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A Car going at some speed hydroplanes: does it speed up, stay the same, or slow down?

  1. Speed Up

    2 vote(s)
  2. Stay the Same

    4 vote(s)
  3. Slow Down

    10 vote(s)
  1. Oct 15, 2003 #1
    I have an on-going diasgreement and I don't wish to temper my own oppinion into this thread so I will ask the question in an open-ended way...

    If you have a car going at some speed (say 55 MPH) and the car starts to hydroplane, does the car speed up, remain essentialy the same speed, or slow down?

    In words, how would you prove your point?

    <edited to add>
    I should add that we are making some assumptions that this is on a flat level surface and that the only change was the surface of the road from blacktop to a layer of water that caused the hydroplaning to occur.
    Last edited by a moderator: Oct 15, 2003
  2. jcsd
  3. Oct 15, 2003 #2
    first, think of newton's laws. for the car to speed up or slow down, the forces on it must change; there must be a net force either forwards (speeding it up) or backwards (slowing it down).

    now ask the question, if the car starts hyrdoplaning, does it alter the net force on the car? yes it does, in two ways.

    the first is that it decreases the friction between the car and the road. this reduces the force on the car trying to slow it down.

    the second is that it effectively makes the engine less efficient. the tyres don't grip the road very well, so a lot of the engine power is wasted. this means that there is a smaller drive force pushing the car forwards.

    now you need to balance these two effects: if the decrease in frictional force is greater than the decrease in drive force, the car will speed up. if the decrease in frictional force is smaller than the decrease in drice force, the car will slow down.

    a simplified version of this example would be the car running in neutral (down a hill for example). if the car starts hydroplaning, the frictional force will decrease, but there is no driving force at all (only gravity). thus the car will speed up.

    so the answer to your question: it depends on the specific circumstances.
  4. Oct 15, 2003 #3


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    Fuego, you're making it too complicated.

    To a first approximation, the hydroplaning car experiences zero friction -- this means it's not being slowed down by the road, and it's not being sped up by its tires. It's a free body.

    In the absence of atmospheric drag, it'd stay the same speed as long as it hydroplaned. Of course, drag is a huge effect for cars at hydroplaning speeds, so it can't really be ignored. This drag will slow the car down, and, since there's no friction, there's nothing the tires can do about it.

    - Warren
  5. Oct 15, 2003 #4

    Chi Meson

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    When the car hydroplanes, the friction that is lost is the friction that is trying to push the car (static friction between tires and road). The friction that is trying to stop the car is primarily the air resistance, which is still present. In the real world, the car will definately slow down, but at a small rate of deceleration, and without control.

    If anyone thinks it will speed up, identify the force that pushes the car forward and try to explain how this force suddenly increased when the tires lost their grip.
  6. Oct 16, 2003 #5
    When a car hits the water, the friction to force ratio that was driving the car in the first place, CHANGES, the force remains applied but now without any friction, hence the motor revs. This is followed by, under the right conditions the simplicity that the tires will (Sorta) re-attempt to establish their traction, and are now spinning at a much faster rate, so it is quite feasable that the spinning tire will, as it aquires traction, speed the car forward.
  7. Oct 16, 2003 #6

    Chi Meson

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    But this scenario is a different problem.

    As the tires initially hydroplane, the static friction between the tires and the road drops to essentially zero. At this point the two forces that were balanced (force of friction from tires and air resistance from air) are no longer balanced. Acceleration must occur in the direction of the larger force. The larger force is air resistance. THe car slows down.

    Moments later the wheels will be spinning faster, and they may in fact be throwing some water backwards and so doing they will gain some forward thrust, but this will be far less than the old friction. Moments later still, the wheels will come back into contact with the surface, the car will get its frictional force back with the road, and the car will accelerate forward again, but this is after the hydroplaning is over, so its a different problem.

    There might be a sensation in the real world of speeding up, but this has to do with the fact that hydroplaning occurs when we are trying to slow down. We hit a puddle, hydroplane, and then we do not slow down as fast as we were. This can feel like speeding up, but it's just slowing down less.
  8. Oct 16, 2003 #7


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    Strange how you can post what I described in another thread yet you disagreed with me there.
  9. Oct 16, 2003 #8


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    I think this is the main source of confusion. I've heard cops say that hydroplaning causes acceleration (as opposed to deceleration). I think the concept of acceleration is getting confused with the concept of jerk (no, not a person with a disagreeable attitude, but the third order derivative of position with respect to time).
  10. Oct 17, 2003 #9
    That is becasue there is a circumstantalization that must be accounted for, as in what are the road conditions?, what manner of contact of water?, (rain only, large puddle, snow, ice, etc.) and what occurs in the initial timeframes?, later possible timeframes, etc.

    When a driving tire hits the water, it initially hydroplanes, that loss of friction allows for the engines exerted torque to overcome what little wheel resistance is left, thus spinning the tire, that event clears some of the water from the tires track, increasing it's friction while it is now spinning at a greater speed, hence if it re-gains enough traction, it will accelerate the car, if not it won't! As stated, about road conditions, and driving wheels (front or rear or all four) all count enormously and will change the results.

