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Rule #1


Scarabrae
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I am making my way through "A Twist of the Wrist II" for about the 5th time, only this time I believe I am starting to make sense of what Keith is saying, this is because I am asking myself questions to further understand and fill in the gaps I have in my model.

 

For example...Rule #1 "Once the throttle is cracked on, it is rolled on evenly, smoothly, and constantly throughout the remainder of the turn."

 

The question that I am asking myself is particularly about the "constant rolling on of the throttle throughout the remainder of the turn" part.

 

I understand that the tyres contact patch is usually 40/60 in terms of area available for contact with the ground and hence traction. So the cracking open of the throttle will move the static/constant mass of the bike/rider combination from the 50/50 to the 40/60 ideal, matching the contact patch area.

 

If however, the throttle, despite being cracked open is held constant at some point through the turn (not chopped), then the acceleration would decrease...the bike would reach a constant velocity and as a result the distribution of the bike/rider mass return from the ideal 40/60 back to a 50/50 distrubution and therefore no longer take advantage of the ideal contact patch area distribution (traction) available from the tyres.

 

Sorry if this rather complicated and repetative...but its where my understand is at the moment

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I see where you are going, but your not quite there.

 

Basically, holding the throttle constant during a turn will leave the contact patch at around 40/60%.

 

If you were coasting through the turn, you would have closer to 50/50%, as the deceleration without brakes would be compensated by the suspension.

 

When accelerating, the more you open the throttle the more the bias of weight to the rear tyre and opposite way around on the brakes, but holding constant throttle should not be seen as the mid-way point of the equation. The very middle point (50/50%) would be where the throttle is at zero but engine braking isn't involved (as this could be seen as negative throttle). Basically, somewhere between throttle off and coasting. Image coasting without friction of wind slowing you down! that's 50/50%!

 

With the throttle holding constant speed, I believe you are at no less than 45/55%, as the motor is always pushing you forward, hence the motive power is always at the rear wheel. Again, suspension comes into play, but I wouldn't worry about being at 50/50% on constant throttle positioning.

 

Hope that helped?

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I'm not sure I am seeing a question there.

 

I can roll the throttle on and continue to accelerate after I have stopped turning the twist grip...for a short period of time. Then it would maintain a fixed speed. I suspect that is more of a bike specific situation and determined by how your bike carburates.

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I see where you are going, but your not quite there.

 

Basically, holding the throttle constant during a turn will leave the contact patch at around 40/60%.

 

If you were coasting through the turn, you would have closer to 50/50%, as the deceleration without brakes would be compensated by the suspension.

 

When accelerating, the more you open the throttle the more the bias of weight to the rear tyre and opposite way around on the brakes, but holding constant throttle should not be seen as the mid-way point of the equation. The very middle point (50/50%) would be where the throttle is at zero but engine braking isn't involved (as this could be seen as negative throttle). Basically, somewhere between throttle off and coasting. Image coasting without friction of wind slowing you down! that's 50/50%!

 

With the throttle holding constant speed, I believe you are at no less than 45/55%, as the motor is always pushing you forward, hence the motive power is always at the rear wheel. Again, suspension comes into play, but I wouldn't worry about being at 50/50% on constant throttle positioning.

 

Hope that helped?

Yes...thats helped! :) Cleared up the thinking in my head and the pieces fit. Onto the next chapter...

 

Cheers Mate

 

PS: Nice site you have there.....

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Hey Andy,

 

You talk about holding the throttle constant and relating that to weight bias or contact patch, and later you mention holding the speed constant and relating that to contact patch. I'm feeling a little confused. Should I be thinking about the throttle position or my speed in relation to contact patch? :D

 

I can hold the throttle constant and not be accelerating. Or I can hold the throttle constant and be accelerating.

 

In my opinion, I need to relate contact patch to a condition of speed. Accel, decel, or constant.

 

 

Don't you find that you need to have the throttle at least cracked a bit to achieve a 50/50 balance of weight bias? Or constant speed?

