Questions for Keith Code - Round Two

[stangmx13]

There may be an issue with figure 4 in the pdf file. First note that the bike is making a right turn (the force from the pavement is to the right). Here's the conflict, in response to the right yaw (rotation about vertical axis, which is going to be a slow rate, other than very tight slow speed circles), the gyroscopic torque about the roll (lean) axis is trying to stand-up the bike, which is shown correctly in figure 4. However, it's described as "aligning", meaning the rider would be pulling on the right bar (steering further inwards) to compensate, but since the torque is trying to stand the bike up, the rider would need to counter-steer (push on the right bar) to prevent the standing up response.

Tony Foale explains all this a lot better in his book. u should check it out if u havent already, chapter 4. heres one of the relevant graphs. he (and i) agree that gyroscopic torque should be "misaligning" as its trying to steer the wheel into the corner. who knows if that invalidates the other study.

(this simulation assumes 0 tire width)

My main interest was the speed related aspect of this. Getting back to my original question, I'm thinking that the answer is that on a bike that is self-stable, one that tends to stand up with neutral steering input and the rider centered on the bike, that as speed increases, the rate of recovery (how fast the bike tends to stand up) decreases. At high enough speed, the rate of recovery (tendency to stand up) is so slow that it's almost imperceptible, which is what I was noticing at around 100 mph. There was nothing magical about 100 mph, that just happened to be the speed I noticed the effect on the Busa.

notice in the graph about that the resultant torque is 0 or near 0. even w/ the 0 tire width, i think the implication is still valid that there is no "recovery" in that simulated case. Foale doesnt give parameters for the corner, no speed, radius, or lean angle. but most of his other graphs show a high lean angle (>45deg), so we can prob assume its either a fast and/or tight corner.

speed and everything i showed both matter when a racer is entering a corner on the brakes at over 100mph. i suspect that there are far fewer >100mph corners where the rider enters off the brakes vs on the brakes, ie there are many more corners where the rider has adjusted the "capsize speed" dramatically because of their inputs.

that just exemplifies a main point that u should take away from all this. on the race track, im almost never not applying some steering torque. this is not because im not near capsize speed and the bike needs to be held in the corner. its because im braking all the way to the inside apex and i need to "fight" the steering torque resulting from that. for most tracks, theres at most 1 long corner that has any amount of time where im neither on the brakes or the gas. so the feelings described in this convo dont happen much on the track.

 

[rcgldr]

Tony Foale explains all this a lot better in his book. u should check it out if u havent already, chapter 4. heres one of the relevant graphs. he (and i) agree that gyroscopic torque should be "misaligning" as its trying to steer the wheel into the corner. who knows if that invalidates the other study.

It's a different gyroscopic torque axis. the pdf article from above is talking about gyroscopic torque about the roll / lean axis, which is going to be small (it's a reaction to yaw (vertical axis), which is fairly slow).

The Foale article is talking about gyroscopic torque about the steering axis, which is a response to torque about the roll axis. The graph shows this as positive values, but in most cases it's negative, as it dampens (opposes) gravity (trail) induced steering, preventing weaving back and forth due to over-correction. The gyroscopic reaction is a precession that is in the same direction as gravity, but the natural (no other torques) rate of precession is slower than the rate that gravity (trail) tries to steer the bike, so the gyroscopic reaction normally ends up dampening (opposing) the gravity (trail) reaction, unless the lean rate is very slow. Note that at mid turn, in a balanced (coordinated) turn, there is no torque about the roll axis, so there's no gyroscopic reaction.

notice in the graph about that the resultant torque is 0 or near 0.

Other than when changing the steering angle of the front tire, the net torque on the front tire is near zero, otherwise it would be changing direction. The fact that the entire bike is turning means that there is some torque on the front tire, related to the rate of turn of the bike.

I think the implication is still valid that there is no "recovery" in that simulated case.

