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Leaning while turning.

  1. Oct 17, 2008 #1
    The wikipedia article on "Bicycle and motorcycle dynamics" in the "Turning" section
    In order to turn, that is, change their direction of forward travel, bikes must lean to balance the relevant forces: gravitational, inertial, frictional, and ground support.

    Is this right? I think leaning is done for the rider staying on the bike. (balancing the moment of the friction and the moment of the normal force about the C.G) and not for initiating the turn. comments?
  2. jcsd
  3. Oct 17, 2008 #2
    So, how are you suggesting that turns are made? Do you think by turning the handlebars? Go try it - try to turn the bars right, and you will lean over to the left and turn left (as long as you are moving with some speed). If you don't believe this please go try it before replying.
  4. Oct 17, 2008 #3
    So, how are you suggesting that turns are made?
    So there is no need of friction? Just a torque contributed by the normal force is enough? Those silly guys are unnecessarily providing handle bars then. God forgive them!
  5. Oct 17, 2008 #4


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    A bike turns (the center of mass accelerates "inwards"), when ever the tires apply an outwards force to the pavement, which coexists with an equal and opposite inwards force onto the tires. The force produces an inwards acceleration and rolling torque on a bicycle. If the bicycle isn't leaned sufficiently for gravity related torques to counter the steering related torques, the bicycle will roll outwards. Note that the rate of inwards acceleration is equal to the inwards force from the pavement divided by the mass of the bicycle, regardless of any roll reaction.

    Normally the goal is to not fall over, so a somewhat "coordinated" turn is needed, and countersteering, either directly through steering inputs, or body leaning, which results in a brief outwards steering of the front tire is needed in order to initiate the roll to acheive a proper lean. Note that body leaning only works within a speed range.

    Not quite. Turn the bars right, and the bicycle will turn right while rolling left, ending up on it's left side, but having turned right in the process. The side force determines the rate of inwards acceleration, independent of any roll reaction of the bicycle, the caveat here is to note that for a given steering input, the side force will be less if the bicycle experiences outwards roll axis acceleration than if the turn is properly coordinated (or inwards roll axis acceleration).

    Then again, it's difficult to turn a bicycle that is sliding on its side after falling over.
    Last edited: Oct 18, 2008
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