40 Foot Circle

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Let's say you were to lean a bike over and put it into a 40 foot diameter (or radius) circle turn. In a perfect world and no additional steering changes or pressure on the bars, you roll on the throttle just enough to keep the suspension loaded. Would these things happen:

1. Bike increase this 40 foot diameter circle

2. Decrease in ground clearance

3. Increase in lean angle as speed increased

4. Eventually break tire traction

?

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Let's say you were to lean a bike over and put it into a 40 foot diameter circle turn. In a perfect world and no additional steering changes or pressure on the bars, you roll on the throttle just enough to keep the suspension loaded. Would these things happen:

1. Bike increase this 40 foot diameter circle

2. Decrease in ground clearance

3. Increase in lean angle as speed increased

4. Eventually break tire traction

?

If you have a fixed lean position and constant speed going around the circle, I'm guessing basic physics would keep you in the turn not giving for tire slippage and wear, depending on how long you would do it.

To stay in the circle when increasing speed you would have to increase lean angle which would decrease ground clearance.

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Sorry. Double post.

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Let's say you were to lean a bike over and put it into a 40 foot diameter circle turn. In a perfect world and no additional steering changes or pressure on the bars, you roll on the throttle just enough to keep the suspension loaded. Would these things happen:

1. Bike increase this 40 foot diameter circle

2. Decrease in ground clearance

3. Increase in lean angle as speed increased

4. Eventually break tire traction

?

If you have a fixed lean position and constant speed going around the circle, I'm guessing basic physics would keep you in the turn not giving for tire slippage and wear, depending on how long you would do it.

To stay in the circle when increasing speed you would have to increase lean angle which would decrease ground clearance.

Is there any way to do this circle while observing TC Rule #1 and Steering Rule #1?

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Let's say you were to lean a bike over and put it into a 40 foot diameter circle turn. In a perfect world and no additional steering changes or pressure on the bars, you roll on the throttle just enough to keep the suspension loaded. Would these things happen:

1. Bike increase this 40 foot diameter circle

2. Decrease in ground clearance

3. Increase in lean angle as speed increased

4. Eventually break tire traction

?

1) Yes the radius would increase as speed does as long and you don't put an input into the bars.

2) Ground clearance might decrease a little as the speed increases and the suspension loads up. Not 100% sure on this one.

3) As long as you do not put an input into the bars the lean angle should not change.

4) Yup.

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Let's say you were to lean a bike over and put it into a 40 foot diameter circle turn. In a perfect world and no additional steering changes or pressure on the bars, you roll on the throttle just enough to keep the suspension loaded. Would these things happen:

1. Bike increase this 40 foot diameter circle

2. Decrease in ground clearance

3. Increase in lean angle as speed increased

4. Eventually break tire traction

?

1) Yes the radius would increase as speed does as long and you don't put an input into the bars.

That would appear to disagree with TC Rule #1

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Let's say you were to lean a bike over and put it into a 40 foot diameter circle turn. In a perfect world and no additional steering changes or pressure on the bars, you roll on the throttle just enough to keep the suspension loaded. Would these things happen:

1. Bike increase this 40 foot diameter circle

2. Decrease in ground clearance

3. Increase in lean angle as speed increased

4. Eventually break tire traction

?

1) Yes the radius would increase as speed does as long and you don't put an input into the bars.

That would appear to disagree with TC Rule #1

I don't think it would. Increasing speed without increasing lean makes you go wider. Maintaining speed and increasing lean would tighten the circle.

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Maintaining speed and increasing lean would tighten the circle.

...or put him on the ground.

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Let's say you were to lean a bike over and put it into a 40 foot diameter circle turn. In a perfect world and no additional steering changes or pressure on the bars, you roll on the throttle just enough to keep the suspension loaded. Would these things happen:

1. Bike increase this 40 foot diameter circle

2. Decrease in ground clearance

3. Increase in lean angle as speed increased

4. Eventually break tire traction

?

