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40 Foot Circle


Jaybird180

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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

 

?

 

Number one for sure, number four is unlikely.

 

Keith

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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

 

?

 

Number one for sure, number four is unlikely.

 

Keith

 

Jaybird asked a follow up question - what exactly is the mechanism the CAUSES the radius of the circle to increase, if the rider does not add any steering input? Intuitively I thought that if you rolled on the throttle (possibly harder than Jaybird was thinking in his quote above) the increased forces would make the bike want to stand up and go wider, but I am told that this is not true, that with no rider input the bike's lean angle will not change when the throttle is rolled on. So, is that correct? And if so, what IS it that makes the bike travel a larger circle, slippage of the tires? Does this depend on bike setup?

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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...

The perfect tool for this experiment would be a radio controlled motorcycle. Would it have to be one of the expensive hobby types that would allow you to change the trail?

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The perfect tool for this experiment would be a radio controlled motorcycle. Would it have to be one of the expensive hobby types that would allow you to change the trail?

 

Good idea! Do you think my Evel Knievel stunt cycle will work??? :)

I googled it.....I had one of those as a kid....awwwww you brought back memories

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I'd have to keep his bike afterward in case further experiments were required...

 

You with keeping the bike, and JB with his bovine scatology, funny guys.

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Yes I had to reread the "bovine scatology" at first cause it threw me off. lol

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OK, so did Hotfoot get the answer desired?

 

BTW, Hotfoot went racing over the weekend, the first sanctioned event (had done CODERACE), where's the forum update??

 

C

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OK, so did Hotfoot get the answer desired?

 

BTW, Hotfoot went racing over the weekend, the first sanctioned event (had done CODERACE), where's the forum update??

 

C

Hotfoot tried nudging a complete answer out of Keith, but he's being tight-lipped :-)

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OK, so did Hotfoot get the answer desired?

 

BTW, Hotfoot went racing over the weekend, the first sanctioned event (had done CODERACE), where's the forum update??

 

C

Hotfoot tried nudging a complete answer out of Keith, but he's being tight-lipped :-)

 

Apparently Keith isn't so much "being tight-lipped" as he is "being out of town."

 

Yes, I went racing, it was great, pretty happy with a fourth place finish, as there were some much faster bikes mixed in the class. And yes, Cobie, you can take the blame for getting me hooked on this expensive sport. CodeRACE was a great preparation, I am SO glad I did that first. And BTW, where is YOUR answer to this burning question about circles? I might have gotten a podium finish if I had known the answer BEFORE the race. :)

 

I was paying attention to this when I was riding, I had a good long turn with a constant radius to try it, and it sure did FEEL to me like the front tire (maybe the back too, not sure) was losing ground and that was the thing that was making the radius bigger.

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Apparently Keith isn't so much "being tight-lipped" as he is "being out of town."

 

Yes, I went racing, it was great, pretty happy with a fourth place finish, as there were some much faster bikes mixed in the class. And yes, Cobie, you can take the blame for getting me hooked on this expensive sport. CodeRACE was a great preparation, I am SO glad I did that first. And BTW, where is YOUR answer to this burning question about circles? I might have gotten a podium finish if I had known the answer BEFORE the race. :)

 

I was paying attention to this when I was riding, I had a good long turn with a constant radius to try it, and it sure did FEEL to me like the front tire (maybe the back too, not sure) was losing ground and that was the thing that was making the radius bigger.

 

Which turn and where in the turn?

 

C

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Yes, I went racing, it was great, pretty happy with a fourth place finish, as there were some much faster bikes mixed in the class. And yes, Cobie, you can take the blame for getting me hooked on this expensive sport. CodeRACE was a great preparation, I am SO glad I did that first. And BTW, where is YOUR answer to this burning question about circles? I might have gotten a podium finish if I had known the answer BEFORE the race. :)

 

 

Congrats on your fourth place you first time out. Scratching at the podium spots already.

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  • 2 weeks later...
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

 

?

 

Number one for sure, number four is unlikely.

 

Keith

 

Jaybird asked a follow up question - what exactly is the mechanism the CAUSES the radius of the circle to increase, if the rider does not add any steering input? Intuitively I thought that if you rolled on the throttle (possibly harder than Jaybird was thinking in his quote above) the increased forces would make the bike want to stand up and go wider, but I am told that this is not true, that with no rider input the bike's lean angle will not change when the throttle is rolled on. So, is that correct? And if so, what IS it that makes the bike travel a larger circle, slippage of the tires? Does this depend on bike setup?

Well, let's analyze this step by painstaking step.

