Cruise control is one of those 'magical' devices in the modern car that few people understand. It is the perfect candidate for anonymous emails that purport to present an assessment or conclusion by so-called experts.
In the case of cruise control, it's very easy to imagine a rogue computer taking over, speeding your car out of control and leaving you and your car in a smouldering mess. In reality, cruise control has been in everyday use for at least 40 years and existed on cars well before computers. It is one of the most simple, refined and reliable of car accessories and while modern computer power has smoothed out cruise control operation, the basic operating principles haven't changed over that period.
The following is typical of what is being circulated with lifesaving earnestness:
So is this assertion true or false? The final conclusion and advice might be sound advice but not for the reasons given. Aquaplaning (sometimes referred to 'hydroplaning') can happen if there is standing water whether cruise control is in operation or not. (Ed: as far as accelerating when traction is lost -- that's a furphy!) Indeed, far from being the absolute cause, cruise control is almost a red herring in the above scenario.
How does cruise control work?
It was common for cars prior to 1950 to feature a hand throttle. Instead of having to keep your foot on the accelerator for hours on end, you simply pulled out a knob to match the amount of accelerator you needed and lifted your foot. Serious four-wheel drives still have them so your foot doesn't bounce off the accelerator in rough territory.
Using a hand throttle at speed ultimately proved too dangerous as traffic increased. The car could run away downhill, would slow going up hills and would overcome the brakes if you forgot to disengage it. If you forgot that you were using it and pressed the clutch pedal, you could damage the engine as it over-revved.
The hand throttle was best forgotten on road cars and disappeared entirely. The Alfa Romeo GTV6 was the last modern car to offer a hand throttle but it was more often used by owners to allow the engine to run smoothly during warm-up.
Then someone had the bright idea of powering a hand throttle with an electric solenoid or vacuum-powered mechanism which was linked to the speedo. If the car slowed below the pre-set speed going up a hill or into the wind, the mechanism would automatically pull on the throttle cable to access extra power then it would stop as soon as the set speed was reached again. If the car sped up over the set speed, it would lift off pressure on the accelerator until the car returned to the pre-set speed. It was that simple.
Computer controls for today's accelerators make this process even more accurate, smoother and responsive.
It was then a case of attaching a safety switch to the brakes so that as soon as you hit the brake pedal, it would cancel the cruise control. On manual cars, a similar switch was fitted to the clutch pedal so if you tried to change gear while the cruise control was on, it would cancel before the engine could over-rev.
Who invented cruise control?
It is widely acknowledged that Ralph Teetor, a blind and talented US engineer, was prompted by a ride with his lawyer to develop the cruise control. The lawyer was a serious chatterbox who sped up and slowed down according to the conversation, a tendency that disturbed the blind Teetor enough for him to pursue a patent on the device as early as 1945.
Chrysler was the first to fit it to certain upper level models in 1958 followed by Cadillac's across the range fitment in 1960.
Can a cruise control speed up a car beyond the pre-set speed?
No, but basic versions can't apply the brakes if a downhill grade causes the car to speed up. If the engine is not strong enough to hold the car downhill, most cars will go over the set speed even if the cruise control has shut down the throttle completely.
In this case, the car speeds up because a lower gear has not been selected, not because of the cruise control. New grade control logic in the latest automatic transmissions are now reading this situation and changing down automatically so even this scenario will soon belong to the past. Even more advanced systems will actually apply the brakes to maintain a speed downhill.
Equally, if the engine doesn't have enough power to climb the hill at the pre-set speed, the cruise control can’t maintain the speed unless the transmission changes to a lower gear.
Neither of these scenarios fit the situation described in the above email.
What stops it from speeding up a car over the pre-set speed?
This is the key that makes a nonsense of the email. Modern cars take their speedo reading from the driveshaft or transmission. This means the cruise control bases its responses on the speed of the driven wheels, not the car itself. This is an important distinction and fail safe position.
If the driven wheels skid because they lose grip, the spinning wheels will cause the speedo to show a higher reading which will force the cruise control to release the throttle faster than most drivers. Regardless of whether the car itself slows down or speeds up, the cruise control will always reduce the throttle no matter what until the driven wheels slow down back to the pre-set speed.
If the wheels continue to slip under this scenario, this will always leave the car travelling more slowly relative to the road, not faster as described in the email.
The only exception to this would be if the speedo was driven by a road wheel not connected to the engine and transmission. The only car in recent history where this could be an issue was the old Volkswagen Beetle which had its speedo cable connected to a front wheel when the rear wheels delivered the power.
Thankfully, it didn’t have cruise control when in this case, the cruise control would increase power to a skidding rear wheel until the old Beetle's front wheel showed the correct speed. Whether the old Beetle had the power to break traction at highway speeds to create such a scenario is another issue!
What has aquaplaning got to do with it?
This is the key factor in the above email that has been lost in all the discussion about cruise control. Loss of control in adverse conditions can be lethal and probably accounts for more incidents because of the ignorance surrounding aquaplaning.
Modern cars, even small, lightweight models, have much wider tyres than ever before. Coupled with smoother roads that can hold an even film of water without high dry spots, the chances of water getting trapped between the tyre and road surface are higher.
It is similar to the difference between a stiletto heel and a wide flat heel on a shoe. Where a stiletto can punch through a film of water to contact the ground, a wide heel can slip. The tread pattern on a wider tyre assumes critical importance when a vehicle's weight alone is not enough to punch today's wider tyres through a film of water.
Tread patterns on modern tyres act as irrigation channels to sluice away the water under a tyre and allow the tyre tread to make contact with a dry road surface, no matter how wet the road. If these channels are not able to remove the water quickly enough, the tyre can actually loss contact with the road and skim across the film of water just like a skiffle board in the seaside shallows.
This creates the sensation of speeding up because you can no longer slow down, stop or steer. "Flying through the air" is not quite what happens but it certainly feels that way!
Braking and steering actions can be totally lost as the forces of nature dictate where you go next. Scared? You should be when in most cases it is caused by sloppy maintenance, not cruise control.
The mindset that doesn't replace tyres until they are bald is the real killer. Modern tyres have to be replaced long before the tread disappears. Drivers who drive with their tyres worn below the tread depth indicators are driving on tyres no longer capable of clearing the road of water. These drivers are setting themselves up for a serious aquaplaning crash.
Poor wheel alignment is a primary factor when it causes uneven tyre wear and robs the tyre of its grip in the main area where it contacts the road. Worn shock absorbers will also encourage aquaplaning if they cause the wheels to leave the ground after they hit road surface variation, and allow a film of water to collect between the tyre and road.
What has cruise control got to do with aquaplaning?
Not much, but it’s how most drivers react while the cruise control is on that will almost certainly decide whether a nasty aquaplaning situation turns into anything more than a scare.
Here are some examples you should consider...
So what's the bottom line?
The narrow margins applied in enforcing Australian speed limits dictate you should assess whether your use of cruise control is governed more by your need to protect your license than safe-driving practices. Some speed limits may be too high under certain weather conditions while in others, your direct communication with all the car’s controls may be more critical than sitting back with the cruise control on.
Consider some of the following issues and actions when you next engage cruise control...
Prepared from discussions with Holden Special Vehicles Powertrain Engineer, Sam Davis.