Does using cruise control in the rain lead to accidents?

[LURCH]

Hey all, I got this e-mail forwarded to me, and I wondered if any of you had seen it yet. I'm scepticle as to the claims within, because it sounds like an urban legend. What do all of you think?

This was sent to me buy the man in the Bat Cave. [name deleted out of courtesy] We thought is
>needed passed through the battalion.
>
> >
> >Cruise Control
> >
> > A 36 year old female had an accident several weeks ago and totaled her
> > car.
> >
> > A resident of Kilgore, Texas, she was traveling between Gladewater &
> > Kilgore. It was raining, though not excessive, when her car suddenly began
>
> > to hydroplane and literally flew through the air.
> >
> >
> > She was not seriously injured but very stunned at the sudden occurrence!
> > When she explained to the highway patrolman what had happened he told
> > her something that every driver should know - NEVER DRIVE IN THE RAIN
> >
> > WITH YOUR CRUISE CONTROL ON. She had thought she was being cautious
> >
> > by setting the cruise control and maintaining a safe consistent speed in
>the rain.
> > But the highway patrolman told her that if the cruise control is on and
>your
> > car begins to hydroplane - when your tires loose contact with the
> > pavement - your car will accelerate to a higher rate of speed and you
> > take off like an airplane.. She told the patrolman that was exactly what
>had
> > occurred.
> >
> > We all know you have little or no control over a car when it begins to
> > hydroplane. You are at the mercy of the Good Lord. The highway patrol
> > estimated her car was actually traveling through the air at 10 to 15
> > miles per hour faster than the speed set on the cruise control.
> >
> > The patrolman said this warning should be listed, on the drivers seat
> > sun-visor - NEVER USE THE CRUISE CONTROL WHEN THE PAVEMENT
> >
> > IS WET OR ICY - along with the airbag warning.
> >
> > We tell our teenagers to set the cruise control and drive a safe speed -
> > but we don't tell them to use the cruise control only when the pavement
> > is dry.
> >
> > The only person the accident victim found, who knew this (besides the
> > patrolman), was a man who had had a similar accident, totaled his car,
> > and sustained severe injuries.
> >
> > If you send this to 5 people and only one of them doesn't know about
> > this, then it was all worth it. You might have saved a life.
> >
>
> _____

 

[Ivan Seeking]

First, I think we can safely assume that cars don't fly.

Next, I think we can safely say that driving with the cruise control on during bad weather is a bad idea.

How should the car react? What strikes me is that if you hit enough water to hydroplane, the car slows down from the extra drag of moving so much water. This might cause the CC sense the loss of road speed and thus attempt to increase the speed. Now that we have further broken traction and we are really hydroplaning, it still would not see a response so it tries to increase the speed more...

I would think this depends a lot on the car; front or rear drive, automatic or manual, where is the road speed sensed. I can imagine scenarios where this should not happen; depending on the configuration.


Edit: So, I think that what may happen is that the CC adds to the hydroplaning problem. Such a complete loss of control occurs that the driver thinks the wheels have actually left the road...and in effect they have. But the car rides on water not air.

 

[LURCH]

Those are some good points, Ivan. I'm pretty sure the part about the car "flying" might just be a reference to the car going off the road. Most roads are slightly raised above the neighboring ground for drainage purposes. Going off the site of the road at high speed, a motorist could find themselves suddenly airborne for a brief preiod.

But the part that mainly skeptical in the first place was the idea of the cruise control speeding up when the tires lose traction. AFAIK, all cruise controls are governed by the speedometer, which knows only one thing; how fast the tires are spinning. When the tires lose traction and begin to spinning freely, the speedometer will register this as a sudden acceleration, which should cause the cruise control to back off of the throttle.

To counter arguments almost immediately occurred to me. The first has to do with the brief period of time between the moment the tires lost traction and began spinning freely, and the moment the cruise control reacted to that by letting off the gas. During this short time, the engine would rev up and the speedometer would show up a sudden acceleration. Perhaps drivers have observed this and claimed that the cruise control suddenly increased the throttle.

The second possibility is the one that would actually lend some credence to the officer's warning. Using the road as our inertial frame of reference, a tire that turns "sideways" but continues to travel "forward" will encounter the road surface moving diagonally across its tread. But, from the tires frame of reference, "foreword" is the motion that causes pavement to move from the leading-edge of the tire for the trailing edge. Because conservation of momentum means that the pavement is still moving past the tire at the same total velocity, and some of that velocity has been diverted to "sideways" motion (from the tires frame of reference), the movement of the pavement from front to rear is decreased. If the speedometer registers this as deceleration, then the cruise control will indeed increased throttle.

This, of course, would still mean that the idea of the car leaving the road at "10 to 15 mph faster than the speed set on the cruise control" is untrue. Probably just a sensation of panic due to the loss of control.

