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 To accelerate or remain constant, that is the question
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rayg50
Male Moderator
2082 Posts
[Mentor]


NYC, NY
USA

Honda

Shadow Spirit 750DC

Posted - 06/22/2011 :  6:07 AM
While I digest and consider your points let me clarify a couple of points so we can be on the same page.
quote:
but if I read between your words, your style of cornering appears to have the Apex as the point of highest lean. I'll accept that this is the kind of cornering line you wish to discuss


The use of the word Apex carries with it meaning which is why I rejected it's use and invented AMME which should carry no meaning for you (no preconceptions) or me. If your understanding of what apex means differs from my understanding of what it means then our arguments will not be on the same page. I will waste time telling YOU what YOU meant and vice-versa.

I defined AMME as the point at which the greatest traction demand occurs. Prior to it the bike experiences increasing demand after it the bike experiences decreasing traction demand. By agreeing that that word carries that definition we can investigate what happens as traction increases separately from what happans as traction decreases. We can both give a point of reference as to when acceleration should occur. IMO we both agree that accelerating "out of the turn" is normal. Where we MAY differ is the point at which it should occur. If we do not agree on a reference point then examining that point becomes useless.

I also attempted to separate the bikes path of travel versus the physical curve of the road which for my personal understanding of the points you will make is useless. If the roads characteristics provides a constant radius but you ride a decreaing radius path then the forces the bike experiences are not constant radius but rather decreasing radius forces. Make sense?

quote:
It appears that the key difference then, between your preferred constant speed approach and the KC / Lee Parks modest acceleration approach, is the amount of roll on in the pre-Apex phase.
If you read my post to Mikeydude you will see that I am NOT at all talking about the roll on. My interest is in the bike and how acceleration affects it not in discussing how it is achieved. I prefer to discuss this in terms of no acceleration, and acceleration of the bike and not in the amount of wrist movement. Again, make sense?

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rayg50
Male Moderator
2082 Posts
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NYC, NY
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Honda

Shadow Spirit 750DC

Posted - 06/22/2011 :  6:27 AM
quote:
9- The front tire experiences the increased traction demand before the rear tire (in a curve).

Point 9 is I think incorrect. Tip 66 would disagree with you.

If you are dropping feet first into a black hole the difference in gravitational force effecting the soles of your feet and that effecting the top of your head is such that the soles of your feet would be stretched and torn from your body. In effect it would be like being putting your body through a shredder. You can argue that one with the theoretical physicists. It is their theory not mine.

I had wondered if that point would be applicable since the frame holds the 2 pieces tires together so that pulling one pulls the other. I then considered that the traction demands I envision (in a curve) are lateral (sideways) in nature. So the front tire must be experiencing the increased traction demand first. I am willing to run contrary positions through the mental mill.

Edited for clarity.

Edited by - rayg50 on 06/22/2011 6:43 AM
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rayg50
Male Moderator
2082 Posts
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NYC, NY
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Shadow Spirit 750DC

Posted - 06/22/2011 :  6:46 AM
quote:
One of the main arguments FOR modest acceleration is about a less loaded front means more compliant suspension. However what if there's a patch of water? Which cornering scenario is more likely to lead to a front end wash out?

So you're saying that if you are entering a lower traction condition the solution is to reduce traction even more?

Edited for the following:

In rereading the quoted passage I am now reading it as a question rather than a statement. Let me therefore give a position rather than ask a question.

IMO, if you are entering a lower traction condition (water) the last thing you would want to do is reduce traction even more. I think steady speed is preferable in this instance.

Edited by - rayg50 on 06/22/2011 8:37 AM
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CaptCrash
Male Advanced Member
744 Posts
[Mentor]


Nampa, ID
USA

Honda

Phantom

Posted - 06/22/2011 :  7:25 AM
Removed by author. This thread is getting confused and I'm not helping.

Edited by - CaptCrash on 06/22/2011 8:14 AM
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James R. Davis
Male Administrator
17276 Posts
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Houston, TX
USA

Honda

GoldWing 1500

Posted - 06/22/2011 :  9:23 AM Follow poster on Twitter  Join poster on Facebook as Friend  
Guys, there is a repeated assumption going on in this thread that bothers me as it is not true and is getting in the way of coming to a common agreement about fundamentals as well as technique.

