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Old 12-23-2011, 11:46 AM   #76
Yossarian™
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Quote:
Originally Posted by PolloAsesino View Post
The combined center of mass is.
Agreed, but I believe JDK is talking about the COM/COG of the bike itself.

Maybe an extreme illustration would be to postulate a 6' stepladder. It's an A-frame structure, and the COG of the ladder is roughly halfway up the ladder and between the two sides, a point "in space", akin to putting a dot on the center crossbar of the capital letter A.

Now, if you put a person on the lowest rung of the stepladder, has the COG of the stepladder itself changed? (Answer: other than slight changes due to compressive forces, NO.)

How about if the person climbs to the top run of the stepladder? Now has the COG of the stepladder changed? (Answer: same as above.)
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Old 12-23-2011, 12:18 PM   #77
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I believe the biggest problem here is COG and combined COG which are similar but very different things. One being static and the other very dynamic. Not to mention the failure (other than the last couple posts) to differentiate between the two.
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Old 12-23-2011, 12:50 PM   #78
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Quote:
Originally Posted by JDK111 View Post
OK smart guy--
Take 2 identical bikes and 4 identical 100 lb. blocks of concrete..

Bike #1 has two of the 100 lb. blocks of concrete attached solidly to its seat.
Bike #2 has one 100 lb. block attached solidly to each of the riders 2 footpegs.

Explain to me how bike #1 can have a lower center of gravity than bike #2

I think it's you needs to bone up on what cog is.
You're the one who doesn't seem to get it.

The LOCATION of the blocks matters. How the blocks are attached is completely meaningless.
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Old 12-23-2011, 01:09 PM   #79
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Quote:
Originally Posted by JDK111 View Post
OK smart guy--
Take 2 identical bikes and 4 identical 100 lb. blocks of concrete..

Bike #1 has two of the 100 lb. blocks of concrete attached solidly to its seat.
Bike #2 has one 100 lb. block attached solidly to each of the riders 2 footpegs.

Explain to me how bike #1 can have a lower center of gravity than bike #2

I think it's you needs to bone up on what cog is.
If two depleted uranium slugs equal to half your mass were welded to each peg - the COG of the now-modified bike would go be closer to the ground. If these two slugs were bolted to the seat, the cog of this system would go up. This is true - but also irrelevant. The rider is not attached to the bike in any way that would affect its cog. You are a wiggly blob, it is a rigid body. There is no relevance to a center of gravity of the combination of the bike and rider - because they aren't combined in this fashion.

The rider exerts forces on the bike - we do not change its COG. We worry about the bike falling over, not any "bike/rider system". We must learn to "use the Force" to get the most out of our bikes.

Think it through - it has taken me a while to grasp this and I have a master's degree in mechanical engineering (my professors will shake their heads in shame as they read my posts!) I'm far from an expert on motorcycle dynamics, but I think I'm getting a handle of the COG bit.
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Old 12-23-2011, 02:52 PM   #80
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Quote:
Originally Posted by John Smallberries View Post
There is no relevance to a center of gravity of the combination of the bike and rider - because they aren't combined in this fashion.
I wouldn't say no relevance. There are cases where both CoGs matter, as bike dynamics WILL change when you stand up, for example; bike is more likely to wheelie. Or, when you are standing, you can have a bigger side-to-side impact in some cases because your CoG (and thus the net CoG) is higher.

You're on the right track, though!
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Old 12-23-2011, 03:09 PM   #81
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Granted, there are also aerodynamic issues that come into play, but if standing on the pegs lowered your center of gravity, this guy would be standing - and not hanging off the side of the bike, getting as low as he can.
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Old 12-23-2011, 09:36 PM   #82
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Sooner or later the physicists would show up. This talk about gravity is interesting. But it can be too much theory. Too much talk.
But it is always a learning experience.

