Nagging Questiong: Hydroplaning and Cruise Control

[hairball]

Hi all,
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:

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.)

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?

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.

Cheers,

Hairball

[freddo]

This website is usually good for the final say on such issues.

Snopes believes it is true.

[cyswxman]

Welcome to the board, hairball!!

[darkhunter]

Welcome to the board, Hairball!

Cruise conrtol is governed by the speed from the transmision--it "knows" (oversimplified) what gear the car is in, and the drive ratio ot the rear end/transaxle. It acts on the engine RPMs to bring the measured speed to the set speed. Engine speed is related to the speed at the drive wheels, by it isn't a fixed ratio, you have to acount for clutch/torque converter slippage, gear ratios, and gear lash throughout the rest of the drive train.

When you are using cruise control and lose traction (by hydroplaning) it will keep your tires going at the speed it is set at, no matter the actual speed or orientation of the car. When the tires grip again, the car will likely be faceing in a diferent direction. The surge of acceleration and attempts to regain control can quite easily spin the car.

Also, if the car exits the road and get traction again, it will go in whatever direction it's facing at the speed set on the cruise control.

The safest thing is to avoid cruise control during conditions that would lead to slick roads.

[at last--a question I can answer about a subject I'm good at!!]

edit: I think this should actually be in the BLABBing forum....

[George]

There are new developments in road design/materials to help minimize this road hazard that you might find interesting.

PFC (permeable friction course) is the most encouraging I've seen. This is a thin layer of hot mix asphalt with large air void percentages (around 20%). This makes the road porous so the water actually flows beneath the surface and to the road shoulders. The other intended benefit is the large reduction of glare from on-coming traffic. A surprising benefit is the large road noise reduction.

They are testing this in the north and south. It seems to be holding up so far.

[Andreas]

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.

Friction is not what keeps a car on the road (assuming level road and no crosswind), it's what keeps a car steerable and brakable. If you're just going straight some short hydroplaning shouldn't be too much of a problem.

Cruise conrtol is governed by the speed from the transmision--it "knows" (oversimplified) what gear the car is in, and the drive ratio ot the rear end/transaxle.

Really, why would it be so complicated? Why not pick up speed from the wheels directly?

[parejkoj]

There are new developments in road design/materials to help minimize this road hazard that you might find interesting.

PFC (permeable friction course) is the most encouraging I've seen....

Do you know anything about what it is like to bike on this stuff? I know some of the roads here (Ann Arbor, MI), can get quite slick in the rain, and I've had trouble keeping control on some of them, even with fairly sticky tires. But the thing you talk about sounds like it might make for a bit of a rough ride. Or are the pores really small?

Would this stuff cause increased tire wear? Just curious. If it does what you say, it would definitely be a good idea for interstates at least...

[darkhunter]

Cruise conrtol is governed by the speed from the transmision--it "knows" (oversimplified) what gear the car is in, and the drive ratio ot the rear end/transaxle.

Really, why would it be so complicated? Why not pick up speed from the wheels directly?

Already have a reading coming off of the transmission--it's simpler to use that.

For measuing the speed off of the wheels, you need something that is robust enough to handle the suspension motion and road hazards and water. On top of that, having the unsprung wieght will affect the vehicles ride quality, tire wear, etc.

The gear on the transmission is able to be protected to a greater extent and have fewer parts between it and the speedometer and pickup to flex and be damaged by road hazards.

from experience, the fewer parts to break, the better from a reliability standpoint....

[Andreas]

Cruise conrtol is governed by the speed from the transmision--it "knows" (oversimplified) what gear the car is in, and the drive ratio ot the rear end/transaxle.

Really, why would it be so complicated? Why not pick up speed from the wheels directly?

Already have a reading coming off of the transmission--it's simpler to use that.

Ah, of course. I was thrown off by the "it "knows" (oversimplified) what gear the car is in", which makes it sound terribly complicated so that I forgot that you could just grab the speed on the transmission output so that it's irrelevant what gear it's in.

For measuing the speed off of the wheels, you need something that is robust enough to handle the suspension motion and road hazards and water. On top of that, having the unsprung wieght will affect the vehicles ride quality, tire wear, etc.

I was thinking electronic sensors, mechanical speedometers should be obsolete technology by now. Besides, with antilock brakes every wheel needs its own sensor anyway.