1) Yes the radius would increase as speed does as long and you don't put an input into the bars.

That would appear to disagree with TC Rule #1

I don't think it would. Increasing speed without increasing lean makes you go wider. Maintaining speed and increasing lean would tighten the circle.

Perhaps I missed something.

Imagine this:

You're at full max lean. The force applied to the rear tire (acceleration) causes a downward force (relative to the tank, not relative to earth), which presses the tires harder into the tarmac. The ground pushes back, increasing traction. This load must be absorbed and is absorbed by the front and rear springs. Eventually you will either run out of ground clearance due to compressed suspension (which IMHO may or may not happen even in a bottomed suspension, but that's another discussion), run out of traction (based on tires) or run out of engine power. Why would the turn radius increase? What would put the rider on the ground? What am I missing?

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You're at full max lean. The force applied to the rear tire (acceleration) causes a downward force ...What would put the rider on the ground? What am I missing?

JB;

I'll try to answer your last question in practical terms (not in theory) because theory gets scant attention when your sliding on your butt across the track.

If you're at maximum lean angle and you add any appreciable acceleration (or braking force) - you will break the rear (or both) tire(s) loose. Key word here is maximum meaning the limit of lean for a given speed, suspension settings, surface conditions, temperature and tire wear, etc.) I have lowsided at what turned out to be maximum lean by adding throttle once and braking twice. None of these were in theory and all caused me much grief.

Kevin

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You're at full max lean. The force applied to the rear tire (acceleration) causes a downward force ...What would put the rider on the ground? What am I missing?

JB;

I'll try to answer your last question in practical terms (not in theory) because theory gets scant attention when your sliding on your butt across the track.

If you're at maximum lean angle and you add any appreciable acceleration (or braking force) - you will break the rear (or both) tire(s) loose. Key word here is maximum meaning the limit of lean for a given speed, suspension settings, surface conditions, temperature and tire wear, etc.) I have lowsided at what turned out to be maximum lean by adding throttle once and braking twice. None of these were in theory and all caused me much grief.

Kevin

That's why I asked the question about disagreeing with TC rule #1. As I understand it, once in the turn the throttle is rolled on smoothly evenly and throughout the remainder of turn. This 40 foot question is a hyper example of a sweeper, is it not?

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You're at full max lean. The force applied to the rear tire (acceleration) causes a downward force ...What would put the rider on the ground? What am I missing?

JB;

I'll try to answer your last question in practical terms (not in theory) because theory gets scant attention when your sliding on your butt across the track.

If you're at maximum lean angle and you add any appreciable acceleration (or braking force) - you will break the rear (or both) tire(s) loose. Key word here is maximum meaning the limit of lean for a given speed, suspension settings, surface conditions, temperature and tire wear, etc.) I have lowsided at what turned out to be maximum lean by adding throttle once and braking twice. None of these were in theory and all caused me much grief.

Kevin

That's why I asked the question about disagreeing with TC rule #1. As I understand it, once in the turn the throttle is rolled on smoothly evenly and throughout the remainder of turn. This 40 foot question is a hyper example of a sweeper, is it not?

I am getting pretty far afield of my area of expertise here but I wll try. TC rule #1 says that once the turn is initiated the throttle is rolled on smoothly and consistenly doesn't it? That doesn't mean wacked WFO, it just means smoothly and consistenly even if it is done in small measures. A sweeper requires that you continually roll on (or you will lose speed) but it is done in relative terms to the conditions that you're in. When you asked about turning in a circle I inferred that to mean a circle with out an entry, an apex or an exit and you would be riding at max lean angle; adding speed in that circumstance will put you on the ground IMHO.

