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Static, kinetic and rolling friction when rounding a flat curve

  1. Sep 18, 2010 #1
    Alright, everyone, I have some questions in regards friction when rounding a flat curve and was hoping to get some help with it. For that intent, I've borrowed a couple of posts from another old thread on the topic of centripetal force. Hope the authors don't mind.

    The textbook I'm using actually says the vehicle wouldn't skid, but would just negotiate a turn not as steep as if it would've gone slower. Does that mean that the vehicle will, should it exceed the allowed speed, NOT go out of the turn tangentially, but rather in a curve that will, due to being a part of a circle with a bigger radius, eventually lead it off the road.
    Oh, and could someone shed more light on the difference between those three types of friction in a flat curve. Say you're travelling slowly. Then the only friction involved is static, right? But what happens when you exceed that speed limit and your wheels keep on turning? Is the static friction still the one that's applied? Or does kinetic friction enter the frame here? If yes, does it substitute static friction or just supplement it? And the direction would in any case still be towards the centre of the (bigger) circle, correct?

    And one last question. If we suppose rolling friction isn't neglibile, is its direction also towards the centre (because while I suppose the wheels are in fact turning forward, that is tangentially to the circle, the change in their velocity points towards the centre) and not tangential to the circle?

    Thanks in advance, guys and girls.
     
  2. jcsd
  3. Sep 18, 2010 #2

    rcgldr

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    Both static and kintetic (dynamic, skidding) friction will accelerate a car inwards and slightly backwards, depending on the orientation of the tires versus the path the tires are following. Rolling resitance is normally considered to generate a force opposing the path the tires are following, or opposing in the direction the tires are oriented.
     
  4. Sep 18, 2010 #3
    So rolling friction is tangential to the circle the car is following then? And can static and kinetic act at the same time? Or does breaking the treshold of maximum static friction cause the kinetic friction to step into action since the car is now moving away from the radius of the orbit it was supposed to follow at first? If the latter is true, is there a jerk right at the breaking point since kinetic friction is usually smaller than static?
     
  5. Sep 19, 2010 #4

    rcgldr

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    Because of tire deformation, the actual path radius is slightly larger than the geometrical radius calculated from tire orientation. There's usually some combination of both static and dynamic friction within the contact patch between tire and road.

    For most tires, there is is a jerk like loss of traction when the limits of the tire are exceeded. Some bias bly racing slicks minimize this effect. Radial racing tires also try to reduce this effect, but they are not as "forgiving" as the bias play racing slicks. There are other advantages to radial racing tires (less slip angle, and usually more maximum grip).
     
  6. Sep 19, 2010 #5
    Alright, thanks for the answers. And if anyone else wants to add anything, please do so :smile:
     
  7. Sep 19, 2010 #6
    Oh, forgot to ask one more thing, just to make it clear. Static and kinetic friction, regardless of whether they are both acting at the same time, then have the exact same direction? Am I getting this right? Say that your wheels jam and you start skidding. Does kinetic friction then still accelerate you towards the centre of the circle, whose radius is dependent upon the magnitude of said friction? And if you have to make a bigger turn because there isn't enough friction, does friction point toward the center of the smaller circle, which you want to be following, or the center of the bigger one, which you in fact will be going to follow?

    Sorry, I know it's a lot of questions, but I really want to get this figured out.
     
  8. Sep 19, 2010 #7

    rcgldr

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    The contact patch will have a net direction, but the static and kinetic components will have different directions.

    The kinetic component just opposes the path the car is moving in. Unless you lock up the tires with the brakes, or spin out, as long as the tires are rolling, there's some amount of static friction where the tire deforms at the contact patch, so that even in a 4 wheel drift, the car will still turn but the radius of the turn will be larger if the tires are sliding a lot.

    The radius is related to the centripetal acceleration produced by the tires and the speed2 of the car.

    Friction force divided by mass determines the acceleration. The steering force ends up pointing a bit behind the center of the circle of the path a car is moving. If not for the engine resulting in a forwards force at the tires, a coasting car would slow down as it turns. The energy loss is related to the amount of deformation of the tires due to slip angle as well as rolling resistance, the speed of the car, and other factors related to tire construction.
     
  9. Sep 19, 2010 #8
    So kinetic friction then has the same direction as rolling friction, that is tangential to whatever circular path the vehicle is following or, in other words, in the opposite direction of instantaneous velocity in every such instant?

    Hmm, sorry, but I didn't quite get that. Behind the center of what circle does the steering force point? The "original" one, if we can call it that, or the bigger one that results from the speed being too high and the vehicle not being able to follow the set-out path?
     
  10. Sep 20, 2010 #9

    rcgldr

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    Rolling friction is due to non-elastic deformation of a tire (the force causing the deformation is greater than the force that occurs during recovery, due to hysteresis). The difference between these forces times speed equals the power consumed. Kinetic friction losses are related to energy converted into heat by sliding surfaces. Rolling friction would be in the direction the tire is rolling (not sure if slip angle is considered as part of rolling friction), while kinetic friction opposes the path the tire is taking.

    The larger, or actual circular path. Take the case of a car coasting and cornering. The path will be a spiral with decreasing radius, because there is a backwards (drag) force as well as an inwards force when cornering. A forwards force has to counter the backwards force from conering in order to maintain a circular path. Because of slip angle deformation when cornering, the tires (front and rear) are angled inwards of the actual path the tires are traveling, so the actual path is a circle with a larger radius than the distance between car and the point where the lines through front and rear axles cross.
     
    Last edited: Sep 20, 2010
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