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Why does the front end of the car lift up when accelerating forward?

  1. Dec 3, 2011 #1
    Can anyone explain it to me?

    Why does the front end of the car lift forward when it is accelerating forward?

    Is there a torque that is causing this lift?
     
  2. jcsd
  3. Dec 3, 2011 #2

    clem

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    Science Advisor

    Yes. The forward force is on the four wheels at the bottom of the car. This provides a torque which tends to rotat the front end up.
     
  4. Dec 3, 2011 #3
    But the forward force is parallel to radius of the car, so the cross product would be zero, right?
     
  5. Dec 3, 2011 #4

    AlephZero

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    There are two equal and opposite torques.

    In the horizontal direction, there is the force the road is exerting on the car to accelerate it, and the inertia (mass x acceleration) acting at the car's center of mass which is some height above the road level.

    In the vertical direction, there is the weight of the car, again acting at the center of mass, and the weight distribution between the front and rear wheels.

    When the car accelerates some of the weight transfers from the front to the rear wheels, so the front suspension springs extend and the rear ones compress.
     
  6. Dec 5, 2011 #5
    Also rear-wheel drive cars have torque on the rear wheels, and a counter-tourque which tends to lift the front wheels.

    What others have been describing is the torque which arises due to inertia of the car. The car wants to stay in place at its center of gravity, but it's being accelerated by where the wheels contact the ground. This causes a torque which tends to lift the front.

    If you place a tall glass on a table for example, you will notice that it can easity tip over if you push it horizontally by its very bottom. The harder/faster you push it, the more likely it is to tip. However, if you push it closer to its center of gravity, it won't want to tip as much.
     
  7. Dec 5, 2011 #6
    Only works if you have the CoG of the car on the ground (inline with the forces). Load tranfer depends on height of CoG, and the wheelbase.
     
  8. Dec 7, 2011 #7

    NascentOxygen

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    Staff: Mentor

    With rear wheel drive, the general idea is that the engine rotates the wheels around the back axle. But if you were to cement the wheels into the roadway so they can't rotate, then the engine will endeavour to rotate the car's body about that axle. Most likely it will succeed only in lifting the front a bit before the engine stalls. But where the vehicle has plenty of torque and a lot of weight to the rear, such as you find with a farm tractor, it is too easy to lift the front clear of the ground when starting off under load. With the wheels off the ground, you have no front steering.

    You can see the car body rotate instead of the wheels if you dangle a model car in midair, holding it by one wheel from above, and supply it with power.
     
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