 

In my opinion, coasting at zero throttle thru a flat turn is decelerating. And running wide. Not 50/50.

 

How about you?

 

 

In any case, I agree with your statements, scarabrae. You seem to have it sussed.

 

 

Cheers.

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Hey Racer

 

What you are saying also makes sense..argh, suppose I need to get back on a bike and get to css with this in mind.

 

I dont want to give the impression that I am living just in my head and ignoring the process of actually riding. But knowing how I am in similar situations...I know that if I gather evidence to demonstrate and validify whats happening on a bike...I will have an easier time coaxing the part of me that thinks it has my best interest at heart by firing off my SR's :) After all I dont want to get rid of that part of me and need to honour it's intention.

 

If I truly believe something then I can then act "as if" it were true and then hold the SR's back a little each time...hopefully this will demonstrate to the part of me that believes it is looking after me that what Keith is saying holds true. Boy...placing my trust in you Keith :)

 

I suppose it doesn't help that I have not ridden for 15 years and will not be buying my bike to March..so I am not receiving feedback on the process...yet.

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My understanding is this (corrections welcome :-) ):

 

If you coast into a corner the bike will naturally slow down, purely as a result of cornering. In this case the f/r weight distribution would be as if the bike was at rest as 'you' are not doing anything to move it to the front or rear.

 

To retain a constant speed around the corner you need to apply a little throttle to counteract the cornering forces, this shifts the weight a little to the rear tyre as you are now accelerating the bike.

 

More throttle and you will start to speed up, shifting even more weight to the rear.

 

60/40 applies at the point of maximum lean (it isn't the ideal, but is a good rule of thumb to get an idea of what you are trying to achieve), after which point you should be gradually applying more and more throttle as you pick the bike up out of the corner.

 

Yes?

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Actually Woody, the bike slows down when you get off the gas and begin "coasting". The biggest reason is that your speed is being used to drive the engine around with no fuel in it. Forcing air thru valves and compressing it with no real power stroke to push the motor round except the barest minimum needed to idle and practically working against a vaccuum. This is commonly referred to as engine braking. The road driving the motor as opposed to the motor driving the road? But even without that, gravity and friction with the road and rolling resistance in the drivetrain (wheel bearings, chain rollers, etc) will slow you down to whatever the idle rpm would support. Even if you pull in the clutch, you are still slowing down.

 

Same as driving a car. Take your foot off the gas and your weight is thrown forward, eh?

 

So, no, the bike is not at rest equivalant weight balance while "coasting". Nor does the bike slow down "purely from cornering forces" while drifting or coasting. Hope that helps.

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Actually Woody, the bike slows down when you get off the gas and begin "coasting". The biggest reason is that your speed is being used to drive the engine around with no fuel in it. Forcing air thru valves and compressing it with no real power stroke to push the motor round except the barest minimum needed to idle. This is commonly referred to as engine braking. But even without that, gravity and friction with the road and rolling resistance in the drivetrain (wheel bearings, chain rollers, etc) will slow you down to whatever the idle rpm would support. Even if you pull in the clutch, you are still slowing down.

 

Same as driving a car. Take your foot off the gas and your weight is thrown forward, eh?

 

So, no, the bike is not at rest equivalant weight balance while "coasting". Nor does the bike slow down "purely from cornering forces" while drifting or coasting. Hope that helps.

 

Isn't 'coasting', defined as riding with the clutch pulled in (or not in gear)?

 

Yeah, I understand that the bike will slow down anyway, but when coasting (using my definition above) this slowing occurs by external influences (wind & rolling resistance mainly - ie, not by any input from the rider or engine). Because of this, the 'whole bike' slows at the same rate which means that there is no weight transfer front or rear.

For weight transfer to occur you'd need to brake or accelerate the wheels either by the engine (rolling on or off the gas) or pulling on the brake lever, which as you mentioned (quite rightly) in your reply would cause a weight transfer and affect the stability of the bike.

