I just got this in my in box, and I was wondering if it was true. I searched, but I couldn’t find anything, so I come to you, my fellow dopers…
"I learned a lesson I’d like to pass on to you. You may know this
already- but the highway patrolman told me that you should never drive in the rain with your cruise control on. He said if you did and hydroplaned (which I did) that when your tires were off the road your car would accelerate to a high rate of speed (which it did). You don’t have much, if any control when you hydroplane, but you are totally in the hands of God when the car accelerates. I took off like I was in an airplane. I’m so thankful I made it through that ordeal. Please pass the word around about not using cruise control when the pavement is wet or icy. The highway patrolman said this should be on the sun-visor with the warning about air-bags
Still I have to wonder, does this apply to all cars? Seems to me that the wheels wouldn’t speed up if the speedometer cable was attached to the drive wheels instead of the free ones.
It’s a well known email chain letter that is true, according to snopes and these snopes wanna-be’s. In fact your email appears in full on the 2nd link.
If your tires have no friction contact with the road (more or less the definition of hydroplaning), how is the car going to accelerate? It’s just going to keep sliding until the tires make contact again.
The car’s speedometer determines speed by the rate of wheel rotation. If cruise control remains active, it will maintain this rate. The car, once its tires regain contact, will recover its original speed.
Of course, most people his the brakes when they hydroplane (they shouldn’t, but they do), and braking deactivates cruise control. It’s a safety mechanism.
Nametag, it isn’t that the car accelerates, the tires speed up. This prevents them from getting good traction any time soon. Your other two questions seem kinda valid, though.
They do mention that not all cars will necessarily speed-up while under cruise control while hydroplaning but some will. Still, there were other reasons given why it is a bad idea anyway except on dry pavement.
I don’t doubt that it’s unwise to use cruise in poor traction situations. Without the cruise on, the driver can react very rapidly to wheelspin by lifting off the gas. With it on, it’s going to take longer to achieve that, by tapping the brake pedal or turning the cruise off. That extra moment could certainly make the difference between maintaining control and losing it.
That said, I don’t find this believable: …when your tires were off the road your car would accelerate to a high rate of speed…I took off like I was in an airplane.
A properly working cruise system is not going to get the car’s speed up to more than the speed that was set (give or take a couple mph). The above implies that the car (not the tires, the car) is really going to zoom away on you. And as Nametag mentioned, the system is actually sensing the speed of the drivetrain, not of the vehicle, which significantly limits how much it will try to accelerate without traction.
The point is a good one, but the story strikes me as fairy-tale-ish.
“2) The car’s speedometer determines speed by the rate of wheel rotation. If cruise control remains active, it will maintain this rate. The car, once its tires regain contact, will recover its original speed.”
I’m not an expert, but the cruise control could get its speed information from the speedo, which is normally hooked up to the front wheels.
So in a rear-wheel drive car, if all four wheels aquaplane the front wheels will slow down, the cruise control is fooled into thinking that the car itself is slowing down and gives the free-spinning rear wheels more gas. When traction is regained by the rear wheels, there might well be a moment of high acceleration.
FWIW, I had an '86 Cavalier when I was in HS that had cruise. One time I had cruise activated, and as I was going down a hill, I decided to see how far I could coast. My dumbass didn’t deactivate cruise, and the second I popped it into neutral (it was an automatic) the engine revved like crazy, even though my speed hadn’t changed at all.
So it seem feasible to me that if the tires lost traction, the could spin like mad, and launch you when they regained traction.
Then again, that was an '86. I’d hope they’d make improvments by now.
I’m not an expert, but the cruise control could get its speed information from the speedo, which is normally hooked up to the front wheels.
The only type car I’m aware of that ran a speedo cable from a front wheel was the (rear-drive) VW Beetle. In everything else I’ve seen, including all modern cars, the speedo uses a cable and/or sensor from the transmission output shaft, which is coupled to the drive wheels.
So, front-drive cars normally have the speedo hooked to the front wheels, but it’s because they’re the drive wheels, **NOT[b/] because they’re the front wheels.
Every cruise control system I’ve seen, factory or aftermarket, gets its speed signal from some element of the drivetrain. They do not directly sense vehicle speed, they accomplish their task by sensing (and maintaining) the speed of whatever drivetrain part they’re hooked to (typically a tranny output shaft, front drive axle, or driveshaft).
The point here is that once the wheels are rotating as fast as they normally would when going X mph, the cruise system stops accelerating. The logical effect of wheelspin would be that the cruise control is fooled into thinking the vehicle is going faster than it really is, so it backs off before the actual roadspeed is up to the setting.
The feedback in these systems is not instantaneous, so if there is no load on the engine, as with wheelspin, the engine can briefly rev up high before the system reacts and stops accelerating. But it’s not going to act like it’s continually floored.
Cruise control does take its input off the drive wheels in virtually all modern cars. That doesn’t mean it will act entirely in a safe way, though. Some systems have been known to floor it if you’re climbing a hill or under other circumstances, sometimes hard enough to get an automatic transmission to downshift into second at freeway speeds. This could break the tires loose on slick pavement, and once the tires are spinning freely, the cruise control will hold them at this speed. And they will continue to spin at the same speed until you turn the cruise control off or the car is going fast enough that the tires regain traction.