Therefore, since we got your attention, I have a question to which I haven't gotten an answer from an expert yet:

Where on the motorcycle does an applied force by the rider has more leverage to balance the bike:
a) on the pegs of the bike
b) on the seat of the bike
c) on the handlebars (not talking about counter steering here, because it would be too easy)
d) A combination of some sort
e) None of the above?

Order them in terms of leverage power, highest leverage to lowest leverage.
(I'm assuming leverage is similar to torque here. If you have the right position and the right lever length, the least amount of force will be the most effective use of force.).

Archimedes' work on levers caused him to remark: "Give me a place to stand on, and I will move the Earth."

Well, I'm way over my head here. So please let me know your thoughts.
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Old 12-24-2011, 04:02 AM   #83
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It just goes on and on and on
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Old 12-24-2011, 06:37 AM   #84
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Why is it gravity can reach out millions and millions of miles yet is considered a weak force?
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Old 12-24-2011, 06:38 AM   #85
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Quote:
Originally Posted by Lion BR View Post
Sooner or later the physicists would show up. This talk about gravity is interesting. But it can be too much theory. Too much talk.
But it is always a learning experience.

Therefore, since we got your attention, I have a question to which I haven't gotten an answer from an expert yet:

Where on the motorcycle does an applied force by the rider has more leverage to balance the bike:
a) on the pegs of the bike
b) on the seat of the bike
c) on the handlebars (not talking about counter steering here, because it would be too easy)
d) A combination of some sort
e) None of the above?

Order them in terms of leverage power, highest leverage to lowest leverage.
(I'm assuming leverage is similar to torque here. If you have the right position and the right lever length, the least amount of force will be the most effective use of force.).

Archimedes' work on levers caused him to remark: "Give me a place to stand on, and I will move the Earth."

Well, I'm way over my head here. So please let me know your thoughts.
In general terms, you will gain the greatest leverage by having the longest lever. That means that the farther from the bike's COM that you apply your force, the more leverage you have, but the greater the travel of your force.

So, it really depends upon the bike. But looking at an MX bike with very high handlebars, the bars are the rider's point of contact which give the most leverage. On a sport bike with very low clip-on bars, there's less leverage at the bars. Make sense?

But, since we are just balancing the bike, the best way to do so is to move the rider's mass independently of the bike, such as shifting rider's hips left to move the bike to the right (equal and opposite reactions, etc.) We have to remember that every force applied to the bike while riding it also has a counteracting force.

If you are talking about balancing the bike while standing next to it, then do so by applying your force at the highest practicable point of the bike.
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Old 12-24-2011, 07:36 AM   #86
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Quote:
Originally Posted by Yossarian™ View Post
In general terms, you will gain the greatest leverage by having the longest lever. That means that the farther from the bike's COM that you apply your force, the more leverage you have, but the greater the travel of your force.

So, it really depends upon the bike. But looking at an MX bike with very high handlebars, the bars are the rider's point of contact which give the most leverage. On a sport bike with very low clip-on bars, there's less leverage at the bars. Make sense?

But, since we are just balancing the bike, the best way to do so is to move the rider's mass independently of the bike, such as shifting rider's hips left to move the bike to the right (equal and opposite reactions, etc.) We have to remember that every force applied to the bike while riding it also has a counteracting force.

If you are talking about balancing the bike while standing next to it, then do so by applying your force at the highest practicable point of the bike.
Makes sense. Seems like we are going somewhere with this conversation.
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Old 12-24-2011, 08:01 AM   #87
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Quote:
Originally Posted by Yossarian™ View Post
In general terms, you will gain the greatest leverage by having the longest lever. That means that the farther from the bike's COM that you apply your force, the more leverage you have, but the greater the travel of your force.

So, it really depends upon the bike. But looking at an MX bike with very high handlebars, the bars are the rider's point of contact which give the most leverage. On a sport bike with very low clip-on bars, there's less leverage at the bars. Make sense?