 

[Ivan Seeking]

It depends I think on whether we are talking about water or ice. I really only spoke to the case of water. In this case, I see the car loosing speed if it hits enough water to hydroplane. Once we start to hydroplane, esp with an automatic transmission, I would expect the wheels to turn less quickly. I can then see the CC reacting to increase speed. This could help to further decrease control of the car. Since CCs responds rather slowly, it may require too much time to recover to be helpful.

On ice, I can only guess that any response from the CC could be bad. Anyone who drives on ice knows how critical throttle control can be.

I think your idea about the two vectors makes good sense...I don't see how else to account for 10-15 mph.

 

[kat]

http://www.snopes.com/autos/techno/wetroad.asp

 

[Doc]

After working with aftermarket cruise controls for a number of years I can say that it IS possible for the car to lurch forward faster than the speed the cruise was set at.

1st) The cruise control measures the speed via the drive train. It senses it from WHEEL SPEED.

2nd) The cruise is set up in such a way so it adjusts the throttle based on the speed TAKING INTO CONSIDERATION THAT THE WHEELS ARE ON THE GROUND. In other words, jack the driving wheels up into the air and try to set the cruise. It WILL surge.

3rd) If the driving wheels come off of the road for a split second then the wheel speed will increase quite a lot because the engine is 'unloaded' so to speak. Now in a perfect world, the cruise would react to this instantly and keep the speed the same. In the real world it doesn't happen this way. See item #2.

4th) A split second after the wheels leave the road they come back down again to pavement which brings the traction back up and 'loads' the engine. Keep in mind that the wheels are at this instant in time doing quite a few MPH faster than actual road speed.

I imagine what happened is similar to doing a 'neutral drop', if anyone here knows what that is.

Was she actually flying through the air? Not a chance. Did she have a wild ride? Quite possible.

 

[Zantra]

ahhh the neutral drop. Brings back childhood memories of frying trans in junkermobiles.. hehe

 

[LURCH]

Originally posted by kat
http://www.snopes.com/autos/techno/wetroad.asp

Thanks Kat, that link was helpfull.

 

[Mr. Robin Parsons]

Priginally posted by Doc
3rd) If the driving wheels come off of the road for a split second then the wheel speed will increase quite a lot because the engine is 'unloaded' so to speak. Now in a perfect world, the cruise would react to this instantly and keep the speed the same. In the real world it doesn't happen this way. See item #2.

Uhmm, when the wheels start to hydroplane, they spin faster, causing the CC to adjust down, which can, and will, lock the wheels, (or cause skidding, as they have little friction) which will result in the CC flooring the engine to get the car back up to speed, resulting in, probably, side spin of the car as the rear wheels spin faster then the car is actually going. (and if they catch acceleration!)

They work on a combination of wheel speed and engine vacuum, the engine vacuum being what is used to sense engine speed.

On ice, your rear wheels might very well end up trying to drive the car faster, then the car, is traveling due to slippage of the wheels.

Used to drive a salt/snowplow truck, it had CC, worked really well!
(but that truck weighed in at 8 metric tons, empty, has eight driving wheels, and is usually loaded with about 10-12 metric tons of salt. way better contact patch then your average car)

 

[megashawn]

I have nothing to offer on the debunking of this, I think that's been done 3 or 4 times already.

We all know you have little or no control over a car when it begins to hydroplane. You are at the mercy of the Good Lord.

It is due to statements like this that I assert that all drivers should attend a credible driving school before getting there license.

Hydroplaning can be nasty, but is controllable to a person who knows how to drift and what not.

Hydroplaning is a result of the gaps in your tires filling with water, thereby creating a pillow of water between your tire and the road. Same thing happens in ice. Your treads fill up with ice, and you no longer have treads, but something more comparable to a drag slick.

The best advice that can be given for avoid hydroplaning is get good tires. I had 15" wheels on my Kia when I first got it. It was horible for breaking traction in rain/ice.

I put some 17" Katana with Ventus tires and my car does not hydroplane, I don't care if its 1/2 inch of water of 1/2 a foot. I can't wait for the snow to see how much better they do. And anyone keeping up with the weather knows I've been swimming here in NC lately.

 

[Mr. Robin Parsons]

Originally posted by megashawn
(SNIP) Same thing happens in ice. Your treads fill up with ice, and you no longer have treads, but something more comparable to a drag slick. (SNoP)

Slippery YUP, but the action of 'tire on ice' is somewhat similar to ice skates, and ice, pressurization/friction/heat and (melt)water.
If you skid a tire on ice, or spin your wheels trying to go forward on ice, your tire polishs the ice surface even smoother, and creates a very thin layer of water, on the surface of the ice, making it very slippery.

Have you ever gone sideways, just because of the can't of a banked corner, (~5-8°) because it was coated in a sheet of solid ice, was being rained upon, and your 22 metric tonne truck (48,400 lbs) just would'nt stay stopped? I have, you should have seen the look on the peoples faces as I slid towards their car, them in the far left, me sliding in from the emergency lane, on the far right. Musta finally stopped about ten feet short of hitting their car.

