Can Weight Shift Theory be debunked?

[Jaybird180]

I have some concerns about the discussions on "weight shift" theory. Albeit not telling the entire story, by summarily dismissing it sans a thorough review or questioning of their effect on the motion or change of motion on a motorcycle we could find ourselves lulled into a mindset similar to religious cultism. Is that healthy?

It can be readily observed that if a rider moves his/her body to one side of the seat that the bike tends to lean (and change vector) and the rider has to compensate for such tendencies. It is possible to explain it away that the rider is making an unintentioned bar input, however the experience of riding with a pillion (passenger) would seem to betray that logic and provide a rationale that something else is occurring now that we have an asymmetrical weight distribution about the longitudinal axis.

Is it possible to exert a force (weight) at a lever point away from the center of rotation and have a predictable outcome? A discussion on Gyroscopic Precession may be able to provide some clues

 

[Lnewqban]

In order to communicate with the same terms, are you refering to the rolling motion of the motorcycle?

Is your question limited to the reactions of the bike and steering and trajectory following a lateral weight shift of the rider?

Sorry, I couldn't clearly understand your question.

[Jaybird180]

Yes. Hang off to one side and the bike rolls (per your diagram above).

[Lnewqban]

Unless you have a fixed fulcrum to exert leverage against, you move the bike away from you (roll it a little) as you move your body off in the opposite direction.  The total center of gravity (yours plus bike's) remains along the vertical line that crosses the imaginary horizontal line that connects both contact patches.  If the steering is kept perfectly fixed and aligned with the contact patches, the bike does not have a reason to turn.

If instead the steering is free to adjust by itself, the geometry of the front tire and angle of suspension, combined with the total weight and gyroscopic reaction (please, refer to your video and see that a left roll of the bike induces a left steering) , will  slowly turn the steering towards the side upon which the bike has rolled (only if steering angles, tire's profile and pressure are neutrally set, so there are no over or under-steering tendencies).  That slight counter-steering will induce a balancing slow roll towards the side upon which the rider is hanging off and the bike will commence a turn.  That is the same self-balancing principle that allows a rider-less bike keep going for a while while speed is relatively high.

That is a very different situation than exerting "a force (weight) at a lever point away from the center of rotation".  We are starting from an out-of-balance situation.  In that case, the bike will be forced to roll due to the moment created by the total center of gravity being initially far away from the line that connects both contact patches.  Either or not the self-balancing capability of the steering will be strong and fast enough to compensate for that initial lack of balance depends on several factors, such as magnitude of off-set weight, weight's lever, mass of front tire and linear speed of the bike.

 

[Jaybird180]

Grab a folding chair and sit backwards so you have somewhere to put your feet. The mass of the system equals yourself and the chair.

Scoot over far enough and the chair tips. I’m willing to ponder that a motorcycle behaves similarly, even with those black gyroscopes spinning.

[Hotfoot]

Nice post, Lnewqban.

Jaybird, what is it that you are trying to fix or figure out? We know from riding our no BS bike at the school that you can get a bike to drift to one side by hanging weight off to one side - as Lnewqban addresses quite well above. But we also know that it is slow and imprecise, and anyone who has ridden the no BS bike recognizes immediately that they are not in control of the motorcycle when their hands are on the fixed bars. Given a slow enough speed and enough time and space to accomplish it, you can get the bike to turn, but it is hardly effective enough to get one around a racetrack or avoid an obstacle. 

You can see a clear demonstration of this in the Twist II DVD, you can see the effects of weight shift, how the the bike reacts and how the bars react. 

I'm not quite clear whether you are trying to address hanging weight off to the inside, or talking about weighting one peg without moving the Center of Mass, the effects are different. 

More importantly, what challenge are you facing in your riding that has you asking about this? (Or is it all just an academic dicussion ? )

[Lnewqban]

On 4/21/2018 at 6:40 PM, Jaybird180 said:

Grab a folding chair and sit backwards so you have somewhere to put your feet. The mass of the system equals yourself and the chair.

Scoot over far enough and the chair tips. I’m willing to ponder that a motorcycle behaves similarly, even with those black gyroscopes spinning.

