tzrider

Tim, I trust my own eyes. I still disagree with you. Have a look at the following video of a guy countersteering quickly to the right, as viewed from in front of the bike: Video [1] Watch the centerline of the bike and note how the front tire visibly swerves the the rider's left before the bike turns the the right. You will see that the front tire swerves at least four inches to the left of the original line of travel before the bike cuts right. The vantage point of the video does not let us see clearly how far out from under the bike's center of mass the wheels steer, but it will be around 12 inches. Observing the phenomenon isn't simple, because the moment the bike leans, it begins to turn and you lose your vantage point. It's helpful to remember that the bike leans by having its tires steered out from under its center of mass, or roll axis if you like, not out of the line of travel. I make the distinction because the roll axis moves toward the inside of the arc as you lean the bike and this does make it hard to see the changing orientation of the contact patches and roll axis of the bike. Turning quickly is the only practical way to see this, as a great deal of lean angle change takes place before the bike peels off into the turn. [1] The original link can be found on this page: Link. I don't agree with everything the author says, but he's mostly right.

It sounds like we may now be on the same page about two things when entering a right turn: 1) Turning the front wheel left makes the bike lean right. 2) When you stop turning the front wheel to the left, it turns back to the right to follow the path of travel. The only disagreement we have is whether precession makes the bike lean. I say not only is it not the primary force, it actually doesn't help the bike lean at all. Consider the following: You know that if you hold a bicycle wheel in front of you by the axle ends and spin it forward, then push the right end of the axle forward the wheel will both turn left on a vertical axis and tip right on a horizontal axis. It's the classic demonstration of precession. If you then pull back on the right end of the axle, the reverse will happen; the wheel will turn right on a vertical axis and tip back left on a horizontal axis, returning to vertical. With that in mind, consider countersteering. You're going straight and enter a right hander. You push forward on the right bar which turns the wheel left and the bike leans right. Once you stop pushing the bar, the wheel turns right to follow the path of travel. If precession were the driving force that leans the bike, the bike would stand back up. We both know that doesn't happen. And yet, precession doesn't work in only one direction or just when we want it to. Precession results whenever the wheel turns, but the resulting torque on the bike isn't enough to make it do much of anything. The wheel is held captive by a much larger mass. It is the act of steering the tires out from under the bike to the left that makes the bike lean right.

Tim, do you believe that during an entire right hand turn your front wheel remains pointed to the left?

You are correct that in the case above when you turn the bars right the contact patch moves to the left. If the bike were not moving, it would indeed tip to the right. When the bike *is* moving, the front wheel veers to the right, as that's the direction it's pointing. If you don't do anything other than turn the bars, the bike would lean left. At low speeds, many riders are accustomed to initiating lean with body movement and then turning the bars into the arc. Whether you initiate lean with body movement or by consciously countersteering, once the bike is leaned, you do have to turn the bars (or allow them to turn) into the arc. I would guess that when you turned the bars, you also moved your upper body a little bit, since you expected the bike to lean in the direction the bars were turned. At such a low speed, your body weight would be sufficient to initiate lean. Note that this is not the same thing as saying that countersteering does not work below a certain speed. If the wheels are rolling, countersteering will work. However, at low speeds, you can initiate lean with body weight shifts that would be ineffective at higher speeds. To refer back to Will's example, if you have leaned the bike a little farther than you meant to in a low speed turn and find it about to fall to the inside of the turn, without putting a foot down, what would you do to save it?

The above is a key point. If precession drove lean angle changes, then not only would the bike lean over when you countersteer, but when you stopped countersteering the bike would stand right back up when trail repositioned the wheel on the path of travel. That doesn't occur.

That's for sure. A few other observations to support your point: If you get lighter wheels for your bike you will find that it countersteers just as effectively, but with greater ease. If angular momentum were a primary force in initiating lean, lighter wheels should not be able to steer the bike as effectively. If you observe someone countersteering quickly from behind, you will see that the bike rolls (leans) on an axis that is somewhere near or above the midpoint of the bike's height. Precession would create a torque at the axle and if precession were the primary cause of lean angle changes, the roll axis would be at the height of the axle. Ski bikes reportedly countersteer. They don't have wheels; there is no precession involved.