Keith said yes to #1 but no to #4. WHAT?????

Hmmmmm.....so the turn radius increases, but not because the tires are slipping. Is that about right?

 

Humbly, Mr. Code. I'm going to have to ask that you dress that chicken a bit more before you send it across the road.

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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

 

?

 

Number one for sure, number four is unlikely.

 

Keith

 

Jaybird asked a follow up question - what exactly is the mechanism the CAUSES the radius of the circle to increase, if the rider does not add any steering input? Intuitively I thought that if you rolled on the throttle (possibly harder than Jaybird was thinking in his quote above) the increased forces would make the bike want to stand up and go wider, but I am told that this is not true, that with no rider input the bike's lean angle will not change when the throttle is rolled on. So, is that correct? And if so, what IS it that makes the bike travel a larger circle, slippage of the tires? Does this depend on bike setup?

Well, let's analyze this step by painstaking step.

Keith said yes to #1 but no to #4. WHAT?????

Hmmmmm.....so the turn radius increases, but not because the tires are slipping. Is that about right?

 

Humbly, Mr. Code. I'm going to have to ask that you dress that chicken a bit more before you send it across the road.

 

He just got back from Vegas, but had to go right to another track today. I'll let him know about this thread, see if we can get him here in a day or 2.

 

Best,

CF

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I asked Will this question:

What MECHANICALLY makes the radius change when you increase the speed? It is not really a discussion of the CSS type roll-on for stabilizing the bike (although it may have started that way), it is the more theoretical discussion of what happens if you keep your lean angle, and increase your speed a lot, the radius of the circle will change, but what MAKES it do that, just the tire sliding more?

 

And here is his response:

 

It is too simple for most to realize it is the answer, Trail. What trail does is balance the bike against the pull of gravity. The rider sets the lean angle by displacing the bars against the force of trail and making the bike lean. Once released trail points the front wheel to balance the bike against gravity. Trail doesn't care about the speed of the bike, it just balances it at whatever angle it is at ( lean angle). If you increase speed without changing lean angle trail will simply "steer" the bike into a new bigger radius as the force to balance the bike is already there, or vise versa into a smaller one as speed drops while maintaining the lean angle as set by the rider.

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That makes sense now.

 

Last time I checked my specialty wasn't physics. I'll leave that up to those who love the subject. Here is what can be observed: a rider at 50 degrees lean angle in a 40 mph turn and a rider at 50 degrees lean in a 100 mph turn. The radius of the 100 mph turn is larger than the radius of the 40 mph turn yet the lean angle is the same. I know why it works like that but you all can find that out as well by looking at books like Tony Foale's "Motorcycle Handling and Chassis Design" to name one. In other words get the real data and not my interpretation of it.

 

From a rider's perspective, you want to get around the turn and stay on a line you like and allow for as few steering/lean angle changes and as good throttle control as you can get and as predictable a line as possible. The reason that fundamental throttle control contains the idea of getting the gas back on as soon as your line is set (and not before) is because the line does widen as throttle is added. Now you are looking at the actual art of cornering: the rider's ability to predict that ever widening arc as throttle is added is what makes his line "predictable". Some riders see and feel this better than others. Some just notice that the line widens and are afraid of it and miss the importance of noticing just how much it widens at different lean angles and speeds and throttle application.

 

Keith

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I asked Will this question:

What MECHANICALLY makes the radius change when you increase the speed? It is not really a discussion of the CSS type roll-on for stabilizing the bike (although it may have started that way), it is the more theoretical discussion of what happens if you keep your lean angle, and increase your speed a lot, the radius of the circle will change, but what MAKES it do that, just the tire sliding more?

 

And here is his response:

 

It is too simple for most to realize it is the answer, Trail. What trail does is balance the bike against the pull of gravity. The rider sets the lean angle by displacing the bars against the force of trail and making the bike lean. Once released trail points the front wheel to balance the bike against gravity. Trail doesn't care about the speed of the bike, it just balances it at whatever angle it is at ( lean angle). If you increase speed without changing lean angle trail will simply "steer" the bike into a new bigger radius as the force to balance the bike is already there, or vise versa into a smaller one as speed drops while maintaining the lean angle as set by the rider.

That makes sense now.

Makes sense to me too.

 

Last time I checked my specialty wasn't physics. I'll leave that up to those who love the subject. Here is what can be observed: a rider at 50 degrees lean angle in a 40 mph turn and a rider at 50 degrees lean in a 100 mph turn. The radius of the 100 mph turn is larger than the radius of the 40 mph turn yet the lean angle is the same. I know why it works like that but you all can find that out as well by looking at books like Tony Foale's "Motorcycle Handling and Chassis Design" to name one. In other words get the real data and not my interpretation of it.

