Jump to content
Fajita Dave

Body Position And Cog

Recommended Posts

This question doesn't have so much to do with cornering or sportbike riding but more of a general motorcycle physics question. On another forum that I visit the debate whether standing on the pegs lowers or raises the COG comes up often. Since there are a few people on here who have a good understanding of physics and more importantly the math behind it I figured we might be able to come up with some answers!!! There might be to many variables to come up with a good answer.

 

So does standing on the pegs lower or raise the COG??? Obviously this one has plenty of variables. Standing as high as you can would surely raise the COG (like you would on a dirtbike) of the whole bike/rider system but what kind of effect would it have? Or how about putting 100% of your weight on the pegs while still keeping your body as low as possible (as you would on a sportbike) like you're crouched behind the windsheild?

 

I don't know calculous but I try to have a good basic understanding of physics. Personally I would think moving where the rider's weight is placed on a motorcycle frame from above the engine at the top of the bike, to bellow the engine near the bottom of the frame would have a massive effect on the COG of the bike and change its handling pretty dramatically! This could make the bike much more stable and change lean angle / direction much quicker which could be usefull riding through a rock garden on a dirtbike. However, the rider will need to be standing for that situation which would raise the overal height (and COG). Resting weight on the pegs instead of the seat I would also think could help keep the front tire on the ground while powering out of a corner on the extremely powerfull modern sportbikes. That would let you twist the throttle a little bit more and power past your opponents down the straight away.

 

So what kind of effect do you think weighting or standing on the pegs has?

 

P.S. Changing how you hold yourself on the bike anywhere between your ankles to your hands can change how the bike reacts in any given situation. I think its even a make or break part of the riders techique to keep the motorcycle under control. That makes almost limitless variables so I'd rather keep that out of the equation for now!

Share this post


Link to post
Share on other sites

I'm a crappy rider but I may have a grasp on some physics concepts.

 

Lets have a go at a simple thought experiment.

 

Imagine a rider that has no weight on the seat. All weight is on their feet on the pegs. The rider is hanging off like a monkey to the inside of the turn.

Would the hanging off then be for nothing? In other words, would the hanging off be pointless?

 

 

I drew this up for you... Hopefully it makes sense. The circle that looks like a BMW logo is the symbol for C.G.

cg_zpse8e6c092.jpg

 

As you can see, C.G. is not effected. It doesn't matter where the "brace" for the weight is. It only matters where the weight ACTUALLY is.

 

If I take your thinking a bit further - the tires are contacting the ground right? So is the C.G. at ground level? Nope.

 

besides, why would you want to lower your C.G.? It's not a car... In a car, lower is better because high C.G. causes sway / lean.

 

On a bike, a higher C.G during a corner is better flat out. I'm a crappy rider but I know my physics. Because the tires are not infinitely thin, during a lean, you end up riding on the inside of the tire which actually shifts the CG a bit to the right... ooops!!!

 

I drew this up for you:

cg2_zps3dd45ac3.jpg

 

Case #1 does not exists. As you can see, the black line in reality does not make contact with the ground.

Only the red and blue line touch the ground.

 

Case #2 you can take X corner at 50mph

Case #3 you can take X corner at 60mph

 

 

Hanging off the bike is just gravy, but nomatter what, higher CG is better during the turn.

 

On the other hand higher CG is not good when flicking the bike from one direction to another. So for that, like in a chicane, perhaps it's best to lower your upper body as much as possible - bike will flick quicker. But once in the turn, higher CG is better. If you can manage to hang off the bike and be way up high at the same time - that is most ideal. Physics talking, not me...

Share this post


Link to post
Share on other sites

So does standing on the pegs lower or raise the COG???

Fajita,

 

As Noamkrief properly explains above, standing on the pegs doesn't do much for the vertical location of the CG of the bike+rider system, especially for light riders and heavy bikes.

Where the weight of the rider is supported by the bike is related to internal forces of that system, which don't change the distribution of masses (the center of mass or CG of the whole system remains around the same point in the space).

 

I don't know calculus but I try to have a good basic understanding of physics.

 

In order to understand the physics behind the dynamic changes that happen when we stand on the footpegs, one needs to understand the concept of "moment of inertia".

 

"In classic mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass, is a property of a distribution of mass in space that measures its resistance to rotational acceleration about an axis...........and is the rotational analogue to mass. Mass moments of inertia have units of dimension mass × length2.