It has been said that riding in a banked curve INCREASES TRACTION. It has been said that accelerating on a banked turn INCREASES TRACTION.

That is simply not true.

Back to basics, okay?

Remember 'Coefficient of Friction'? Does anybody here actually believe that the Coefficient of Friction between the bike's tires and the roadway surface changes as a result of speed? It does not.

What certainly happens is that the rider 'feels' heavier in the turn than when riding in a straight line. It is similarly true that the bike's suspension compresses as speed increases. But that is NOT traction.

I asked a poster here earlier if a 100 pound tire was at rest on that banked curve, how much pulling force (in-line with the roadway) would it take to cause the tire to skid. If it took 100 pounds of pull, then the Coefficient of Friction (static) would be 1.0 and it would take 1g of acceleration or braking to cause it to skid.

Then I asked if that tire was rolling at 100 mph in a curved path along that banked roadway, how much pull it would take to cause it to skid - and I proposed that it would still take 100 pounds of force to do so. The traction did not change one bit based on the super-elevation.

What is true is that it would take more than 100 pounds of force to make the tire SLIDE (move sideways relative to path of travel). But that is not increased traction, either.

Instead, the banked roadway accepts more and more of the centrifugal force resulting from speed in a curved path into increased weight pressing into the roadway (not straight down, but in the direction of the lean angle) as the bank angle got steeper or the speed increased.

It still would take 100 pounds of force in the direction of the bank angle to cause the tire to SLIDE, or 100 pounds of force from acceleration or braking to SKID, regardless of how much centrifugal force is involved.

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rayg50
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NYC, NY
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Shadow Spirit 750DC

Posted - 06/22/2011 :  8:12 PM
Oh great, I had just gotten over the CoF discussion nightmares.

=====================

Fearing that I was misremembering, I did a search for "falling into a black hole" and found the following

"The gravity at your feet is stronger than the gravity at your head, as long as you fall in feet first. ... You feel this difference in gravity between your feet and your head as a tidal force, which pulls you apart vertically in a process called 'spaghettification,' " Hamilton writes.

Since in the past we have requested links to the source so that the quote or reference can be read in context, here you go.

http://edition.cnn.com/2009/TECH/sp...e/index.html

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Andrew Dressel
Male Standard Member
244 Posts


Milwaukee, WI
USA

Moto Guzzi

California Special

Posted - 06/23/2011 :  3:53 AM
quote:
Originally posted by James R. Davis

...if a 100 pound tire was at rest on that banked curve, how much pulling force (in-line with the roadway) would it take to cause the tire to skid. If it took 100 pounds of pull, then the Coefficient of Friction (static) would be 1.0 and it would take 1g of acceleration or braking to cause it to skid.



I'm with you so far.

quote:
Originally posted by James R. Davis

Then I asked if that tire was rolling at 100 mph in a curved path along that banked roadway, how much pull it would take to cause it to skid - and I proposed that it would still take 100 pounds of force to do so. The traction did not change one bit based on the super-elevation.


I was curious about how the numbers would work out for this part, so I tried some. To do the math, we need two more constants: the radius of the curve and the super elevation angle.

The numbers suggest that you are approximately correct, which is probably your goal, but the exact values depend on the radius of the curve.

With a 100,000 ft radius at 100 mph (147 ft/s), the normal force between the road and the tire actually decreases as the bank angle is increased or decreased. Below 100,000 ft the normal force increases as the bank angle increases and so increases the maximum friction force possible. At a 1000 ft radius the normal force increases by about a pound per degree of increased bank: just 1%.

quote:
Originally posted by James R. Davis

What is true is that it would take more than 100 pounds of force to make the tire SLIDE (move sideways relative to path of travel). But that is not increased traction, either.


Yes, at 100 mph, 1000 ft turn radius, and a coefficient of friction of 1, the force required to slide the 100 lb tire increases by about 1 lb per degree of positive bank angle.

quote:
Originally posted by James R. Davis

Instead, the banked roadway accepts more and more of the centrifugal force resulting from speed in a curved path into increased weight pressing into the roadway (not straight down, but in the direction of the lean angle) as the bank angle got steeper or the speed increased.