Considering that poor technique can account for differences in lean angle to negotiate a turn, can we really consider this a good example? Can we deduce as an absolute that turn radius is directly proportional to the factors of lean angle and speed? I can think of an aviation example (3-dimensional world) where it is not, but since were not talking 2-dimensional, I’m not so eager to agree.

 

From a rider's perspective, you want to get around the turn and stay on a line you like and allow for as few steering/lean angle changes and as good throttle control as you can get and as predictable a line as possible. The reason that fundamental throttle control contains the idea of getting the gas back on as soon as your line is set (and not before) is because the line does widen as throttle is added. Now you are looking at the actual art of cornering: the rider's ability to predict that ever widening arc as throttle is added is what makes his line "predictable". Some riders see and feel this better than others. Some just notice that the line widens and are afraid of it and miss the importance of noticing just how much it widens at different lean angles and speeds and throttle application.

 

Keith

Thank you for taking the time to write this; means a lot and frankly is some clean and clear stuff. On one level it appears to speak directly to the discussion, and on another appears to be a non-sequitur. And it is on the non-sequitous part that it invites new questions. Thankfully, as I allow time for the questions to be properly formed, I am comforted by your use of the term ‘art of cornering’. This very well could be the thing to save me from having a premature linguistic ejection :blink: .

 

Thanks for the good stuff.

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Last time I checked my specialty wasn't physics. I'll leave that up to those who love the subject. Here is what can be observed: a rider at 50 degrees lean angle in a 40 mph turn and a rider at 50 degrees lean in a 100 mph turn. The radius of the 100 mph turn is larger than the radius of the 40 mph turn yet the lean angle is the same. I know why it works like that but you all can find that out as well by looking at books like Tony Foale's "Motorcycle Handling and Chassis Design" to name one. In other words get the real data and not my interpretation of it.

Considering that poor technique can account for differences in lean angle to negotiate a turn, can we really consider this a good example? Can we deduce as an absolute that turn radius is directly proportional to the factors of lean angle and speed? I can think of an aviation example (3-dimensional world) where it is not, but since were not talking 2-dimensional, I’m not so eager to agree.

 

 

Poor technique has nothing to do with it, the 50 degrees is an example of a lean angle at 2 different speeds! no matter what your lean angle is a 100 mph turn will always be a bigger radius than a 40 mph turn!

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OK, so did Hotfoot get the answer desired?

 

BTW, Hotfoot went racing over the weekend, the first sanctioned event (had done CODERACE), where's the forum update??

 

C

Hotfoot tried nudging a complete answer out of Keith, but he's being tight-lipped :-)

 

Apparently Keith isn't so much "being tight-lipped" as he is "being out of town."

 

Yes, I went racing, it was great, pretty happy with a fourth place finish, as there were some much faster bikes mixed in the class. And yes, Cobie, you can take the blame for getting me hooked on this expensive sport. CodeRACE was a great preparation, I am SO glad I did that first. And BTW, where is YOUR answer to this burning question about circles? I might have gotten a podium finish if I had known the answer BEFORE the race. :)

 

I was paying attention to this when I was riding, I had a good long turn with a constant radius to try it, and it sure did FEEL to me like the front tire (maybe the back too, not sure) was losing ground and that was the thing that was making the radius bigger.

Hotfoot-

Do you think that what's now posted would have helped you during your race weekend? Why or why not?

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Last time I checked my specialty wasn't physics. I'll leave that up to those who love the subject. Here is what can be observed: a rider at 50 degrees lean angle in a 40 mph turn and a rider at 50 degrees lean in a 100 mph turn. The radius of the 100 mph turn is larger than the radius of the 40 mph turn yet the lean angle is the same. I know why it works like that but you all can find that out as well by looking at books like Tony Foale's "Motorcycle Handling and Chassis Design" to name one. In other words get the real data and not my interpretation of it.

Considering that poor technique can account for differences in lean angle to negotiate a turn, can we really consider this a good example? Can we deduce as an absolute that turn radius is directly proportional to the factors of lean angle and speed? I can think of an aviation example (3-dimensional world) where it is not, but since were not talking 2-dimensional, I’m not so eager to agree.

 

 

Poor technique has nothing to do with it, the 50 degrees is an example of a lean angle at 2 different speeds! no matter what your lean angle is a 100 mph turn will always be a bigger radius than a 40 mph turn!

 

Can you lean a bike to 50 degrees at 30mph, 60mph, 90mph?