Yeah, you're supposed to be mad at racer. Don't worry about why, just be mad at him. ab

We have a winner! ab

Knowing what your speed is before you enter the turn doesn't compromise the question, which is: What will happen to your speed if you turn the bike and maintain constant throttle? ab

Let me ask Cobie's question a different way. If I get it wrong, he can jump in: You're riding on a road in Kansas -- not a hill or camber change for 100 miles in any direction. You're on a straight section of road, holding a steady 60 mph with the throttle partway open. You turn your bike into a constant radius corner while maintaining the exact same throttle opening. What happens to the bike's speed? ab

I don't offend and you're being nice anyway. No worries, my friend. ab

Racer, I don't know if you've gotten an answer to your question of where the fulcrum is. M1 said something about it a few posts back, but your subsequent posts make it seem you're not certain where the fulcrum would be. Getting agreement on this is going to be a key to understanding the rest of the discussion, I think. M1 stated that the roll axis is somewhere between the CoM and the contact patch. For practical purposes, I think the roll axis is almost exactly at the CoM. Wherever it actually is, this point is the fulcrum you've been asking about. When you apply force to a lever connected to a free body (such as the handle of a hammer balanced on your hand), the fulcrum will be the roll axis. BTW, as you do, I like to keep it real too. Analogies are good though, because they can help isolate various pieces of a more complex system to understand the dynamics of each aspect independently. You've used a few analogies yourself along the way and seem to find them useful within limits too. In your trip-the-running-man example, I would suggest that the fulcrum is his CoM. The lever end is the point at his ankle where you trip him. This continues to be interesting. Any comments on where the fulcrum is?

Not as simple as that. Your equation doesn't address the height of CoM, does it? I couldn't find your ealier post with this formula, so I don't know for sure what all your variables are above, though most of them are intuitive. I'm not sure what x and d are. Can you clarify? Even so, your formula doesn't address lean angle at all; I suspect it addresses equillibrium of traction vs. lateral acceleration. This formula does deal with lean angle: m * g * cmh * sin(theta) = m * a * cmh * cos(theta) Variables: m is mass g is gravity cmh is height of CoM theta is lean angle from vertical a is lateral acceleration (force tending to push the bike to the outside of the turn) Remember that whatever formula you use for this must take "cmh" into account with respect to both gravity and lateral acceleration. Your earlier post on this thread used F1 cars as an example, but the example doesn't apply, as F1 cars don't lean into the corner. To summarize my point, the height of CoM affects some things, but it does not affect lean angle required in a given turn at a given speed. Hanging off does, because you are moving the CoM of the bike/rider towards the inside of the turn. This certainly allows you to stand the bike up a little.

Racer, I agree with you that datadan could have chosen a better word than "highside" for what he was saying. I interpreted his words to mean that the lateral force tends to stand the bike up, while gravity tends to make it fall in. The two vectors should zero each other out. Have a good evening out and I'll look forward to what more you have to say. On the idea that we're overthinking this, I'm with you, in that the discussion may not help your riding. Being a little nerdy, I like the subject anyway.

I'm a little confused by your last couple of posts -- not sure what issue you're addressing. If you happen to be talking about lean angle while in a turn, I don't know that reading up on gyroscopic force will help. Here's an example that may help distinguish the notion of equillibrium while leaning in a turn from anything to do with gyroscopes. Imagine that you are standing on the outer edge of one of those big turntable things you sometimes find in a child's playground. You know, they're about 12 feet across, have handrails on them and you can run around to get them spinning and then ride them until you vomit. Let's say you're riding one of these while you're spinning and decide to balance a yardstick on your finger. If you could manage to balance the yardstick (let's pretend that the breeze wouldn't make this next to impossible), would the yardstick be dead vertical or would it be leaning in some direction? Why?

In my example above, I'm assuming the same speed and same turn radius.

I understand your point above, but would point out that "easier" might not always equal "quicker." I assume we agree that the roll axis is at or near the center of mass when we're changing lean angle and that the lean angle change comes about as a result of deflecting the tires one way or the other under the CoM. With a longer moment from the contact patch to the CoM, there is greater leverage on the mass of the bike/rider to make it roll, but the tires must also deflect farther to cause a given amount of lean angle change. By contrast, lowering the CoM will give you less leverage on the mass of the bike, thus making it feel harder to turn, but will also mean that the tires don't have to deflect as far to cause the same lean angle change. You could probably reach a point of diminishing return either way by raining or lowering the CoM too much. When you lower the CoM, the bike should turn more quickly, assuming the rider is stong enough to work with the reduced leverage. When you get to the point that the rider cannot press the bars hard enough to turn the bike faster, you are completely right that raising the CoM would allow the rider to turn the bike faster. The height of the CoM really doesn't matter at all. Assuming you don't shift weight from side to side, as in hanging off, the only variables left that determine lean angle are speed and turn radius. The speed of the motorcycle squared divided by the radius of the turn equals the lateral acceleration of the vehicle. When a motorcycle is leaned into a turn, lateral acceleration is acting on the CoM to stand the bike up, while gravity is acting on the CoM to make the bike fall into the turn. The bike is in equillibrium when the lateral acceleration and gravity are equal. See the following diagram: On the left part of the diagram, the leaned object has a high CoM, which lengthens both the moments of gravity and lateral acceleration. The diagram on the right shows a lowered CoM at the same lean angle, and you see that the moments of both gravity and lateral acceleration are shorter. If you'd prefer to look at this mathematically this thread on the Cycle World forum has a good proof from "DataDan:" Link Here is an excerpt from the relevant post:

The height of CofG makes no difference at all to the lean angle required to get through the turn, assuming the speed and line remain the same. Lowering the CofG on a motorcycle makes it easier to turn quickly by reducing the moment arm from the CofG to the contact patches. Kind of the same thing manufacturers talk about with regard to mass centralization; the closer all the mass is to the roll axis, the more easily you can turn the bike.

Here are three advantages to being square (not crossed up) on the bike: 1) Having your body parallel to the bike gets more of your weight down and to the inside of the bike. Keith mentioned this too and this is the primary reason to hang off in the first place. Riders who cross up lose some or all of this advantage. 2) When you are crossed up, your hips rotate around the tank. This forces your outside knee off the side of the tank and prevents you from using your knee against the tank as an anchor. Most riders can't fiind a substitute for that very critical anchor and end up latching onto the bars because they feel like they're about to fall off. 3) If you are going around a right-hand corner and you cross up, your body is rotating to the left. In order to look into the corner (to the right), you need a neck like an owl. ab

A couple of months ago, Cobie Fair, Chief Instructor for the California Superbike School, called to ask if I might have some time in the middle of June to work a school. He?d need me for a week. It hasn?t been easy to take much time off for the past couple of years, so I asked, ?A week? Where is it?? ?Rio de Janeiro.? ?Right. Lemme see what I can do.? A few weeks later, Keith and Judy Code, James Toohey, Cobie and I were piled into an airport limo amid our gear, listening to the many fascinating things James had learned about Brazilian women prior to the trip. He?d learned quite a lot. So much, in fact, that it took most of the next 20 hours of travel time for him to finish briefing us. We had also heard that getting through Customs might be a lengthy ordeal, taking up to several hours. Stories were circulating about frustrated travelers making obscene gestures to officials and incurring fines upwards of $13,000. Sounded serious. When I arrived at the Customs desk, the fellow behind the glass wanted to see my passport and declarations. He looked them over for all of 15 seconds, handed them back and smiled, ?Have a nice day.? That was the whole process. It was easier than getting into Canada! The event organizer, D?cio, did an outstanding job of planning this event and provided students and staff with every amenity. He has in fact raised the bar for services provided at the school. He served incredible catered meals, hired a masseuse to work people over if they were feeling ?a little tight on the bike,? and provided a mechanic who gassed the instructors? bikes after each session. I?m including that last bit in the thin hope that Will, the CSS mechanic, might read this and draw some inspiration. For gassing the bikes, not giving me a backrub? Brazil?s population is about 160 million people and the wealth distribution is very uneven, being second only to Bangladesh. 10% of the Brazilian population owns 50% of the nation?s wealth, while the poorest 50% of the population holds only 10%. People who ride motorcycles on a racetrack in Brazil are in the top economic tier. As we drove from the airport to our posh accommodation, we saw vast slums that served to remind us upon whose backs this economy rides. Some of the things that go along with this are a bit surreal to a Norte Americano. D?cio?s cars are bullet-proof. This is not eccentric paranoia; there is a whole industry in South America based on kidnapping members of moneyed families for ransom. We heard of one family that has had 27 family members kidnapped over time. Kidnappers don?t hold out for extremely high stakes; the demands are typically low enough to be a trivial decision to buy the safe return of a family member. Thus, the industry thrives. As far as I could tell, tourists aren?t at much risk for kidnapping, but there are some places you don?t want to go. In some areas of the city, bandits will throw a tourist on the ground and rifle through his pockets. There is no real safety in numbers, as the bandits have lots of friends. If not, one guy with a gun can be persuasive too. Not that any of us experienced this first-hand. We were too busy getting sand on our tongues at the beach. Brazilians seem quite proud of their women, as well they should be. The bathing suit of choice for women is a variant of the bikini, called a tanga. The tanga is a very attractive garment that has the secondary benefit of being quite economical. You can make about three of them out of an ordinary dinner napkin and a roll of dental floss. The next morning, we went to the Nelson Piquet raceway to get oriented. When we arrived in the vicinity, our first impression was of a high, bleak concrete wall encircling the place. The facility was very prison-like in appearance and the image was heightened by the presence of armed guards at the gate and throughout the premises. The track was first constructed in 1975 and was rebuilt in 1995. The layout was very fast and the pavement looked wonderful. Crews were on the track cleaning it and painting the curbs in preparation for the Brazilian round of MotoGP. The scenery from every angle of every corner of the track was breathtaking. We did off-track drills with the students that afternoon, which laid a good foundation for the next two days of on-track instruction. D?cio had provided three translators; one translated Keith?s classroom briefings, one was