 

Attempting to summarise, what I am saying is by pulling in the clutch and coasting, relying on wind/ rolling resistance to slow the bike won't cause a weight transfer (well, not a significant one anyway - ignoring minor resistance from the drivetrain), but slowing one part of the bike (the wheel) will.

 

Make any sense?

 

[edit]

Thinking a little more though, when coasting the wind resistance will only slow the parts of the bike it hits (the front). The rear of the bike will still try to continue on at the same rate which will inevitably cause the weight to shift forwards. The only way to counteract this would be to have a little bit of gas to keep the speed of the bike constant. This keeps the weight distribution the same as if the bike was at rest.

 

Bring a corner into the equation and with the same amount of gas the bike will slow down, purely because of the cornering forces. The key here is that the f/r weight distribution won't change, even though the bike is slowing, as the slowing forces are now being transmitted toward the outside of the corner, through the wheels and tyres...

Edited by Woody
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Racer, you have a very good point in that you have to think in terms of acceleration, deceleration or constant speed rather then throttle position when it comes to throttle control. I agree with you and thanks for putting it in such easy to understand terms. I definitely think in those terms when it comes to throttle control, but I seldom verbalize it that way.

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You're quite welcome, Stuman. And thank you.

 

 

 

OK...My mistake Woody. With the possible exception of a sudden seizure of a 2 stroke GP bike, or getting stuck in a false neutral, or the 'coasting races' I've heard about (conducted by nuts in neutral gear with the motor shut-off going downhill), I cannot think of any situation in which I would intentionally "coast" through a corner with the clutch pulled in. Hence, my assumption that you meant coasting in gear. That being said...

 

If one attempts to coast through a corner leaned over with the clutch pulled in, then, yes, the "cornering forces" will dominate and generate frictional resistance at the tire/pavement contact patch(es), slowing the bike. However, inertia (the tendency for the bike to want to continue in a straight line toward the outside of the turn), and the fact that you are now slowing down, will also bias that resistance (weight) forward as you attempt to negotiate the turn. Right up until you run off the track...

 

So...No. Coasting thru a corner (with or without the clutch in) will not result in a "constant speed" balanced 50/50 weight distribution. Or it's equivalant.

 

Once the bike is turned, one must "crack the throttle" to stop decelerating and shift the weight bias off that front tire. And continue accelerating smoothly to redistribute the weight bias to the rear wheel in order to maintain positive control and negotiate the turn as fast as possible. Which is MY entire agenda on track. Here's an interesting thought. I've heard it's possible, after getting the bike leaned over and back on the gas, to complete a turn with the front wheel in the air...while still leaned over. :blink:

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If one attempts to coast through a corner leaned over with the clutch pulled in, then, yes, the "cornering forces" will dominate and generate frictional resistance at the tire/pavement contact patch(es), slowing the bike. However, inertia (the tendency for the bike to want to continue in a straight line toward the outside of the turn), and the fact that you are now slowing down, will also bias that resistance (weight) forward as you attempt to negotiate the turn

 

Not sure I fully understand this bit or why the weight is biased more towards the front, but I suppose I can live with not understanding everything. The most important thing is I understand why I need to be on the gas once the bike is turned.

Edited by Woody
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You seem to have a good understanding of the idea that slowing down while traveling forward in a straight line will shift weight forward.

 

And you seem to understand that "cornering forces" will slow a bike in a corner.

 

Well, even in a corner, the bike is still traveling forward. Right?

 

So, if you are slowing down in a corner, the weight still shifts toward the front wheel.

 

Get it?

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Interesting thread. Let me toss out one question to you guys to see if you have this clearly sorted out: if traveling in a straight line, with constant throttle, what is the weight bias? Then, what happens to the speed when you lean into a turn (and the weight bias)?

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Interesting thread. Let me toss out one question to you guys to see if you have this clearly sorted out: if traveling in a straight line, with constant throttle, what is the weight bias? Then, what happens to the speed when you lean into a turn (and the weight bias)?

