Center of gravity and body position

[doc47]

Dumb question?:

 

When I stand on the pegs while riding what happens to the COG compared to sitting? Lets assume I'm staying loose and light on the bars, allowing the machine to move under me.

[onmyrt]

Lets see, most of your body mass is in your torso, so I would guess that as your body rises from the sitting position,

the COG of the whole machine rises, and probably moves forward just a few inches too.

[upflying]

I think COG would be adversely raised if you sit rather than stand on the pegs. That's why off-road riders stand..to lower the COG and let the bike wiggle and wander beneath them.

[Firefight911]

Standing effectively lowers the COG.

 

View it as the weight is either primarily on the seat through your butt or on the pegs through your feet.

 

Standing is more effective in off road control by way of this lowered COG in conjunction with the bike being allowed to move more freely and quicker as you stand.

[VinnyR11]

I think COG would be adversely raised if you sit rather than stand on the pegs. That's why off-road riders stand..to lower the COG and let the bike wiggle and wander beneath them.

 

That's a very common misnomer and is often used incorrectly in articles and books pertaining in particular to off road riding. Standing on the pegs vs. sitting in the saddle raises the COG. A way to look at COG is that it is the averaged position of the overall mass of the bike including the rider. Raise any part of the mass and you raise the COG.

 

The thinking that standing on your pegs lowers COG comes from the belief that your weight transfers to the pegs. Viewed that way, you can say all the weight of the bike and rider is already on the contact patches of the tires and doesn't change.

 

In any case, if you move any mass on a bike the COG will follow along in that direction. Standing vs. sitting raises it.

 

There are other reasons why standing on the pegs helps off-roaders handle their bikes, but it's not due to lowering COG.

[ShovelStrokeEd]

I'm with Vinnie on this.

Standing up will raise the COG of the entire assembly. The main advantage of standing is to disconnect a substantial portion of the mass from the bike, allowing it greater freedom to pivot about whichever axis you choose.

[Woodie]

COG of the bike+rider by definition would go up. However, by removing the fixation between the bike/rider (the seat/tank grip), it allows for the COG of the bike and the rider to separate, and therefore change the behavior of the whole.

 

Knees now flex, absorbing shocks, allowing the bike (and it's COG) to move separately from the COG of the rider.

 

too slow.

[T__]

Dumb question?:

 

When I stand on the pegs while riding what happens to the COG compared to sitting? Lets assume I'm staying loose and light on the bars, allowing the machine to move under me.

 

doc47, that depends on what COG you are looking at.. If looking at the static COG of the rider/machine “assembly” when sitting still you would raise the COG by standing..

 

BUT, while riding it doesn’t happen that way as by standing you lower the EFFECTIVE COG as you allow the bike to move around under you.. If you stayed solidly coupled to the motorcycle standing would raise the COG but in reality the dynamics of loose bent arms & bent knees allow the riders body to react separately from the motorcycle..

 

On my dirt bike I can stay standing straight up & push the bike way over into a heavy lean so my weight stays centered over the tire contact patch in loose gravel or loose dirt..

 

Twisty

 

[Huzband]

 

The thinking that standing on your pegs lowers COG comes from the belief that your weight transfers to the pegs. Viewed that way, you can say all the weight of the bike and rider is already on the contact patches of the tires and doesn't change.

 

 

And so based on that, road racers weighting the outside peg is an exercise in futillity. I don't think so.

 

I have more on this, but it's getting late here on the East Coast. I'll get back to you tomorrow.

[onmyrt]

Maybe this will help explain the misconception.

[jfremder]

 

The thinking that standing on your pegs lowers COG comes from the belief that your weight transfers to the pegs. Viewed that way, you can say all the weight of the bike and rider is already on the contact patches of the tires and doesn't change.

 

 

And so based on that, road racers weighting the outside peg is an exercise in futillity. I don't think so.

 

I have more on this, but it's getting late here on the East Coast. I'll get back to you tomorrow.

 

I've read a bit on peg weighting on the street/track. Found a nice summary here: Peg Weighting

[VinnyR11]

 

The thinking that standing on your pegs lowers COG comes from the belief that your weight transfers to the pegs. Viewed that way, you can say all the weight of the bike and rider is already on the contact patches of the tires and doesn't change.