This effect is almost twice as large. The friction force required to keep the tire in the 1000 ft radius turn decreases by about 1.8 lbs per degree of bank and so the reserve friction (maximum possible friction force - friction force used to maintain radius) increases by almost 3 lbs per degree. This may not sound like much either, but the friction force required to maintain the radius with zero degrees of bank is about 67 lbs so the friction in reserve is only about 33 lbs and so the percent increase in reserve friction climbs nicely. Just 5 degrees of bank increases reserve friction by 43%. I think I remember reading that 6 degrees is some kind of standard, and that yields a 52% increase in the friction in reserve: from 33 lbs to 50 lbs.

quote:
Originally posted by James R. Davis

It still would take 100 pounds of force in the direction of the bank angle to cause the tire to SLIDE, or 100 pounds of force from acceleration or braking to SKID, regardless of how much centrifugal force is involved.


So I don't quite draw the same conclusion. For braking and accelerating, at right angles to the lateral friction force required to maintain the radius, we should look at the friction ellipse. We can simplify by assuming a friction circle so that the square of the total available friction equals the sum of the squares of the friction in the lateral and longitudinal directions. In that case friction available for braking or accelerating increases by about 18.5% with a positive 5 degree bank angle; 21.5% with 6 degrees.

Here's the graph:

If I missed your point or have gone off on a tangent, I apologize. I just thought this was an opportunity to make a fun graph. I hope I got all the terminology correct, or at least made clear what I mean.

(Edited twice! to fix a blunder I made in Excel. I mistakenly used the speed in mph (100) instead of ft/s (147). With that corrected, a radius of 1000 feet gives about the same results as originally posted. I've also now double checked from scratch in MatLab, so I think I've removed all the errors.)

Here's a more-general pair of graphs that support various forward speeds and turn radii.

A couple points look correct at first glance for 100 lbs and a coefficient of friction of 1:
  • at 1 g of lateral acceleration and zero degrees of bank there is no static friction left for braking
  • at 1 g of lateral acceleration and 45 degrees of bank a maximum of 141 lbs is available for braking
  • at zero gs of lateral acceleration and zero degrees of bank, the maximum of 100 lbs of static friction is available for braking
  • at zero gs of lateral acceleration and zero degrees of bank, the minimum of zero lbs of static friction is available because all available friction is required just to keep from sliding down the bank.

Edited by - Andrew Dressel on 06/23/2011 8:40 AM
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rayg50
Male Moderator
2082 Posts
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NYC, NY
USA

Honda

Shadow Spirit 750DC

Posted - 06/23/2011 :  6:26 AM
Andrew, thank you for joining the fray. I swore to myself I would stop adding posts as I prepare for work because they come out poorly expressed but here I go, again.

First thank you for your addition.

I am translating your post to say that if you have a positive bank angle more speed would be required than a flat surface to cause the bike to slide off. I would therefore assume the reverse to be true. A negative bank angle would require less speed.

Since I have tried to provide the basis for my thinking, for review, let me add my thought process here for your comment. It would be VERY much appreciated if as a non-biased observer if you would kind of kick the tires. I had considered the point you are raising and prepared to attempt to remove it as a consideration when raised by Rob. I will reference speed only because I think it is easier for me and the casual reader to get my point and draw their own conclusion.

If a curve, taken at 40, allows you to remain on the road but when taken at 41 has you sliding into the sunset, does the bank angle really matter? At 40 you get to hug your kids at 41 they get to cry, leaving mom (or dad if mom is the rider) figuring out how to now make ends meet.

Again as an independent would you be good enough to comment on my use of the effects of dropping into a black hole. I am trying to point out that as you approach "a force" the closest object experiences the effect first and to a greater degree before a trailing object.

I do this so that we can examine just the front tire on the approach to the increased traction demand and whether or not reducing it's traction is a good thing. IMO if the front tire slides out it will take out the back tire with it. If it slides off the road what the rear tire is doing will not much matter. Yes I am way oversimplifying but my question to you is does this thinking of mine hold water?