Kevin

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I am getting pretty far afield of my area of expertise here but I wll try. TC rule #1 says that once the turn is initiated the throttle is rolled on smoothly and consistenly doesn't it? That doesn't mean wacked WFO, it just means smoothly and consistenly even if it is done in small measures. A sweeper requires that you continually roll on (or you will lose speed) but it is done in relative terms to the conditions that you're in. When you asked about turning in a circle I inferred that to mean a circle with out an entry, an apex or an exit and you would be riding at max lean angle; adding speed in that circumstance will put you on the ground IMHO.

Kevin

...TC rule #1 says that once the turn is initiated the throttle is rolled on smoothly and consistenly doesn't it? That doesn't mean wacked WFO,

Whacked open not implied here

A sweeper requires that you continually roll on (or you will lose speed)

Hence my example. A sweeper is a semi-circle, is it not?

... but it is done in relative terms to the conditions that you're in.

That could be a key phrase in your reply.

When you asked about turning in a circle I inferred that to mean a circle with out an entry, an apex or an exit and you would be riding at max lean angle; adding speed in that circumstance will put you on the ground IMHO.

Kevin

Agreed. Geometric circles don't have an entry nor exit. Obviously in this example you'd have to begin SOMEWHERE and quit the exercise SOMEWHERE, presuming that you don't land on your butt. Why do you think you'd fall? Would it be lowside or highside? What is the cause?

-Disclaimer- I'm not challenging your knowledge on this subject it's just my way of illustrating to a point of absurdity for understanding and clarity.

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Jay, your example does not relate well to a corner on a race track. Corners have a beginning and and end, circles do not. I think it is a futile exercise to try to apply a technique that was intended for a corner (throttle control) to a circle.

In short corner your roll on the gas (throttle control) can be fairly aggressive.

In a longer corner your roll on must be more gradual, or you will run wide of your intended line.

Speed + Lean = radius

If you lean does not change and your speed increases the radius will increase.

If you are at the limit of traction and you push on the bars to add lean (to maintain a given radius) you will loose the front.

If you are at the limit of traction and you give the bike more throttle, you will loose the rear.

If you continue to add throttle while going around in a circle you will eventually reach a limit.

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Jay, your example does not relate well to a corner on a race track. Corners have a beginning and and end, circles do not. I think it is a futile exercise to try to apply a technique that was intended for a corner (throttle control) to a circle.

There is a school (won't name names) that has it's students tracing circles in a parking lot as a foundation to cornering on a racetrack. I don't know if this is all they teach but I had an extensive discussion with it's founder and noted author (hint, hint). Are you saying that this has no merit?

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I assume you are referring to Lee Parks' school.

I'm saying that riding around in a circle would not be a good way to teach throttle control.

We have our students ride around in a circle for the the lean bike, but we work on body position, not throttle control. Throttle is constant during these drills.

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Jay, your example does not relate well to a corner on a race track. Corners have a beginning and and end, circles do not. I think it is a futile exercise to try to apply a technique that was intended for a corner (throttle control) to a circle.

There is a school (won't name names) that has it's students tracing circles in a parking lot as a foundation to cornering on a racetrack. I don't know if this is all they teach but I had an extensive discussion with it's founder and noted author (hint, hint). Are you saying that this has no merit?

I agree totally with what Stuman says in his post above. To me, maintaining a constant radius circle is a LOT different from going around a corner; it intuitively makes sense that you cannot increase throttle indefinitely on a fixed circle without eventually reaching the limit of tires and/or available lean angle (unless, of course, you are in one of those cool circle cages where you are making loops without having to lean the bike...). On a racetrack there is an exit to the corner, so you can gauge your roll-on rate to match the corner such that you are WFO as soon as possible to get best exit speed, which is, presumably, the goal. My confusion in your question is I don't quite get what you are trying to find out, or how you wish to apply the information, which makes it a bit harder to answer the question in any helpful way. Is there one particular aspect of this that you are trying to apply on the track?

