Centre of gravity of a car shifting with speed

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Main Question or Discussion Point

Hi. I have heard that with high speed of a car, its centre of gravity (CG) shifts backward. Isn't the CG a function of mass and dimension? Pl. explain. Thanks.
 

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  • #2
phinds
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"I have heard ... " is not an acceptable reference on this forum. Please post a citation.
 
  • #3
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Why would it need a reference? It's not a statement of fact, it's a question.

OP, CoG is dependent on mass and the geometry. What happens when you accelerate? And would travelling at any speed matter?
 
  • #4
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Who ever stated that, probably got CoG and CoP mixed up.

At a steady speed, the CoG is static, regardless of the speed.

The CoP changes according to the aerodynamics of the vehicle and the speed at which it is travelling.
 
  • #5
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Why would it need a reference? It's not a statement of fact, it's a question.

OP, CoG is dependent on mass and the geometry. What happens when you accelerate? And would travelling at any speed matter?
Well, I am unable to imagine the effect during acceleration of a rigid body. My body bends back when I accelerate my bike because the bike and I are not rigidly connected. But for the bike, will the acceleration do anything?
 
  • #6
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Who ever stated that, probably got CoG and CoP mixed up.

At a steady speed, the CoG is static, regardless of the speed.

The CoP changes according to the aerodynamics of the vehicle and the speed at which it is travelling.
Hi Kozy. Can you pl. explain what as CoP is? Thanks.
 
  • #7
AlephZero
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The OP's statement (wherever it came from) might be a garbled explanation of the fact that the weight distribution between the front and back wheels of a vehicle changes when it is accelerating. That does NOT mean that the CG position changes. The basic reason why the weight distribution changes is because the CG is at some distance above the ground level. The higher above the ground level, the bigger the effect (which is one reason why high performance cars are designed to be as low to the ground as possible).

Assuming CoP means the aerodynamic center of pressure, I don't see what relevance that has to the OP's question. In any case, acceleration has no effect on the CoP either. The CoP position might change by a small amount as the speed varies, but speed is not the same as acceleration.
 
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Well, I am unable to imagine the effect during acceleration of a rigid body. My body bends back when I accelerate my bike because the bike and I are not rigidly connected. But for the bike, will the acceleration do anything?
Some things to think about.

1. Is a car a rigid body?

2. You tilt back when you accelerate on your bike, accelerate hard enough you pull a wheelie. Why should a car act any differently?

3. Even in a rigid body, load transfer effects still take place. In a non rigid body you have load and weight transfer. (you can look up the difference on wiki)
 
  • #9
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The OP's statement (wherever it came from) might be a garbled explanation of the fact that the weight distribution between the front and back wheels of a vehicle changes when it is accelerating. That does NOT mean that the CG position changes. The basic reason why the weight distribution changes is because the CG is at some distance above the ground level. The higher above the ground level, the bigger the effect (which is one reason why high performance cars are designed to be as low to the ground as possible).

Assuming CoP means the aerodynamic center of pressure, I don't see what relevance that has to the OP's question. In any case, acceleration has no effect on the CoP either. The CoP position might change by a small amount as the speed varies, but speed is not the same as acceleration.
Hi AlephZero. I think then what I heard was wrong. I agree that there will be a shift of weight between front and rear axles during acceleration / retardation. If I am right, it is because of the moment created by the wheel traction force and the distance between CG and wheel contact. Thanks a ton!
 
  • #10
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Some things to think about.

1. Is a car a rigid body?

2. You tilt back when you accelerate on your bike, accelerate hard enough you pull a wheelie. Why should a car act any differently?

3. Even in a rigid body, load transfer effects still take place. In a non rigid body you have load and weight transfer. (you can look up the difference on wiki)
Hi Chris!
1. Though the car is not perfectly rigidly, it is still rigid enough to assume as a theoritically rigid body.
2. I tilt back when acclerating because there is no rigid connection between the bike and me. Secondly, a human body is highly flexible.
3. I agree. I am clear now that CG doesn't shift.
 
  • #11
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Hi Kozy. Can you pl. explain what as CoP is? Thanks.
Center of Pressue. Ratio of front to rear downforce or lift.

Not relevant here after all.
 
  • #12
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Hi Chris!
1. Though the car is not perfectly rigidly, it is still rigid enough to assume as a theoritically rigid body.
2. I tilt back when acclerating because there is no rigid connection between the bike and me. Secondly, a human body is highly flexible.
3. I agree. I am clear now that CG doesn't shift.
The body is connected to the wheels via the suspension, i.e the stiff body sits on 4 springs that are far less stiff. So the body is flexible in relation to the wheels. So it's conceptually no different to you on a motorcycle.

The CoG does acutally move. It just doesn't move very far at all.
 
  • #13
AlephZero
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In a non rigid body you have load and weight transfer. (you can look up the difference on wiki)
My heart usually sinks when I see a wikipedia page like http://en.wikipedia.org/wiki/Weight_transfer that starts with an assertion like
Weight transfer and load transfer are two expressions used somewhat confusingly to describe two distinct effects:
... often, "here be logic-chopping dragons" ... but actually that page isn't so bad.

There is one important (if obvious) difference between bikes and cars here. The weight transfer caused by the rider moving around relative to the bike is an essential part of bike riding technique. On the other hand, high performance cars are designed to minimize weight transfer of both the car and the driver as much as possible.
 

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