    Doc you were right, partially, and I did miss that one. (sorry!)
  11. Oct 17, 2003 #10

    Chi Meson

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    WHen the wheel does re-contact the road in this scenario, the wheel surface will be skidding against the road surface. THis means the friction that returns to propel the car will be of the kinetic form. We want static friciton to propel the car (the kind when the wheels aren't skidding). Kinetic friction is never greater that static friction, so there is no way under normal conditions that a greater force will be applied to the road. The wheels will not "grab" the road until the tangential speed of the wheel's surface is equal to the speed of the car (whic has now slowed down). Once the wheels re-establish static friction, the car will again accelerate positively, but it has already slowed by this time.
  12. Oct 17, 2003 #11
    applies to the reddened part How do you establish that it has "now slowed down" when the engines speed has NOT been backed off of, (it actually increased due to the loss of traction/resistance) and the wheels are still having the same, or greater, amount of torque being applied to them by that motor?
  13. Oct 17, 2003 #12

    Chi Meson

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    No matter what the engine does, and no matter how much torque is produced, the car will have no forward propulsion force unless the wheel are contacting the ground. The maximum force that that car can apply to the road is equal to the maximum static frictional force between the road and the tires.

    Take your car on to very slick ice. Same car, same engine, same "torque," but very little forward force because the static friction is near zero. This is also why your car cannot work when its wheels are off the ground.

    If you are driving along with the accelerator to the floor and the ground drops away from you, can you imagine a situation where you would speed up? (In the forward direction I mean, of course you would start to fall down and speed up that way).
    Last edited: Oct 17, 2003
  14. Oct 18, 2003 #13
    You seem to be dealing with this as a static surface, it isn't it is a tire tread on/in water, the rotational speed of the tire increasing, will cause the removal of more of the water from underneath the tire's tread, hence it will begin to re-establish traction coefficients at a higher rate of rotation, thus speeding the vehicule forward.......Doc's right for the circumstance(s)..........
  15. Oct 18, 2003 #14

    Chi Meson

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    You need to brush up on the forces of friction. Traction coefficients is a term I am not familiar with. You you mean coefficients of friction? In Doc's earlier post he used the term "friciton to force ratio" : this sounds like the coefficient of friction to me, but that is the "frictional force to weight ratio," nothing to do with the force or torque of the engine.

    As for thinking it is a static situation, you need to reread my post. It is specifically the loss of the static situation that causes a sudden loss of propulsion force.

    If a tire is spinning on the surface of anything, it is a kinetic friction situation. If and when the car reestablishes contact with the surface of the road, the kinetic friction will be the same no matter what rate the wheel is spinning (assuming that the temperature of the rubber is not changing). Kinetic friction does not depend on the relative speeds of the two surfaces.

    Doc is not right on this one. Newton is. Sorry.
  16. Oct 19, 2003 #15
    I have a suspicion that you think that the wheel re-engages at a fixed contact pressure, it doesn't, it's re-conection with the road surface (traction/friction) is an even unto it'self inasmuch as the tire and the road surface will gain in friction/resistance relatively quantified upon the amount of water V expulsion of that water, the amount of pressure upon that wheel, at that time, relative to the inertial mass distribution over time, the "backwash trail" it might be following, (for rear wheel drives) or slips out of, the braking resistive forces, acting on the front end of the car, that may steer, and decelerate the front end, such that re-establishing simply the preset road speed might seem "The Acceleration" from the deceleration that occured upon entry to the hydroplaning event site, etc. etc.
  17. Oct 19, 2003 #16

    Chi Meson

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    I could not understand your last post.

    This is the last post for me in this thread. After this, I’ll just be spinning my wheels. In the macroscopic world, Newtonian mechanics is valid. Quantum theory does allow for small, but non-zero possibilities for almost anything to happen, so ignoring all quantum fluctuations, we must examine forces.

    What are all the forces that act on the car (internal forces cannot act on the car; see Newton’s first law)? The static frictional force of the road on the car’s tire (this is the reaction to the force of the car’s tires on the road; see Newton’s third law) , the resistance of the air (this is a function of the car’s relative speed with the air), and the resistance of the water in the puddle as the car enters the puddle.

    There is one significant downward force on the car, its weight. The supporting force on the car from the ground or (whatever is underneath it) is the "normal force." These forces are balanced when the car is ona flat surface.

    Only one of these forces is in the forward direction, the static friction. While traveling at a constant speed this force is balanced by the resistance force. When the car hydroplanes, the static friction is lost (because "mu" goes to nearly zero) . Within this instant, there is no significant forward force (If the tires regain contact with the road, then the car is no longer hydroplaning).

    Now the car is hydroplaning; its wheels are spinning, throwing water behind it. The forces that are trying to slow the car down are: the resistance of the air and the resistance of the water in the puddle. The only force that is trying to push the car forward might be a small reaction force to the car throwing the water backward.

    So the net forward propulsion force has dropped nearly to zero, while the resistant forces probably have actually increased. If the negative forces are more than the positive forces, then acceleration will be in the negative direction (see Newton’s second law).

