Cruise control + rain = bad?

[WRXtranceformed]

Just got this email from a friend of mine:

NEVER KNEW THIS BEFORE.
> I wonder how many people know about this?
>
> 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 excessively, 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 lose 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.
>
> 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 driver's seat sunvisor - 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.

I've never really thought about that before, is it true? Traditionally I've rarely used cruise control, but lately since I commute a lot on 81 I set it pretty much every day. Haven't used it in the rain yet though... if this is the case I'll have to remember it before I do!

[Apoc]

Take off like an airplane? LOL!

Should you begin to hydroplane it's likely to exacerbate the situation by continuing to spin the wheels but it won't make you take flight.

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[WRXtranceformed]

Yeah, I thought "flying" was a little overexaggerated, but I could definitely see how accelerating could potentially make the situation worse.

[Feersty]

Never heard of this before. I suppose it is common sense not to set it because drivers are going to be going a variety of speeds in the rain and some people are going to be overly cautious. For this reason it is not wise to set cruise control in these driving conditions. Your foot or at least mine hovers over the brake in the rain as it is.

[HAULN-SS]

I dont see how this happens? Your car doesnt know how fast it's actually going...so why would it think you're slowing down when you hydroplane? The entire drivetrain would still be moving at the same speed - even if the car was lifted up and was hanging mid-air

[Eclipsor]

The cruise control uses the speed sensor (usually mounted on the tranny), so wouldn't wheelspin make the system think that the speed is above where it should be? The cruise control should close the throttle plate at that point.

I think this is entirely false and it really IS an urban legend. This has been passed around the Internet over and over since 2002. As we all know, hardly anyone in the general public knows anything about how a car works... let alone the cruise control system.

Is it good to use your cruise control in the rain? Of course not. I just don't think it will cause the car to accelerate unabated when the tires lose traction.

Here's a quotation from popularmechanics.com:

URBAN LEGEND
The legend has been related to us by all manner of people, including a couple of state police officers. Using the cruise control in rainy or slick conditions will make your car speed up uncontrollably, until you lose control and crash. It's a myth. Engaging the cruise will not make the car speed up. The cruise will attempt to maintain a steady speed. If the wheels lose traction and the car starts to slow down, the cruise will speed up the engine to attempt to accelerate back to the set speed. This will make the drivewheels spin more briskly. The vehicle speed will go slower, regardless of how furiously the wheels spin. If you have a rear-wheel-drive car, the effect is to destabilize the vehicle, and you probably will spin out unless the vehicle regains traction in a reasonable length of time. A front-wheel-drive car with spinning tires will attempt to continue in a more or less straight line--which will make steering control dodgy at best.

Heavy rain or snow makes use of the cruise control inadvisable, but such conditions certainly won't make your car accelerate to ludicrous speeds while you hang on for dear life. If the cruise is engaged and you perceive a loss of traction, the best bet is to tap the brake to disengage the system, and then add just enough throttle to maintain steering until the vehicle slows down to a more appropriate speed.

[.RJ]

I dont see how this happens? Your car doesnt know how fast it's actually going...

I suppose them newfangled speedometers are just for show, right?

[BLINGMW]

I don't know enough about how cruise works obviously. Maybe in some vacuum driven only system, the drivetrain would accelerate if the wheels lost grip? But in a modern system, where the computer knows your (wheel) speed, and keeps you there, if the wheels lost traction.... they'd just keep that same speed and the car would start to slow down. Take off like a rocket my ass.

I think if the road's so wet that you might hydroplane, who the hell would be using cruise control anyway? Only a person who doesn't understand how to drive, and thinks "it's in god's hands now", anytime they lose traction would get called out by this.

[Eclipsor]

I don't know enough about how cruise works obviously. Maybe in some vacuum driven only system, the drivetrain would accelerate if the wheels lost grip?

I believe that even the older systems still used a reading from the speed sensor to maintain a consistent pace. Also, I think modern systems still use vacuum to open the throttle plate. The vacuum is used purely for mechanical purposes, not for speed sensing purposes.

[.RJ]

The cruise just modulates the throttle opening based on feedback from the speedometer sensor. Its a really rudimentary system.

Say the driven wheels lose traction, and hydroplane. The ECU thinks the wheels are slowing down, so it adds more happy pedal, and makes the situation worse.

[Eclipsor]

No. When the wheels lose traction and start spinning, the computer would think the car is SPEEDING UP.

[BLINGMW]

yeah, that's all I knew cc to be, but I think what he's getting at is if the speed sensor was on the non-drive wheels, when it slowed down, ECU would floor it!

...are there any cars like that though?

[Eclipsor]

but I think what he's getting at is if the speed sensor was on the non-drive wheels

...are there any cars like that though?

Not that I know of. I believe it uses the same input as the speedo, which comes from the speedo sensor. That's most often mounted on the transmission.

[BLINGMW]

yeah, trans or diff... I can't imagine any reason why it wouldn't be on the drive wheels I guess. So yeah, BS on this. :lol:

[Eclipsor]

Even if you could find a car like that (one that used a speed reading from the non-drive wheels), though, it wouldn't actually accelerate to an insane speed. It may spin the tires some, but it would still hold at whatever speed the CC was set at.

So all this is a bunch of crap...

> But the highway patrolman told her that if the
> cruise control is on and your car begins to hydroplane --
> when your tires lose 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.
>
> 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.

Plus, you can tell from the language that's used. Supposedly the "car will accelerate to a higher rate of speed and you take off like an airplane." I mean, really... WTF is that?

[ViPER1313]

Total BS (at least for my car) - the cruise control is operated off the vehicle speed sensor (which is driven from the transmission as mentioned before.) If the wheels broke traction, the VSS would detect a speed much higher than the actual speed of the car and lessen throttle input in response to try and keep your set speed - not increase it. The ONLY way I could ever see this happening is if the cruise control based its speed off the rotational speed of the cars non drive wheels (rear if FWD, front if RWD) - this is not how cruise control works though.

[white_2kgt]

The cruise just modulates the throttle opening based on feedback from the speedometer sensor. Its a really rudimentary system.

Say the driven wheels lose traction, and hydroplane. The ECU thinks the wheels are slowing down, so it adds more happy pedal, and makes the situation worse.

Are you just playing dumb? Sometimes it is hard to tell. As it has been pointed the car really DOESN'T know how fast it is going. It only knows how fast the driveshaft is turning (which is why when you change your tire height or final drive ratio you have to recal your speedometer. In my case I changed trans ratio's and I had to recal the way my computer caculates speeds in various gears, pretty easy to do w/ my EEC software but still.

So in fact, it would be IMPOSSIBLE for the wheels to 'spin out of control' on a wet road because the cruise control would stop adding throttle at a certain speed and the vehicle would start to slow down.

[.RJ]

So in fact, it would be IMPOSSIBLE for the wheels to 'spin out of control' on a wet road

SO.... where did I say the cc is going to make the car spin out of control?

[HAULN-SS]

.RJ - what I meant was that the car doesnt know it's relative speed to anything - all it knows is how fast the tires are turning, and how far it's going (and only knows that because it's been calibrated for that particular wheel/tire combo and gear ratio), and also, i'm pretty sure hydro-planing doesnt actually make the "wheels" slow down

[Ryan T]

My cruise control doesn't even work, lucky me.