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 Motorcycle Safety
 Physics and the theoretical
 Zero speed balance
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twowheelsbg
Male Junior Member
50 Posts


Burgas, Burgas
Bulgaria

Suzuki

Posted - 03/02/2014 :  2:56 AM                       Like
Hi Guys,

I've been recently trying to improve my slow speed control, doing some popular exercises, one of which is keeping upright as much as possible at slowest pace. I tried also with no speed at all, holding the bike stationary. Not to fool you, this task seemed beyond my competence. My touring enduro bike is both tall and heavy and I don't expect to master such skill completely, but anyhow the why-and-how remains. Trial guys do it upright and with handlebar well off center. Okay for the lifting the center of gravity, it facilitates the balance. Why handlebar off center ... I will try it on a bicycle.

Approaching it theoretically, I checked David Hough's book Proficient Motorcycling. At p.42, section Mass Shift, there is some reasoning. Steering to the right, shifting mass to same side. Is it correct? I did it with both my feet on the ground, then it happens so. But with feet on the pegs and bike leaning slightly right, my intuitive brisk move to keep instant balance is handlebar to the right. I did something else also, got off the bike, kept it upright gently supporting handlebars and when I turn sharply to right - front end goes there, but the rear one tilts to the opposite side.

Might be anyone here would share BMX experience before we start drawing vectors

Andrew Dressel
Male Standard Member
244 Posts


Milwaukee, WI
USA

Moto Guzzi

California Special

Posted - 03/03/2014 :  10:09 AM
quote:
Originally posted by twowheelsbg

...I tried also with no speed at all, holding the bike stationary. Not to fool you, this task seemed beyond my competence. My touring enduro bike is both tall and heavy...


The high center of mass of a tall bike should make things easier, not harder. The angular acceleration of a pendulum, inverted or otherwise, is inversely proportional to its length, so a taller bike will likely lean more slowly than a shorter bike.

quote:
Originally posted by twowheelsbg

...I tried also with no speed at all, holding the bike stationary. Not to fool you, this task seemed beyond my Trial guys do it upright and with handlebar well off center. Why handlebar off center ... I will try it on a bicycle.



I can't vouch for what trial guys do, but I find bicycle track stands pretty easy to do, and the handlebars are turned so that any movement forward and back translates into lateral movement of the front end. On a track bike, or anything fixed-gear, the rider can simply pedal forward or backwards. On bikes with a freewheel, the rider can still pedal forward, and can create backwards motion with an appropriate slope in the pavement or by lurching the body back against momentarily locked brakes.

Finally, there's always this: "one other way that a bike can be balanced, with or without locked steering, is by applying appropriate torques between the bike and rider similar to the way a gymnast can swing up from hanging straight down on uneven parallel bars, a person can start swinging on a swing from rest by pumping their legs, or a double inverted pendulum can be controlled with an actuator only at the elbow." Of course, the heavier the bike and the lighter the rider, the harder it is.

Edited by - Andrew Dressel on 03/03/2014 10:15 AM
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gymnast
Moderator
4260 Posts
[Mentor]


Meridian, Idaho
USA

Harley-Davidson

Sportster Sport

Posted - 03/03/2014 :  2:57 PM
From a stop, hold the front brake locking the wheel and turn the bars full lock (as taught to me by National Number 27, Elliot Schultz). Proficiency requires practice. Elliot could do a headstand on the seat of a stopped bike as well as walk around on it (however there was another "trick" to that involving a strong rubber band).
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twowheelsbg
Male Junior Member
50 Posts


Burgas, Burgas
Bulgaria

Suzuki

Posted - 03/04/2014 :  2:20 PM
Thank you, it makes sense. Sure I would try those two options on my bicycle.
But as you supposed, motorcycles weight ratio bike/rider
( in my case approx. 3/1 ) makes it difficult.