 

The moment of the inertia force on a particle around an axis multiplies the mass of the particle by the square of its distance to the axis, and forms a parameter called the moment of inertia.

 

The moments of inertia of individual particles sum to define the moment of inertia of a body rotating about an axis."

 

http://en.wikipedia....ment_of_inertia

 

http://en.wikipedia....ents_of_inertia

 

So what kind of effect do you think weighting or standing on the pegs has?

 

The concept of moment of inertia sounds difficult, but it is not so much.

The bike+rider system always rotates around the combined or system's CG, either during hard braking-acceleration or during quick flicking.

 

Sitting on the bike, our body has a somehow rigid connection with the bike and; hence, there is one moment of inertia, which has a high value (the system has more inertia to be rotated).

 

Standing on the pegs, our body has a flexible connection with the bike (via our legs, knees and hips); hence, there are two masses connected by a flexible link.

That flexibility reduces the total moment of inertia because the smaller mass of the bike can rotate around its CG with certain margin of freedom respect to the mass of the rider.

 

Note that the value of the moment of inertia depends on the value of the mass, but also depends on the square of the distance that separates the CG and the pivot around which it rotates.

Standing on the pegs drastically reduces the moment of inertia or rotational inertia to flicking the bike or to perform a wheelie.

Edited by Lnewqban

Share this post


Link to post
Share on other sites

 

The concept of moment of inertia sounds difficult, but it is not so much.

The bike+rider system always rotates around the combined or system's CG, either during hard braking-acceleration or during quick flicking.

 

Sitting on the bike, our body has a somehow rigid connection with the bike and; hence, there is one moment of inertia, which has a high value (the system has more inertia to be rotated).

 

Standing on the pegs, our body has a flexible connection with the bike (via our legs, knees and hips); hence, there are two masses connected by a flexible link.

That flexibility reduces the total moment of inertia because the smaller bike of the bike can rotate around its CG with certain margin of freedom respect to the mass of the rider.

 

Note that the value of the moment of inertia depends on the value of the mass, but also depends on the square of the distance that separates the CG and the pivot around which it rotates.

Standing on the pegs drastically reduces the moment of inertia or rotational inertia to flicking the bike or to perform a wheelie.

 

That is the answer I've been having such a hard time figuring out on my own! Primarily on my dirtbike I've always noticed that the bike seems to handle quite a bit differently while standing on the pegs. I mostly noticed this after I started trail riding recently instead of the high speed motocross I was used to since I was 9. It simply "felt" like the center of gravity of the motorcycle on its own was lower, however it makes perfect since that what I've been feeling is just a result of me "detatching" myself from the motorcycle it reduces the moment of inertia. Just to make sure I have this perfectly clear, the CG doesn't have any significant change you're simply changing the pivot point of which the motorcycle leans around. If I locked my legs to the motorcycles making the bike/rider system ridged again (like sitting on the seat) it will raise the MOI which is exactly what it feels like in practice on my dirtbike.

 

I definitely know you don't want a low CG on a motorcycle for cornering. Its way past my normal bed time so I'm having trouble thinking of how to explain it but cornering G's and gravity go hand in hand. When you maintain a lean angle that is the exact equalibriam of gravity pulling you to the ground and the cornering G counteracting gravity to keep the bike upright. Having a higher CG gives gravity a bit more leverage for pulling you to the ground while leaned over so less lean angle would be required for any given corner compared to a lower CG. Thats the basics of it anyway....

 

Thats why I come to this forum even though I don't post much. People let the facts do the talking instead of their egos!

Share this post


Link to post
Share on other sites

Like I said - putting your body weight on the pegs will not change the CG of the bike +rider system assuming that you still hold the same body position...

 

I can see why in offroading putting your body weight on your legs and footpegs is helpful. You basically gain another set of shocks!!!

 

I think the human body is a superior suspension system of any ohlins and HP4 dynamic dampning control out there.

I don't think technology has reached the level of the human body acting as suspension...

 

For example - I challenge anyone to build a suspension system for a GoPro that you can mount on a bike, and it would have no vibration, up big ups and downs during bumps... Even with the latest designs that electronically adjust 1000 times per second, try mounting your gopro on top of your helmet... Smooth as silk!!! :)

Share this post


Link to post
Share on other sites

Previous post edited to correct the statement:

 

"That flexibility reduces the total moment of inertia because the smaller mass of the bike can rotate around its CG with certain margin of freedom respect to the mass of the rider."