 

To answer the first part of the question...I still have a notion that the bias is towards the back of the bike, take wind drag for example, I have a sense that this would load the rear to a degree.

 

The second part well...I'm not sure.

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Interesting thread. Let me toss out one question to you guys to see if you have this clearly sorted out: if traveling in a straight line, with constant throttle, what is the weight bias? Then, what happens to the speed when you lean into a turn (and the weight bias)?

 

 

I would think in a straight line the bike would be 40/60.....when turned, and assuming the constant throttle, the bike would still be 40/60..or close to it..??? Actually, I'm just guessing on nthe turning part, cuzz if I recall, going into T7 at the Streets of Willow I've been on the gas from T5 and 6..so there's no slowing down for 7, although it's uphill...it seems to me that the bike would be biased toward the rear...is that correct.?

 

40 being the front...

Edited by Hammer 4
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This is what I don't quite understand myself.

 

Assuming a constant throttle, I don't understand why the weight distribution would change whether cornering or running in a straight line.

We're all happy that the bike would slow when you start cornering, but what I still don't quite 'get' is why this results in a change to the f/r weight distribution (the explanation that this occurs simply because the bike is slowing is a little too simplistic I am afraid and doesn't really explain why). My thought is that the energy spent (that slows the bike) when cornering passes through both tyres toward the outside of the corner. Why would this result in a change in the f/r weight distribution?

 

This is all pretty much academic of course and I only want to know for interest and to try and understand what is going on.

 

One thing is certain, the practical stuff is far more fun than the theory...

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Newton's First Law of Motion. Also known as the law of inertia.

 

Webster's definition: a property of matter by which it remains at rest or in uniform motion in the same straight line unless acted upon by some external force. Though, from a physics standpoint, it would be more accurate to say, "unless acted upon by an unbalanced force."

 

Here's a cool website to check out:

 

http://www.glenbrook.k12.il.us/gbssci/phys...laws/u2l1b.html

 

Actually, if you google up "inertia" there is tonnes of stuff regarding this subject.

 

In the meantime, try this...

 

Imagine the corner is steeply banked. In this scenario, the bike's path would still be a straight line as seen from directly above. So, relative to the pavement, the bike is still straight up and down. Right? Well, as it relates to f/r weight distribution under deceleration, there's really no difference in a flat corner. The bike is still moving forward, whether straight up and down or in a corner, makes no difference. Inertia rules hand in hand with gravity. Tell me this Woody...What would happen to your weight if you applied the brakes in a corner? Would it be evenly distributed over both wheels toward the outside of the corner? Or would it be distributed in a straight line forward toward the outside of the corner?

 

The funny thing is...I'm working all of this backwards. What I mean is, I didn't learn this from a book and then go riding. I learned it by riding (and crashing) a motorcycle or bicycle. And now I'm going backwards to explain it. Which makes me want to ask a question...

 

What kind of bike do you ride, Woody?

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http://www.glenbrook.k12.il.us/gbssci/phys...laws/u2l1a.html

 

This page is titled "The Big Misconception" and is absolutely awesome.

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Braking or engine braking are a little different as these require positive inputs from the rider, inputs that we have all done and experienced the effects of. I wouldn't have thought that many people would have much experience of the effects of keeping a constant throttle from a straight to a corner (aside from gentle bumbling, which doesn't really count), which is why this is something that is a little harder to grasp...

 

I think I know what you are trying to say. I'll have a go (and probably fail) at explaining it a little more plainly.

 

Assuming a constant throttle and also assuming that the f/r weight distribution moved forwards whilst cornering, then taking this to the extreme, if we could make the corner sufficiently tight enough it should be theoretically possible to result in 100% of the motorcycle's weight over the front wheel (ignoring tyre limitations etc), at which point the motorcycle would stop.

The equivalent of running into a wall, (which is after all just a corner with an infinitely small radius).

 

What kind of bike do you ride, Woody?

A trike...