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Andrew Dressel
Male Standard Member
244 Posts


Milwaukee, WI
USA

Moto Guzzi

California Special

Posted - 06/23/2011 :  8:16 AM
quote:
Originally posted by rayg50

I am translating your post to say that if you have a positive bank angle more speed would be required than a flat surface to cause the bike to slide off. I would therefore assume the reverse to be true. A negative bank angle would require less speed.


Yes, I believe you are correct.

quote:
Originally posted by rayg50

If a curve, taken at 40, allows you to remain on the road but when taken at 41 has you sliding into the sunset, does the bank angle really matter? At 40 you get to hug your kids at 41 they get to cry, leaving mom (or dad if mom is the rider) figuring out how to now make ends meet.


Well, as you describe the scenario, the bank angle only matters if it changes. A slight increase may let the bike stay on the road at 41 and a slight decrease may send it off at 40. The same is true, of course, for the coefficient of friction between the tires and the pavement. The opposite should then be true for longitudinal friction expended for acceleration or deceleration. According to the friction ellipse model, laying off the brake or the throttle should allow additional friction to become available for centripetal acceleration.

quote:
Originally posted by rayg50

Again as an independent would you be good enough to comment on my use of the effects of dropping into a black hole. I am trying to point out that as you approach "a force" the closest object experiences the effect first and to a greater degree before a trailing object.


You've lost me here. I don't see what you are considering to be closer to what.

quote:
Originally posted by rayg50

I do this so that we can examine just the front tire on the approach to the increased traction demand and whether or not reducing it's traction is a good thing. IMO if the front tire slides out it will take out the back tire with it. If it slides off the road what the rear tire is doing will not much matter. Yes I am way oversimplifying but my question to you is does this thinking of mine hold water?


I believe the bigger issue is control or the loss of it. I read that it is preferable to have the rear wheel start to slide before the front. There is even an entire sport built around letting the rear wheel slip while keeping the front wheel pointed in the desired direction of travel. I think this picture illustrates the point: I don't mean to promote or even discuss racing, on road or off, but merely show a type of riding that successfully navigates turns with sliding friction at the rear wheel. I don't know of an example that illustrates the opposite.

I don't believe we can discuss the costs and benefits of an action that may reduce traction of the front tire without knowing the total available traction of both tires and the demands for traction at both tires.

(edited to fix typo)
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James R. Davis
Male Administrator
17276 Posts
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Houston, TX
USA

Honda

GoldWing 1500

Posted - 06/23/2011 :  9:12 AM Follow poster on Twitter  Join poster on Facebook as Friend  
Andrew,

You are absolutely correct, as usual. My blunder was failing to consider the traction demand of being in a curve at any speed.



At point 'A', where there is no bank angle, the apparent weight of the bike is 100 pounds, but as point 'B' shows, the amount of braking force available is only about 75 pounds (not 100 as I proposed). Actually, the amount of braking force available does not reach 100 pounds until the bank angle is about 10 degrees.

A bank angle does cause available braking forces to increase in a turn - and that means that traction increases with a bank angle.

I withdraw my assertion and stand corrected. Thank you.

It was nice of you, Andrew, to give me a little 'wiggle room' as to my intentions, but on this site if you make declaratives, you need to be prepared to defend them. I made several and I cannot defend them because I was wrong.
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gymnast
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Meridian, Idaho
USA

Harley-Davidson

Sportster Sport

Posted - 06/23/2011 :  10:45 AM
For illustration purposes only! Going sideways and traction reserves, dipping into that last 1% of whats available. Do not try this at home.
"Beezers" in the "the old days" http://www.youtube.com/watch?v=IOV5...ture=related
And, the newer stuff, http://www.youtube.com/watch?v=py-Z...ture=related

Edited by - gymnast on 06/23/2011 10:50 AM
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rayg50
Male Moderator
2082 Posts
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NYC, NY
USA

Honda

Shadow Spirit 750DC

Posted - 06/23/2011 :  12:57 PM
quote:

I believe the bigger issue is control or the loss of it. I read that it is preferable to have the rear wheel start to slide before the front. There is even an entire sport built around letting the rear wheel slip while keeping the front wheel pointed in the desired direction of travel. I think this picture illustrates the point:

That is part of what I have been trying to get to, but it has been a slow go. I will probably give a safety consideration recap for the riders reading this thread and then leave it as an agree to disagree.

