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Weight Shift at speed (acceleration and braking)

  1. Dec 28, 2006 #1
    Let's say you have a body in motion, a car, an airplane or hovercraft - does not matter, looking for basic laws of physics here.

    1. You know the weight.

    2. You know the center of gravity at rest.

    3. You know the center of lift (if applicable).

    Let's say our object weighs in at 600 lbs.

    Lets say our object is going 30 mph.

    How much does the center of gravity or weight shift back?

    Lets say I need to start or stop very quickly, how do I calculate the location shift of CG backwards and forwards?

    I think the key is taking the the actual weight and turning it into mass, then take into account speed the horizontal force, which in turn may reduce the gravity force or weight over a given time peroid.

    An actual example would help, then I can apply it to my situation.
  2. jcsd
  3. Dec 28, 2006 #2


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    I'm not sure I understand you. Say you have a car with known properties and start accelerating at a given rate. Are you looking for how the normal forces on the two axles change due to that acceleration? I suppose you could define an "effective" center-of-mass to be the position where the COM would have to be for the dynamic weight distribution to be the static one.

    If this is what you're interested in, you can derive the result once the suspension motions have been damped out by calculating the forces and torques on the vehicle by the ground. The total torque should vanish, and the total force should be ma for a vehicle with mass m accelerating at rate a. If such a vehicle has center-of-mass height h from the ground and wheelbase L, the change in normal force on the front tires can then be shown to be

    \Delta N_{f}/mg = - h (a/g) /L

    You can work out everything else pretty easily from that.
    Last edited: Dec 28, 2006
  4. Dec 28, 2006 #3
    The car example is getting too complex, my suspension is an air cushion - this should be easier because there are no axles.

    I have a hovercraft, but you may want to think of it as a boat on plane or planing if you are more familiar with that.

    Even though I used calculations and a pipe to balance the craft, this balance is at static hover, not at speed.

    I added too much weight in the back, but how do I determine the center of weight/gravity at speed?

    http://www.hoverclubofamerica.org/forum/uploads/1165881286/gallery_181_6_20081.jpg [Broken]
    http://www.hoverclubofamerica.org/forum/uploads/1165881286/gallery_181_6_238757.jpg [Broken]

    Full thread of project:
    http://www.hoverclubofamerica.org/forum/index.php?showtopic=962&st=0 [Broken]
    Last edited by a moderator: May 2, 2017
  5. Dec 28, 2006 #4
    Perhaps I've tried to find a universal solution that just does not apply.

    Example of what I meant.

    Weight Transfer-1
    http://www.msgroup.org/TIP074.html [Broken]
    Stern/rear hovercraft skirt drag might be a version of torque or braking power applied.

    1. Car/Motorcyle; weight and center of gravity stay the same unless braking torque is applied or aerodynamic forces have an influenence.

    2. Boat; weight and center of gravity are the same until displacement mode is replaced by planing mode, then there is a shift to the "center of lift". This is countered by tabs at the rear which induce drag counterforces and angle changes.

    3. Airplane; weight and center of gravity are the same until speed increases lift on the wings causing a shift which is countered by the elevator (typically at the back).

    4. Hovercraft; once it's over the "hump speed" which is also a boat term with a slightly diffenerent context, it is in level flight. Weight is centered until torque (paracitic skirt drag) is encountered in the form of wave impact and aerodynamic resistance.
    Last edited by a moderator: May 2, 2017
  6. Dec 28, 2006 #5


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    No, the key is drawing a free body diagram.

    The CG does not move anywhere. If you found Stingray's words too confusing, try a picture....

    Attached Files:

    • bike.jpg
      File size:
      34.7 KB
  7. Jan 2, 2007 #6
    Thanks....................it also take some imagination to visualize not just a line across a body in motion (right/left and front/back), but it's distance above a plane in 3D (up/down).
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