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Riding a motorcycle in a circle, power slide

  1. Nov 29, 2017 #1
    Say you (a skilled motorcycle racer) ride a motorcycle in a big empty parking lot and ride in a big circle of constant radius and slowly go faster and faster. If you are careful as you go faster will you all of a sudden go from very little power slide to a lot of power slide like the video below? As you go faster around the circle is it a gradual transition to power slide or more abrupt? I wonder if it is a smooth transition or more abrupt.

    So to quantify this plot speed around the circle versus the difference in distance between the radii of the front tire contact point to the center of the circle versus the radius from the center of the circle to the rear tire contact point. With extreme power slide this difference is greatest.

    Assume you have a racing motorcycle with racing tires. Please see video below.


    Note the fresh rubber being clearly laid at 1:08 and 2:28.

    Last edited: Nov 29, 2017
  2. jcsd
  3. Nov 29, 2017 #2


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    Staff: Mentor

    The rear tire stepping out in the video is from acceleration, not during a constant speed turn. You can drift the motorcycle on purpose like they do in flat track racing, but that is usually done on dirt. With the extra grip of asphalt, drifting turns is not usually the fastest way around the track. And flat track racers typically wear a steel sole on their inside foot so that they can support themselves during the power slide. You're not going to be putting a foot down at a sportbike/superbike racetrack during your power slide...

  4. Nov 29, 2017 #3
    Thanks Berkeman, I will think about what you wrote. I seem to remember reading that the great flat track racer Kenny Roberts who went on to dominate motorcycle road racing for a time used the power slide through turns to go much faster then the competition at the time? A technique that required even greater sized testicales then the huge ones road racers need.

  5. Nov 29, 2017 #4


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    Staff: Mentor

    Interesting, I did not know that about him..

  6. Nov 29, 2017 #5
    Yes, Kenny changed the style of MotoGP type racing by introducing the power slide, much as some suspect Marc Marquez will change the style by others following his great knee/elbow saves in MotoGP.

    To answer your question about the abruptness of the slide; it depends on the tyres. Most high quality rubber will allow a lot of feel and the slide will be quite predictable. The opposite may be true of lower quality tyres. Some modern motorbikes have slide control as part of their electronic aid suite, i.e. the traction control system will allow a little slide and wheel spin.

  7. Nov 29, 2017 #6


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    An ordinary touring street bike will definitely have an abrupt transition to the slide. In my approx. 100 000 miles of street riding I've only had to do it once, when on a mountain road at night there was suddenly a tree in front of me. Fortunately I was on a small bike and 'stepped' a slide around that 90 degree corner. Avoid if possible.

    EDIT: I just realized that my heart rate went up just making this entry.
  8. Nov 30, 2017 #7
    So from a physics point of view does that mean that the static friction coefficient is similar to the sliding friction coefficient for such tires?

    I had to search for elbow saves, don't try this on your first lap. He looks like he is on one wheel at 0:21?

  9. Nov 30, 2017 #8
    Tyres are designed with a quantity called slip angle. Up to the slip angle the coefficient of friction increases and from there decreases. So sliding and wheel spin increase the friction a little. Some people give rules of thumb figures of 5 - 10% increase. From this you can see that some sliding will increase your corner speed, allow more throttle at corner exit, and allow you you turn tighter.

    Marc is a fantastic rider, and since he has done this more than once one has to believe that it has become a technique for him, and probably others will follow. I imagine it takes a lot of strength and good leathers.

  10. Nov 30, 2017 #9
    That is interesting, something like the below?


    Or more like this,

    Last edited: Nov 30, 2017
  11. Nov 30, 2017 #10
    When you are in a power slide the bike, or a car, actually points a bit inward from the actual path of the center of gravity, you steer a bit opposite to the turn, so that some of the thrust of the tire is inward helping you make the turn? Is that where some of the increase comes from?

  12. Nov 30, 2017 #11
  13. Dec 3, 2017 #12
    Sorry I haven't replied but our weekends come a day earlier than some and we are out of synch. If you imagine in a car you are turning into a corner the car may experience understeer, that is the front tyres start to slide forward rather than turn. The slip angle is literally the angle of the front tyres relative to straight on where the friction force starts to drop off. I see from your reading that you already have a good idea of what ids going on.

  14. Dec 4, 2017 #13
    Thanks Cosmik Debris. So I think I am missing something fundamental about tires and cornering at high speed. From the graph above aircraft tires will slow the plane down with greatest deceleration when there is significant slip. From this one can take the inverse problem and it looks like the greatest acceleration for a car or motorcycle in a straight line is when there is also significant slip. From this can I conclude that when cornering at the highest speed possible that both the front and rear tire are slipping to produce the greatest cornering acceleration with the power slid being a technique to increase cornering acceleration? From the graph above it looks like even modest deceleration (or acceleration) whether in a straight line or in a corner causes tire slip?

    So, it looks like generally only under conditions of zero acceleration do tires not slip?

  15. Dec 4, 2017 #14
    Yes I think so.

    I hope not.

  16. Dec 4, 2017 #15

    Attached Files:

  17. Dec 6, 2017 #16
    OK, very interesting, thanks.
  18. Dec 6, 2017 #17
    Very reasonable,
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