Edited by Woody
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:D ok mate...please forgive me and my frustration as i cling to my own perceptions of so called reality... :D:D

 

I'm gonna have to get back to you when i stop laughing...trike...good answer mate :lol:

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"a little more plainly", eh?

 

hey mate, don't call me surely... :D

 

alrighty then, can you come to auckland in feb for a speights on me or do i have to swim there for a foster's?

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ok mate...please forgive me and my frustration as i cling to my own perceptions of so called reality...

Rather yours than mine, mate.

 

 

alrighty then, can you come to auckland in feb for a speights on me or do i have to swim there for a foster's?

Fosters! :angry: I feel like spitting! Wish I could mate, booked for Tassie instead though.

You live in NZ, or just visiting?

I'm planning a trek over there next summer for a couple of months (so long as I can sort out some shipping for the ... er, trike). Speights sounds good, mate. ;-)

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Oh...you were serious about the trike...I thought you were being politely offended by my question and poking fun at me. I'm new to this computerized form of communication without aural/visual cues like facial expressions or tone of voice, etc. So, forgive me again and please bear with me. I'll try to give the most serious and sincere effort at explaining my "perception of reality". But I don't have time to get into it just now. Vacation is over and real life beckons...

 

In the meantime, think about road bicycle riders/racers like the Tour de France. They tend to coast through the sharper corners and when they bite it, it's the front wheel that washes out, eh?

 

As for NZ...I was planning to head back down under from the states this month for work/play/friends' wedding/etc. but might get 'stuck' here.

 

So, we'll have to play it by ear. In any case, I've never visited Australia. If I can swing it, I'd love to come there.

 

Sorry about the Fosters. It was the only OZ beer I could think of. I know how testy you boys can get about your second most favorite thing... :P

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  • 2 weeks later...

Hey there Woody,

 

Don't know if you are still out there, but...

 

I was going back over some of this thread and came across a couple things I didn't respond to before.

 

I'm trying to figure out what you mean by a difference in braking because of positive inputs. And I think I'm getting a handle on that...but you tell me.

 

You mentioned before about brakes slowing the wheels and that being different than just decelerating.

 

Well, it is different. In that the contact patch is certainly being stressed more. Not only is it being affected by the lateral cornering forces, but, the brakes are trying to make it lock and skid as well. So, yeah. It definitely puts more stress on that contact patch. However, in terms of weight distribution or bias, deceleration is deceleration. It's not the brakes or the fact that the wheel is being acted upon, it's that the whole deal is decelerating by whatever force that makes the weight want to keep going forward. I don't know if that helps.

 

And I don't know for sure if you were kidding about the trike or not. But if you are trying to relate this info to a three wheeled vehicle, I can see where there would be some difficulty in applying it.

 

Now, I'm not sure about the infinite radius thing being a vertical wall...the more I think about it the more my head hurts. But if you locked the steering head, and had infinitely sticky tires, sure, you could stand it on the front wheel with cornering force. IN fact, I believe there is a model gear train like those euro mountain climbing deals in switzerland, cog wheel i think they call it and this model actually did stand and flip in just that sort of scenario. The cog wheels being essentially infinite grip...if that helps...??? Or even if it was just a silly question with a silly answer...

 

As far as how many folks have done the constant throttle scenario, it's sort of hard to tell from the way Cobie asks his question just what he means by constant throttle and there are several conditions which are unclear...

 

Is the road level? Are we on a race course or a public road?

 

Like are we doing turn one at Brainerd? The dogleg at Daytona? The exit of the chicane at Daytona heading up onto the banking? The bend as you peel over the top into the gravity cavity at Road Atlanta? All constant throttle but...not quite the same with elevation changes creating wide variables I can't begin to deal with in terms of weight bias.

 

Or are we leaning it into a bend on the interstate while cruising at a constant velocity of say 75 mph?

 

Thinking I know what he is looking for, I would assume the level interstate scenario as this would make the most sense.

 

In which case, I think most folks have done that lots.

 

So, what's to do on vaca in Tasmania?

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