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Can wheel locking occur with engine breaking?

  1. Mar 9, 2008 #1
    Engine breaking seems to me as dependent of relative angular velocity of the wheels to angular velocity of the engine which is connected to the gear. Engine breaking comes from drag forces that occur inside cylinders and act on the pistons moving inside fluid. So in this sense there is no breaking when there is no relative velocity, so wheel locking cannot occur no matter how icy the road is.
     
  2. jcsd
  3. Mar 9, 2008 #2

    pam

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    The danger from wheel locking is that static friction is overcome, and the tire slides over the ice. Static friction can be overcome in engine breaking as well, with the same skidding of the tire. The tire may still be rotating, but the skid will be the same.
    That is why you should (in desperation) apply a bit of acceleration to try to recover static friction again. Then you can try to brake, but not too hard.
    When encountering ice, don't take your foot all the way off the gas.
    Keep static friction as well as you can.
    Incidentally, that is also why you shouldn't give too much gas in starting on ice or snow.
    Slow and steady moves the car. Otherwise you're just spinning your wheeels.
     
  4. Mar 9, 2008 #3

    Stingray

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    With the throttle closed, the engine will want to return to its idle speed. If you're in gear, that will always try to slow the car down. I don't know what relative velocity you're talking about, but engine breaking certainly can "lock" tires. I've managed to do it before.
     
  5. Mar 9, 2008 #4

    rcgldr

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    It won't stop the tire completely, but it will cause driven tires to skid on the pavement. If your car is a rear wheel drive car with an open differential, then downshifting on ice will only cause one of the rear tires to skid, leaving the other 3 available for steering. I only experienced iced over roads one time on a business trip with a rental, and downshifting into 1st gear is what I did to slow down the car whenever I encountered ice.

    With some types of racing cars, the engine braking is used to induce lift throttle oversteer, which slows the car down as well as turning it inwards (the rear end slides outwards a little), allows a higher speed on corner entry.
     
  6. Mar 9, 2008 #5
    plus, even with the engine stalled, due to the compression of the engine if you are in a low gear the wheels require a lot of torque to rotate. Try pushing a standard car in first gear with the clutch out. Not easy.
     
  7. Mar 9, 2008 #6

    rcgldr

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    Forgot to mention, engine braking is mostly due to friction and movement of air underneat the cylinders. The air above the cylinders is fairly elastic, acting more like a spring than a source of significant drag. If you shut off a motor while going downhill, the engine braking effect stays about the same regardless of throttle position.
     
  8. Mar 10, 2008 #7

    chroot

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    The wheels can't lock completely, because that would require the engine to also be stopped, since the car is in gear. The wheels can, however, slide across the pavement, turning more slowly than they need to for proper rolling.

    - Warren
     
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