Here is the basic discussion as I see it:

Robsalvv feels that mild acceleration through a curve is preferable to maintaining a steady speed. I will not speak for him and explain his position. I would do a poor job of it since I have not been able to even present my own case in a coherent fashion.

I contend that maintaining a steady speed is preferable. IMO acceleration shifts weight to the rear. This decreases traction reserve in the front and increases it at the rear. The lessening of traction reserve at the front is happening at a time when traction demand is increasing sooner at the front than at the rear. In effect you are shifting your bikes traction focus from the immediate threat to the future one. IMO a mistake that can cost you.


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Andrew Dressel
Male Standard Member
244 Posts


Milwaukee, WI
USA

Moto Guzzi

California Special

Posted - 06/23/2011 :  1:13 PM
quote:
Originally posted by rayg50

The lessening of traction reserve at the front is happening at a time when traction demand is increasing sooner at the front than at the rear.


I don't get you here. Why do you say that "traction demand is increasing sooner at the front than at the rear?"
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rayg50
Male Moderator
2082 Posts
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NYC, NY
USA

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Shadow Spirit 750DC

Posted - 06/23/2011 :  4:12 PM
quote:
Originally posted by Andrew Dressel

quote:
Originally posted by rayg50

The lessening of traction reserve at the front is happening at a time when traction demand is increasing sooner at the front than at the rear.


I don't get you here. Why do you say that "traction demand is increasing sooner at the front than at the rear?"


Let me frame the statement first and then say it differently.

Just before you enter a curve you have a certain amount of traction demand. As you move into the curve and begin to turn, the traction demand changes. I contend that traction demand increases as you move along the curve (constant radius has been cited as an exception). At some point the traction demand reaches it's highest demand and then the demand begins to decrease until you exit. With that background in mind let me rephrase what I wrote.

When traction demand is increasing, the front tire experiences the increased demand sooner than the rear tire because it is further into the curve than the rear tire.


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Andrew Dressel
Male Standard Member
244 Posts


Milwaukee, WI
USA

Moto Guzzi

California Special

Posted - 06/24/2011 :  6:01 AM
quote:
Originally posted by rayg50
I contend that traction demand increases as you move along the curve (constant radius has been cited as an exception). At some point the traction demand reaches it's highest demand and then the demand begins to decrease until you exit. With that background in mind let me rephrase what I wrote.

When traction demand is increasing, the front tire experiences the increased demand sooner than the rear tire because it is further into the curve than the rear tire.


Hmmm. That is a tricky one. I'm not sure the front and rear wheels experience significantly different forces due a change in the turn radius of curvature, especially during legal street riding. If you rapidly applied a strong torque to the handlebars, to avoid an obstacle for example, the front wheel would momentarily experience more lateral acceleration than the rear, but I don't think that is what you are talking about.

The bigger factor, I suspect, is the different turn radii that each wheel has even in a constant radius turn, but even that is a very small difference. For example, if we pick a 0.5 g turn (about 26 degree lean angle) at 65 mph, my crude kinematic calculations suggest that we need a 0.5 degree steer angle, the front wheel will have a quarter inch larger turn radius, and so will be traveling 2 hundredths of a mph faster. Together that yields one hundredth of a foot/s^2 more centripetal acceleration.

The same 0.5 g lateral acceleration (and lean angle), however, at 15 mph requires a 9 degree steer angle (ignoring the effect of the lean angle on steering kinematics and the effect of different tire widths on rear frame yaw angle, among others I haven't thought of), a four inch difference in wheel track, a 0.2 mph difference in wheel speed, and a 1.25% larger lateral acceleration at the front wheel than the rear wheel.

My guess is that these small effects are overwhelmed by differences in normal force on each wheel due to center of mass location and load transfer.

