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Negotiating a bend

  1. May 5, 2010 #1
    Hi there!

    When I had first learnt about the situation of a vehicle taking a turn around a bend, I was quick to conclude that the vehicle would bend inwards (towards the center of the circular path) as I had the image of the MotoGP racers who bend to the extremes to negotiate sharp bends. But I was later satisfied by the mathematics of the problem which showed clearly that reality is just the opposite..

    But my initial doubts still remain.. What about the bikers..?
    Even I have noticed this effect while riding. Though the classroom derivation was about 4 wheelers probably (we imagined a bending street and the back of a car that was about to take the turn, the normal reaction could only be zero on the "inner" wheel), but out on the streets it would be absurd to think that the physics of the bike and the car around the bend is different..
    Where am I getting it wrong?(Physics cant be different that way).
    Is the Inertia of Direction factor playing a greater role for the bike than the car? Well why so..?
    (Is the center of gravity of the car situated lower than that of the bike in a horizontal frame? If so then what bout a truck carrying 9 elephants? Where would the center of gravity be in such a case? In which direction will it turn in this problem? Or does this location of center of gravity really matter in this case?)

    Ideas Please...
  2. jcsd
  3. May 5, 2010 #2


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    Hi Starwanderer1! :smile:

    The car can stay level because it has two lines of wheels, but the bike has to stay in line with the total force (in the rider's rotating frame).

    The driver of the car has a rotating frame and experiences gravity downward, and a "fictitious" centrifugal force outward: total force out-and-down, he's sitting on a horizontal surface, so he will slide outward.

    The rider of the bike experiences the same forces, and the same total force, but in his case, he's sitting on a surface that is perpendicular to the total force (otherwise the bike will fall over), so he won't slide. :wink:
  4. May 5, 2010 #3
    I get the fact that if the bike would have behaved like the car then it would fall over. But even the biker is in a rotating frame. So he too would experience a centrifugal force. Further if I make a bike with extra wide wheels (it's still a bike, it still has one row of wheels, just it looks wierd), what would be it's behaviour? What do you think about the role of inertia of direction in this situation..?
  5. May 5, 2010 #4


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    For a car not to roll, the net force must lie within the track of the vehicle. For a bike not to fall over, the force has to be almost exactly through a line through the contact points of the two wheels.
  6. May 5, 2010 #5


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    Yes, and that centrifugal force is the same as in the car.

    But remember, he won't feel the centrifugal force individually, he'll only feel the total force on him.

    And in both cases, as sophiecentaur :smile: says, the total force must go through the track of the vehicle.

    At normal speeds, the bike wheel would have to be very wide for that to work with the bike staying vertical.

    Of course, if it did stay vertical, the total force on him would have a sideways component, and he would tend to slide on the seat.
    what do you mean? :confused:
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