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Jaybird180

TC Rule #2 and Braking

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Jaybird180    30

In Twist 2, Chapter 6 it says "Throttle control rule number two: in any fast-entry turn, calculate the roll-off as carefully as you would a roll-on."  IIRC there's also a reference in Twist 1 about a smooth roll-off, but I don't have it available to me at the moment.

With new information being presented in L2 about braking, does this change anything regarding this rule? In the class, we were presented with graphs of rider input on the brakes, showing that it spiked to near maximum input nearly instantaneously, Dylan said it was about 0.3s. If it's the case that riders are instantly getting to maximum braking and then trailing them off at a rate of about 1.5 - 3s it would also seem to make sense that being diligent with throttle roll off would be less critical.

I have observed that many racers nearly instantly shut off the throttle at the initiation of the braking zone before getting hard on the binders. I've also observed myself doing this with no overt ill-effects.

Is there anything that can be considered about this rule?

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Hotfoot    48

Notice that it says "in any fast-entry corner", so this is not the same situation as coming off a fast straight to a sharp 40mph corner.  You may be braking lightly or not at all. 

What effect would it have on the bike if you abruptly chopped the throttle from full on to full off, at high speed? How would this effect a turn entry? 

At high speed, what is the primary external force acting upon the bike that would make it slow down when you let off the gas? 

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Jaybird180    30

I agree that there's a qualifier in the statement, but the bike and front tire do not know a difference (nor care) about the source of the added load. According to Dylan's telemetry data, riders are routinely doing this and not getting into any sort of trouble.

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Jaybird180    30

Anytime the speed is decelerating, there is an extra load being placed on the front tire, agree? Assuming "yes", the source of the load could be brakes or from rolling off, yes? The speed with which one does this (transfers load to the front), if I understood what Dylan was communicating is not as important as the transfer of load to the rear of the motorcycle, hence the introduction of the Brake Release Drill to control the removal of the forward load before getting back on the gas.

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Hotfoot    48

At high speed wind drag is a major source of deceleration. That is NOT the same effect as using the front brake. Wind drag is a tremendous factor at high speed, and practically a non-factor at slow speeds. Wind drag is proportional to velocity SQUARED so increasing speed causes a huge increase in drag. Not saying the whole throttle control rule #2 is about wind drag, but I AM saying you can't assume a hard-braking slow corner is the same as a slow or non-braking corner at very high speed. The radius of the corner will be substantially different so the steering effort is different, the forces on the bike are different, what the rider needs from the geometry is different, use of the brake is different, amount of slowing required is different, etc. 

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Jaybird180    30

@Hotfoot,

Thanks for your response. I also received your PM and I think that the reference used may have an unintended consequence. Allow me to explain:

Often, when we learn something new, we must compare that new thing to the old thing that we knew or thought we understood. One of them or both of them must then be updated as we get closer to the truth about any subject or find new, relevant data.

As I thought about the drill and particularly the classroom session, I began to think about a reference in one of the twist books about a smooth roll-off, however I couldn't quite find the one that I wanted but the one quoted was closest to it, so I went with that one, introducing an unintended consequence of discussing the specificity about high-speed corners. My real question is more accurately about heavy braking zones and the tendency of riders to dump the throttle and grab and handful of brake. I then desired to generalize about the roll-off technique itself. In all academic advice prior to Dylan's class the action of initial hard braking was verboten, but Dylan observed that riders are routinely doing it, and the data confirms so there must be something to it. I agree.

Assuming that this is correct that the load on the front tire isn't critical when straight vertical, it has the beneficial potential to allow the rider to keep some attention in reserve, thus spending it more wisely.

 

 

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Jaybird180    30
21 hours ago, Jaybird180 said:

Anytime the speed is decelerating, there is an extra load being placed on the front tire, agree? Assuming "yes", the source of the load could be brakes or from rolling off, yes? The speed with which one does this (transfers load to the front), if I understood what Dylan was communicating is not as important as the transfer of load to the rear of the motorcycle, hence the introduction of the Brake Release Drill to control the removal of the forward load before getting back on the gas.

