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The physics in cars

  1. Feb 5, 2012 #1
    1) when we steer a car, the front wheels point to the direction we steered in. why does steering change a car's velocity?

    does it have something to do with the angular velocity, momentum, or even torque?
    -i tried to think of the individual forces that act on each wheel, and all i have is the normal force, gravity, and perhaps friction. (not sure if there is some other forces)

    2) and sometimes, we can oversteer and understeer. can someone explain to me how and why this occurs?

    i know understeer is kinda like drifting, but how does it happen? why does the locking of the wheels cause us to lose control of the car? (lose control as in the car slides sideways when the wheels lock)

    3) why is it right to brake when entering a curve, and then accelerating out of the curve? why can't we just apply throttle throughout the entire turn?
  2. jcsd
  3. Feb 5, 2012 #2


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    Welcome to PF, Physi123.
    I'm too tired right now to answer more than the first question; the rest are more complex. As for it, though, acceleration is defined as a change of either direction or speed. Steering obviously changes direction.
  4. Feb 5, 2012 #3


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    Static friction, the tires roll in the direction they are pointed and when the front and rear tires don't point in the same direction, the car turns (unless there's no friction).

    There are multiple definitions for oversteer and understeer. One definition is which end of a car slides first (or tends to slide first) in a turn. Another definition is which end of the car has more slip angle at the tires. Wiki article:


    If the tires are locked up the car slides in a straight line or downhill if the car is on hill or banked pavement.

    Depends on the shape of the curve and the entry speed. If the curve has an increasing radius, or the entry speed is lower than what the curve would allow, you would be applying throttle throughout most or all of the turn. For a constant radius turn approached at high speed, the normal racing line is to brake until the apex and then use throttle from apex to exit. If it's a high speed turn and the car has little or no acceleration at that speed, then the racing line is just the inside of the turn which is the shortest path.
  5. Feb 6, 2012 #4
    You brake before the turn because obviously you need to slow down, otherwise you will fly off the road. You accelerate out of the turn because after the turn you obviously don't need to be going slowler anymore.

    The reason why you don't brake or accelerate WHILE turning, for the most part, is because the tire only can only hold the road a limited amount before it slips. It doesn't, for the most part, care if it's holding the road sideways (turning) or back/ forth (braking/ accelerating), or a combination of these. So if you're turning as hard as you can, and you still ask the tire to accelerate, the tires are going to slip. If you're braking as hard as you can and still ask the tire to turn, the tires will also slip. If you're coming out of a turn and don't have to turn as hard anymore, however, you might have some traction to spare for a little extra acceleration and you should give it some gas.

    The key is to always ask the tires for as much as you can, but no more. If you're not right at the cornering limit, then you should also be either accelerating or braking. Keep in mind the whole braking-braking/cornering-cornering-cornering/accelerating-accelerating procedure should be smooth and seamless so as not to upset the balance of the car. Also keep in mind that I'm no racecar driver, and am spewing mostly theory.
  6. Feb 7, 2012 #5

    jack action

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    You need to understand the principle of the friction circle.

    Basically, based on the friction circle theory, if the front axle looses lateral grip before the rear axle (which still «holds» the car to the ground), you have an understeer condition and, similarly, if the rear axle looses lateral grip first, you have an oversteer condition.

    Still based on the friction circle theory, you can analyzed a GG diagram to see that to be able to do the fastest lap, you need to keep the car constantly to the maximum acceleration (in any direction), hence the maximum tire friction force (in any direction).
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