Center of Gravity - Standing -vs- Sitting


102 replies to this topic
  • bg10459

Posted December 22, 2005 - 06:26 PM

#1

Another thread on this forum started meandering toward the direction of Center of Gravity. There seems to be some discussion as to whether the CG will be raised or lowered by standing rather than sitting. I thought it deserved a thread of it's own. Let's hear waht everyone has to say. Here's my take:

When you stand, you definitely raise the CG of "THE SYSTEM" This is a law of PHYSICS AND GRAVITY!!!
However, part of "the system" has built in shock absorbers (legs and arms) which are not permanently fixed to the bike and a constantly adjusting counter balance (your body). So, while the CG of the bike is generally fixed, the masses of the rider are continually changing locations. The perception to the rider is based on the adjusted CG of the BIKE per the locations of the loads (rider + position) placed upon it, and not necessarily the CG of the system as a whole. Therefore, standing will lower the CG of the BIKE, and sitting will raise it.
Someone once told me that loosing 1 lb on your bike (mtn bike) is like losing 10 lbs yourself. Your bike will feel lighter if you move weight from the bike to your back (water bottles, tool kits, etc.). Even though the system still weighs the same (and CG is raised), the bike feels lighter and easier to ride.



  • BigIvyI

Posted December 23, 2005 - 04:44 AM

#2

It's a common misconception people have about center of gravity. Center of gravity has absolutely nothing to do with weather most of your weight is on the footpegs or the seat. That's the wrong way to think about it.

It's all the location of the mass in space. A better term is "center of mass".
Sitting lowers the center of mass, standing raises it. It's that simple. :applause:

Now, you have more leverage on the pegs when you stand as compared to when you sit. That changes some things, but it has nothing to do with center of mass.

  • jchammond

Posted December 23, 2005 - 05:04 AM

#3

It's a common misconception people have about center of gravity. Center of gravity has absolutely nothing to do with weather most of your weight is on the footpegs or the seat. That's the wrong way to think about it.

It's all the location of the mass in space. A better term is "center of mass".
Sitting lowers the center of mass, standing raises it. It's that simple. :applause:

Now, you have more leverage on the pegs when you stand as compared to when you sit. That changes some things, but it has nothing to do with center of mass.


Yes, yes, yes. The only reason I attempted to clarify the issue in the previous thread was because it seemed there was a belief that somehow more weight on the pegs lowers the center of gravity. This as you have stated is incorrect when speaking of the bike-rider system.

I think basically what is meant by the "standing lowers CG" statement is that standing, in conjunction with your arms and legs constantly adjusting to the movement of the bike (usually trying to maintiain a neutral balance to keep the bike upright) effectively reduces the rider's influence on the CG and behavior of the system. So, the bike would move more with respect to it's own (lower) CG. If one were to stand, and suddenly lock their knees and elbows and act as a rigid system with the bike, the CG would move right back up closer to the rider, and the results would probably be fairly short lived.

  • barton

Posted December 23, 2005 - 08:19 AM

#4

I think basically what is meant by the "standing lowers CG" statement is that standing, in conjunction with your arms and legs constantly adjusting to the movement of the bike (usually trying to maintiain a neutral balance to keep the bike upright) effectively reduces the rider's influence on the CG and behavior of the system. So, the bike would move more with respect to it's own (lower) CG. If one were to stand, and suddenly lock their knees and elbows and act as a rigid system with the bike, the CG would move right back up closer to the rider, and the results would probably be fairly short lived.

I think the term you're looking for is "de-couple". Standing allows you to operate more indepently of the bike. This somewhat de-couples (but not completely) the two primary mass systems (yours and the bikes). In this mode, you can let the bike's cg move more independently of your cg. This gives you more flexibility in how you approach an obstacle or turn.

When standing, dicing through tight single track my cg can move in more of a straight line, while the bike's can wander more left or right, up and down. Same thing when crossing a log. My cg can remain an almost perfectly straight line, while the bikes cg MUST jump over the log, although the shocks take up some of the bump, my legs compress a lot more.

If I sit down, my cg is now more firmly coupled to the bikes cg, and our independent cg's must now follow more closely congruent lines.

Sitting or standing, our systemic or total cg is the mass-weighted average of the independent cg lines. Obviously sitting lowers the total cg, and standing raises it.

  • rfdonnelly

Posted April 18, 2006 - 05:09 PM

#5

I still don't think this was clarfied enough.

  • Standing raises the center of mass.
  • Sitting lowers the center of mass.