The above statement is incorrect. Applying the front brake puts load on the front tire. Rolling off the throttle, while decelerating the motorcycle the REAR tire would be a source of friction, decelerating the motorcycle. Hotfoot also pointed out wind drag, a parasitic force. It would not add load to the front tire, but it could add load to the rear due to leverage if the wind is exerted on the riders' erect torso.

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Hotfoot    48

So here is how I take it: the rule says "calculate the roll-off as carefully as you would a roll-on" and I take that to mean, make a plan on how you want to do it. I don't think it means it always has to be slow, and maybe in certain circumstances (good traction, straight up and down) it might not even need to be smooth, but it does need to be considered. If it is too slow, it could delay braking, and if it is too fast it could upset the chassis. I learned the hard way that just snapping the throttle off is not always a good idea; when riding in the rain, going fast, I rolled off abruptly (as I had often done in the same exact place) but the sudden slowing and slick conditions made the back end come around. OOPS, not what I wanted to have happen! I also found out at COTA in the rain that BMW actually accounts for this in the traction control, in Rain mode. If you abruptly go from full throttle to off throttle and decelerate too fast for conditions the bike will feed some power back to the rear wheel to regain traction. A little shocking to experience that but it's a good lesson in the importance of calculating your roll off, instead of getting in the habit of just snapping it off every time.

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Jaybird180    30

From your exegesis I posit that the rider should have a plan for every corner, every ride. But keep in mind the oft-repeated military saying, "No battle plan survives contact with the enemy."

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Lnewqban    11
On 5/18/2017 at 10:56 AM, Jaybird180 said:

In Twist 2, Chapter 6 it says "Throttle control rule number two: in any fast-entry turn, calculate the roll-off as carefully as you would a roll-on."  IIRC there's also a reference in Twist 1 about a smooth roll-off, but I don't have it available to me at the moment.

........

Is there anything that can be considered about this rule?

This is the way I understand throttle control rule number two in Chapter 6:

Fine modulation of the throttle helps you read the forces that you feel more accurately.

The advantage of that is that your entry speed will be more consistent and appropriate than if you grossly decelerate in a hurry (charging the curve), just to find out that your entry speed (at the end of that precipitate deceleration) is lower than it should be (because your senses were overwhelmed, you are erring on the safe side of entry speed).

The error about the entry speed is more significant for any fast-entry turn, especially due to the aerodynamic drag explained by Hotfoot above.

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Lnewqban    11
On 5/18/2017 at 4:38 PM, Jaybird180 said:

My real question is more accurately about heavy braking zones and the tendency of riders to dump the throttle and grab and handful of brake.

 

 

When a bike is decelerating, due to front brake (stress on the front tire's patch), throttle-off or rear brake (stress on the front tire's patch), a forward inertial force appears applied at the height of the CG.

The height difference between the forces pointing aft and forward makes the bike initiate a rotation around its axis of pitch.

http://www.motorcyclistonline.com/blogs/roll-pitch-and-yaw-drawing-line

That rotation is resisted by the moment of inertia (angular mass or rotational inertia) of the bike, which slows down the max loading (vertical force) of the front tire's patch and the min loading of the rear's.

https://en.wikipedia.org/wiki/Moment_of_inertia

In other words, even when it is quick, it takes some time for the nose to dive and the tail to stand up.

At the beginning of that process, the weight distribution is close to 50/50 on both tires, while at the end, it could be as close to 100 front/0 rear as the rate of deceleration is closer to 1G.

It is only natural to roll-off first when the rear tire still has max traction for braking (50/50 condition), to follow with quick but gradual application of front brake (while bike starts nosing down/tailing up), to end with desired pressure on the front brake (once the nose-diving is complete and the front patch reaches max traction).

