Column Shift

Jimmy H. asks: Can you give me a basic ex­planation of how cruise control works and why sometimes it seems like there’s a little “jump” in the speed when it is set?

Matt F asks: How does the new collision-preven­tion cruise control work?

More than one reader has asked about cruise control. Similarly, some of the readers have asked what cool or new features they might be able to get on a new car purchase. Let’s combine these thoughts and look at all things cruise-control re­lated. Let’s also consider what we really want cars to do for us. Cruise control is a form of speed control for machines. You might think that this is a simple thing, but machines pre-dated useful electronics, so controlling the speed of a machine had to be mechanical. Imagine an engine that is spinning a generator to make electricity. If the engine were large the danger from its running too fast and then starting to break apart could be quite real. Rotating parts have a lot of energy that can trans­late

into force if they break off and hit a person or something expensive, so controlling a machine from running too fast was an early challenge. One way to control the speed of a machine used in early days was what is basically called a centrif­ugal clutch. Metal balls connected to springs were used to control the rotational speed of equipment. As speed increased, the balls would move away from the centerline. They were connected to a de­vice

that would choke the engine if they moved too far away from the center. They would then move back toward the centerline a bit and the engine’s speed could be mechanically controlled in this fashion. “Balls out” and similar expressions start­ed from this type of equipment. Modern car designs use sensors that compare the speed the driver selects and the speed as mea­sured by the speedometer system. If more throttle is required to maintain the speed, it is automati­cally provided. If less throttle is required to maintain the speed — say, as you head downhill — the throttle is reduced or cut. Generally, the brakes are not applied, but read on for some exceptions.

Cruise control in cars was invented in the 1940s by Ralph Teetor. Despite the challenge of being blinded when he was young, he earned a mechanical engineering degree and then became an inventor working on steam turbines and other equipment. Eventually he found his way to the auto industry.

Although cruise control was invented in the 1940s, it did not become available as an option on cars until the late 1950s and was not widely avail­able until the 1960s. Today it is available on almost all cars, and is standard on many of the most popular models. I have been in very inexpensive rental cars that had cruise control, but had manual seats covered in cloth. So it has become an option not really asso­ciated with luxury. Mainstream cruise control has two main uses. The first is to make highway cruising a bit easier on the driver. The second is to save fuel. It is gen­erally accepted that using cruise control will save fuel for the average person driving in a normal fashion. I only mention this because more and more people are considering extreme measures to save fuel. Many of these involve expensive equipment added to a basic car. Using cruise control is one way to cost effectively save fuel in real-life driv­ing. Most readers already know how the system works. You first enable the cruise control by hit­ting

an “on” button. Then, when you are at the speed you want to stay at, you hit a “set” button which will allow the cruise contol to take over the control of the speed. Sometimes there is a slight nudge as it takes over. This has to do with how quickly you take your foot off the gas and it is dif­ferent from model to model. I suspect some car makers actually program in a slight nudge to alert you that cruise control is now engaged. Once set, you can gently increase the speed or decrease the speed with up and down buttons usually found on the right side of the steering wheel. This is useful for long trips as you adjust for changes in speed limit or to allow space for other cars also on cruise control. Most cars will actually shift an automatic transmission to keep up the set speed on steep hills.

That is sort of fun if you haven’t tried it. Try it on the next steep incline you come across on the highway. It is safe, but the car making all those adjustments on its own is a little weird and unset­tling. Hitting the brake, or clutch if there is one, shuts off the system. Using the gas does not. Most systems allow you to hit the gas to pass and then when you lift your foot off the accelerator the sys­tem lets the car go back down in speed to the set point. You may have heard about adaptive cruise control, or if you have an expensive car, you might have it. This is a variation on the theme that in­cludes

some form of radar or other sensing that will allow you to follow a car at a set distance in front of you as it speeds up and slows down. Your cruise control “adapts” to the situation and allows you to keep a good spacing in traffic without hav­ing to constantly readjust the speed setting. The best use of this is on long stretches, like the Mass. Pike west of Route 495, for example. There, regular cruise control can be frustrating as you constantly creep up on the same car for mile after mile and then they return the favor if you pass. I have found that if you do this long enough you will findyourself next to that person in line at the next rest area buying coffee. You might as well introduce yourself! Some new cruise control systems will actually slow the car to the point that it will stop if the car in front stops. That is a major departure from old­er systems, which never used the brakes to control speed. They would only watch the set point and either supply more throttle to speed up, or cut the throttle to allow the car to slow down from wind resistance and rolling resistance. A variation on this is even newer systems that watch what is ahead of you all of the time, not just when cruise control is setting the pace. These sys­tems will go so far as to apply the brakes firmly and stop the car if warranted. I think this new accident prevention system is a little too much. There is no harm in the system being able to do this, other than added cost. However, I think if you actually use it the car should email you a polite note asking if it is time to possibly consider the bus next time. After all, if the car had not stopped itself, what would have happened?

Please send me a question at

john@columnshift.com.