[Humphrey]

Thing is, with all new cars the point of cruise controll and hydroplaning is moot. The second you put your foot on the clutch or brake, the cruise controll diengages. So you have only to worry about pumpuing your brakes to slow you down and gain traction.

For examply both my former car (2000 hyundai tiburon) and my current one (2002 Ford Mustang) have a cruise controll that disengages when the clutch or brake was pressed.

[Andreas]

Thing is, with all new cars the point of cruise controll and hydroplaning is moot. The second you put your foot on the clutch or brake, the cruise controll diengages. So you have only to worry about pumpuing your brakes to slow you down and gain traction.

New cars only? I assume this was an essential part since it's too dangerous without.

And all that does not make the point moot. The point being that active cruise control might worsen the situation and even if it doesn't do so immediately, you have to remember to disable.

These are hectic situations where power must be reduced and braking is a very stupid idea. Cruise control just means that power is applied longer (and maybe too long), since getting your foot on the brake and carefully pressing it enough to deactivate cruise contrl certainly takes longer than just taking your foot off the gas pedal.

[Humphrey]

True, i agree with you Andreas. :-)

[George]

There are new developments in road design/materials to help minimize this road hazard that you might find interesting.

PFC (permeable friction course) is the most encouraging I've seen....

Do you know anything about what it is like to bike on this stuff? I know some of the roads here (Ann Arbor, MI), can get quite slick in the rain, and I've had trouble keeping control on some of them, even with fairly sticky tires. But the thing you talk about sounds like it might make for a bit of a rough ride. Or are the pores really small?

Would this stuff cause increased tire wear? Just curious. If it does what you say, it would definitely be a good idea for interstates at least...

Sorry I'm slow at catching this question.

PFC material (also called "porus friction course" as it is easier to say) comes in at least two "flavors". A modified hot mix blend and a new patented European method called Nova Chip. Both use a smaller than normal rock but use few "fines" in the blend (to help the permeability). Both are suppose to be an open gradation mix to allow the water into the pavement where the water then travels to the road shoulders. I have seen pictures and first hand the big difference this makes during a rain. Water does not stand and glare is greatly reduced.

It rides very smooth and quiet. Bicycling should be no problem. Roller blading may or may not notice a difference. You would not know you were driving your car on it except once you get on you immediately hear noticeable reduction in tire noise (unless you are using tire chains in Michican ).

The Nova Chip method is unique in that it applies special liquid emulsion (Koch Oil), with strong polymers, directly in front of the hot mix paver screed. "Hot mix" refers to the rock/asphalt blend of material from the asphalt plant. The normal temperature of the hot mix is around 300 deg. F. Emulsion is usually about 1/3 water and 2/3 oil (asphalt). So not only do they "mix water and oil" but then they apply it to 300 deg F hot mix. Strange brew, right? Oddly, the results are quite good. The flashing of the water to vapor helps create the voids. This allows the process to be essentially a one machine operation plus roller and tanker. Nova Chip substantially minimizes the time it takes for traffic to be restored.

Either Michigan or Wisconsin has been doing PFC testing in the last year or two. Georgia also has been doing some.

PFC isa bit counter intuitive as the real purpose (normally) of asphalt roadways is not to add any strength to the road (which it doesn't) but to keep water off the lower layers of the road and to act as a good wearing surface with skid resistance. Good existing asphalt or concrete pavements must exist before this can be done and sometimes special shoulder drainage must be added to keep the water flowing.

If PFC will hold up to the freeze-thaw cycle and be cleanable to maintain permeability, we may all see many major larger roadways improve nicely. The costs are not much more (25% currently) than conventional. (Cost is down some due to the thinner application than conventional 1.5 to 2" lifts [layers]).

What you are experiencing with "slick roads in the rain" is possibly "bleeding" or a worn down road surface. Designers try to keep this from happening but many things have to happen right to eliminate it. If it is very oily, complain and they might at least sand it or maybe seal coat it.

I am a road equipment house specializing in asphalt equipment but a piece of my heart is astronomy. Hard to blend the two and never figured I'd be talilng asphalt here. 8)

Now it so happens that there is evidence that hydrocarbons come from space.....hmmmm, maybe I can connect more. We'll see....

[Russ]

Since I'm an industrial control engineer and deal with this type of problem every day, Ill kick in my two cents here.