Not sure if this helps, but it's what I can come up with today.
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rayg50
Male Moderator
2082 Posts
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NYC, NY
USA

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Shadow Spirit 750DC

Posted - 06/24/2011 :  12:46 PM
quote:
Not sure if this helps, but it's what I can come up with today.

It helps my understanding. I definitely thank you.

I will repeat back what I am understanding. There is a difference between front and rear tire traction but it is small enough to not be worthy of note (in a constant radius turn). The greater portion of that difference is caused by the front tire traveling a wider arc.

Andrew thank you. Any correction to my summary of understanding would be appreciated.

Ray

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(Deleted or Lost)

Posted - 06/24/2011 :  9:01 PM
I thank Andrew for entering the discussion. I've always had a healthy deference and respect for his deeper physics understanding on things (even spotted some edits he has authored on Wikipedia)


quote:
Originally posted by gymnast

For illustration purposes only! Going sideways and traction reserves, dipping into that last 1% of whats available. Do not try this at home.
"Beezers" in the "the old days" http://www.youtube.com/watch?v=IOV5...ture=related
And, the newer stuff, http://www.youtube.com/watch?v=py-Z...ture=related

For further illustration of traction increasing in a banked turn I contend http://www.youtube.com/watch?v=Oq1QnZPRp20 (Do not try this at home)




My comments in red:
quote:
Originally posted by rayg50


Robsalvv feels that mild acceleration through a curve is preferable to maintaining a steady speed.
>>> and so does James, Lee Parks, Keith Code, MSF and almost all RTO's that teach intermediate and advanced riding techniques - this is NOT a unique proposition.

I will not speak for him and explain his position. I would do a poor job of it since I have not been able to even present my own case in a coherent fashion.
>>>Cheers Ray. I've explained several times the benefits I perceive and has been explained to me and in writing in books and other literature.


I contend that maintaining a steady speed is preferable. IMO acceleration shifts weight to the rear. This decreases traction reserve in the front and increases it at the rear.
>>>However in a banked turn available traction increases (& yes so does demand). So on the basis that all right thinking motorcyclists enter corners at a safe speed, does a little bit of acceleration in a banked turn reduce front wheel traction to less than the vertical picture?? I don't believe so. Experience tells me no. I believe the discussion that Andrew has so kindly added says no.


The lessening of traction reserve at the front is happening at a time when traction demand is increasing sooner at the front than at the rear.
>>>I don't believe that's correct. BOTH wheels are experience the same speed increase. Both wheels are responding to that increase based on the particulars of their design, shape, wear, suspension etc I believe based on your response to Andrew you don't hold this view to be true any longer.

With a little acceleration there is a little oversteer, so unless the front rides through an oil puddle or similar low traction patch, then all things being equal, the rear will let go first - EVEN IF what you say was happening at the front were true.



In effect you are shifting your bikes traction focus from the immediate threat to the future one. IMO a mistake that can cost you.
>>>If the previous logic is flawed as shown, then the conclusion drawn must be flawed too, yes?


Slight change of tack.

Let's put the constant radius example aside.

Given your discussion of increasing and decreasing traction after a certain point, you MUST be talking about a changing radius line through a corner (e.g. in wide, out tight). SO that reduces the considerations down to two cornering scenarios. "In wide, out tight" with a constant speed scenario till exit phase, or "In wide, out tight" with a modestly accelerating picture throughout.

You seem focussed on the front tyre traction picture on the basis that the modest acceleration scenario is demanding more traction from the front tyre in comparison to the constant speed scenario. I'm going to take it that in the latter case, the less loaded front is experiencing a slightly increasing traction demand in comparison, BUT the only scenarios you can envisage where this causes a safety issue are a) if a rider already approaches at a speed near the corners traction limits, or b) the rider passes over a lower traction patch on the road.