Talking about chairs, it has occurred to me that we can discuss the actions of monkeys (passengers) in sidecars races.  By moving around for each corner, they do what you describe about your folding chair: they relocate the total or combined center of gravity as far from the motorcycle or as close to the rear tire as possible.  Rather than trying to make the motorcycle and sidecar roll, they compensate the natural rollover tendency during fast cornering as much as possible.  That rollover tendency is induced by the combination of centrifugal effect and height of the center of gravity respect to the road.

 

A regular sidecar could be comparable to the situation that you have pictured above: a motorcycle with a dramatic asymmetrical weight to its side.  Would the bike yield to the induced roll?  Let's say that thanks to the third wheel, that weight does not roll the bike over and instead keeps it vertical.  If we weld the steering to the frame keeping the steering bar perpendicular to the bike and then make the bike and sidecar gain speed on a straight trajectory, the contraption will describe a straight line.

As the bike happily cruises along, if we suddenly remove the sidecar wheel, even with the stability induced by the two remaining main gyroscopes of the contraption, that asymmetrical mass or weight will be able to roll the bike until the sidecar axis hits the ground (the lateral balance will be lost).  The bike, even while leaned over, will try to keep going along the straight line (assuming no dragging forces from that dragging axis) because the steering has not changed.  

Riding with a Motorcycle Sidecar:

http://www.steves-workshop.co.uk/vehicles/bmw/sidecar/riding/sidecarriding.html

Yes, a substantial weight with some lateral leverage is able to roll a motorcycle in movement or tip the stationary chair of your example over.  Nevertheless, without the complicity of the steering capability, the bike will not turn, even if leaned over.  The following video shows that the steering capability of a motorcycle, with or without a sidecar, has a powerful influence regarding directing it onto either a straight or a circular trajectory in a precise and controlled manner ....... and what it seems more important: combined with speed and rider's skill, it is able to lift that asymmetrical weight and keep it balanced at will, even on a left turn, in which the centrifugal effect tries to take the chair down.  The maneuver is known as "flying the chair".

 

https://www.youtube.com/watch?v=k6ZSSPY32Jk

 

[Jaybird180]

With effective leverage only basically parallel with the road and feet in an awkward (forward) position, this guy can describe to you pretty readily what makes his bike change direction

This guy however, with his bodyweight over the steering stem may have a different perspective on what makes the bike change direction

When I was at the school last year, Dylan explained in a way that I thought was brilliant (for me at least) what Keith actually described but didn't use the same words in TOTW. Keith used the word "countersteer" and then described the action of the front wheel tracking the trajectory of the turn. Dylan used the words, "countersteer" and "prosteer" in the same sentence and I had an "Aha! moment".

A read through some writings dismisses as negligible any effect of bodyweight on the influence of steering. I posit that there are too many riders doing things with their weight on the bike to dismiss it and I think some of the absolutism may be obscuring the effects we have on the ability to steer a motorcycle through different intentional means.

The reason I started this thread was because I didn't want to muddy another thread where I saw glimpses of the poster perhaps getting some traction in this vein until the thread got derailed and rather than reopen a can of worms, I settled on a cleaner path.

As anecdotal proof of combined effort, (that's often cited) Dani Pedrosa is known for using the Hook Turn technique at corner entry and throughout the remainder of the turn. He uses it in combination with his steering input to get the desired result.

Being a school, I understand (and support) clearly the many reasons why it would be desirable to teach and emphasize countersteering then (relaxing input and) prosteering (by the bike's geometry) as a primary means of control.

I hope that my intentions are now made clear.

 

[Hotfoot]

That helps a lot, yes. 

[Jaybird180]

Thanks @Hotfoot and @Lnewqban for allowing me to work through this. I realized after re-reading this and a few other threads, thinking and letting it all sink in that I had been conflating separate movements of the bike. So I'll wrap it up thusly and perhaps bring this thread to a close.

I've been thinking about this new chapter (I could almost swear it wasn't in my book before- LoL) and it seems that there's a jewel in there about the bike steering about the rear wheel (once leaned over).

I realized that I'd been conflating the concept of a longitudinal rolling motion (leaning) the bike and the result of changing the bikes directional vector (turning). We can do all types of things to try to get the bike to follow a corner, but ultimately it's the result of the smaller circular circumference of the rear tire that makes the nose of the bike point in the direction with the lean, just like a coin standing on it's edge rolling in a circle.

