I just read that a woman had her car suddenly accelerate while driving in the rain with the cruise control on. I am under the impression that vehicle speed is monitored at the drive shaft, and do not understand how hydroplaning would cause the cruise control to apply more throttel? I wonder if this is not another of the bogus Audi “unintended acceleration” stories?

All owner’s manuals tell you NOT TO USE the cruise control in traffic, on snow and on rainy surfaces. It’s very easy to spin out on a slippery surface with the cruise on. I’m somewhat sceptical about sudden “acceleration” with the cruise on but the driving mechanism is not the air speed of the car.

The drive may differ for different makes, but the idea is to keep the car’s speed the same by measuring the speed of the driving wheels and adding throttle as needed. When going into a skid with the cruise on, the first impulse is to steer into the skid, but one should tap the brake first to disengage the cruise.

During such a panic, the driver may claim just about anything happened. I hit an icy patch on an uphill grade, and the car went into a skid with the cruise on. It’s a scary experience.

Yes, this bogus story has been going around on the Internet for some time.

It is not a good idea to use cruise control during inclement weather, but cars do not leap into the air and fly off the road.

Different cars get their speed readings from different sites. If a car’s speedometer is based on a sensor on a front wheel and that front wheel hydroplans then the speed of the wheel will go down, while the actual speed of the vehicle stays the same.

If the cruise control is “on” it will interpret the slowing wheel as a slowing car and apply more gas. If the car is rear wheel drive those wheels are in contact with the road and will have the traction to increase the speed of the car.

I guess in theory it could happen.

[i]Here’s Part Of A Theory That Supports This Story.[/i]

Remember my story about the hydroplaning Beetle? If not and you’re interested in my theory, read it again at this link and then come back: http://community.cartalk.com/posts/list/2119076.page

OK, You’re back. We know that the VW speedo runs off a front non-drive wheel. Beetles were rear-wheel drive. What would have happened if the Beetle was on cruise control and was front-wheel drive?

Let’s now switch to a modern front-drive car with a front-drive reading speedo.

Also, we know that if you are “stuck” in your car and are gassing it to get unstuck, the spinning wheel turns at twice the indicated speed. The speedo says 60 and the spinning wheel is going 120! This is dangerous. You can blow things up.

So what happens if you’re driving with the cruise control set and you hit water or slush starting one or two driven tires hydroplaning? Does a wheel stop or slow? Does this cause a wheel to accelerate? Does the speedometer indicate this acceleration? What happens when the car all of a sudden regains traction?
My theory is that some of this is going on, I’m just not exactly sure what, but I can see where things could happen to make the driver become homesick.

There are all kinds of discussions and warnings about cruise and rain on the web.


I believe this is what happens.
Let’s say that are driving along on cruise control and you hit a slippery spot which causes the wheels to lose traction. The load has been quickly taken off the engine, so it revs up out of control with lots of wheel spin until the cruise control can react and shut it down (or the driver reacts and steps on the brake).

The bottom line is that the car did not accelerate out of control, but it sure sounded as if it did.

A mechanical cruise accelerator is working against existing resistance and, the same as when it’s your foot on the pedal, At the point there is NO resistance…zzzzzzzzzoom. When it’s your foot, you let off the pedal. When it’s the cruise, it doesn’t know to let off.

This is absolutely true. Check it out on an urban legend site, like snopes.com.

If you lose speed due to water in the road, you know enough to not try to regain it right away. Cruise control doesn’t know why the car slowed down. Yes, you could lose control. Stories are told in strange ways by people who like to overstate things but they are still true to some degree.

I don’t know about hydroplaning but I had a RWD Olds with posi-traction and if you wanted a wild ride all you had to do was use the cruise on a slippery road.

PDV2, What’s Your Theory?

I think we may be on the same page. I can’ tell. You say, “Cruise control doesn’t know why the car slowed down.” How does the car know it slowed down? It gets its info from speed sensors in the drivetrain, not from a GPS. Seems like drivetrain speed would increase. Are you saying wheel(s) are slowing down or speeding up or some of each?

That Posi-Traction Must Have “Walked” The Whole Rear-End All Over The Place.