Regarding using a circle as an exercise, I think it's a great thing to do to really get comfortable with the concepts we are discussing. After all, all you need is a smooth parking lot and you can go out, start on a circle, then roll on a bit, then roll off a bit, and see exactly how the bike reacts - circle gets bigger, circle gets smaller. It's also a good way to get comfortable with being leaned over and a good way to practice body position; you can do it at CSS on the lean bike, too, of course. So, I think it is a very useful learning tool, since being able to isolate the concepts and test them directly, without other distractions, is extremely helpful.

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If you lean does not change and your speed increases the radius will increase.

How does the radius increase on a bike leaned to the inside of a turn when no bar input has been applied (assuming the tires are not sliding)?

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If you lean does not change and your speed increases the radius will increase.

How does the radius increase on a bike leaned to the inside of a turn when no bar input has been applied (assuming the tires are not sliding)?

It just DOES, dammit.

OK, I'll take a stab at this, but I'm fearful it will start one of those scary physics discussions. And before I even start, let me make the point that it's probably not realistic to assume the tires are not sliding - they pretty much always are, to some degree, when you are turning, right...?

I THINK that as you roll on the throttle, you are increasing the force that is making the bike want to go straight (linear momentum?) AND stand up (less lean angle), so unless you actively prevent it from happening, the bike will stand up as you roll on and thus your circle will get bigger. If you roll on AND push on the bar to try to keep your lean angle constant, you will be adding more load on the tires and could lose traction.

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If you lean does not change and your speed increases the radius will increase.

How does the radius increase on a bike leaned to the inside of a turn when no bar input has been applied (assuming the tires are not sliding)?

It just DOES, dammit.

OK, I'll take a stab at this, but I'm fearful it will start one of those scary physics discussions. And before I even start, let me make the point that it's probably not realistic to assume the tires are not sliding - they pretty much always are, to some degree, when you are turning, right...?

I THINK that as you roll on the throttle, you are increasing the force that is making the bike want to go straight (linear momentum?) AND stand up (less lean angle), so unless you actively prevent it from happening, the bike will stand up as you roll on and thus your circle will get bigger. If you roll on AND push on the bar to try to keep your lean angle constant, you will be adding more load on the tires and could lose traction.

Yes, this IS an academic discussion.

I recall in Twist 1 that it says that a motorcycle in a turn will continue to turn unless the rider's countersteering action stands it up.

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Yes, this IS an academic discussion.

I recall in Twist 1 that it says that a motorcycle in a turn will continue to turn unless the rider's countersteering action stands it up.

OK, I retract my prior statement, as a very knowledgeable friend tells me that rolling on the throttle (even rolling it on a LOT) does NOT, in fact, make the bike stand up, that if the bike is standing up under acceleration it's because the rider is steering it up. So now this question is bugging me, too...

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If you lean does not change and your speed increases the radius will increase.

How does the radius increase on a bike leaned to the inside of a turn when no bar input has been applied (assuming the tires are not sliding)?

Here is a quote from Wikipedia that's interesting:

Because real tires have a finite contact patch with the road surface that can generate a scrub torque, and when in a turn, can experience some side slipping as they roll, they can generate torques about an axis normal to the plane of the contact patch.

One such torque is generated by asymmetries in the side-slip along the length of the contact patch. The resultant force of this side-slip occurs behind the geometric center of the contact patch, a distance described as the pneumatic trail, and so creates a torque on the tire. Since the direction of the side-slip is towards the outside of the turn, the force on the tire is towards the center of the turn. Therefore, this torque tends to turn the front wheel in the direction of the side-slip, away from the direction of the turn, and therefore tends to increase the radius of the turn.

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If you lean does not change and your speed increases the radius will increase.

How does the radius increase on a bike leaned to the inside of a turn when no bar input has been applied (assuming the tires are not sliding)?

Here is a quote from Wikipedia that's interesting:

Because real tires have a finite contact patch with the road surface that can generate a scrub torque, and when in a turn, can experience some side slipping as they roll, they can generate torques about an axis normal to the plane of the contact patch.