 

[Doc]

Originally posted by Mr. Robin Parsons
Uhmm, when the wheels start to hydroplane, they spin faster, causing the CC to adjust down, which can, and will, lock the wheels, (or cause skidding, as they have little friction) which will result in the CC flooring the engine to get the car back up to speed, resulting in, probably, side spin of the car as the rear wheels spin faster then the car is actually going. (and if they catch acceleration!)

They work on a combination of wheel speed and engine vacuum, the engine vacuum being what is used to sense engine speed.

On ice, your rear wheels might very well end up trying to drive the car faster, then the car, is traveling due to slippage of the wheels.

Used to drive a salt/snowplow truck, it had CC, worked really well!
(but that truck weighed in at 8 metric tons, empty, has eight driving wheels, and is usually loaded with about 10-12 metric tons of salt. way better contact patch then your average car)

One of the main points of my thread was to point out that the cruise does not react instantly as you describe. The cruise WILL try to compensate by lowering the speed but by the time it does that the wheels could easily be back on the road spinning 20 or more MPH faster than previously.

I have NEVER heard of a cruise control that applies brakes (reference to wheels locking in your post) to control the speed. The only thing they use engine vacuum for is to control a servo which pulls the throttle cable. Newer ones don't even do that, they are actuated by an electric motor. Incidentaly, engine vacuum is a very poor way to sense engine speed. If the cruise control uses the vacuum in ANY other way than moving the throttle cable, it may use it to sense engine load. Engine load is directly proportional to engine vacuum. A high reving engine can have little to no vacuum with a heavy load on it. But it can have high vacuum if there is no load on it. The same way at low rpm, an engine can have high vacuum with no load (idle) or little to no vacuum if it is heavily loaded (extremely) and bogged down.

How was the salt/snowplow truck cruise control set up if I may ask and if you remember? What type engine also?

 

[Mr. Robin Parsons]

Originally posted by Doc
One of the main points of my thread was to point out that the cruise does not react instantly as you describe. The cruise WILL try to compensate by lowering the speed but by the time it does that the wheels could easily be back on the road spinning 20 or more MPH faster than previously.

I have NEVER heard of a cruise control that applies brakes (reference to wheels locking in your post) to control the speed. The only thing they use engine vacuum for is to control a servo which pulls the throttle cable. Newer ones don't even do that, they are actuated by an electric motor. Incidentaly, engine vacuum is a very poor way to sense engine speed. If the cruise control uses the vacuum in ANY other way than moving the throttle cable, it may use it to sense engine load. Engine load is directly proportional to engine vacuum. A high reving engine can have little to no vacuum with a heavy load on it. But it can have high vacuum if there is no load on it. The same way at low rpm, an engine can have high vacuum with no load (idle) or little to no vacuum if it is heavily loaded (extremely) and bogged down.

How was the salt/snowplow truck cruise control set up if I may ask and if you remember? What type engine also?

The vacuum is used as you state, but is adjudicated by the speedometer, that is how/why they speed up going uphill, and slow going downhill.

When the wheels hydroplane there is no resistant to them, when resistance had previously been there, hence they spin faster, CC can compensate, but as the wheels have little or no resistance, when the engine speed is backed off, the wheels can pretty much lock up in comarison to the cars actual forward velocity/speed.

One time, when I was transiting on my motorcycle, I went to pass a car by changing lanes, I ran into one of the small depressive 'wheel ruts' that occur on some highways, due to there traffic volumes, the rut was water filled, my bikes engine started screaming, with speed, as the back wheel went into an accelerating spin because, the ratio of power to resistance had changed.

It was an extremely dangerous situation becuase, as I very well knew, if I backed off of the throttle to quickly, the back wheel would lock up, because of the decelerative forces of the engine, not balancing properly, due to the LACK of friction available from the road surface, simple words, had I dropped the throttle down quick, the bike would have "slid out", down onto the pavement, real quick.

Thankfully, as I had by then several years of Motorcycle driving experiance, I backed off ot the throttle very slowly, and finally got the wheel to the right speed as to re-engage the asphalt without incident. Scary!

As for the truck(s), One of them was an 98 Ford LouisVille with a 475 hp Cat engine and electronic CC, some of the others simply had locking throttles on them, Macks 350 hp, 275 hp and several other kinds, waaaay further back in my life.

 

[Mr. Robin Parsons]

Originally posted by Mr. Robin Parsons
Have you ever gone sideways, just because of the can't of a banked corner, (~5-8°) because it was coated in a sheet of solid ice, was being rained upon, and your 22 metric tonne truck (48,400 lbs) just would'nt stay stopped? I have, you should have seen the look on the peoples faces as I slid towards their car, them in the far left, me sliding in from the emergency lane, on the far right. Musta finally stopped about ten feet short of hitting their car.

Gotta fix this, it's a 20 metric tonne truck, 44,000 lbs roughly, and YA it makes a difference, it's close to two SUV's.