1) The sensor for the speedometer in most cars is on the Power Take Off (PTO) of the transmission. This means the drive shaft(rwd)/axle(fwd) of the car. It also means that the speedo does not have "know" what gear the transmission is using as it picks up axle speed directly.

2) The cruise control (CC) compares a desired speed, called a setpoint (SP) with the measured speed or Prossess Variable (PV) to come up with the signal error (SE). The SE is then applied to what's called a Proportional-Integral (PI) equation which calculates the amount of positive + or negative - fuel flow demand or Control Output. It then sends an analog signal to what's called a continuously variable solinoid valve to supply more or less fuel to the engine as appropriate. SP-PV=SE SE x P x I = CO.

3) Hydroplaning almost always involves the front tires only as the rear tires travel in the channel created by the fronts and (generally) have clear pavement. This becomes not true if the car assumes an orientation other than straight wrt the direction of motion.

In a FWD car, hydroplaning tends not to be a problem on straight road as the thrust vector of the car is at the front and the car will tend to continue in its direction/speed of motion until acted upon by an outside force. Even in a moderat curve one can keep the car headed in the desired direction/orientation.

In a RWD car this is not the case as the thrust vector is at the rear and if the front (hydroplaning) tires are not linear wrt the rear tire thrust vector, the car will tend to skid and spin. The CC will tend to agrivate this situation by keeping the rear tires spinning at the SP speed. As the rear tires exit the channel created by the front tires, they, also, will start to hydroplane and all control is lost.

In short, as soon as you think you're hydroplaning with the CC on, LIGHTLY tap your break to disengage the CC and allow the vehicle to slow on its own until it stops hydroplaning. :wink:

[SirThoreth]

Interesting point - adds another reason for me to dislike cruise control.

On a similar note, my friend's GF was driving her father's Jeep Wrangler recently, when it started raining. Since it's so rare that it rains out here, the first few minutes of rain often works loose all the oils, etc., that build up on the road. You have to be careful, because it's quite easy to put your car into a skid, or to hydroplane, under these conditions. Now, that said, she was driving around a curve at around 20-25 mph when she went into a spin. What she didn't know at the time was that her father had left the Jeep in 4WD, as opposed to 2WD.

"Why is this important?" you ask? Well, I thought about this afterwards. When you're in a FWD car, you're supposed to steer into the direction of your skid to regain control of your vehicle. A RWD car (like my '71 LTD or '73 VW Thing) is the exact opposite, where you steer in the opposite direction of your skid to regain control. What about a 4WD?

I decided to ask my father, who, many moons ago, used to work for American Motors Corporation, and was one of the first people to drive a Jeep Cherokee, and its predecessor, the Wagoneer. He'd told me in the past about taking them to the track with the testers, or even just going out into the parking lot and doing donuts, etc. So, I asked him which direction you steer a 4WD in when you go into a skid. His answer? "Play it by ear, and hope for the best." You have to figure out which half, the front or the rear, is causing the skid, and drive accordingly. Front half is skidding? Steer out of the skid. Rear half skidding? Steer into it.

Now, why bring this up? Because, with the large number of SUVs on the road today, it could be very handy info to have.

And, yeah, I agree, prolly best to move this thread to BABBling

[Russ]

Interesting point - adds another reason for me to dislike cruise control.

I decided to ask my father, who, many moons ago, used to work for American Motors Corporation, and was one of the first people to drive a Jeep Cherokee, and its predecessor, the Wagoneer. He'd told me in the past about taking them to the track with the testers, or even just going out into the parking lot and doing donuts, etc. So, I asked him which direction you steer a 4WD in when you go into a skid. His answer? "Play it by ear, and hope for the best." You have to figure out which half, the front or the rear, is causing the skid, and drive accordingly. Front half is skidding? Steer out of the skid. Rear half skidding? Steer into it.

Now, why bring this up? Because, with the large number of SUVs on the road today, it could be very handy info to have.

And, yeah, I agree, prolly best to move this thread to BABBling

An interesting story. If you skid a Jeep while in 4WD you have to play it by ear.... I have a Chevy Blazer and have never been able to put it into a skid while in 4WD.!??!?? :wink:

[Charlie in Dayton]

Having some little experience in this, in the SCCA ProRally series, where you're deliberately trying to throw the vehicle sideways a large amount of the time, and having just returned from the Lake Superior Pro Rally, the season ender where friends and acquaintances won event/national titles, I hereby sound off...