One such torque is generated by asymmetries in the side-slip along the length of the contact patch. The resultant force of this side-slip occurs behind the geometric center of the contact patch, a distance described as the pneumatic trail, and so creates a torque on the tire. Since the direction of the side-slip is towards the outside of the turn, the force on the tire is towards the center of the turn. Therefore, this torque tends to turn the front wheel in the direction of the side-slip, away from the direction of the turn, and therefore tends to increase the radius of the turn.

Yes more centrifugal force will cause the bike to start running wider creating a bigger radius. Nice definition you found to explain it there Hotfoot.

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Here is a quote from Wikipedia that's interesting:

Because real tires have a finite contact patch with the road surface that can generate a scrub torque, and when in a turn, can experience some side slipping as they roll, they can generate torques about an axis normal to the plane of the contact patch.

One such torque is generated by asymmetries in the side-slip along the length of the contact patch. The resultant force of this side-slip occurs behind the geometric center of the contact patch, a distance described as the pneumatic trail, and so creates a torque on the tire. Since the direction of the side-slip is towards the outside of the turn, the force on the tire is towards the center of the turn. Therefore, this torque tends to turn the front wheel in the direction of the side-slip, away from the direction of the turn, and therefore tends to increase the radius of the turn.

... generate a scrub torque, .... generate torques about an axis normal to the plane of the contact patch.

...such torque is generated by asymmetries in the side-slip along the length of the contact patch. The resultant force of this side-slip occurs behind the geometric center of the contact patch, a distance described as the pneumatic trail...

I usually like wikipedia, but I'm screaming bovine scatology on that one.

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Simple question, but with some not-so-simple considerations. The obvious part of the answer is that increased speed increases the apparent "centrifugal force". ("Centrifugal force" is a misnomer that physicists will challenge, but it works fine for this discussion.) In a "balanced turn" the lean angle and speed are such that the centrifugal force acting to tip the bike upright is exactly balanced by the gravity force trying to tip the bike over onto its side. At a fixed lean angle, if speed is increased the tipping force due to centrifugal force becomes stronger than the tipping force due to gravity, and the bike wants to tip upright-- i.e. to less lean angle.

Since the original question stipulates that there is no rider input to the steering, my initial thought was that the bike would have to tip upright, and the forks would do whatever they had to in order to make the tire happy-- i.e. produce the least scrubbing action, which means that the steering angle would decrease as the turn radius increased.

However, there is another input to the steering than rider input and tire input, and it caused by steering trail. Suppose you had a bike on its centerstand (racers can ask tourers what that is), resting lightly on its front wheel. With its steering centered, push on the left side of the bike tank. You'll see the steering shift right, because the front tire contact patch is behind the point where the steering axis meets the pavement. I think that, as speed increases in a turn, the trail-induced forces tend to steer away from the direction of turn, which would act to increase the lean angle. Therefore, I suspect that a bike with enough trail might actually lean more into a turn, and actually decrease the turn radius, as speed increases. On the other hand, a bike with an intermediate amount of trail might actually tend to increase lean angle exactly enough to maintain a constant radius turn as speed increased-- with zero rider input to steering!

The latter condition sounds like nirvana-- you could fool around all you want with the throttle (within the traction limit), and not affect your line through a turn at all! (But, being a ######, Nature probably exacts some nasty other form of penalty for such a virtue.)

It would be really interesting if one of our Ducati bretheren with the switchable rake/trail bearing cups would do an experiment for us, to see if changing trail changes how a bike with no steering input behaves during throttle roll-on in an initially balanced turn. Of course, we would have to remove his throttle from the handlebar to make sure he REALLY wasn't providing steering input, and we'd have to immobilize all his joints with duct tape (except for right wrist-- we could tape the twist grip into his mouth) so that he couldn't influence the experiment with body motion, and I'd have to keep his bike afterward in case further experiments were required...

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