What I meant as trial guys advice,
could be seen at the beginning of this video:
http://www.youtube.com/watch?v=y700zd7TC7M
This brought my attention over to what happens with mass center
when bike is stationary upright making only handlebar adjustments.
Does it really goes same side as the handlebar?

Of course my front feels and is heavier,
it is not easy to make such short and brisk inputs also.
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Andrew Dressel
Male Standard Member
244 Posts


Milwaukee, WI
USA

Moto Guzzi

California Special

Posted - 03/05/2014 :  8:18 AM
quote:
Originally posted by twowheelsbg

What I meant as trial guys advice,
could be seen at the beginning of this video:
http://www.youtube.com/watch?v=y700zd7TC7M
This brought my attention over to what happens with mass center
when bike is stationary upright making only handlebar adjustments.
Does it really goes same side as the handlebar?


Sure, if the trail is non-zero then rotating the handlebars while the bike is stationary will cause the contact point of the front wheel to move laterally with respect to the midplane of the bike.

If the trail is positive (the contact point is behind the intersection of the steering axis with the pavement, as is probably the case for all commercially available bikes) then steering to the right displaces the contact point to the left relative to the midplane of the bike, and since the contact point of a locked wheel is relatively stationary with respect to the ground, that actually pushes the front wheel to the right. Finally, this motion of the front wheel to the right, under the center of mass, should tend to cause a rolling moment to the left about the center of mass.

Cossalter has a figure which shows part of this on page 24 of his Motorcycle Dynamics. The size of this effect will depend upon not only the size and rate of the steering motion and the current lean angle, but also on several of the bike's parameters: trail, steering axis angle, wheelbase, and location of center of mass.

The assertion made by Ryan Young near the beginning of his video of the need to start with a non-zero steer angle may be based on his belief in the need to reduce the effect to avoid over controlling, similar to his assertion that the body needs to remain motionless.

The lateral displacement of the front wheel also causes a yawing motion of the bike, and this, combined with the gyroscopic effect of spinning engine parts, if they are rotating with the right orientation, may also produce moments about the roll axis.
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twowheelsbg
Male Junior Member
50 Posts


Burgas, Burgas
Bulgaria

Suzuki

Posted - 03/05/2014 :  2:16 PM
quote:
Originally posted by Andrew Dressel
If the trail is positive (the contact point is behind the intersection of the steering axis with the pavement, as is probably the case for all commercially available bikes) then steering to the right displaces the contact point to the left relative to the midplane of the bike, and since the contact point of a locked wheel is relatively stationary with respect to the ground, that actually pushes the front wheel to the right. Finally, this motion of the front wheel to the right, under the center of mass, should tend to cause a rolling moment to the left about the center of mass.



Would you, please, be more specific here about the reason for roll motion leftward - is it the front wheel motion to the right? Or do you mean the ground push to the right, as a reaction to steering moment?

quote:
Originally posted by Andrew Dressel
The assertion made by Ryan Young near the beginning of his video of the need to start with a non-zero steer angle may be based on his belief in the need to reduce the effect to avoid over controlling, similar to his assertion that the body needs to remain motionless.



Might be. I tried zero steer, small non-zero, more non-zero ... and got confused, no control at all.

quote:
Originally posted by Andrew Dressel
The lateral displacement of the front wheel also causes a yawing motion of the bike, and this, combined with the gyroscopic effect of spinning engine parts, if they are rotating with the right orientation, may also produce moments about the roll axis.



Should be expected, I will limit myself to the bicycle so far,
until I master some degree of control. At this stage it is senseless to cope with the motorbike.
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Andrew Dressel
Male Standard Member
244 Posts


Milwaukee, WI
USA

Moto Guzzi

California Special

Posted - 03/06/2014 :  8:01 AM
quote:
Originally posted by twowheelsbg

quote:
Originally posted by Andrew Dressel
If the trail is positive (the contact point is behind the intersection of the steering axis with the pavement, as is probably the case for all commercially available bikes) then steering to the right displaces the contact point to the left relative to the midplane of the bike, and since the contact point of a locked wheel is relatively stationary with respect to the ground, that actually pushes the front wheel to the right. Finally, this motion of the front wheel to the right, under the center of mass, should tend to cause a rolling moment to the left about the center of mass.