 

 

And so based on that, road racers weighting the outside peg is an exercise in futillity. I don't think so.

 

I have more on this, but it's getting late here on the East Coast. I'll get back to you tomorrow.

 

No, that's not what I said. Never mentioned the futility of weighting the outside pegs.

 

I said standing on the pegs raises the overall COG. That's all. And it does.

[Caddis]

When I stand on the pegs while riding what happens to the COG compared to sitting?

 

The COG of what? As others have pointed out, the COG of the "system" that consists of both the bike and the rider goes up, and probably a bit forward. This is a fundamental physics question, and by definition, moving some of the mass of the system in one direction moves the COG of the system in that direction.

 

Lets assume I'm staying loose and light on the bars, allowing the machine to move under me.

 

Ah, here is where the disagreements and confusion come in. If a rider were completely stiff, and connected rigidly to the bike, then things would be simple. But obviously, that isn't the case. When sitting on the seat, the rider can "pivot" around a bit at the point where he is connected to the bike (the seat). When the rider stands up, the "pivot point" where the bike and rider are connected (I'm ignoring the handlebars for now since that is a lot less important) is at an even lower location - the pegs. This means that it is easier to tip the bike over more rapidly, since less of the rider's weight has to move in the direction of the seat. But that isn't because the CG of the "system" is any lower, it is just because the bike and the rider are not rigidly connected.

 

So is the CG of the "system" higher when a rider stands up? Absolutely. By definition.

 

But when the rider's connection to the bike moves from the seat down to the pegs, it lets the two parts of the system move more independently, and that is what helps.

 

[Huzband]

I think Vinny has it right on one level. Standing on the pegs does raise the CoG. But, it lowers the center of mass.

[ESokoloff]

This same discussion (or simular) has gone around at least once in the past.

[Caddis]

Just to be clear, for those of us who live on the surface of the Earth, "center of gravity" and "center of mass" are the same thing.

 

http://en.wikipedia.org/wiki/Center_of_mass

[Ron_B]

I'm with Vinnie on this.

Standing up will raise the COG of the entire assembly. The main advantage of standing is to disconnect a substantial portion of the mass from the bike, allowing it greater freedom to pivot about whichever axis you choose.

 

Ed, does this mean that the rider, sitting down, is "sprung weight," and becomes "unsprung weight" upon standing up?

[ShovelStrokeEd]

Nope, unsprung weight is the wheels, tires, brake discs, sprocket or rear drive, calipers and fork lowers.

 

The chassis, including the rider is sprung weight. It would be interesting and educational to somehow balance a bike on a pair of scales and measure the actual change in weight distribution between the rider sitting on the seat and standing on the pegs.

[VinnyR11]

I'm with Vinnie on this.

Standing up will raise the COG of the entire assembly. The main advantage of standing is to disconnect a substantial portion of the mass from the bike, allowing it greater freedom to pivot about whichever axis you choose.

 

Ed, does this mean that the rider, sitting down, is "sprung weight," and becomes "unsprung weight" upon standing up?

 

I find a simple way of picturing sprung vs. unsprung weight is to envision someone pushing down on the part/item in question and then ask yourself, "if they pushed hard enough would it deflect the springs"?.

 

For example if a friend pushed down on your shoulders while you were on the bike, it would deflect the springs. That doesn't matter whether you were up on the pegs or sitting on the seat.

 

Now picture the list of unsprung items Ed listed above. No matter how hard you pushed down on the tires, calipers, brake discs, etc. the springs are never going to compress.

 

Manufacturers and riders try to minimize unpsrung weight and one reason racers go to such extreme$$ to get light wheels.

[bobbybob]

I submit that the COG is in constant motion when the rider stands on the pegs.

 

1) If the rider is able to stand straight up and remove his hands from the grips, the COG is lowered by his weight on the pegs.

 

2) But as soon as he grabs the grips and leans in a corner while standing, then his weight is acting against the upper bike (i.e. grips, not pegs) and the COG moves upwards to different degrees.

 

And all the varying degrees of loose arms/stiff arms/tight grip/loose grip affect the results.

