BLOG: Cruise-control hoax returns

Oh, dear, here we go again! Like most internet-driven hoaxes, this one began several years ago, faded away, then returned. Already I’ve seen the story popping up on Facebook, circulated by email, and even appear in a car club magazine. The trouble is, people read it and, with all good intentions, pass it on to everyone they know; who in turn pass it on to others, and away it goes. Soon there are a lot of people believing the hoax.

There have been various versions of the story – the original was set in America, then it moved to Australia, with the latest version said to have happened to a Wollongong resident – but the basics of the story are the same.

A woman was driving in the rain, and had the cruise-control turned on. Suddenly the car began to aquaplane and then it, “flew through the air” and crashed. The attending highway patrol officer told the woman that she should never drive in the rain with cruise control on because, “If your car begins to aquaplane, your car will accelerate to a higher rate of speed and you will take off like an aeroplane.” 

When I first read the original story about seven years ago, I shook my head at the lack of logic. But people were taking it seriously and it soon spread. I remember being amazed that no-one seemed to be picking up on how silly this was! Eventually the NRMA published an item in its Open Road magazine denouncing the warning as a hoax. The story is rubbish! Anyone with even half an understanding of how cruise-control works would realise that this is nonsense. Let me explain.

The principle of cruise control, as we all know, is to have the car maintain a constant speed. If the car slows, it will increase the throttle to speed it up. If the car speeds up it will close the throttle to slow it down. It does this by monitoring the output from the gearbox; or in simple terms, how fast the driving-wheels are turning.

Now, let’s look at what happens when a car aquaplanes. Aquaplaning happens when the tyre can’t remove the water from the road and a film of water forms between the tyre and the road surface. The tyre loses contact with the road and so loses traction. 

When this happens to the driving wheels, the tyres are able to spin much more freely than they would if they were gripping the road and pulling the car along, so the wheels spin more quickly. (Of course the car itself doesn’t speed up, because the wheels are just spinning on the water.) 

Now, when the wheels turn faster the cruise control interprets this as the car going faster; (the faster a car goes, the faster the wheels turn, right?). So what does the cruise control do when it detects the car going faster? It releases the throttle to slow it down. So having the cruise control on would, if anything, tend to make the car slow down when it aquaplanes; it definitely would not make it speed up! The story is nonsense!

However, despite the nonsense of the scenario described, there is some good advice here: it is better not to drive with the cruise control on when it’s raining. That much is true; just not for the reasons stated.

One reason is that if something starts to go wrong while driving – such as the car beginning to lose control or skid – many people’s first reaction is to hit the brakes. This is even more so when cruise-control is engaged because most people know that it can be disengaged by braking. And if a car aquaplanes, the one thing you should NOT do is hit the brakes!

Another reason involves cars that are rear-wheel-drive. If the front wheels aquaplane, the car loses steering. What you should do in this instance is to gently lift off the throttle, allowing the car to gently slow and the tyres to (hopefully!) regain traction. However if cruise control is engaged it just keeps the throttle applied, driving the car along at the same speed. Not only that, but if the driver realises this is happening, hitting the brakes to disengage it can make the situation worse. 

(ABS on modern cars would tend to negate the problem of braking, but on older cars it would be an issue).

So the recommendation to not drive with cruise control on when it’s raining is generally a good one: but the story is rubbish!

For further information here are a couple of links:

An article on the original story: http://www.raa.com.au/media_news.asp?TerID=83&RefID=363

A very detailed article on the story, cruise control and aquaplaning: http://www.carpoint.com.au/advice/2006/cruise-control-and-aquaplaning-the-facts-7413

Elwyn Jordan is a musician and full-time music teacher. Besides music, his interests include technology and motoring. He runs a motorcycling website called The Old Bloke. http://theoldbloke.homestead.com. You are welcome to get in touch with Elwyn at elwyn.jordan@yahoo.com.au

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