rear wheel with a bigger one: you are simultaneously slowing down the rpm's of the engine and increasing the resistive torque, moving the operational point of the engine back over the curve of HP to a state in which it can deliver higher torque by breathing better. In extreme cases, when the engine is unloaded too much, even when the delivered torque cannot push that wheel (downhill, for example), it will reach the rpm's limiter, which is designed to prevent the auto-destruction of the engine due to excessive forces of its internal alternating parts, cutting the ignition and temporarily killing its strength. In essence, the proper diameter for your theorical wheel (sprocket and/or transmission selection in practical terms) should make your engine rotate at the optimum average rpm's (between top torque and top HP) demanded by the track conditions. https://m.youtube.com/watch?v=3idIpc0Bv_I

    Welcome, Kneedragger727 What the video shows is very close to be true. They show two different engine's rpm's (in and out) for the same speed of the bike. If you carefully watch between 3:33 and 4:00 times, you can detect an error: entering and leaving speed/engine rpm's ratios must be exactly the same when the bike is perfectly vertical. Why do the RPM increase without throttle input? Because Newton's first law of motion: "In an inertial frame of reference, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force." Because the masses of bike and rider try keeping a constant velocity, the engine "is forced" (see further explanation below) to spin faster by a smaller rear wheel that is forced to spin faster. Rear wheel and engine are solidly connected by a gear train (there is no relative slipping or jamming). In order to follow the constant speed or inertia of the bike, the leaned rear wheel must cover the same linear distance in the same period of time as when vertical, having now a smaller diameter and perimeter. The only way for the wheel to achieve that is by spinning faster (increasing its angular velocity) in the same proportion in which the diameter gets reduced (10% reduction in diameter induces 10% rpm's increase). Velocity of bike = Radius of wheel x Angular velocity of wheel Further explanation: When the throttle is full open, the engine is not really forced to spin faster. There is still enough pressure in the combustion chambers as for the engine keep pushing the rear wheel to rotate, although not at peak torque. The inertia phenomenon explained above unloads the engine (less torque applied over the rear wheel is needed), which performance point moves over the torque curve to a new state of lower delivered torque and higher rpm's. Nevertheless, if the leaned situation would last long enough, the speed of the bike and the rpm's of the engine will go down some. Because of all that, at the end of the curve the leaving speed is slightly lower than the entering speed of the bike. The whole story is that the engine and the gear train have their own rotational inertia and tend to conserve it due to the very same first law of motion........"unless acted upon by a force." A portion of that force is provided by the impulse or the change in momentum of the moving masses of bike and rider (the rest of the force is provided by a weaker engine off the peak torque). Impulsing the engine to spin faster plus the additional internal friction losses of engine and gear train cost energy. The kinetic energy (read speed) of the bike pays for that energy demanded by additional rpm's of the engine. If rather than moving at high speed, your bike with a wheel of smaller diameter (or a much bigger rear sprocket) would start from repose, the final speed of the bike and the the rpm's of the engine would end up being lower at full open throttle.
  14. Dirt vs Asphalt riding styles and technique

    Revisiting your #1 question, asphalt riders push the bike under them sometimes. Both types of riders are improving the agility of the bike to lean over the desired side when they "disconnect" the mass of their upper bodies from the mass of the bike. Yes, the result is an exaggerated final lean angle, but that could be beneficial on asphalt as well as the front tire turned at full lock will describe a smaller radius with a greater lean angle. The first five minutes of this video show the dramatic differences in steering inputs, accuracy of lines and available traction between dirt and asphalt: https://m.youtube.com/watch?v=BzF_q5ivlKE
  15. Dirt vs Asphalt riding styles and technique

    1) The traction is so marginal that the front tire cannot force the bike to turn as easily as tires on asphalt can. Sometimes, the rider tries digging the front tire into the loose surface in order to gain a traction that depends more on surface material building up over the sides of the tire than on pure friction. He/she achieves that by transferring the weight forward, by moving the body forward in the saddle and by extending one leg forward. When the described above is not sufficient to turn the bike as quickly as the next racer can, he/she increases lean angle which makes the rear tire step out of line. That achieves two things: the torque on the rear tires pushes the front tire to stay more or less in track by sliding less out of the turn and the material building up on the out side of the tire helps improve its traction. 2) Making the most from the marginal available traction is the priority. Those are their racing lines. Outside-inside-outside lines are for reducing the length of the curve and increasing the radius when traction is plenty and surface is firm enough to grant a precise line.