 

And as this "interface" with the bike is a dynamic situation, the COG is constantly changing as he moves. This makes complete sense to me, I can visualize it easily, and nobody can talk me out of it.

[Huzband]

Just to be clear, for those of us who live on the surface of the Earth, "center of gravity" and "center of mass" are the same thing.

 

http://en.wikipedia.org/wiki/Center_of_mass

 

The center of mass is often called the center of gravity but this is only true in a system where the gravitational forces are uniform

 

And therein lies the key to Wikie's statement. On the surface, it seems logical. But if you've ever riden a dirt bike, you'll recognize that the first half of the statement is whacked.

 

There's a distict difference from CoG & CoM.

 

Wiki's wrong, simple as that.

[tallman]

Ed,

the real answer to this question is a very complicated (except for Mitch) set of formulas which I won't try to confuse myself or anyone else with.

http://www.ugrad.math.ubc.ca/coursedoc/math101/notes/moreApps/mass.html

Because the body of the rider moves and is not rigid when standing (or moving off the saddle to the side or down) the math becomes even more involved that if the rider simply stood.

The COG is one aspect of the answer, but I also think the rider is applying torque when standing and moving which results in steering and handling changes.

YMMV.

[Joe Frickin' Friday]

The center of mass is often called the center of gravity but this is only true in a system where the gravitational forces are uniform

 

And therein lies the key to Wikie's statement. On the surface, it seems logical. But if you've ever riden a dirt bike, you'll recognize that the first half of the statement is whacked.

 

There's a distict difference from CoG & CoM.

 

Wiki's wrong, simple as that.

 

Did you deliberately omit the next sentence from the Wikipedia article?

 

"For example, on the Earth where the differences in the pull of gravity may safely be ignored."

 

The Wikipedia article is not wrong; it's dead on balls accurate.

Here on Earth, within the range of height of a motorcycle and rider, gravity is pretty much constant, and so center of gravity and center of mass are equivalent. And as others have noted, when you raise up part of the mass of a system (as by standing on the pegs), you also raise up the center of mass/gravity. the center of mass/gravity may move fore or aft, depending on whether the rider is leaning forward or backward, respectively.

 

 

(and now, some idle speculations from someone with virtually no experience in off-road riding.)

But by standing, you've removed the ass-to-saddle connection, changing the way in which your body's mass is coupled to the bike's mass. The bike now has more freedom to lean rapidly to either side as needed, and also to shift (without leaning) side to side.

 

You've also added your legs as a suspension component, helping to push the bike down in the dips to maintain traction (and preventing the saddle from jamming your hips up to your shoulder blades on the rises). I'd expect this to make a significant difference on motocross bikes, where your body is about 40-50% of the mass of the whole system.

 

One other thing to note:

tall things (i.e. things with a high CoG) are easier to balance than short things. Try to balance a pencil in your hand, then try to balance a broom; the broom is much easier. If you make your body tall in the saddle, when you+bike are off-balance you+bike will fall over more slowly, giving yourself more time to correct and get the contact patches back under you.

[doc47]

Mitch, I'm assuming Huzb is still extracting his tongue from his cheek, seeing as dirt bikes don't stay "on the surface of the earth" for very long. (At least I thought he was being facetious...)

 

Your point about "tall things" is a good one and is certainly part of the equation.

 

What I think is happening is that rider-on-seat is more of a single COG system. Rider standing becomes more of a system of two coupled CsOG that shift relative to each other with torques being applied mainly through the foot pegs. (Let's assume negligible force through the bars, just for purposes of this discussion.) The COG may be less important than the lever arms and direction of force affected by where the rider's COG happens to be relative to the pegs at any given moment.

Perhaps it is that the forces are applied through the footpegs rather than through the seat making the moment-arm shorter and lower.

In dirt riding the rider often stands or semi-stands and weights the outside peg, using his COG to apply force over and down through the peg, forcing the contact patch straight down. Now, why this doesn't apply to street riding I'm not sure. All speculative, of course. Maybe it also has to do with the fact that asphalt doesn't move around under the tire, etc.

[Ron_B]

Hey Doc, still instigating trouble?