If that sequence of control's input becomes a habit, no much attention about slowing down is consumed.

For straight up bike / dry track conditions the sequence is quicker, for leaned bike / wet track the sequence is slower.

Similar principle can be applied to acceleration or to quick deceleration-to-acceleration transition, but with the pitching effect in reverse.

Pitch-Roll-Yaw.png

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Jaybird180    30

@Lnewqban I'll give this some thought about moments of inertia, but I also desire to put a pin prick on your emphasis on gradual application of front brake, but might be more appropriate in another thread. But to be succinct, there's no benefit to gradual brake application (despite the years of indoctrination we've all be subjected to about it).

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Hotfoot    48
On ‎5‎/‎22‎/‎2017 at 6:44 AM, Jaybird180 said:

there's no benefit to gradual brake application (despite the years of indoctrination we've all be subjected to about it).

OK I read back through this whole thread, and yes, bringing in Throttle Control Rule #2 seems to have added some confusion, as I personally got stuck on trying to discuss the throttle control rule. After reading back over it all, it seems that the OP's real questions was, does it really cause any problem to let off the gas fast or pull in the brake abruptly, it seems like people do it all the time.... I think the answer to that is situational, depending on how much traction you have and how much you are willing to upset the chassis. And also, of course, how fast you need to slow down, and how accurate you need/want to be when setting your entry speed.

Example, if you are riding your S1000rr in first gear, wide open throttle, 12,000 rpm and you instantly chop the throttle you will feel a very abrupt change in the bike, potentially enough to affect your accuracy in an upcoming turn. However, if you are in sixth gear at 4000rpm, wide open throttle, and let off abruptly, that's not going to be such a violent change, it may not bother you to rock the chassis that amount.

The weight shift is even more pronounced with braking, of course; but it is a matter of priorities. Upsetting the chassis with abrupt brake application may very well be worth it, especially if you need to get it slowed down in a hurry. For sure I can think of corners where I am coming down from high speed to low speed, traction is good, I am straight up and down, where I let off the throttle and bring in the brake as fast as I can - upsetting the chassis is OK with me, I know the front will dive down hard, but my priority is braking in the shortest possible distance and the majority of braking is done at the beginning and I need time for gradual release as I enter the corner. But I can also think of places where I am entering a corner leaned over, or the surface is bumpy, and I apply the brake more gradually to maintain the best suspension/traction scenario, to avoid bottoming the forks or overloading the front tire.

One thing we have been taught is that if you slam the front brake on so fast that there is no time for the weight shift to the front tire to increase your traction, you can slide the front tire. Is that a problem? Maybe not, if you are going in a straight line and don't scare easily. Lord knows we see pros doing some really scary things on the brakes, things most of us don't really want to have happen on a Sunday ride (like stoppies, rear wheel hop, back end wagging around, back end stepping out, etc.)

I'm going to run this question by Dylan and/or Keith and see what exactly Dylan was trying to communicate and get more details on different scenarios - braking while leaned over, wet conditions, etc. 

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powayTom    4

Great thread, keep the info coming.

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Jaybird180    30
12 hours ago, Hotfoot said:

One thing we have been taught is that if you slam the front brake on so fast that there is no time for the weight shift to the front tire to increase your traction, you can slide the front tire. Is that a problem? Maybe not, if you are going in a straight line and don't scare easily. Lord knows we see pros doing some really scary things on the brakes, things most of us don't really want to have happen on a Sunday ride (like stoppies, rear wheel hop, back end wagging around, back end stepping out, etc.)

I'm going to run this question by Dylan and/or Keith and see what exactly Dylan was trying to communicate and get more details on different scenarios - braking while leaned over, wet conditions, etc. 

This is exactly the thing that the laws of physics says cannot happen.  There is no time period necessary for weight transfer, it's simply the result of the decelerative forces. The contact patch flattens as a result of the load increase as it has not any ability to resist.