 

For the mass of the rider's body not to affect the system's moment of inertia much, the hands, ankles, knees and hips should try to rotate the rider's CG much less than the bike's CG.

 

This video shows two bodies with similar mass, but different mass distribution, rotating at different speeds under the same force:

 

 

An off-road rider stands on the pegs to speed up the nose-up / nose-down rotations of the bike (pitch rotation around the bike's CG), almost completely liberating the bike+rider system of the rotational inertia of the rider (CG of the rider doesn't rotate much thanks to the pivot created by the ankles).

The system becomes "lighter to rotation", which allows the tires to better follow the great changes on the ground or track surface.

That is why trial bikes have the shape they have:

 

 

For a bike that weights three times the weight of the rider, if the rider lifts his CG (about where his belly button is) 3" above the original location, the combined or system's total CG will only move 1 " higher than its original location.

 

Relocating the rider's CG sideways via the hang-off technique has a more pronounced effect on the lean angle that the bike needs to take (lean angle of the system (bike+rider) remains the same to balance the lateral and vertical forces induced by the turn).

Share this post


Link to post
Share on other sites

I'm a crappy rider but I may have a grasp on some physics concepts.

 

Lets have a go at a simple thought experiment.

 

Imagine a rider that has no weight on the seat. All weight is on their feet on the pegs. The rider is hanging off like a monkey to the inside of the turn.

Would the hanging off then be for nothing? In other words, would the hanging off be pointless?

 

 

I drew this up for you... Hopefully it makes sense. The circle that looks like a BMW logo is the symbol for C.G.

cg_zpse8e6c092.jpg

 

As you can see, C.G. is not effected. It doesn't matter where the "brace" for the weight is. It only matters where the weight ACTUALLY is.

 

If I take your thinking a bit further - the tires are contacting the ground right? So is the C.G. at ground level? Nope.

 

besides, why would you want to lower your C.G.? It's not a car... In a car, lower is better because high C.G. causes sway / lean.

 

On a bike, a higher C.G during a corner is better flat out. I'm a crappy rider but I know my physics. Because the tires are not infinitely thin, during a lean, you end up riding on the inside of the tire which actually shifts the CG a bit to the right... ooops!!!

 

I drew this up for you:

cg2_zps3dd45ac3.jpg

 

Case #1 does not exists. As you can see, the black line in reality does not make contact with the ground.

Only the red and blue line touch the ground.

 

Case #2 you can take X corner at 50mph

Case #3 you can take X corner at 60mph

 

 

Hanging off the bike is just gravy, but nomatter what, higher CG is better during the turn.

 

On the other hand higher CG is not good when flicking the bike from one direction to another. So for that, like in a chicane, perhaps it's best to lower your upper body as much as possible - bike will flick quicker. But once in the turn, higher CG is better. If you can manage to hang off the bike and be way up high at the same time - that is most ideal. Physics talking, not me...

 

excellent post man

Share this post


Link to post
Share on other sites

I'm a crappy rider but I may have a grasp on some physics concepts.

 

Lets have a go at a simple thought experiment.

 

Imagine a rider that has no weight on the seat. All weight is on their feet on the pegs. The rider is hanging off like a monkey to the inside of the turn.

Would the hanging off then be for nothing? In other words, would the hanging off be pointless?

 

 

I drew this up for you... Hopefully it makes sense. The circle that looks like a BMW logo is the symbol for C.G.

cg_zpse8e6c092.jpg

 

As you can see, C.G. is not effected. It doesn't matter where the "brace" for the weight is. It only matters where the weight ACTUALLY is.

 

If I take your thinking a bit further - the tires are contacting the ground right? So is the C.G. at ground level? Nope.

 

besides, why would you want to lower your C.G.? It's not a car... In a car, lower is better because high C.G. causes sway / lean.

 

On a bike, a higher C.G during a corner is better flat out. I'm a crappy rider but I know my physics. Because the tires are not infinitely thin, during a lean, you end up riding on the inside of the tire which actually shifts the CG a bit to the right... ooops!!!

 

I drew this up for you:

cg2_zps3dd45ac3.jpg

 

Case #1 does not exists. As you can see, the black line in reality does not make contact with the ground.

Only the red and blue line touch the ground.