 

Sheeesh, my brain hurts when I over-analyze. (insert Mr Gumby here)

 

Ride dirt bike - Stand on pegs = good.

[Quinn]

So when do we get to the bird flying around inside an airplane? How about jumping up at the last millisecond as the elevator crashes to nullify the slamming effect?

 

The reminds me of the "towel bar diet" that I've been on for years. I've noticed that if I rest my hand on th towel bar while weighing myself in the bathroom I can usually lose about ten pounds. And I can eat anything I want!

 

---

[pmdave]

Concerns about the elevation of the CoG assume that the elevation has something to do with control. Obviously, standing on the pegs raises the rider's CoG, and that means the combined rider/motorcycle CoG is higher.

 

What's more important is mass. Gravity always pulls straight down on all the little bits and pieces. The Center of Mass may be at the same theoretical point as the CoG, but mass involves stuff like inertia and kinetic energy. And the physics becomes very involved with the bike at speed. Note that the lean angle of a motorcycle/rider in a curve (same radius, same speed, same total weight) is almost constant regardless of the elevation of the combined CoG. THat's why a guy on a GS Adventure can corner at the same lean angle as a guy on an RT. But, let's call it "standing on the pegs" when we're really saying "partially disconnecting your mass from the bike."

 

"Standing on the pegs" does several things. First, it (sort of) disconnects the rider's mass from the motorcycle's mass. That means the bike can roll side-to-side quicker without having to drag the rider's mass at the same rate. You don't have to be actually standing up on the pegs, you can just put more of your weight on the pegs to loosen up the Velcro between your butt and the seat and let the saddle slide around under you.

 

That's just as important for sport riders as for dual sporters: consider that when rolling into a turn the bike's CoG/CoM has to drop. lifting the bike up out of a sharp turn, the tires have to lift the whole CoM up off the pavement. That's easier if the rider's butt isn't glued to the saddle.

 

And, while it might appear that the main reason for a roadracer hanging off to the inside is maximizing leanover clearance, there are other benefits, including neutralizing steering feedback--which maximizes traction.

 

Also, "standing on the pegs" allows the rider to adjust the lean angle of the bike (more or less) separately from the rider's mass. For instance, on a slippery surface, you could hang off or counterlean to keep the wheels perpendicular to the surface, or when making a tight U-turn you could weight the outside peg and lean the bike over farther (to make a tighter turn).

 

"Standing on the pegs" also allows the rider's legs to serve as shock absorbers, which helps maintain traction on an uneven surface. Imagine cresing a whoop-de-do at speed while strapped tightly to the bike. Your mass wants to keep going straght ahead, so your enertia could actually slow the bike's losing altitude (and regaining traction). Sliding forward or aft helps manage weight bias on the wheels as well as how the mass reacts to acceleration/deceleration/pitch/yaw.

 

For those with an interest in the physics, I suggest Tony Foale's book "Motorcycle Handling and Chassis Design." But be forewarned that trying to hang with Foale intellectually is like tring to hang with Rossi on the track.

 

pmdave

[ShovelStrokeEd]

Imagine cresing a whoop-de-do at speed while strapped tightly to the bike. Your mass wants to keep going straght ahead, so your enertia could actually slow the bike's losing altitude (and regaining traction).

 

Dave,

That is just plain wrong. The total mass of the bike and rider will fall at the same speed regardless of the way the bike and rider are coupled. Standing on the pegs won't make a bit of difference. The time to return will strictly be determined by the velocity of the mass (its momentum, not inertia) and the angle of departure. The only advantage standing on the pegs will give the rider is that he will be able to change the pitch angle of the bike and thus determine the angle of contact when the bike returns to the ground.

[Albert]

Imagine cresing a whoop-de-do at speed while strapped tightly to the bike. Your mass wants to keep going straght ahead, so your enertia could actually slow the bike's losing altitude (and regaining traction).

 

Dave,

That is just plain wrong. The total mass of the bike and rider will fall at the same speed regardless of the way the bike and rider are coupled. Standing on the pegs won't make a bit of difference. The time to return will strictly be determined by the velocity of the mass (its momentum, not inertia) and the angle of departure. The only advantage standing on the pegs will give the rider is that he will be able to change the pitch angle of the bike and thus determine the angle of contact when the bike returns to the ground.