    That is all there is to it. The car slows down.
  18. Oct 19, 2003 #17


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    Ok Chi, get your sh!t together before you quote me. Parsons and I had an argument in a totally separate thread concerning cruise controls. I have never said anything about friction to force or whatever.

    Here is the scenario:

    Car is cruising along at say 60 MPH. It hits water that is standing on the road. The cars driving wheels start to hydroplane. Virtually INSTANTLY the driving wheels will speed up considerably because the engine is unloaded. The wheels no longer meet the road. At this point it would be like the wheels are jacked up off of the ground. Now there is nothing to keep the car from going ahead except it's forward momentum. Now in a split second, several things happen. First, the wheels speed up. Second, the car does actually slow down. BUT, how much can a car slow down in a split second when we haven't applied the brakes? So now we have the driving wheels spinning much faster than previously and are actually throwing water away from them. Sooner or later that water will be pumped out from under the wheels and the vehicle will come back down onto the road. So now we have a situation where we are dropping a spinning wheel down onto a road when the difference in wheel speed compared to road speed is quite possibly 20 or more MPH. So tell me what's gonna happen. Were you ever a kid and did fun things with old (or new) vehicles? Well I was. And I can tell you that when spinning wheels leave gravel (less traction) and meet pavement (more traction) the vehicle speeds up. What's the difference? I'll grant you, the vehicle probably slows down for a very brief period, but the effect most of the time is that it puts you back in your seat when the wheels regain traction with the road. It's funny how you go round and round with Parson about this in the same way that he and I did several weeks ago. Newton was right, Doc was right, Parson saw the light. Why can't you? BTW, is THIS one of the reasons why it seems like everyone drives like an idiot? We have a bunch of people that are on the road who think they know everything about driving because they know, or at least think that they know something about physics?
  19. Oct 19, 2003 #18

    Chi Meson

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    Since you are getting insulting, I have to break my word on not continuing on this thread. I did make a mistake. It was Mr. Parsons who used the term "friction to force ratio." I accidentally attributed to you. As for your analysis of the situation, I will say, for the third time, that when the tire reengages contact with the road, it is no longer hydroplaning.

    And again, when the wheels are spinning faster than the road, the friction upon contact will be the slipping sort of friction which is less force than the static kind. Upon contact, will the car again accelerate? Sure. But the car must already have slowed down. Not enough time you say. Well let's agree that if there is enough time for a wheel to increase its rotation, there is enough time for the car to slow down.

    Even if it only a few microseconds, while the car is hydroplning, the car is slowing down.

    And BTW. the bit about "think that they know something about physics"? I really believe Newton is right.
  20. Oct 19, 2003 #19


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    Yeah, you've said it. Why do you insist I am wrong? I FLAT OUT F***ING SAID that when the wheels come off of the ground the car will slow down. AM I WRONG?!?!?!?!

    No, let's NOT agree. At least lets say that the wheel speed can speed up alot more than the vehicles road speed can slow down. Drive down a clear dry stretch of highway when it isn't too busy. Get the speed up to about 60 or whatever. Keep your foot on the accelerator and DO NOT move it. Now, push the clutch in and leave it in for about a half a second. I repeat, DO NOT move your foot on the accelerator. Keep it at the same position. Now after a half a second has gone by, slide your foot off of the clutch pedal and let it pop back up. Don't lift your foot off of it, let it pop. This simulates the condition of the driving wheels coming down on the road, but less so because the wheels, axle, and driveline are still traveling at road speed instead of engine speed. Depending on a number of things, the car may or may not speed up to its ORIGINAL speed. BUT, like I said, it WILL put you back in your seat. It IS enough to make you lose control in some conditions. I can see it now:

    Chi is talking to the cop after hydroplaning, losing control, and slamming into another vehicle. You argue that it WASN'T really the hydroplaning (which a sign 1/4 mile back warned of) that caused the wreck. It was that darn re-entry onto the pavement. Heck if I could have just 'surfed' all the way down that next traffic light I would have been fine.

    Yeah, I already covered this earlier in this post.

    I actually covered this in the last post. When did I say Newton was wrong? I just said you haven't seen the light. And I also said:
    I NEVER said specifically that you 'thought you knew'. I included both. You CHOSE ON YOUR OWN to 'think that you knew'.

    Wanna keep going? You seem to keep arguing (at least with me) just for the sake of argument. Oh, I know. You wanted the last word after I told you to get your stuff together. I really think we are on the same wavelength. You just refuse to see it.

    You might want to think twice about that experiment out on the highway though, I might see it and add it to my mental list of idiot drivers. Nah, just kidd'n.
  21. Oct 19, 2003 #20

    Chi Meson

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    Please please calm down. My arguments were with Mr Parsons and not with you. I did get one of the earlier posts confused, and I mistook the "stuff together" post as a refutal of my previous post. This is clearly not the case, and I am sorry for the confusion. Your prior post was insulting to me though, veiled or not.

    So let's both drop this thread, it's going nowhere and someone might succumb to road rage. See you elsewhere, peace.

    [edited to fix syntax]
    Last edited: Oct 19, 2003
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