The distinction @Dylan Codemade was that that time to get to desired brake pressure is irrelevant but what invites trouble comes is in exceeding that specific pressure that will cause the tire to overload. As I thought about it, I agree and I'd call Dylan the MythBuster.

Moreover, as he expounded, as we lean over the pressure that will cause the tire to break traction is less than if we're straight up, because there is less remaining due to some of it being used by cornering forces. In any scenario (with level pavement) the theoretical limit of brake use is 1G, after that the front brake becomes a lever whereby the rear-end can be lifted. If that's not a concern, then by all means pull harder, but if you care about having sufficient load on the rear then you may not wish to exceed 1G.

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Jaybird180    30
On ‎5‎/‎22‎/‎2017 at 9:44 AM, Jaybird180 said:

 ...there's no benefit to gradual brake application (despite the years of indoctrination we've all be subjected to about it).

Actually there is one benefit: Unknown available traction. We tend to sneak up on brake pressure because of uncertainty about where the limit is. But if the limits were known and we were accurate with our application of the brake lever, then we can get right to the business of braking up to that maximum level.

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Hotfoot    48
2 hours ago, Jaybird180 said:

This is exactly the thing that the laws of physics says cannot happen.  There is no time period necessary for weight transfer, it's simply the result of the decelerative forces. The contact patch flattens as a result of the load increase as it has not any ability to resist.

The distinction @Dylan Codemade was that that time to get to desired brake pressure is irrelevant but what invites trouble comes is in exceeding that specific pressure that will cause the tire to overload. As I thought about it, I agree and I'd call Dylan the MythBuster.

Moreover, as he expounded, as we lean over the pressure that will cause the tire to break traction is less than if we're straight up, because there is less remaining due to some of it being used by cornering forces. In any scenario (with level pavement) the theoretical limit of brake use is 1G, after that the front brake becomes a lever whereby the rear-end can be lifted. If that's not a concern, then by all means pull harder, but if you care about having sufficient load on the rear then you may not wish to exceed 1G.

Ah, now I see where your question is coming from, this makes more sense. I agree that the danger of applying the brake too fast is in applying it TOO MUCH. If you are really, really good and know exactly how much brake pressure is needed and have good enough control to get right to it instantly, then YES, go for it, that makes sense.

However, not everyone is that good so it is understandable that riders are taught (or learn through experience) not to snatch the front brake.

There are things that happen that do change with time, as you can observe and experience directly for yourself. The front shock compresses as you shift weight forward, this takes time - not a lot, but it is not instant, because the forks do actually move and that takes some time. Same with front tire flattening; it's very fast, but it ain't instant, and a slow motion camera would show this, the tire has to actually flex. Then there is the rider - a time lapse video would show the effect of deceleration/weight transfer on the rider. Because those things are occurring, the situation can and does change over time - for example, a hard grab of the brake can cause the rider to tense up the arms and fall forward, and the effect can worsen as the front end of the bike dives down, so what should NOT have been a front tire slide (based on brake pressure alone) can become one due to the increased load on the front due to the rider's weight and restriction of the bar movement. This is where it gets complicated to reconcile theory and real world experience - it is difficult, with theory alone, to account for all of the variables created by the bike and rider. There is a great discussion of this in The Soft Science of Motorcycle Racing, I will have to look and see what chapter that is in.

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Jaybird180    30

Yes, this is getting more complex. As you mentioned suspension- most modern suspension systems have separate hi and low speed valving that should be able to account for braking and having the forks collapse. I wonder if @Dylan Code can show us some graphs of front fork travel under the same "instant" braking scenarios (heavy braking zones) to see if the collapse of the forks correspond to what I see on TV - the forks dive instantly and don't rebound until the rider is done with the turn (perfectly timed brake release) and back on the gas.