But, since we are just balancing the bike, the best way to do so is to move the rider's mass independently of the bike, such as shifting rider's hips left to move the bike to the right (equal and opposite reactions, etc.) We have to remember that every force applied to the bike while riding it also has a counteracting force.

If you are talking about balancing the bike while standing next to it, then do so by applying your force at the highest practicable point of the bike.
Variant translations:
Give me a place to stand, and I shall move the world.
Give me a fulcrum, and I shall move the world.
Give me a stick long enough and a pivot and I shall move the world.

Enjoying this thread as I can. Getting a clear understanding of terms would help. Sometimes posters do not understand what 'center of mass', 'center of gravity',etc means and the relationship of desired actions using these terms. Regardless of the length of the lever or the placement and support of the fulcrum, the vectors (directions of travel) of the applied force are critical too. Tony Foal proved that one could stand on a handlebar grip and the bike will not largely change direction. This is due to the force being mis-applied in direction. Yet, a very slight push on the bar grip with one's little finger will have a very large effect on bike attitude (countersteering is based on this). The understanding of the forces envolved using the idea of 'equal and opposit reaction' is critical to understanding COG and COM in a dynamic model.
Archemedes' lever requires a fulcrum strong enough to support the transfer of force. The lever/fulcrum/load 1/load 2 need to be applied in a very specific alignment for the desired action to occur. Accounting for all the vectors and loads in a dynamic model can be tough.Then there is the problem of the 'tethered and supple' connection between the rider(operator of the lever) and the bike during a dynamic moment when the lever(s) has to be applied 'on the move for a spepecific time interval'. Getting all this explained in an accurate yet simplified way such that any damn fool can understand it and use the info properly without killing himself and/or damaging the environs is the goal. Let us be careful and kind along the journey.
Archemedes was concerned with only one-way motion. As bikers, once we induce a given motion, we also have to bring the motion to a halt and recover stability in a new configuration. Only to do this so many times as to appear to be doing it as a constant effort.
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Old 12-24-2011, 11:16 AM   #88
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i enjoy it when the guys obviously educated in physics chime in . Provolks some interesting thoughts . While I think I understand the physics ,i could not explain like they do in a million years.
That fact that they all disagree with each other makes it so amusing . I think maybe the physics off riding 2 wheels are complex ,Maybe more so than we think.
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Old 12-24-2011, 09:02 PM   #89
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Quote:
Originally Posted by Lion BR View Post
I have a question to which I haven't gotten an answer from an expert yet:

Where on the motorcycle does an applied force by the rider has more leverage to balance the bike:
a) on the pegs of the bike
b) on the seat of the bike
c) on the handlebars (not talking about counter steering here, because it would be too easy)
d) A combination of some sort
e) None of the above?

Order them in terms of leverage power, highest leverage to lowest leverage.

.
The 'order' answer to your question is ... it depends what the rider is doing with the bike at the time
The rider's mass is not bolted solidly to the bike - so his/her mass can be used to affect's balance differently depending on what the bike is doing - traveling straight down a road, racing around a corner (like N Hayen above) or 20 ft. in the air on a motor cross track or 1000 other possibilities.
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Old 12-24-2011, 09:09 PM   #90
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Originally Posted by high dangler View Post
I think maybe the physics off riding 2 wheels are complex ,Maybe more so than we think.
in the context of this discussion, it is actually very simple.

if two masses are locked together (the bike and the rider), and one mass (the bike) moves, the other (the rider) has to go with it...making it impossible for the 2nd mass (the rider) to counter the first mass's (the bike's) movement and maintain the balance of the overall system.

if the two masses are decoupled, and one mass (the bike) moves, the other (the rider) can move in the opposite direction to maintain the balance of the overall system.

that's why standing makes the whole system "more stable"...it decouples (to a much greater extent than sitting on the seat) the rider's mass from the bike's mass and allows the rider to counter the bike's movements to maintain the balance of the bike/rider system. that's really all there is to it.
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