All of this depends on which end of the car is sliding. You're better off with some traction to the steering axle to change directions. If the front end is sliding, basically you're screwed. We're going to assume that all these are rear end skids and the front will still more-or-less do what you're telling it. That being said...

RWD - steer in the direction of the skid, and let off the throttle maybe halfway (not totally). That allows the rear tires to slow down their rotation through the range where they will grab the road and start to follow the front end which you have pointed in the right direction.

FWD - steer in the direction of the skid, and accelerate a bit. The front wheels will pull the rear end back into line (if there's adequate traction and the fronts grab).

NOTE - in both these situations, the recovery is quick and can go the other direction. Those long smooth slides seen in race cars are different situations, in that the slides are deliberately being extended.

AWD/4WD (not the same thing - one's designed to keep some torque going to all four wheels, and one's designed to walk up the side of a building -- but generally and for purposes of this argument, we'll equate them) - This one's tough. There are elements of both here. Steer in the direction of the skid. Accelerate gently. The road forces acting on the front wheels are in line with the plane of tire rotation. The road forces acting on the rear wheels are at an angle to the plane of rotation, causing a frictional drag across the tread surface. Even with the acceleration, the forces on the rear end are higher than the front, and the front again will pull the rear into line.

Read the note again - the unprepared driver is the one you see with the car penduluming back and forth from a skid, and they just can't catch it and recover.

In all cases, DO NOT touch the brakes until the car is straight and under control.

There are methods to deliberately brake under acceleration to throw the car into a controlled skid for certain purposes. Practice if you like on a snow-covered empty parking lot, but let's not have any 'bootlegger's turns' on the street, ok?

[SirThoreth]

Interesting point - adds another reason for me to dislike cruise control.

I decided to ask my father, who, many moons ago, used to work for American Motors Corporation, and was one of the first people to drive a Jeep Cherokee, and its predecessor, the Wagoneer. He'd told me in the past about taking them to the track with the testers, or even just going out into the parking lot and doing donuts, etc. So, I asked him which direction you steer a 4WD in when you go into a skid. His answer? "Play it by ear, and hope for the best." You have to figure out which half, the front or the rear, is causing the skid, and drive accordingly. Front half is skidding? Steer out of the skid. Rear half skidding? Steer into it.

Now, why bring this up? Because, with the large number of SUVs on the road today, it could be very handy info to have.

And, yeah, I agree, prolly best to move this thread to BABBling

An interesting story. If you skid a Jeep while in 4WD you have to play it by ear.... I have a Chevy Blazer and have never been able to put it into a skid while in 4WD.!??!?? :wink:

Heehee, you don't have to worry about skidding in a Blazer - ya just roll over, much like an Explorer.

[darkhunter]

RWD - steer in the direction of the skid, and let off the throttle maybe halfway (not totally). That allows the rear tires to slow down their rotation through the range where they will grab the road and start to follow the front end which you have pointed in the right direction.

FWD - steer in the direction of the skid, and accelerate a bit. The front wheels will pull the rear end back into line (if there's adequate traction and the fronts grab).

NOTE - in both these situations, the recovery is quick and can go the other direction. Those long smooth slides seen in race cars are different situations, in that the slides are deliberately being extended.

AWD/4WD (not the same thing - one's designed to keep some torque going to all four wheels, and one's designed to walk up the side of a building -- but generally and for purposes of this argument, we'll equate them) - This one's tough. There are elements of both here. Steer in the direction of the skid. Accelerate gently. The road forces acting on the front wheels are in line with the plane of tire rotation. The road forces acting on the rear wheels are at an angle to the plane of rotation, causing a frictional drag across the tread surface. Even with the acceleration, the forces on the rear end are higher than the front, and the front again will pull the rear into line.

I second (thrid, fourth?) the motion this be moved to BABBing..

For front wheel drives, if you get it backwards and react as if it were rear wheel drives, you will succeed in getting even less control--trust me: the first front wheel drive I ever had I tore up the front end by reacting if it wwas a rear wheel drive....(front tire blow uots will ruin your day)

I have managed to get my exploder to break loose in four-wheel-drive--but I had to find a parking lot with a thick coat of glare ice to do so... Never even come close to rolling it though.