I hypothesize that front tire flex would be at the same equivalent rate as brake application. I say "equivalent" because brake systems have leverage ratios built into the master cylinder that allows the rider more or less pressure at the pads than the lever travel might suggest.

I don't account in the above for tightening on the bars, as I put that in the category of rider error.

I look forward to what jewels you dig up in Soft Science. It's been awhile since I've read that one.

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Jaybird180    30

@Lnewqban I now know what was giving me trouble about your post: it's the center of axes of rotation in the graph. The Pitch axes should be about the front and rear axles (braking and accelerating), the roll axis should be at a point just above the axles but IIRC not above the top of the tire, and the yaw axis would be about the steering stem.

A motorcycle doesn't behave like an aircraft where the centerline of rotations are through the centers of symmetry and intersect at the Cg.

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Lnewqban    11
On 5/24/2017 at 0:58 PM, Jaybird180 said:

@Lnewqban I now know what was giving me trouble about your post: it's the center of axes of rotation in the graph. The Pitch axes should be about the front and rear axles (braking and accelerating), the roll axis should be at a point just above the axles but IIRC not above the top of the tire, and the yaw axis would be about the steering stem.

A motorcycle doesn't behave like an aircraft where the centerline of rotations are through the centers of symmetry and intersect at the Cg.

On the axis of pitch:

All the three axis of rotation must go through the center of mass: that applies to a motorcycle, an airplane or any body with a mass that is forced to rotate.

Of course, that is respect to the ground (an inertial point of reference); if you set a camera on the front wheel's axle, you will see the forks rotating some around it.

On the need for smooth application of the brake:

As you know, the maximum friction force that resists the tire from skidding is proportional to the force acting perpendicularly to both surfaces in contact, which is the weight being transferred on the contact patch (50 front/50 rear when coasting, 100 front/0 rear when wheelie).

When you abruptly apply pressure to the front brake lever, the only traction that you have on the front contact patch at that very first moment is about 50% of what you have at the verge of a stoppie, which is 100%.

Is that 50% traction enough to initiate the deceleration process?

As explained above, that is circumstantial: you either lock the tire and lose steering or you force the non-instantaneous transfer of weight, which will allow you to use more traction for deceleration.

Braking at 1G means that there is a force pulling the contact patch aft that is equal in magnitude to the combined weight of motorcycle and rider and that is decelerating the bike in 32 feet/second per each second (similar to gravity).

At the very beginning of front brake application, when the weight distribution is still close to 50/50, you cannot brake above 0.5 G.

The magnitude or intensity of a good efficient front braking goes from 0.5 G to 1 G in a short period of time, but never instantaneously.

Copied form Chapter 24:

"Snapping on the brake lever too quickly is not productive except for photos.  Everyone has the feeling they can abuse the front brake whether they have ever locked it up or not. But there are only two real rules of front brake use and abuse:
1. Don't snap it on too quick.
2. If the front wheel locks up, loosen up on the lever."

 

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Jaybird180    30
11 hours ago, Lnewqban said:

On the axis of pitch:

All the three axis of rotation must go through the center of mass: that applies to a motorcycle, an airplane or any body with a mass that is forced to rotate.

Of course, that is respect to the ground (an inertial point of reference); if you set a camera on the front wheel's axle, you will see the forks rotating some around it.

I've got a few minutes to digest and work on this...

When accelerating, where is the force being applied from the engine? It's not at the center of mass. The drive chain applies a force to the rear sprocket and it's about the sprocket that the force is applied, in an upward oblique vector. It's what accounts for the bike's anti-squat capabilities if the swingarm is at the proper angle (in race configuration - most streetbikes aren't setup this way). This pulling up and the reaction of Newton's First Law is what causes a ham-fisted rider to lift the front of the motorcycle on an aggressive start where the rear wheel stays in exactly the same place, looping the bike. Some stunters can be seen doing this on purpose and they use super-large sprockets to gain more leverage.