If this is true, the act of leaning and turning are 2 separate behaviors and are about different planes of motion. The leaning would be a roll about the longitudinal axis and turning would be a lateral movement or yawing motion about what is often referred to as Z- Axis. But these are still 2 separate changes of state of motion.

I apologize if I caused anyone distress (other than myself). Eventually...I suppose, I'll figure the Motorcycle Theory of Everything (MToE).

[Lnewqban]

You are welcome, Jaybird  

What you have been analyzing and trying to understand is very complex dynamics, reason for which most riders don't even bother learning the "why" of these things.  The books that explain the whole interconnection of steering, wheels, masses, forces, etc. in a motorcycle are very dense to read and difficult to comprehend.  

I believe that there is value in understanding the basics of the Physics behind riding a motorcycle in a proficient way.  It is difficult to explain those principles to inexperienced riders without going too deep into the subject and causing confusion.  Most mentoring/teaching is limited to "do this to achieve that and go practice it".  The experienced rider has the advantage of having tested what works and what does not, of having felt those forces and the reactions of the machines during enough time to make sense of those principles. 

If serious about this, by persistent observation during thousand of miles, an educated rider becomes more aware and more sensitive about the dynamics of riding and develops a finer input of all the controls and sense of balance.  The Physics then becomes less abstract and more in harmony with our senses and minds.

In order to function as a motorcycle rather than as a bag of potatoes, all the forces and moments acting over a motorcycle in different directions must be in balance.  If our control inputs or road conditions break that balance, a brief transition period follows, during which the machine does its magic to self-adjust to a new state of balance.  If that state is not physically achievable, a fall will follow.  Counter-steering is a clear example of that: the rider intentionally steers the bike out of balance (out of its rectilinear path), inducing many reactive forces, movements and moments for a very brief period of time, forcing the machine into a new state of balance (onto a curvilinear path).  

If the machine continues on in one of the two states of balance, the rider is doing nothing or too little to modify those, like it happens in the No BS bike demonstration.  If the machine is upset by incorrect control inputs from the rider, like closing the throttle during a big rear tire slide, the machine can go from stable cornering balance to unstable transition to out of balance (highside fall) really quick.

The speed of the motorcycle is very influential about the steering, gyroscopic reactive forces, rolling and balance, reason for which counter-steering is so powerful in a superbike at high speeds, but almost negligible for a trial bike at walking speeds.

http://www.dynamotion.it/eng/dinamoto/8_on-line_papers/effetto giroscopico/Effettigiroscopici_eng.html

 

 

[jcw]

On 4/25/2018 at 12:19 PM, Jaybird180 said:

If this is true, the act of leaning and turning are 2 separate behaviors and are about different planes of motion. The leaning would be a roll about the longitudinal axis and turning would be a lateral movement or yawing motion about what is often referred to as Z- Axis. But these are still 2 separate changes of state of motion.
 

What a coincidence! This popped into my mind as well just this past week. And I was ruminating on it as well...

Roll and Yaw are two distinct types of motion on the motorcycle. Most people think of roll (lean angle) when we think of cornering. But, what we are really looking for is yaw- getting the motorcycle to go around a turn. Because of the way our physical universe exists, the most efficient (perhaps only) way to achieve this yaw results in roll.

 

Do you lean to turn or do you turn to lean. LOL.

 

Anyway, I found this interesting technical, analytical real world article measuring this yaw and roll and various rider inputs here.

http://bicycle.tudelft.nl/schwab/Bicycle/evertse2010riderMinApp.pdf

I had to read it a couple times and only understand part of it, but it clearly shows the countersteering inputs applied in getting the motorcycle into a turn, the yaw rates and direction, the roll rates and direction, the steering torque and direction and the actual steering direction.

It goes into gyroscopic moment (which might be what your original question was referring too) and how that initial countersteering input can itself contribute to the roll but minimally.

 

Fascinating stuff.

[Johnno down under]

I have had precisely one set of road tyres ( 1980s super sport radials) that seriously exaggerated the pro steering component. This meant the bike sat far more vertical through hard turns. They also taught me the value of being loose on the bars. Countersteer hard at turn in then let the bikes front end do its thing.then gas on at the right point. Easy as.