The recovery wouldn't be clear in the graph. The net torque goes to near zero when the front tire stops changing steering angle relative to the bike. If this final steering angle is more than needed to maintain a lean, then the bike will tend to stand up. On any bike that is self-stable, there's always some tendency to stand up, but it may be a very small tendency (slow rate of recovery). As mentioned in my prior posts, the rate of recovery decreases as speed increases.

capsize speed

I've since changed my mind about this. Self correction tendency gets reduced as speed increases, but with real tires (not razor thin ones), I doubt it goes negative at high speeds. With real tires, the contact patch is offset inwards from the center of the tire, which results in a slight outwards roll torque, probably enough to keep the self-stability from going slightly negative which would cause the bike the fall inwards at an extremely slow rate (which would be capsize mode).

braking while turning

This depends on the bike and tire profile, but since the contact patch is offset from the center during a lean, then front braking will try to steer the front tire inwards, and has to be opposed with counter-steering to keep the bike from standing up. Trivia - back in the late 1960's and early 1970's this was a question on the California DMV test, asking what happens if braking is done while turning, and the correct answer was "it stands up" (versus one of the alternate answers that the cornering radius decreases, which would be the actual correct answer, assuming the rider holds the lean angle by counter-steering). If I recall correctly, the California Highway Patrol policy was against counter-steering, although I'm sure many or most of the bike guys were counter-steering. The later MSF schools taught counter-steering, but were leery of explaining that counter-steering out tracks the tires from under the bike to lean it. At least one MSF school in my area is now teaching this. Link to image:

http://rcgldr.net/misc/cntrstr0.jpg

I suspect that there are far fewer >100mph corners where the rider enters off the brakes vs on the brakes.

The classic example is Daytona. Two of the banked turns leading onto the two short straights of the tri-oval track are used along with the infield track. The superbikes exit the banked turns onto the straights at over 180 mph, flat out on the throttle, and nearly horizontal at turn exit. It takes a huge amount of counter-steering effort to get the bikes back to vertical. Skip to 1:10 into this video:

[YOUTUBE]kFbv0NCexvU[/YOUTUBE]

 

[code]

Rider inputs at lean

A good shot of Lorenzo with a perfectly relaxed left hand resting on the bar.


http://www.bayarearidersforum.com/forums/attachment.php?attachmentid=501870&stc=1&d=1510857861

 

[rcgldr]

A good shot of Lorenzo with a perfectly relaxed left hand resting on the bar.

Cool picture. I agree that if a rider leans or hangs off to the inside, then a bike will not tend to stand up. My issue was about what happens if the rider remains centered on a bike. It would seem that any bike that is self stable is going to tend to stand up with the rider centered and no torque applied to the handlebars, but on a bike with very little trail this tendency may be very small with a slow rate of recovery.

The answer to my primary question is that there was nothing magical about 100 mph, just the general trend that the rate of recovery (tendency to stand up) decreases as speed increases, but with real tires (not razor thin ones) that the stability never goes negative at high speed (where the bike would tend to fall inwards at a very slow rate). I'm sorry that I took so long and so many posts to come to this conclusion.

 

[code]

Cool picture. I agree that if a rider leans or hangs off to the inside, then a bike will not tend to stand up. My issue was about what happens if the rider remains centered on a bike. It would seem that any bike that is self stable is going to tend to stand up with the rider centered and no torque applied to the handlebars, but on a bike with very little trail this tendency may be very small with a slow rate of recovery.

The answer to my primary question is that there was nothing magical about 100 mph, just the general trend that the rate of recovery (tendency to stand up) decreases as speed increases, but with real tires (not razor thin ones) that the stability never goes negative at high speed (where the bike would tend to fall inwards at a very slow rate). I'm sorry that I took so long and so many posts to come to this conclusion.

rcgldr-

Since most corners don't last long enough to experience a long drawn out stand up it has little relevance to rider training. On a related note, if the rider moves their butt over before a corner, there is a small amount of counter steering pressure necessary to eliminate the bike's tendency to drift over in the direction of the weight bias they have created. It's a very light pressure.

In my experience a rider straight up in the seat (not counter leaning) has a negligible affect on the lean angle provided only that the throttle control is OK.

It's also interesting to note that a slightly opened throttle with no roll on gives the impression of holding a steady state speed whereas data acquisition shows us that the speed decreases while holding the throttle steady.

This tendency is amplified by lean angle. More lean, more slowing with a steady throttle position.

