Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Centre of gravity of a car shifting with speed

  1. Dec 17, 2013 #1
    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.
     
  2. jcsd
  3. Dec 17, 2013 #2

    phinds

    User Avatar
    Gold Member
    2016 Award

    "I have heard ... " is not an acceptable reference on this forum. Please post a citation.
     
  4. Dec 17, 2013 #3
    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?
     
  5. Dec 17, 2013 #4
    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.
     
  6. Dec 17, 2013 #5
    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?
     
  7. Dec 17, 2013 #6
    Hi Kozy. Can you pl. explain what as CoP is? Thanks.
     
  8. Dec 17, 2013 #7

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    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.
     
  9. Dec 17, 2013 #8
    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)
     
  10. Dec 17, 2013 #9
    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!
     
  11. Dec 17, 2013 #10
    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.
     
  12. Dec 18, 2013 #11
    Center of Pressue. Ratio of front to rear downforce or lift.

    Not relevant here after all.
     
  13. Dec 18, 2013 #12
    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.
     
  14. Dec 18, 2013 #13

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    My heart usually sinks when I see a wikipedia page like http://en.wikipedia.org/wiki/Weight_transfer that starts with an assertion like
    ... 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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Centre of gravity of a car shifting with speed
Loading...