at course control in the event they needed to talk with a rider and one was in the hot pit to translate conversations when instructors pulled students off the track. It worked very well. The mix of students included moto-journalists from the top four motorcycle publications in Brazil, a few current racers and some enthusiasts. Everyone was very eager to learn and most students were unfamiliar with the topics we teach. Still, some of the guys were pretty fast to begin with and doing reasonably well in their racing. Students did a nice job with our drills and some of our techniques and material were quite dramatic to them. The one thing everyone kept saying was how shocked they were at how fast our instructors turn their motorcycles. To varying degrees they all learned to flick their bikes faster, but this was a very new concept to most of them. Another drill that occurs late in the first day focuses on getting riders to relax on the bike. At a point in the day where riders are beginning to tire, this is typically a popular exercise. On-track instructors ride with students during that drill, modeling a relaxed posture and pantomiming loose, relaxed arms by flapping their elbows as they corner. As usual, every student improved, but some of the results were stunning. One racer came to the school in hopes of improving his times by 2 or 3 seconds; he finished the second day running times 9 seconds faster than his previous fastest, most desperate race lap! He wouldn?t stop grinning. It would be fascinating to see his next race; his competitors are going to wonder what the heck got into him. For our part, we found the track very entertaining. It was partly the sheer speed; most of the corners are fast and the back straight is the longest on the GP circuit at over 1000 meters. The MotoGP bikes will be hitting over 200 MPH on that straightaway. Even our 600?s were hitting the rev limiter in top gear. While the pavement had seemed perfectly smooth when we were puttering around, at speed some turns were quite bumpy. The grip levels seemed good when the sun was out, but in the shadow, the grip went off. I?ve read since returning home that the course is not renowned for its high level of grip. Anyway, it made fun riding for us and the upcoming MotoGP will be outrageous. On the morning of the second day, one of James? students had a bee fly into his helmet. He was in turn eight or nine and suddenly had this critter buzzing around in front of his eyeballs for the remainder of the lap. He sat up on the bike and started to freak out a bit, death-gripping the bars and wobbling around. Who should happen by at this moment but James, who saw all the shenanigans and decided he?d better help the guy out. He passed the rider, got his attention and started flapping his elbows, ?Loosen up, dude!? D?cio punctuated the event at the end of the second day of classes with a party. Before he let any of the instructors start drinking, he had us each answer a brief interview question for national TV. I think he was wise to do things in that order, now that I have seen how we all behave drunk. Everyone was included in the party. Between the track workers, paramedics, officials and riders, most socio-economic groups were represented. We all participated in group photos and festivities, and it meant a lot to all of us to have contributed something to this event. Students, support staff and instructors, all went to dinner afterward to celebrate well into the night. We indulged in everything you might expect from people who know they have absolutely no obligations the following day. Indeed, we awoke around noon and ambled down to the beach to get a little more sand on our tongues, body surf and drink coconut milk out of coconuts while lying in the fine, white sand. Later, D?cio?s good friend Mario took us on a driving tour of one of the local mountains. It wasn?t until we were high on the mountain looking out over the vast city that I really understood how amazing Rio de Janeiro is. The scale is immense, the layout beautiful and the geology bizarre. Huge granite batholiths rise out of the shoreline to heights of over 2000 feet. These break the city into segments, so it isn?t possible to see the whole city from any part of the city itself. Keith may have said it best when he said, ?This is like San Francisco on steroids.? It?s an apt description from both the standpoints of the difference in scale and geographical relief. On our last day, we decided to go shopping. Some of us were under orders to bring something home. Requirements included: Must be Brazilian, must not be a tanga. That?s OK, none of us guys were any too eager to be seen in the local Speedo-thingy the men wear. We ended up in the largest mall in South America. In many respects, this could have been any large mall in the US. One of the few remaining clues that we were still in Brazil was the presence of many stunning Brazilian beauties. As we strolled through the mall, James risked whiplash time and again. Sometime later, we noticed that the girls in various shops were making a sport of watching men watching women. Whatever else you can say about Brazil, it makes no pretense of being gender-neutral about anything. That afternoon, we said our goodbyes and began the long trip home. I?ve had most of a week since to reflect on the warmth and genuineness of everyone we met, the enthusiasm and gratitude of our students and the grace, thoroughness and thoughtfulness of our hosts. It was a special and memorable trip in which we were treated to many wonderful things and got to provide a few Brazilian students with the best rider training on Earth. Andy Burnett