Would you, please, be more specific here about the reason for roll motion leftward - is it the front wheel motion to the right? Or do you mean the ground push to the right, as a reaction to steering moment?



First, I must admit that I don't have a good source for this exact material. Cossalter goes into a lot of the geometry and kinematics of bikes, but does not discuss this situation specifically in the material I have seen. Second, I have not done any physical experimentation other than my own riding. Thus, everything I am saying is based on extrapolation, supposition, and application of basic mechanics rules. Bikes have proven, however, to be quite resistant to such simplistic back-of-the-envelope analyses so far. See the published debunking of the need for gyroscopic effects and trail for details.

In any case, to answer your question, it is my opinion that when the front wheel turns to the right, about a steering axis that intersects the ground in front of the contact patch, the wheel is displaced to the right, and a rightward ground reaction force is necessary to cause that displacement. This rightward ground reaction force, acting on the bike in the ground plane, and so below the center of mass of the bike, creates a rolling moment, counter clock wise from the rider's point of view, about the center of mass of the bike, and should tend to cause the bike to roll to the left.
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The Meromorph
Male Moderator
834 Posts
[Mentor]


White House, TN
USA

BMW

R1100RT

Posted - 03/06/2014 :  2:17 PM
Jeez Andrew, I actually followed that last paragraph, and thought it made perfect sense.


You must have got it wrong...
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twowheelsbg
Male Junior Member
50 Posts


Burgas, Burgas
Bulgaria

Suzuki

Posted - 03/18/2014 :  1:20 PM
quote:
Originally posted by Andrew Dressel
Bikes have proven, however, to be quite resistant to such simplistic back-of-the-envelope analyses so far. See the published debunking of the need for gyroscopic effects and trail for details.


Absolutely.

quote:
Originally posted by Andrew Dressel
In any case, to answer your question, it is my opinion that when the front wheel turns to the right, about a steering axis that intersects the ground in front of the contact patch, the wheel is displaced to the right, and a rightward ground reaction force is necessary to cause that displacement.



Thank you, I had similar thoughts. I thought also of the interaction between front and rear of the bike - pushing handlebar to the right/ clockwise responds as pushing the rear of the bike counterclockwise and thus opposite leaning tendency due to leftward ground reaction force at rear contact. Probably this one is quite inferior compared to that created at front tire due to the great ratio front/rear trails.

Anyhow I continue experimenting with any bike at hand, made a few seconds progress on a bicycle balance, had a chance on a 170kg enduro ... no way. On my 240kg touring enduro I can balance only with periodical increments of forward motion along the steering corrections.

And this dude http://www.youtube.com/watch?v=ww5hsY-8LUo
drives me crazy
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gymnast
Moderator
4260 Posts
[Mentor]


Meridian, Idaho
USA

Harley-Davidson

Sportster Sport

Posted - 03/18/2014 :  9:39 PM
"And this dude http://www.youtube.com/watch?v=ww5hsY-8LUo
drives me crazy"

Note his use of his front brake lever on the left handlebar.
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scottrnelson
Advanced Member
6881 Posts
[Mentor]


Pleasanton, CA
USA

KTM

990 Adv, XR650L

Posted - 03/19/2014 :  8:38 AM
quote:
Originally posted by gymnast

Note his use of his front brake lever on the left handlebar.

That's where they've always put them on bicycles.

I tried swapping them on my mountain bike so that the front brake would be consistent with my motorcycles, but decided to switch it back. Because I've ridden both bicycles and motorcycles for so long, it seems that I automatically know which one goes to the front depending on what I'm riding.

I don't think I could adapt to left brake, right shift like on some motorcycles from the 60's, though. I'm not even sure I could even adapt "GP style" reverse shifting on a motorcycle either.
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