On a slow speed stoppie, you can observe (and stunters are really good with this) the point where the calipers lock onto the rotors, making the front axle the fulcrum point and the entire rear end coming up and rotating around that front axle.

My first encounter with the "strange" longitudinal axis placement was on a day directly behind a faster rider watching him snap the bike into turns. I observed that while the top of the bike tipped in, the tire's contact patch displaced some distance in the opposite direction from where the input was made. Observe this for yourself, but there is a thread somewhere here discussing this phenomenon.

Yaw axis: the clearest example would be the rear end coming around or a tank slapper where the rider has a death grip on the bars. Steering dampers work around this axis to break the oscillation and possibly save the bike from flicking the rider off.

11 hours ago, Lnewqban said:

 

On the need for smooth application of the brake:

As you know, the maximum friction force that resists the tire from skidding is proportional to the force acting perpendicularly to both surfaces in contact, which is the weight being transferred on the contact patch (50 front/50 rear when coasting, 100 front/0 rear when wheelie).

When you abruptly apply pressure to the front brake lever, the only traction that you have on the front contact patch at that very first moment is about 50% of what you have at the verge of a stoppie, which is 100%.

The formula for Coefficient of Friction doesn't have a time component. So as pressure on the mating surfaces increase, so does CoF in proportion. At any point in time, you have a maximum CoF at the tire's contact patch because it changes and the key element of braking is to not exceed that specific value. As the contact patch spreads, you have more rubber in contact with the pavement. The question is: what causes the contact patch to spread?

If you could apply a downforce to the front tire in an infinitesimally small amount of time (approaching instant) would the contact patch spread or would there be some delay in the contact patch spreading? I think you'd agree with the obvious answer is that as an inanimate object it would spread in direct proportion to the force applied to it.

Having the braking limit at 1G is very convenient because we travel across the surface of the earth, which acts on all objects toward its center at a force of 1G (9.8m/s/s or 32ft/s/s) - the opposing force. Disturb that (on a flat surface) and you get to do some fun math with orbital mechanics.

11 hours ago, Lnewqban said:

Copied form Chapter 24:

"Snapping on the brake lever too quickly is not productive except for photos.  Everyone has the feeling they can abuse the front brake whether they have ever locked it up or not. But there are only two real rules of front brake use and abuse:
1. Don't snap it on too quick.
2. If the front wheel locks up, loosen up on the lever."

 

I'm going to take a stab at interpreting this section in concert with understanding more about the physics of decelerating objects.

Point 1 is a point of practicality akin to Keith's advice on the use of the rear brake. The average person doesn't have the requisite skill to use it to maximum effect so for the benefit of most people, he advises in his publications to use and become proficient with the front brake. But since this is the internet and not a racetrack, we can wax strong about the perfect world of being able to apply exactly the right amount of brake pressure every time and at all times. Rear and Front.

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Lnewqban    11

There is no more weight transfer once the bike does a wheelie or a stoppie: it is 100% of the total weight on the tire in contact with the surface; no more, no less.

The wheelie and the stoppie are beyond the transient state of initiating braking that we are discussing.

That is the state during which the bike pivots around its center of mass or axis of pitch (just like a seesaw or teeter-totter), progressively pressing more on the front suspension and front tire (which is the reason for which the nose dives and the tail rises up simultaneously).

This link explains it better than I could:

https://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics#Braking_according_to_ground_conditions

Again, with good rubber and on dry track, you can get away with suddenly stabbing the brake lever: the weight transfer will happen during the time dictated by the rotational inertia, but it will not make a difference, the contact patch will fiercely grip.

Doing the same on public dirty greasy roads, right after start raining, will lock the tire and you will go down very quickly, unless you are quicker releasing that pressure on the lever: that is the whole point behind bikes equipped with ABS brakes.

With no ABS brakes, you may be surprised of how hard you can brake in those conditions if you apply increasing pressure on the front brake lever in proportion to the rate of weight transfer.

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