 

Case #2 you can take X corner at 50mph

Case #3 you can take X corner at 60mph

 

 

Hanging off the bike is just gravy, but nomatter what, higher CG is better during the turn.

 

On the other hand higher CG is not good when flicking the bike from one direction to another. So for that, like in a chicane, perhaps it's best to lower your upper body as much as possible - bike will flick quicker. But once in the turn, higher CG is better. If you can manage to hang off the bike and be way up high at the same time - that is most ideal. Physics talking, not me...

 

You had me until that last diagram... now I'm a bit confused...

 

I learnt a fair bit about this topic from a couple of different pages on this site, good if folks want to have a look at maths and technical stuff: http://www.stevemund.../leanangle.html

 

Note this one paragraph:

 

What's the real lean angle?

 

The concept of the lean angle is not as trivial as one would imagine. If we had infinitely thin tires then it'd be obvious: Just draw a line from the center of mass to the contact point and measure the angle between that line and the horizontal or vertical. But we have fat tires. The actual lean angle must be measured from the center of mass to the center of the contact area, which will be to the inside of the tire. So the apparent lean angle — using a line parallel to the forks, say — will be greater than the actual lean angle, using the center of the contact area of the tire. Note that this means that a motorcycle with a lower center of mass will lean more than a motorcycle with a higher center of mass, for a given corner radius and speed.

 

Remember that more lean at the same speed is not a good thing... it just means you will run out of lean angle at a lower speed. Therefore a higher center of gravity is desirable to maximise lean capacity and ability for faster cornering.

 

Maybe I'm getting this mixed up with body position? Changing body position to create a lower C.G. is one way to reduce the actual lean angle. I'm sure this image isn't new to many here:

 

46139713-O.jpg

 

Which basically demonstrates what you're saying about how the different C.G. affects lean angle. But now I can't reconcile this with the passage I quoted above? Ahh... help? Edit> Just realised that the rider photos above don't accurately show the "actual" lean angle, only the bike lean angle. I will have to have another look at this in the morning...

 

 

I also recall from previous discussions that people have mentioned taller C.G (to a point) providing quicker/easier steering?

Share this post


Link to post
Share on other sites

..........But now I can't reconcile this with the passage I quoted above? Ahh... help? Edit> Just realised that the rider photos above don't accurately show the "actual" lean angle, only the bike lean angle. I will have to have another look at this in the morning...

 

I also recall from previous discussions that people have mentioned taller C.G (to a point) providing quicker/easier steering?

 

Mugget,

 

A friend and mentor told me once that when in doubt, I could analyse things in ridiculous extreme conditions.

 

That is what I did in the attached schematic, using AutoCAD, so the dimensions for angles are provided by the software.

 

You can see that I fixed the turn's speed and radius, which gave me a dynamic lean angle (angle between the vertical, the center of the contact patch of the tire and the rider-bike combined center of gravity) of 45 degrees.

 

The differences in tires' cross sections and height of the centers of gravity have been exaggerated in order to obtain clear results.

 

In real life, the differences are not that big: the CG of the upper portion of the body can only change around 3" in height.

For a rider of 200 lb on a bike of 400 lb, that change induces around 1/2" of height variation in the combined CG (100 lb upper body / 600 lb combined weight).

 

If we compare a sport 125 and a 1000 cc custom chopper, the cross sections of the tire would have more important influence on the lean angle of the frame of both bikes during identical turns.

 

In summary, high CG and thin tires help the suspension work better because they keep the frame of the bike a little more upright.

 

I would say that a lower CG helps flicking, if we consider the roll-resistant moment of inertia discussed above.

post-23333-0-20320700-1361392957_thumb.jpg

Share this post


Link to post
Share on other sites

Just checking in on this, and a bit late...but in the above drawings, the rider does not have his upper body off the bike the same amount...this will change the lean angle of the bike/rider unit.

Share this post


Link to post
Share on other sites

As a rider , to me it doesn't matter why, just need to know what works in the real world. Based on the information I see here there is a conflict between what I see the pros/motogp doing in that they hang off the inside and keep the torso low as opposed to upright.

 

It's interesting that throughout history there have been many theories about why something works that at one point is generally accepted but in the future a better explanation comes along and get accepted. One thing that remains the same through the changing explanations is that it works.

 

I leave this to the engineers to figure out while I ride.

 

 

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

×
×
  • Create New...