Using Cruise Control - Safety

[ralphsulser]

This sounds reasonable, anyone else heard about this?
SUBJECT: EVERY ONE SHOULD KNOW 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 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 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. We all know you have little or no control over a car when it begins to hydroplane.

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. Th e 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.

It may be a preventive action to pass this one if someone hasn't heard abput this. You might have saved a life.

 

[Tim Folkerts]

I don't know about the speeding up, but I had a similar problem a year ago on a road with icy patches:mg:. Cruiese control can produce a dangerous positive feedback loop. If the wheels start to slip, you lose power and the car slows down, which makes the cruise control kick up the RPM, which makes the wheels slip more! Spinning wheels on ice makes the car want to fishtail.

The other problem is that to disengage the cruise control, what do you do? Tap the brakes. But applying the brakes also tends to make the wheels slip, which means a loss of control.

I was lucky. I just slipped into the ditch on the right side (at 45-50 mph!) coasted up the bank on the other side of the ditch which steered the car back toward the road. The car slowed as it erapproached the road, so I gunned the engine and got back on the road.

I haven't used the cruise control on slippery roads since.

Tim F

 

[Steve Prevette]

I don't know about the speeding up, but I had a similar problem a year ago on a road with icy patches:mg:. Cruiese control can produce a dangerous positive feedback loop. If the wheels start to slip, you lose power and the car slows down, which makes the cruise control kick up the RPM, which makes the wheels slip more! Spinning wheels on ice makes the car want to fishtail. Tim F

Something doesn't seem to make sense here - how does your car know how fast it is going? From the RPM on the tires, I assume (though what if the drive tires are going at different RPM's might be a question). How would your car "know" it was slowing down due to wheel slip?

 

[Tim Folkerts]

Something doesn't seem to make sense here - how does your car know how fast it is going? From the RPM on the tires, I assume (though what if the drive tires are going at different RPM's might be a question). How would your car "know" it was slowing down due to wheel slip?

Good point - I hadn't thought about that! Perhaps the sensor is not in the drive wheels. Perhaps the cruise was simply trying to maintain the original speed in the the drive tires even though the car was actually going slower. Perhaps the cruise just didn't react quickly enough and "kept its foot on the petal" well after it started slipping. In any case, the tires did certainly start to slip!

Tim F

 

[Justin]

Something doesn't seem to make sense here - how does your car know how fast it is going? From the RPM on the tires, I assume (though what if the drive tires are going at different RPM's might be a question). How would your car "know" it was slowing down due to wheel slip?

Let me help you clear this up.

The brain of a cruise control system is a small computer that is normally found under the hood or behind the dashboard. There are many inputs to this brain. The most important input is the speed signal; the cruise control system does a lot with this signal.

So, YES, your car does know how fast it is going.

And the reason you will hydroplane on a slippery surface is because:

If the car starts to slow down, the cruise control can see this acceleration (slowing down and speeding up are both acceleration) before the speed can actually change much, and respond by increasing the throttle position.

As far as your car is concerned, when you start to hydroplane your car thinks it has stopped moving so gives it throttle. Yee Haw !

 

[Steve Prevette]

Interesting. Thanks, I didn't know that cars had accelerometers but it does make sense now.

 

[Laura M]

Traction control

This doesn't happen with traction control though, right? I think TC shuts off cruise. I'm not a big user of cruise control anyway - just think it makes me pay less attention to driving.

 

[somerqc]

I almost experienced this same thing except my car has traction control. If the traction control engages, it automatically disengages the cruise control.

In my case, I drove over a bump that caused one of the tires to lose contact with the road. This caused the traction control to briefly trigger thus turning off the cruise. Thankfully, nobody was too close behind me as I did slow down about 10km/h (or ~6mph for those imperial measure users).

I will definitely keep this in mind now that winter is starting to show up here (Toronto area).

Keep safe all.

 

[David Hartman]

Actually the cruise control does not "increase" the wheel or vehicle speed in these instances, it just does its job of attempting to "maintain" the selected speed. When on a high-friction surface (pavement) your cruise control senses through drive wheel sensors the rpm of the drive wheel(s) and adjusts throttle to maintain the "selected" rpm. When moving to a reduced-friction surface the cruise attempts to "maintain" that selected rpm which results in slippage (you can actually look at your speedometer and see the wheel speed being maintained at your select point - although due to the slippage the vehicle itself is slowing down).

As a kid (just last year in-fact), I used to mash the throttle to the floor on take-off while on ice just to see how far I could get the speedo swing (I have seen 90 mph on ice while only moving forward about 2 feet - same principle as witnessed with the cruise on).

 

[Jim Wynne]

As a kid (just last year in-fact), I used to mash the throttle to the floor on take-off while on ice just to see how far I could get the speedo swing (I have seen 90 mph on ice while only moving forward about 2 feet - same principle as witnessed with the cruise on).

The same principle as in a car on a dynomometer. While the best advice (don't use CC when the road is slippery) has already been given, keep in mind that tapping the brake, accelerator or clutch pedals will disengage CC. The latter is the best idea when it's available.