This won’t produce runaway acceleration, but it could momentarily get you to skid or hydroplane.
By my reading of the Snopes cite, it looks like the cruise control might increase the danger of driving in rain etc. but I doubt it will cause ‘instant’ acceleration. One, the wheels in a hydro-planing situation are per se not in contact with the gorund, two, most cars at highway speed don’t accelerate all that quickly anyway.
OK let’s take this apart one absurd sentence at a time.
(For the record I do qualify as an expert, as I teach this stuff for a car company)
OK I call bullshit. Hydroplaning is defined as when a tire has a thin layer of water between it and the road resulting in a zero traction condition. This is why BTW good tires are so important in the rain. Deep tread can channel away more water, and are therefore more resistant to hydroplaning. Anyway back to the quote. If the drive tires have NO traction (they are off the ground after all, being supported by a thin sheet of water) then how in the name of all things automotive can the car accelerate? The car has NO traction. No traction= no acceleration. Also for the record, when you are hydroplaning you have no steering and until your tires regain touch with Mother Earth no brakes. Now the tires can (and may) accelerate, but since the tires aren’t on the ground, Mr. Newton gets some time off.
Up until the word hydroplane the sentence is correct, and quite possibly understating the effects of hydroplaning. However after the comma I call double bullshit. As I have explained with no traction you cannot have acceleration.
Now a word about how cruise control units work. In most if not all modern cars the cruise gets a vehicle speed signal electrically from either the vehicle speed sensor or possibly the ABS system (after all ABS knows the speed of all the wheels) or possibly as a truput from the speedometer. I am unaware of any car that still uses a speedo cable, there may be one but I sure don’t know of it. The cruise unit uses this speed signal to regulate the throttle either open or close to maintain the set speed. If the tires were to lose traction, then any small movement of the throttle would result in a huge change in engine RPM, however no change in vehicle speed (No traction = no acceleration) But wait you say I can press down on the gas pedal and only cause a 100 RPM gain, why does the crusie casue the engine to race? Simple, a movement on the throttle so small that only causes a 100 RPM shift in neutral would not be noticed when the cruise is set. The cruise makes much coarser adjustments.
At least some of the cars on the road have safeguards built into the cruise unit. for the last 12 years the cars I work and teach on have the following safety disconnects
[list=1]
[li] Brake pedal depressed (the usual way to disengage cruise)[/li][li]acceleration of the wheels exceed a predetermined rate (this would disconnect the cruise if the car hydroplaned, or if the cruise control was set while a technician had the car on a rack with the wheels in the air. The designers know exactly how fast a car can accelerate, and any observed acceleration of the wheels above that rate would cause a disengagement.[/li][li] If the unit says Hi Opal! this will cause a disengagement[/li][li] the tires slow down beyond a predetermined rate (In case the brake pedal switch is dead, this is a backup.)[/li][li]Transmission shifted to neutral (or in the case of a stick, the clutch pedal depressed (this addresses what happened to Gilded Lily)[/li][li] actual speed is less than 75% of actual speed (hey if the cruise can’t keep the car near the set speed, they want you driving)[/li][li] abnormal speed signal (unit does not like the signal and therefore will refuse to operate)[/li][/list=1]
On the older cruise systems If a cruise control was inadvertently set while the car was on the rack with the wheels off the ground the engine will go to redline in nothing flat. This is due to there not being any drag on the driveline. A very small application of throttle will do it. This is the same that would happen if cruise were set and the car hydroplaned, the tire would start to slow down and the cruise (thinking the car was slowing) would open the throttle, causing the wheels to spin at very high speed. This could (and probably would) cause the driver to shit a brick. However it would not cause the car to accelerate, as the tires have no traction. Same thing would happen if the car were to be shifted into neutral, car slows down, CC opens throttle engine go to redline, gets driver’s attention.
So what happens when your tires regain traction wouldn’t that cause the car to take off like a shot? IMHO no. Let say that we jack up the back of a RWD car (could be FWD, but RWD is easier to explain) with a floor jack. Tires are off the ground. You get in and press the throttle gently until the car is registering say 80 MPH. You probably have the throttle pressed down somewhere between 1/4-1/2 of an inch to get the wheels turning at this speed. If I were to release the floor jack and drop the drive wheels back on the ground. The tires would probably chirp and impart a couple of miles per hour to the car and then the tires and the ground would be at zero slip and then the car will accelerate at the same rate it would with 1/4"-1/2" of throttle because, after all, you only have the throttle pushed down that far.
Now with all of that said I would agree that using cruise control in poor weather is probably not a Nobel prize winning idea. When the weather is truly crappy you need to be in touch and in control of your car. When things go stupid, they can go real stupid real quick. Driving with cruise control tends to keep you away from one aspect of the driving experience and could lead to an accident due to a fraction of a second of inattention.