On a) acceleration is only added if the rider thinks it's safe to do so - I repeat, it's not cornering by numbers. A rider in this scenario has a high chance of losing it from many points of view, a tiny amount of inadvertent roll off which throws weight forward, bumps in the road, tar snake etc. So I contend that it's not helpful to explore this scenario - the rider is at risk from many angles and arguing the modest acceleration technique is wrong because of this high speed cornering scenario is specious. The rider has a more fundamental technique problem, their vision or attitude needs adjustment!

on b) assuming the traction demand of the less loaded front tyre is slightly higher in the modest acceleration scenario, then the front has a higher chance of experiencing a possible slide, BUT, if the rear tyre has the higher influence over a bike once settled in a corner and this tyre is now experiencing a higher traction reserve and the bike is experiencing oversteer, doesn't this somewhat reduce the likelihood of a slide?

Leaving the water patch aside, the with modest acceleration the front (and rear for that matter) suspension is more compliant so road imperfection which could otherwise destabalise the bike are better handled. Doesn't this directionally decrease the likelihood of a slide in a corner? Isn't that benefit ALONE worth pursuing?



quote:
Originally posted by rayg50

If you read my post to Mikeydude you will see that I am NOT at all talking about the roll on. My interest is in the bike and how acceleration affects it not in discussing how it is achieved. I prefer to discuss this in terms of no acceleration, and acceleration of the bike and not in the amount of wrist movement. Again, make sense?
Fair enough, but for the most part in this discussion, constant speed has been equated with constant radius, which means ONE throttle adjustment at or after tip in to account for cornering losses. However we're both now discussing changing radius lines, therefore constant speed MUST INVOLVE rolling on throttle.

This means that the practical difference between the two techniques is how much one moves their wrist. I found it intriguing Ray that you are in actual fact only a fraction of a roll rate away from routinely shifting weight rearward.

I'll take a total leap of speculation from this point - I'd hazard that since you're not watching your speedo in every instance of cornering AND because your preferred line must involve rolling on throttle to counter cornering losses throughout the entire way into a corner, you've PROBABLY already experienced moments of acceleration from inadvertently rolling on more than you need, perhaps even through whole corners, and as a result benefited from the performance improvements. (sorry if that was a little cheeky of me, but you have to admit my speculation is definitely a possibility!)




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rayg50
Male Moderator
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NYC, NY
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Shadow Spirit 750DC

Peer Review: 1

Posted - 06/25/2011 :  1:12 AM
As I previously posted I am withdrawing from this thread but I will make some parting comments.

I respect the opinions of others but I do not let their thinking replace my own. One can name everyone who agrees with them as often as they want, it holds no sway with me. I have never felt majority vote should replace my thinking. I did not enter this thread to prove or disprove anything but rather to examine it. Had my desire been to "win" rather than to learn I could have starting naming known riders that prefer roadcraft. I saw no point in turning this into a bridge game with each seeking the higher trump card. Quite honestly a movie star endorsing a politician does not mean a whole bunch to me. I still need to look under the hood and decide for myself.

I find I am not following the context created, for example the fixation on constant radius. If it has some unique property then perhaps that is exactly the one to not use. If it is typical then why single it out? Equally confusing was the wanting to discuss the impact of bank angles. My reaction tended to be so what? I agree there is an impact. But, the impact would be the same to both riders using the 2 different methods when on the same curve. I will continue to read this thread so hopefully I will learn the relevance. Then there is my inability to get across the difference between acceleration and wrist movement. I also posed a scenario of 2 riders entering a curve at exactly the traction limit with the rider who accelerates failing to negotiate the turn. No rebuttal just a dismissal. I cannot gain understanding from nonresponse.

This could have been a great mental exercise with new understanding for me. It's not happening and I feel incapable of turning it around with continued participation.

quote:
I'll take a total leap of speculation from this point - I'd hazard that since you're not watching your speedo in every instance of cornering AND because your preferred line must involve rolling on throttle to counter cornering losses throughout the entire way into a corner, you've PROBABLY already experienced moments of acceleration from inadvertently rolling on more than you need, perhaps even through whole corners, and as a result benefited from the performance improvements. (sorry if that was a little cheeky of me, but you have to admit my speculation is definitely a possibility!)


I never said whether I do or do not twist my wrist on a curve. We never got that far in the exchange. We never came to agreement on a reference point (AMME). For all I know I "accelerate out of the turn" before KC does.