Raising your the center of mass is a bad thing for a single rigid body. Think top heavy SUVs. They flip easier than a sports car.

But a motorcycle with rider can be simulated as two rigid bodies hinged together. They will be illustrated as a line.

| <--- rider
| <--- motorcycle
The rider is directly above motorcycle. The center of mass is centered front to back, side to side. It is in equilibrium.

/ <--- rider
\ <--- motorcycle
The rider leans to the right while the bike leans to the left. The center of mass is in equilibrium again, keeping the rider and motorcycle from falling.

\ <--- rider
.\<--- motorcycle
The rider leans to the left and the bike leans to the left. The center of mass is no longer in equilibrium, it is to the left of the motorcycle inducing a fall.

While standing raises your center of mass, it allows you to shift more weight from side to side, front to back, thus allowing the bike to move more side to side, front to back, keeping the center of mass in equilibrium, enabling you and the bike to stay upright and keep from falling.

While sitting, you can't shift as much weight so then the bike cannot be moved as much to prevent a fall.

I hopes this clears things up a bit more!

  • Dwight_Rudder

Posted April 18, 2006 - 10:40 PM

#6

Standing effectivly lowers the CG because you now have all your weight on the pegs and not on the seat. I still sit when I can and stand when the bike tells me to and only sit again when the trail difficulty lessens.
Dwight :thumbsup:

  • BigIvyI

Posted April 19, 2006 - 03:22 AM

#7

Sorry Dwight, your all wet here! :thumbsup:

Imagine a skyscraper, a tall tree, the statue of liberty, or anything else tall. By your theory these things all have a low center of gravity, because the weight is resting on the ground. :thumbsup:

  • 40oz

Posted April 19, 2006 - 04:11 AM

#8

:thumbsup:
Do what ever you like the best, and allows for you to race your max speed for that section.

  • 2fst4u

Posted April 19, 2006 - 04:20 AM

#9

to my belief and understanding dwight is right, think of a stack of blocks like jenga blocks! if all the weight is concentrated to the center of the tower, the bottom will still carve out and topple over, hence: "top heavy", where theres nothing to stabilize the bottom of the tower.

place the weight on the bottom of the tower and hold the top steady (like a rider on a bike standing on the pegs) you equally distribute the load of weight from top to bottom and in the middle, thus stabilizing all parts of the tower.

so sitting on a bike transfers all of the weight to the seat of the bike, and the seat is in the middle of the bike, so nothings holding the bottom of the bike rigid.

standing transfers all of your weight conservitively to the pegs through the whole bike to keep it stable.

and. as we all know stable means handling.

i probably didnt make anysense to you but get a ruler out or a stick. hold it in the middle, and push the bottom out. easy. hold the bottom and try to push the bottom out, harder. thats the principal for gravity. all about weight.

  • CBus660R

Posted April 19, 2006 - 05:29 AM

#10

barton and rfdonnelly hit on the key point in that when you are standing, the rider's weight is not fixed to the bike like when you are sitting. When you are sitting, your mass is along for the ride and has to follow what the bike is doing and vice versa. Think of the affect a bump has on your body when you are sitting compared to when you are standing and can absorb the impact with your legs. This aspect makes talking about CG of the system realtive to sitting and standing an almost irrelevant argument. Let experience on the bike tell you when to sit or stand and not what looks good from a theoretical standpoint.

  • harjp1959

Posted April 19, 2006 - 08:27 AM

#11

I think the term you're looking for is "de-couple". Standing allows you to operate more indepently of the bike. This somewhat de-couples (but not completely) the two primary mass systems (yours and the bikes). In this mode, you can let the bike's cg move more independently of your cg. This gives you more flexibility in how you approach an obstacle or turn.

:thumbsup:

  • brutis1984

Posted April 19, 2006 - 09:04 AM

#12

Center of gravity can also be referred to as the center of mass. Which is a balanced point where all mass may be rotated about that point in equilibrium. There exists three different values to assess that point and they exist in the x,y and z coordinates, each being different. On top of that the loading is kinetic so it becomes difficult to calculate the exact COM without taking a snapshot of an exact position and analyzing the weights and finding that rotational point or centroid which really boils down to calculating the moment of inertia. Complicating this even more the riders ability to move in three dimensions counters and changes the moment of inertia constantly reiterating the need to calculate a fixed position. In the two scenarios described (sitting or standing); since the rider is able to change the moment of inertia then the somewhat fixed MOI of the bike could be used to calculate with loading positions as described, on the pegs or on the seat.

