first off, I'm a first time poster but long time fan of the site. I really like the all the interesting little science bits that I pick up from the site.
I have one of those odd science questions that pop into daily life from time to time, and I thought this would be the perfect place to get a good answer (since you all know so much ).
I first heard it from my wife while we were driving in wet weather with cruise control on, it went something like this: "I heard somewhere that people are having accidents when using cruise control while it is raining. Apparently the car can't tell how fast it is going when the wheels start to hydroplane and that make the cruise control speed up the wheels. Then the car gets traction again and the driver can't control the sudden acceleration so they fly off the road."
When she first told me this I laughed because it sounded ridculous. But then I started getting curious as to whether this could actually happen or not.
I had thought I had worked out the problem in my head before doing any research. However, research only made things worse. I now have more questions than when I started. But before I get into that, let me start with what I had thought of...
1. First, the subject of hydroplaning. It is my understanding that hydroplaning is essentially like losing all meaningful friction contact with the road because the water has created a middle layer between the road and the tire. With water being a liquid this can create very low (it can't be zero can it?) friction. Since friction is what keeps the car on the road, this is obivously very bad. This also means that if the car begins hydroplaning, and the engine is still suppling the same amount of torque, then the tires are nearly (but not zero...thinking of the little things like possible water friction) spinning free.
2. How does cruise control really work? Again, I'm no expert on cruise control, but the systems I've seen and read about seem to rely on the car's speed rather than RPMs. If this is true, then my next assumption is related to how a car's speed is gathered. I think it that it would be done by some trip meter on a rotating part near the wheel or axle, this seems to be coroborated by comments I have read.
3. If we can assume the above, then we can build a scenario from it. Let's say that when the car is not hydroplaning and using cruise control set at an actual speed of 60mph that the RPM value is 3000 and the spedometer reads 60mph. When the car begins hydroplaning then I would assume it would slow down even if only marginally (there is still friction with the water I believe) but I would like to assume for this purpose a constant actual speed of 60mph. At the same time, the 3000 RPMs would produce more than enough torque for the tires to spin freely which would result in the spedometer reading greater than 60mph. So if cruise control works by what the spedometer tells it, then the cruise control would begin lowering the RPMs so that the tires of the car rotate at what the crusie control thinks is 60mph. This sets up for when the car stops hydroplaning and makes useful friction contact with the road again. If we assume that the car is still travelling at roughly 60mph and the tires have not sped up beyond 60mph due to the cruise control then there should be few if any problems. Concievebly, a hypothetical situation could be setup where a car began hydroplaning on a very straight road and did so long enough for the car's actual speed to drop significantly. Then when contact was again made with the pavement the car might jerk forward unexpected or uncontroably, but this seems unlikely to me.
After working the above out, I checked on the net to see what I could find... I came up with a couple of pages actually relating the subject : http://www.uawlocal1292.org/hydroplaning.htm . That was well and fine but it didn't explain much other than some of the origin. I then came across http://urbanlegends.miningco.com/lib...control-m3.htm which really confused me since noone could seem to agree. The comments by some readers were very convincing. Especially the following:But even this just doesn't feel right... if the (note: rear wheel drive) car slows down almost imperceptibly then shouldn't it also speed up almost imperceptibly? And is that enough to spin the car? And if you are in a situation where your front tires have no traction (or are just being slowed I guess) and your rear tires have enough traction and torque to cause a serious rotational unbalance like that, isn't the subject of cruise control the least of your worries?When my brother-in-law discussed the accident with an engineer friend of his, the friend asked if the cruise control was engaged. My brother-in-law said yes. The friend then explained that when the front wheels hit an accumulation of water, no matter how deep, the water slows the vehicle, almost imperceptibly. However, it slows enough for the cruise control to "kick-in" and apply power. The rear wheels race ahead causing the vehicle to spin out of control. It all happens in a split second. (The reaction of a front-wheel drive vehicle, of course, is not the same.)
I would really be interested in anything anyone has to say on this. I am leaning towards cruise control not being a factor in accidents on slick pavement, but I still have doubts as to the accuracy of my assumptions. I am still interested in opinions, but obivously hard facts would be great.