 

 

I think pmdave is correct. If you visualize a single mass system traversing a path described by a sine wave (whoop de do's). There will be a component of momentum that tends to propell the mass off the crest of the peaks. If you now divide the system into 2 masses coupled by a spring/damper (the rider's legs), depending on the amplitude of the sine, the upper mass (rider) could quite eaily describe a horizontal path relative to the sine. The overall momentum of the system will follow a path of a sine at a reduced amplitude to the single mass system. Downhill skiers display this same tendency when traversing a siries of moguls.

[tallman]

Newton's 2nd Law might come into play.

A bullet fired parallel to the ground and one dropped from the same height and all that...

[pmdave]

ShovelStroke,

 

In discussions like this it not uncommon to have disagreements. My suggestion for responding is to say, "In my opinion you are wrong on that issue", or "It's my understanding that it doesn't work that way." I am very likely wrong about many things, but I will stand corrected when someone convinces me with a valid argument.

 

I suggest that the term "momentum" is misused by a great many motorcyclists, including magazine journalists. I'll accept "political momentum" but not "momentum turn." In physica terms, momentum refers to the period of time it takes to bring a moving body to rest when under the action of a constant force or moment. Sounds like braking, eh?

 

Inertia is a property of matter by which an object remains at rest or in uniform motion unless acted upon by some external force. Since inertia is not a force, we have a lot of trouble describing how it works. The best description I've found is to say a motorcycle "wants" to keep going straight ahead.

 

So, I suggest it is inertia that causes a speeding motorcycle to attempt to travel straight ahead even when the road is dropping away under it. And, since the rider also has inertia, he or she can adjust the amount of force applied to the speeding motorcycle by rising or dropping in relation to the bike's CoM.

 

Note that race bikes generally run very light wheels, to reduce the inertial resistance to suspension movement.

 

pmdave

 

[EddyQ]

Imagine cresing a whoop-de-do at speed while strapped tightly to the bike. Your mass wants to keep going straght ahead, so your enertia could actually slow the bike's losing altitude (and regaining traction).

 

Dave,

That is just plain wrong. The total mass of the bike and rider will fall at the same speed regardless of the way the bike and rider are coupled. Standing on the pegs won't make a bit of difference. The time to return will strictly be determined by the velocity of the mass (its momentum, not inertia) and the angle of departure. The only advantage standing on the pegs will give the rider is that he will be able to change the pitch angle of the bike and thus determine the angle of contact when the bike returns to the ground.

 

Ed, I think in pmdave's case, the bike did not leave the ground. It is all suspension and legs with appropriate forces to preserve a good contact patch. BTW, inertia was the correct term. Inertia is the resistace to change velocity.

 

Momentum is the mass velocity product. If two objects with different masses and different velocities, but with the same momentum were subjected to the same force for the same amount of time, then they would remain at the same momentum. But as you probably know, a good lump in the road can increase the bike vertical momentum (due to high suspension forces overcoming the bike/rider inertia) such that if the rider is sitting, he to will experience these higher than gravity forces and will be thrown off the seat.

[ShovelStrokeEd]

Dave,

To start, I don't need lectures about net etiquette from you or anyone else. I think I've been around long enough to know what is what. I'm gruff sometimes, that's how I became the Village Grouch.

 

Now, let's get back to your information. I mentioned nothing about turning, momentum turns or any other thing. I merely pointed out that nothing about the motorcycles tendency to travel straight ahead or the rider's ability to effect that. Let us review the section I quoted.

 

"Imagine cresing a whoop-de-do at speed while strapped tightly to the bike. Your mass wants to keep going straght ahead, so your enertia could actually slow the bike's losing altitude (and regaining traction)."

 

You referenced cresting a whoop and the bike/rider wanting to continue upward or straight ahead. This is momentum, not inertia. It is a vector quantity referring to an object's mass and velocity. The outside force, in this case, is the force of gravity. It will act on the entire mass (bike and rider) regardless of the location of the center of gravity/center of mass. There is no way to slow this effect, nor can you speed it up. This is why I mentioned the angle of departure. If the bike is heading upward at the time it leaves the ground, it will certainly take longer to return to earth but it will lose vertical velocity at exactly the same rate. 9.81 m/sec/sec near the average surface of the earth.