It may have thrown in some extra horsey power, too. Giddy-up!

Not bogus.

If one hits a puddle at speed the tire can encounter sudden increased resistance at the same time as sudden loss of traction. That can result in the wheel suddenly slowing. This sudden slowing is recognized as the cruise control system as a loss of speed (it does not know the tire has lost traction) and it activates the system to accelerate to try to regain speed. This can cause the wheel to spin and when it hits pavement again…zoom!

Horse hockey! BTW, the Snopes article says that you can lose control, not speed up. When you lose traction from hydroplaning, the driven wheels will still spin only at the set speed. The cruise control will NOT goose the throttle- the speed sensors in all modern cars measures the speed of the driven wheel(s) as far as I know. Therefore, the driven wheels continue to spin at the set speed. Since traction is lost, the vehicle actually slows down. What happens is a change in PERCEPTION: when you lose control of the vehicle, it FEELS like it’s speeding up. But it isn’t. No way, no how.

Just Like I Witnessed My Beetle Doing In The Mountains A Few Decades Ago, Eh?

The wheel must have stopped turning altogether.

I disagree. The sudden increase in resistance will cause the wheel to want to slow down. Combined with the sudden loss of traction it wil do so, even of the car stays the same speed.

I’m not talking about a wet road here, I’m talking about a puddle.

You haven’t noticed when you hit a deep puddle without cruise that your car wants to suddenly slow down? Haven’t you noticed that it takes more power to get the car through at speed?

Try it on a bike. Try hitting a deep puddle at speed. You’ll be a believer.

How About This:

Did You Read CSA’s Believe It Or Not? If not I’ll wait.

OK, Remember cars have differentials. Hold a wheel on one side and the other wheel will spin at twice the speed indicated. True story.

Now, what happens if the cruise is set on 60 mph and hits water or slush and one wheel stops turning like what happened in the CSA adventure. Will the other wheel go the equivalent of 120 mph (It’s still getting Horse Power deliveries!) because the cruise won’t disengage (It only disengages when you hit cancel, turn it off, or hit the brake pedal) just because the car hits water?

The 120 mph wheel is the one getting traction. Buckle in dudes. Here we go! (I swear my car has punched it before like this and even hit passing gear!)

I’m sipping my evening wine, but I think this theory is bulletproof. Before you say Horse Hockey (Give this some serious consideration), what say you?

In case the answer is “horse Hockey!” again, is there anybody out there buying into my theory? TSM? McP? Tester? FoDaddy? Mike In NH? Docknic? Unncle Turbo? JT Sanders? Bing? PDV2? MarkMast? Ken Green? Come on guys!

Played with a horse pucky…

Anyway, there are some major holes in your theory. First, if you hold one wheel, the other does NOT spin at twice the speed. And with an open differential, it’s the wheel with LESS traction that gets the power.

MB, your logic is also a little faulty also (sorry, old buddy!). Forward motion is impeded, but the wheel itself spins MORE easily due to a decrease in friction. If the friction decreases enough, the wheel skates right on top of the water (or nearly so), which is, of course, the definition of hydroplaning. The wheel breaks traction, and spins at the speed set on the cruise control.

Yes, the unstopped wheel does spin at twice the speed. Go back and study how a diffrential works. An open differential delivers equal torque to both wheels. Lift one wheel off the ground and it takes zero torque to spin it and the wheel on the ground also gets zero torque.

Let’s assume that people only use cruise control at 55 MPH ad faster; everything happens at high speed. If only one drive wheel breaks loose, the other will maintain proper speed. That means the cruise control system will not slow to allow the free wheel to re-attach itself to the road. Since the free wheel doesn’t propel the car forward anymore, the other drive wheel will turn the car towards the free wheel. At least until the free wheel re-attaches to the roadway and turns the car in the opposite direction. How fast does it turn? Who knows? When does it stop? Who knows? It’s a loss of control by a system that is not set up to accommodate it. The system is programmed to maintain speed, and nothing else. A person driving a car can react to a loss of traction by releasing the throttle and waiting for the free wheel to mate with the road. Cruise control can’t.

Howzat, CSA? BTW, you’re forgiven for misspelling my name. You bad doggy, you!