I intuitively disagreed that the method you recommend was better, and wanted us (the forum, inclusive of you) to look at the dynamics. I know increasing speed throughout a turn carries greater risk even to the highly skilled rider. Just watch a pack of racers on a turn and have only one of them slide out. They were probably following your performance improvement recommendation and that one rider turned his wrist a hair more than the others (sorry if that was a little cheeky of me, but you have to admit my speculation is definitely a possibility!).

I will follow the thread with great interest. Perhaps without me to muddy the waters greater understanding will result. I actually did enjoy participating.

Cheers Ray

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Andrew Dressel
Male Standard Member
244 Posts


Milwaukee, WI
USA

Moto Guzzi

California Special

Posted - 06/25/2011 :  2:58 AM
I wish I could create a graph that would resolve this issue once and for all, but I suspect that we have gotten as far as we can with hypothetical scenarios and a few back-of-the-envelope calculations. This stuff quickly gets crazy complicated, and so researchers turn to instrumentation and simulation. I propose we try to take advantage of their work.

For example Limebeer, Sharp, and Evangelou, writing in the Proceedings of the Institution of Mechanical Engineers in 2001 state:
quote:

The results show that the wobble mode of a motorcycle is significantly destabilized when the machine is descending an incline or braking on a level surface. Conversely, the damping of the wobble mode is substantially increased when the machine is ascending an incline at constant speed, or accelerating on a level surface. This probably accounts for the pleasingly stable 'feel' of the machine under firm acceleration.


I added the emphasis, and I found a copy online with a Google Scholar search at http://pic.sagepub.com/content/215/...ull.pdf+html

Unfortunately, they are not writing about accelerating in a curve and specifically state 'firm acceleration' instead of 'modest', but perhaps this phenomenon is why the technique of accelerating out of a turn is popular, and it has nothing to do with available traction at the front and rear wheels.

By the way, I believe it is Robin Sharp who is credited with coining the names of the 'weave', 'wobble' and 'capsize' modes in his 1971 paper on 'The Stability and Control of Motorcycles' published in the Journal of Mechanical Engineering Science

Anyway, I hope this detail sheds a little more light on the topic.
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Andrew Dressel
Male Standard Member
244 Posts


Milwaukee, WI
USA

Moto Guzzi

California Special

Posted - 06/27/2011 :  4:48 AM
quote:
Originally posted by Andrew Dressel

I propose we try to take advantage of their work.


cricket ... cricket ...

Sorry for putting a damper on such a fun conversation. I'm sure it just ran its course. In any case, I've found another interesting paper that sheds a different light on this topic, this time from the crew in Padua, Italy headed by Vittore Cossalter, the author of Motorcycle Dynamics.

In a paper titled "Evaluation of Motorcycle Maneuverability With the Optimal Maneuver Method", published in the proceedings of SAE's Motorsports Engineering Conference and Exposition in Dearborn, Michigan in 1998, the authors describe attempting to evaluate different motorcycles objectively by replacing the human rider with an "optimal controller". One of the maneuvers is a U-turn test with deceleration towards the apex and acceleration away from it. They report:

quote:

As is shown, with reduced adherence, the velocity at which the curve is done decreases. In this case, it makes more sense to drive straight during braking and acceleration phases yielding a path with a sharp curve.


So, since the goal is to maximize performance (minimize time) through a maneuver, among other things, this may be better suited towards racing than street riding, and it says little about accelerating out of a constant radius turn. This may, however, explain part of the motivation for accelerating out of turns at all, as espoused by various authors.

Lest someone say, "Well, duh", remember that this is just one scrap out of a paper intended to introduce an objective method for evaluating motorcycle performance. On one hand, this particular simple example can be seen as a case of "good, the optimal controller does what we would expect," and on the other hand, it is interesting to see the actual numbers for the ideal deceleration, acceleration, roll angle, roll rate, etc., all worked out for for any given coefficient of friction.

This paper doesn't mention physical validation specifically, but they have plenty of other papers that do address that topic. Here's a link to the PDF: http://www.dinamoto.it/dinamoto/7_M...0SAE3022.pdf
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