 

I also did state that a standing rider can easily alter the pitch angle of the bike to return the rear or front wheel first. This will have no effect on the momentum of the system, just which wheel touches down first. Once the bike leaves the ground, it will lose forward velocity in proportion to its aerodynamic drag and lose vertical velocity at 9.81 m/sec/sec.

 

 

[onmyrt]

Note that race bikes generally run very light wheels, to reduce the inertial resistance to suspension movement.

"to reduce the inertial resistance to suspension movement" ???

 

Back when I was racing, the primary reason for running lighter wheels was because they had less rotational mass.

Meaning that you could go deeper into a corner before braking, and accelerate out of the corner more quickly.

[EddyQ]

Back when I was racing, the primary reason for running lighter wheels was because they had less rotational mass.

Meaning that you could go deeper into a corner before braking, and accelerate out of the corner more quickly.

 

I've never raced, but I cannot imagine the rotational mass (inertia) has too much contribution to bike acceleration/deceleration. But, it has significant effects on suspension because it is unsprung. It also will have an effect on steering feel and control since the "flywheel" effect is part of the stability equation of all bikes. So with lighter wheels I can see how a bike may steer quicker. Maybe that is what you are saying here . . .

[onmyrt]

Back when I was racing, the primary reason for running lighter wheels was because they had less rotational mass.

Meaning that you could go deeper into a corner before braking, and accelerate out of the corner more quickly.

I've never raced, but I cannot imagine the rotational mass (inertia) has too much contribution to bike acceleration/deceleration. But, it has significant effects on suspension because it is unsprung. It also will have an effect on steering feel and control since the "flywheel" effect is part of the stability equation of all bikes. So with lighter wheels I can see how a bike may steer quicker. Maybe that is what you are saying here . . .

Maybe you can imagine it if you consider that it takes more energy to slow the rotation of a heavier rotating mass

than it does to slow the rotation of a lighter rotating mass of the same diameter. It's all in the 'inertia'.

 

And you are right about one thing, the steering is quicker with a lighter wheel, for the same reason.

[VinnyR11]

Note that race bikes generally run very light wheels, to reduce the inertial resistance to suspension movement.

"to reduce the inertial resistance to suspension movement" ???

 

Back when I was racing, the primary reason for running lighter wheels was because they had less rotational mass.

Meaning that you could go deeper into a corner before braking, and accelerate out of the corner more quickly.

 

Not sure about the effects of less rotational mass, but the primary reason for lighter wheels in racing (cars and bikes) is to reduce unsprung weight.

[CoarsegoldKid]

It's like a Snickers Bar. Chocolate, caramel and peanuts all in one. You're both right, shake hands.

[Jake]

it's dead on balls accurate.

 

Is that a technical term?

[pmdave]

Please excuse me while I spend some time paying more attention to the advice of Robert Heinlein.

 

pmdave

[tallman]

Well, at times this is a stange land, but don't be a stranger.

I appreciate your contributions.

[Indy Dave]

Please excuse me while I spend some time paying more attention to the advice of Robert Heinlein.

 

pmdave

 

always enjoy your input Dave. Thanks also for the prompt to google Robert Heinlein's quotes.

[philbytx]

Hey!

Disappointed to see that nobody got into the whole arm/leverage equation as this would come into play when you stand on the pegs instead of sitting on the seet !

[tallman]

Nobody?

see above

"the real answer to this question is a very complicated (except for Mitch) set of formulas which I won't try to confuse myself or anyone else with.

http://www.ugrad.math.ubc.ca/coursedoc/math101/notes/moreApps/mass.html

Because the body of the rider moves and is not rigid when standing (or moving off the saddle to the side or down) the math becomes even more involved that if the rider simply stood.

The COG is one aspect of the answer, but I also think the rider is applying torque when standing and moving which results in steering and handling changes.

YMMV."

[philbytx]

Duh!

Sorry Tim

[tallman]

It's OK.

Just let me ride your bike next time I'm over that way and I'll demonstrate.