Cruise control and rain
 

At the risk of sounding like an idiot as well as redudant - how many of you did not know this? And who can explain for me what the WRX's awd system behavior is in a cruise control situation?

- - - -
Subject: This is something worth knowing about driving in the RAIN....

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 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 15 people and only one of them doesn't know About this, then it was all worth it. You might have saved a life.

When I bought my subie....they mentioned that to me. Pretty important anyway.

I'm gonna have to call B.S. on this one.

Holy physics-defying riddles, Batman!

If your drive wheels begin to hydroplane on standing water they have slightly over zero traction, so how are they going to accelerate you to a higher speed? Not to mention, "take off like an airplane"? Umm, no. As far as I know most cars don't have a set of fold-out wings, so they're not going to be departing the ground unless you drive it off a cliff. A cruise control usually takes its cue from the vehicle speed sensor (VSS); again, if your drive wheels hydroplane the drive wheels will start turning faster than they should be, which the VSS will read as an increased speed - I would think this would cause the cruise control to actually cut back the throttle to try to meet its target speed. I would guess what might happen is the system could be slower to respond to changing traction levels than you as the driver are, so if you hit a slick spot it might lag behind and keep applying constant throttle even as the tires are starting to spin and the vehicle is rotating into a spin.

That makes sense, but during the suabru introduction meeting, they did warn everyone not to do it. Maybe because of safty reasons or something. Dunno. :?:

Dammit Austin.... the database broke and lost my reply that was along the same lines.

For the record, I wouldn't want the cruise on in the wet just because I wouldn't want it to apply throttle with disregard to the current traction conditions... basically, I trust myself to see the puddle up ahead and coast more than I'd trust the cruise to just happen to be off the throttle at that point.

The only way I can think of the cruise speeding up in a puddle is in an AWD car... lets say the front hits a puddle and the tires lose traction and slow down... the speed sensor slows down and the cruise opens the throttle. The traction control sends the power to the rear tires that still have traction and the car accelerates. The only problem is that the speed sensor shouldn't read off just the front tires... it's usually in the transmission and reads it there.

I Drift with the cruise on! 8)




JK, I'm an idiot and almost never use my cruise.

I don't remember that from my meeting, what I do remember is the emphasis on the parking light switch on top of the steering wheel to avoid turning them on inadvertently and running the battery down -- and the special Subaru gasket that their svc dept uses when you take your vehicle to them for oil changes. :lol:

Ok, well interesting feedback from Austin and Scott. Thanks guys. I might do a search on nasioc to see what else I can turn up.

Actually I don't think even this scenario is very realistic, for a couple reasons. If the front drive wheels hit a puddle and lose traction, shouldn't they start spinning faster than the rears which still have traction? Assuming the front wheels are dominating the center diff. and driving the VSS, it would cause the cruise to reduce throttle to control the rising speed input.

Second, even if you assume the original scenario above is plausible, if you're going 65mph that translates out to 1144 inches/second. Take an average wheelbase of 100 inches, and that means it only takes 0.0874 seconds to travel the distance of the wheelbase at that speed. That's well within the response time of a computer, but I don't think the entire electro-mechanical system that constitues a cruise control is going to have a response time near that, let alone have time to accelerate the car any meaningful amount before the rear tires also hit the puddle, which leaves you with no tires with any traction, and no possible acceleration.

Whoa. I couldn't even imagined trying to figure that out. :shock:

:lol: I said I didn't think it made sense...

I was just trying to come up with a fractionally logical explaination, and the best I could do was to come up with something regarding traction control and partial loss of traction.

The biggest flaw is that the car wouldn't even see the speed sensor change due to a partial loss of traction since the speed sensor is in the tranny (I know, I broke mine once ;)) and it would read the speed of the transmission not an individual tire. So if even one tire was slower, the speed sensor would still have the speed of the fastest tire or some sort of average based on the way the differentials worked.

All that said, even if the story is bunk, there's still a reason not to use cruise in adverse conditions: you're not in control of the car!! If something happens, the car could be accelerating instead of coasting, and that's real bad news on slippery conditions.

If you hit a long puddle the car would slow down from the increased resistance especially if it's deep. The cruise control would certainly apply more gas to try to get the car to accelerate. Like you said the cruise doesn't react very fast. When you get out of the puddle, the gas is still down but all of sudden you don't have the resistance of the puddle. Off goes your car. The cop may have been a bit dramatic, but the scenario is possible.

JC
My cruise doesn't work. 8)

Well, the original "scenario" from the "highway patrolman" is stating that the car hydroplanes, instead of hitting increased resistance. Increased resistance implies that your tires are still in contact with the road, which means you still have traction. Hydroplaning means that the tires float up on top of the standing water, creating essentially zero resistance to your forward movement.

And even if you do plow through a puddle, meet increased resistance and the car slows down, the cruise will apply throttle to compensate, and once you're out of the puddle it will only take you back up to whatever speed you were at before hitting the puddle. Then you're not even in water anymore, the car has full traction again, and it certainly isn't going to "accelerate to a higher rate of speed and take off like an airplane." :lol: I don't know if the story is bogus or not, but I think the explanation certainly is.

Actually that's a pretty simple calc, it's mostly unit conversion.

Converting mi/hr to in/sec --> (65mi/hr)*(1 hr/3600sec)*(5280ft/mi)*(12in/ft) = 1144 in/sec

Since Speed = Distance / Time, Time = Distance / Speed --> (100in) / (1144in/sec) = 0.0874 seconds :wink:

Math > Me

Actually I always hated math... and I ended up doing years of calculus in engineering school. Go figure...

:lol: me too.... 'cept my "years of calculus" was for only 3 quarters worth of classes.

well, I would have to say it is posible to spin out in a puddle with cruise control on. when your tires loss traction the computer tells the car to slow down....problem being when it tries to slow down the rpm's go up generating more torque and losing traction make'in you feel like the car is going faster

but what do I know?

I like math when it uses the $$$ sign. :)

Interestingly this topic WAS on nasioc.
http://forums.nasioc.com/forums/show...hreadid=416899
The last poster put some links of the searching he'd done -- and I found the original email/letter above had been embellished from the supposed 'original' account.

http://urbanlegends.about.com/librar...isecontrol.htm
http://www.snopes.com/autos/techno/wetroad.asp
http://www.breakthechain.org/exclusives/cruise.html
-----
Long story short -- don't use cruise control in slippery conditions (+ the other cautions in the owner's manual -- not when driving up/down a steep grade, on winding roads or in heavy traffic).

I just dont use cruise unless I am on perfect roads cause of the problems with it. I know people that do it I always tell them not to.