If the bike is slowing there is a forward weight transfer which would tend to stand the bike up.

Also interesting to note is the majority of riders will tell you that rolling on the throttle at corner exit is why the bike comes back up towards vertical where as it is their counter steering action that actually brings it up.

If you had an infinitely long corner it may come up eventually, as you believe is the case, I don't know, there aren't any corners I know of that are long enough to experience that.

Keith

 

[rcgldr]

Since most corners don't last long enough to experience a long drawn out stand up it has little relevance to rider training.

That makes sense.

On a related note, if the rider moves their butt over before a corner, there is a small amount of counter steering pressure necessary to eliminate the bike's tendency to drift over in the direction of the weight bias they have created. It's a very light pressure.

This is one of my arguments against body steering. You often see riders hanging off during corner approach, using steering inputs to keep the bike from turning until they actually actually start the turn in, then they use counter-steering to start the turn in with very little body movement, since they are already hanging off in advance of the actual turn. They aren't using body steering to turn, since they hang off the bike well before they actually start turn in.

This tendency is amplified by lean angle. More lean, more slowing with a steady throttle position.

True, the increased tire deformation is converting the bikes forward energy into heat. Formula 1 race cars are an extreme example this, taking some high speed turns at full throttle and losing 20 mph or more in speed due to the cornering load causing the tires to convert the energy into heat (there are cool looking infrared videos showing the tires heating up during turns).

If the bike is slowing there is a forward weight transfer which would tend to stand the bike up.

That could explain why my demo of a 90 degree left turn on a Norton 850 only required that I stop pressing on the left handlebar to get it to straighten up, since I was coasting in fourth gear with the throttle closed (I wasn't touching the right handlebar).

If you had an infinitely long corner it may come up eventually, as you believe is the case, I don't know, there aren't any corners I know of that are long enough to experience that.

What I've noticed on my bikes is that turns taken around 40 mph require some counter-steering to hold a lean, but this decreases as speed increases. At freeway speeds, it's much less, and at racing speeds probably not noticeable.

 

[tzrider]

What I've noticed on my bikes is that turns taken around 40 mph require some counter-steering to hold a lean, but this decreases as speed increases. At freeway speeds, it's much less, and at racing speeds probably not noticeable.

For what it may be worth, my ST1100 has a strong tendency to stand up if the rear tire is below 40 PSI, even with good throttle control (I'll have to ask you to take a leap of faith that my throttle control is any good). At 40PSI, it holds its line with no bar pressure when I am seated on center.

This seems to be a matter of tire deformation and not rear ride height, as ride height adjustments (I've installed an adjustable rear shock) made little difference. On the other hand, 4 PSI made a dramatic difference.

Through your discussion, I've wondered whether you have played with tire pressure to see if there was a change in handling.

 

[code]

For what it may be worth, my ST1100 has a strong tendency to stand up if the rear tire is below 40 PSI, even with good throttle control (I'll have to ask you to take a leap of faith that my throttle control is any good). At 40PSI, it holds its line with no bar pressure when I am seated on center.

This seems to be a matter of tire deformation and not rear ride height, as ride height adjustments (I've installed an adjustable rear shock) made little difference. On the other hand, 4 PSI made a dramatic difference.

Through your discussion, I've wondered whether you have played with tire pressure to see if there was a change in handling.

Tzrider

Good call Andy.

Keith

 

[rcgldr]

For what it may be worth, my ST1100 has a strong tendency to stand up if the rear tire is below 40 PSI, even with good throttle control (I'll have to ask you to take a leap of faith that my throttle control is any good). At 40PSI, it holds its line with no bar pressure when I am seated on center.

This seems to be a matter of tire deformation and not rear ride height, as ride height adjustments (I've installed an adjustable rear shock) made little difference. On the other hand, 4 PSI made a dramatic difference.

Through your discussion, I've wondered whether you have played with tire pressure to see if there was a change in handling.

Update - on the Busa, I've tried tire pressures at 38 PSI and 42 PSI, it may be making a difference, but not a lot. The tendency to stand up is small in either case, and it's difficult to "feel" a difference with the small amount of counter-steering (or optionally leaning inwards) it takes to hold a lean angle.