  • velosapiens

Posted April 19, 2006 - 09:13 AM

#13

Sorry Dwight, your all wet here! :thumbsup:

Imagine a skyscraper, a tall tree, the statue of liberty, or anything else tall. By your theory these things all have a low center of gravity, because the weight is resting on the ground. :thumbsup:


yep. dwight is wet. standing is better on rough terrain generally, which i assume is the point dwight is trying to make, but it's not because of the center of gravity. rfdonelly's post is very helpful i think. another point about 'decoupling' is that you effectively get extra suspension for your body by using your arms and legs to absorb shocks.

dwight may have also been trying to point out that motorcycles like to have force transmitted through the footpegs, rather than through the seat, since the suspension behaves better that way, and also the bounces don't bounce you up in the air so much (due somewhat to the extra suspension of your legs).

2fst4u, you are unfortunately using a situation to illustrate your point that is exactly the opposite of person standing up.

the center of gravity/center of mass thing is not a debatable issue. whatever your opinions of how a bike works and why, it is absolutely true that standing raises the cg of the rider and of the bike/rider system. there is so much other stuff going on tho that it's not really that relevant, as cbus660r pointed out.

  • Dwight_Rudder

Posted April 19, 2006 - 10:41 AM

#14

Sorry Dwight, your all wet here! :thumbsup:

Imagine a skyscraper, a tall tree, the statue of liberty, or anything else tall. By your theory these things all have a low center of gravity, because the weight is resting on the ground. :thumbsup:



I think you are over thinking things here a bit. I have always been told that standing lowers the EFFECTIVE center of gravity on the bike. Maybe it is better to say applied TO the bike. If you have read enough bike magazines over the years they all say it in this manner. Maybe it isn't 100% correct but in effect it works this way. We are starting to sound like some of the guys in oil forums here. It works use it to your advantage when needed.

IMO,
Dwight

  • 40oz

Posted April 19, 2006 - 11:10 AM

#15

Just go out and try it...see what works better for you.

I personally think and feel dwight is correct, plus it seems he has figured out what works for him....just look at those stats in his signature.

  • harjp1959

Posted April 19, 2006 - 11:37 AM

#16

We are starting to sound like some of the guys in oil forums here.

:thumbsup:

  • Flyin Dub

Posted April 19, 2006 - 11:46 AM

#17

I think the term you're looking for is "de-couple". Standing allows you to operate more indepently of the bike. This somewhat de-couples (but not completely) the two primary mass systems (yours and the bikes). In this mode, you can let the bike's cg move more independently of your cg. This gives you more flexibility in how you approach an obstacle or turn.


Good stuff, Barton. De-coupling is a big part of whipping and scrubbing as well.

  • bg10459

Posted April 19, 2006 - 12:16 PM

#18

.... standing lowers the EFFECTIVE center of gravity on the bike.

I think EFFECTIVE is the key word here. By standing, you have de-coupled yourself from the bike, except at the pegs.

Now, if I do a superman seat grab knack knack triple lindy, what happens to the C of G? :thumbsup:

  • velosapiens

Posted April 19, 2006 - 12:25 PM

#19

I think you are over thinking things here a bit. I have always been told that standing lowers the EFFECTIVE center of gravity on the bike. Maybe it is better to say applied TO the bike.


it does lower the point at which force from your weight is applied to the suspension. i suspect that is what people mean when they incorrectly say anything that uses the phrase 'center of gravity'. of course, if you stay sitting and put all your weight on the pegs, you're not only applying force at a lower location (pegs instead of seat), but you're also not raising your center of gravity (which happens when you stand). i think cg and what happens when you weight the pegs are entirely different concepts. i agree 100% with what you say happens when you weight the pegs, but cg has zip to do with it.

i'm not overthinking, just trained as a mathematician, and i have at least a rudimentary acquaintance with basic physics (very rudimentary). people who don't remember much of physics tend to construct all sorts of weird theories and explanations. fortunately the subject is well-understood and not generally subject to vagaries except by us unedumucated dirtbikers.

  • brutis1984

Posted April 19, 2006 - 12:57 PM

#20

[quote name='velosapiens']but cg has zip to do with it.QUOTE]

cg has everything to do with it for it defines what is and what is not in equilibrium. Without the cg nothing is relative and nothing would be in balance. Center of gravity is also the correct term aka center of mass. :thumbsup:





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