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Hotfoot-

Do you think that what's now posted would have helped you during your race weekend? Why or why not?

 

Yes. I had gotten the answer about trail from Will last week, and it DID help me this weekend, at the track, to recognize that the lean angle only changes if you make it change. I thought the bike would stand up on its own when you roll on, but it doesn’t, and that info helped me to hold the arc a little tighter in some turns, where previously I had been picking the bike up a little early with a steering input. It also helped me stand it up sooner (and more deliberately) in others, so I could get on the gas harder earlier. So yes, any bit of data helps, and anytime my understanding improves, it helps. The part that Keith posted was new to me today, but the point he makes that some riders are afraid of the fact that the arc widens when you roll on would probably have helped me too, as I am a bit afraid of that – if it is a high speed turn and the best line runs out to the edge of the track, I am overly careful to leave a wide margin for error. Next time I will pay more attention to predicting EXACTLY where the bike will want to go, which should give me more room to roll on harder, and therefore have better exit speed.

 

When Keith talks, I listen. Every single time. It's a strategy that is working well for me so far. ;)

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Last time I checked my specialty wasn't physics. I'll leave that up to those who love the subject. Here is what can be observed: a rider at 50 degrees lean angle in a 40 mph turn and a rider at 50 degrees lean in a 100 mph turn. The radius of the 100 mph turn is larger than the radius of the 40 mph turn yet the lean angle is the same. I know why it works like that but you all can find that out as well by looking at books like Tony Foale's "Motorcycle Handling and Chassis Design" to name one. In other words get the real data and not my interpretation of it.

Considering that poor technique can account for differences in lean angle to negotiate a turn, can we really consider this a good example? Can we deduce as an absolute that turn radius is directly proportional to the factors of lean angle and speed? I can think of an aviation example (3-dimensional world) where it is not, but since were not talking 2-dimensional, I’m not so eager to agree.

 

 

Poor technique has nothing to do with it, the 50 degrees is an example of a lean angle at 2 different speeds! no matter what your lean angle is a 100 mph turn will always be a bigger radius than a 40 mph turn!

 

Can you lean a bike to 50 degrees at 30mph, 60mph, 90mph?

 

 

of course, but as I say the 50 degrees is just a figure for example it could be 20 degrees if thats easier for you and at that lean angle or any lean angle for that matter the faster the speed the bigger the radius!

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Last time I checked my specialty wasn't physics. I'll leave that up to those who love the subject. Here is what can be observed: a rider at 50 degrees lean angle in a 40 mph turn and a rider at 50 degrees lean in a 100 mph turn. The radius of the 100 mph turn is larger than the radius of the 40 mph turn yet the lean angle is the same. I know why it works like that but you all can find that out as well by looking at books like Tony Foale's "Motorcycle Handling and Chassis Design" to name one. In other words get the real data and not my interpretation of it.

Considering that poor technique can account for differences in lean angle to negotiate a turn, can we really consider this a good example? Can we deduce as an absolute that turn radius is directly proportional to the factors of lean angle and speed? I can think of an aviation example (3-dimensional world) where it is not, but since were not talking 2-dimensional, I’m not so eager to agree.

 

 

Poor technique has nothing to do with it, the 50 degrees is an example of a lean angle at 2 different speeds! no matter what your lean angle is a 100 mph turn will always be a bigger radius than a 40 mph turn!

 

Can you lean a bike to 50 degrees at 30mph, 60mph, 90mph?

 

 

of course, but as I say the 50 degrees is just a figure for example it could be 20 degrees if thats easier for you and at that lean angle or any lean angle for that matter the faster the speed the bigger the radius!

 

That is the simplicity of it.

 

Keith

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Poor technique has nothing to do with it, the 50 degrees is an example of a lean angle at 2 different speeds! no matter what your lean angle is a 100 mph turn will always be a bigger radius than a 40 mph turn!

 

 

-------------------------

 

 

of course, but as I say the 50 degrees is just a figure for example it could be 20 degrees if thats easier for you and at that lean angle or any lean angle for that matter the faster the speed the bigger the radius!

I'm saying that is is NOT correct that for a fixed lean angle turn radius is ALWAYS directly proportional to speed. I'm saying that there are other factors can vary this, poor technique being just one.

 

How about another?

Dani Pedrosa and Nicky Hayden: same bike, same lean angle, same corner speed...Dani can turn tigher than Nicky.

 

Riders like Capirossi are perfect for this discussion. I've heard that his cornerspeed is usually within 1/10th of a kilometer on successive laps, and that he can hit nickels lap after lap with his RPs. I wonder what he'd say?

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