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Motocross: Physics of the Scrub

  1. Jan 20, 2014 #1
    Motocross: Physics of the "Scrub"

    I was wondering why scrubbing over a jump proves to be faster than hitting the jump upright. James Stewart and many others have given poor, a likely false, explanations and some have provided logical explanations, but I wanted to see mathematical proof as to why it works.

    I've attached a proof that I believe is pretty strong, but please check it out for yourself and let me know if there is anything wrong with it.

    I was surprised to see in my example scenario the substantial difference in speed and time that comes from the technique.

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  2. jcsd
  3. Jan 20, 2014 #2
    I am not sure what scrubbing means, it looks like it is flying over a flatter trajectory. Flatness determines how fast the flight ends: the flatter it is, the less time it takes. If two trajectories start and end at the same points, but one is flatter than another, then the flatter trajectory is faster. Unless I grossly misinterpret the meaning of "scrub", that's all there is to it.
  4. Jan 20, 2014 #3
    I found that for scrubbing


    As voko said, when you go higher, you need more time. Because if the speed of the motorcycle is the same in both cases, to jump higher you need to jump at greater angle and the forward speed will be less.

    [itex]\mathtt{V_{forward}\ =\ \ V_{0}\cos{\alpha} }[/itex]


    So you will be faster with scrubbing.
  5. Feb 5, 2017 #4


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