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Physics of Automobile Drifting Question?

  1. Mar 4, 2012 #1
    Ok, this has been on my mind all day. Sorry if it's a stupid question, I'm still a H.S student trying to figure out to study Mechanical Engineering or Physics.

    But moving on....

    Ok, so the engine propels the vehicle at a constant velocity.

    If the front wheels turn and the back end slides does this mean the angle of the wheel turning balances out to the velocity of the vehicle causing the rear end to be angled but still go at a constant velocity in a straight line?

    Front Wheels= Cause rear end of vehicle to angle

    Engine (RWD)= Cause car to propel foward even when front wheels are turned.

    Thanks Guys
    God bless
     

    Attached Files:

  2. jcsd
  3. Mar 4, 2012 #2
    I'm having a little trouble understanding your question, but here's my understanding of the physics of drifting.

    First let's assume an ideal car: unless someone turns the steering wheel, all four wheels are parallel--let's call the line they're parallel to the y axis, and let the z-axis be toward the sky. (y would be forward, x would be right.) Fix the coordinate system to move and rotate along with the car. (So the car moves in the x-y plane. If it's not sliding or drifting, it always moves in the y direction).

    Assume the wheels rotate without resistance. (I.e., there's no friction against movement in the y direction.) When the steering wheel is turned, the front wheels angle; physically this has the effect of imparting a torque about the z-axis on the car. If you turn the steering wheel clockwise, the front wheels cause the car to turn right, and (using the right hand rule) the torque on the car is in the negative z direction.

    Now there is some finite friction between the wheels and the road, so the wheels can slide in the x-direction. Since there is friction against the rear wheels moving in the x-direction, there's a torque on the car when when the rear wheels are sliding but the front wheels aren't. If the rear wheels are sliding to the left, the rear of the car feels a force to the right, so if the rear wheels slide to the left, the car gets a torque in the positive z direction.

    So if the car is turning to the right and the wheels are sliding to the left (as in a drift), the rear wheels' slide causes a torque which acts against the steering.

    Now the key to drifting, I believe, has to do with the changes in the coefficient of friction that result from the car "peeling out." Here's what I'm talking about: you've seen high-powered cars in drag races where just as the driver hits the accelerator (or pops the clutch), the car basically stands still while the tires spin (usually accompanied by some smoke... "burning out"). This is because once the wheels begin to spin without gripping the road, the coefficient of friction between the wheel and the road drops dramatically. Good drag race drivers want to maximize their acceleration, so they try to reduce "wheelspin" in order to maximize the coefficient of friction between the wheels and the road. (This is basically an example of the difference between static friction and kinetic friction. Kinetic friction is almost always smaller than static friction.)

    Now back to drifting. Since the frictional force of the back wheels sliding works against the steering torque, you can get better turning (overall) torque by reducing the coefficient of friction between the back wheels and the road. As explained in the last paragraph, the wheels have a lower coefficient of friction when they're spinning relative to the road, i.e. "peeling out". So you get more turning torque when your back wheels are peeling out during the drift. You'll notice that drifters don't get a good drift if the back wheels are rolling along with the road (in the y direction), they ALWAYS start peeling out when they need to drift through a tight turn. This is why drifters "tap the clutch" to initiate a drift--putting the clutch in for a moment causes the engine to rev up, and when they release the clutch, the engine makes the rotation of the wheels dramatically increase, so they start peeling out.
     
    Last edited: Mar 5, 2012
  4. Mar 4, 2012 #3

    rcgldr

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    In a drift contest, the front wheels are turned slightly inwards of the direction that the car is moving, so that the car continues to turn while it's moving forwards because the force between front tires and pavement result in the pavement generating an inwards force on the tires which transfer that force to the rest of the car. Even though the rear tires are spinning and sliding, and angled inwards of the direction the car is moving, the force between rear tires and pavement results in the pavement generating a forwards and inwards force on the tires which transfer that force to the rest of the car.

    Since the rear tires are sliding, the front tires will have more lateral (sideways) grip than the rear tires, so the driver has to adjust the steering to reduce the inwards force related to steering to keep the car from getting too far sideways.
     
  5. Mar 5, 2012 #4
    Most drifters use a technique called "E-braking" where they go into a turn at a high speed, push the clutch in, turn hard into the turn, pull the emergency brake (locking the rear differential, making the rear tires stop rotating). Now that the rear tires have lost grip with the road they start to follow the original velocity vector which is tangent to the curve. the front tires have enough friction to hold the car, but The driver reengages the clutch and accelerates the rear tires just to the point where they have some grip (hovering on the coefficient of static friction) causing the car to snap back. for a more sustained slide the driver just alternates pulling the E-brake with the clutch pushed and pushing the power, all the while guiding the car's trajectory around the curve.
     
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