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What Is Power Steering and How Does It Work?

Car and Driver logo Car and Driver 4 days ago Dave VanderWerp
a man driving a car: It's one of the automotive world's best labor-saving devices, and it's evolved into a key high-tech component. © Getty Images It's one of the automotive world's best labor-saving devices, and it's evolved into a key high-tech component.

At its most basic, power steering is a system that reduces the effort required of the driver to turn the steering wheel. Without power assist, the steering of most vehicles would be extremely heavy, particularly during low-speed maneuvers such as pulling into a parking spot, turning a 90-degree corner in the city, or maneuvering in a crowded gas station.

a car parked on a city street: What Is Power Steering and How Does It Work? © Marc Urbano - Car and Driver What Is Power Steering and How Does It Work?

The first power-steering system fitted to a production car debuted in the 1951 Chrysler Imperial, and the competition quickly followed suit. Not only did power steering do the obvious-allow the driver to steer a heavy vehicle with much less effort and greater comfort-but it also allowed engineers to improve steering response, which is how quickly the car changes direction when the driver turns the wheel.

a car parked on the side of a road: 1935 Chrysler Airflow © Bonhams - Car and Driver 1935 Chrysler Airflow

Before power assist became available, cars' steering systems had been geared so that it took many turns of the wheel to negotiate tight turns or to park. This slow gearing gave drivers more leverage against the high effort required steer the front wheels. But the advent of power steering allowed engineers to quicken the steering ratio-how much the steering wheel has to be turned relative to how much the angle of the front wheels changes-because the additional steering effort could now be offset by the new system. In fact, it was more than just offset; steering a car became nearly effortless.

a car parked on the side of a road: 1994 Acura NSX © David Dewhurst - Car and Driver 1994 Acura NSX

However, some of the best-steering vehicles-purebred, lightweight sports cars-have had no power-steering assist, such as the Acura NSX from the early 1990s, the Lotus Elise and Exige, and the Alfa Romeo 4C, which is the last remaining new car that forgoes power steering.

a car parked on the side of a road: 2020 Alfa Romeo 4C © Alfa Romeo - Car and Driver 2020 Alfa Romeo 4C

But these cars got away without it due to their light weight and relatively narrow tires. And, still, it could be quite a chore to turn the wheel in these cars while stopped.

Hydraulic Power Steering

a close up of a gun: What Is Power Steering and How Does It Work? © Roy Ritchie - Car and Driver What Is Power Steering and How Does It Work?

The prevailing type of power steering from the 1950s to the early 2000s was hydraulic assist. Hydraulic power steering uses, as its name indicates, hydraulic fluid that's pressurized by a pump driven off of the engine. Although it served the automotive world well for those 50 years, there are a few downsides to this type of system: there's wasted energy, since the pump is running continuously, even when the vehicle is driving straight and there's no assist needed. Plus, the hydraulic fluid needs to be replaced periodically, and if any of the hydraulic lines spring a leak, it not only makes a mess, but the power assist is lost. However, it's still possible to steer a car without the power steering working.

Electric Power Steering

a vintage photo of a machine: What Is Power Steering and How Does It Work? © Roy Ritchie - Car and Driver What Is Power Steering and How Does It Work?

Electric power steering (EPS) is the norm on today's new cars. There's still a solid metal steering shaft running from the steering wheel to the steering rack, which steers the tires, but the rest is high-tech. EPS uses an electric motor that draws energy from the vehicle's electrical system to provide the steering assistance. This electric motor can be located either directly on the steering rack-this arrangement is more expensive and tends to be used on the sports- and luxury-car end of the spectrum-or mounted to the steering column. Sensors detect the torque, or effort, that the driver is applying at the steering wheel, and a computer decides how much assist needs to be added. In most systems the computer changes the steering effort based on the vehicle's speed: at parking speeds the steering is light and easy to turn, while at highway speeds the effort amps up, giving the driver a feeling of greater stability and control.

EPS Benefits

a person driving a car: What Is Power Steering and How Does It Work? © Sean Rice - Car and Driver What Is Power Steering and How Does It Work?

The benefits of providing assist electrically are multifold: it improves fuel economy by a few percent, as the electric motor only draws power when needed; it eliminates the hydraulic fluid maintenance mentioned above; and it also enables a variety of features. Any driver-assist or convenience feature that involves turning the wheels without steering input from the driver is enabled by electric power steering. Features such as lane-keep assist, automated parking and lane changes, and the ability to guide the car around obstacles all utilize EPS's ability to steer itself when necessary.

Electric power steering is also more tolerant of out-of-spec alignment settings, using software to recognize and compensate the steering pulling to one side. It can also automatically adjust to a cross wind or a crowned road surface that would otherwise require constant steering correction from the driver. What's more, tomorrow's autonomous, self-driving cars will rely on electric power steering, because it allows the car to be steered by an on-board computer system when on automatic pilot. Several systems, like Cadillac's Super Cruise (pictured above being piloted hands-off by a C/D editor), are already capable of steering themselves down the highway under certain conditions.

Electro-hydraulic Steering

Between the hydraulic and electric types of power steering, there's a hybrid of the two systems called electro-hydraulic. It functions like a hydraulic-assist system, only that the hydraulic pressure is created by an electric motor rather than driving the pump off of the engine. This gets rid of the wasted energy complaint noted earlier, but doesn't enable all of the features possible with electric power steering. Only a few vehicles, including some heavy-duty pickup trucks, currently use this system.

If you're interested in a deep dive into the mechanics of how steering assist is created in either hydraulic or electric power steering systems, check out this Car and Driver technical explainer.

Steering Characteristics

Here at Car and Driver, the three major steering characteristics we evaluate in every vehicle we test are effort, response, and feedback. Two of those-effort and feedback-took a turn for the worse in early EPS systems, which didn't replicate the highly evolved, natural feel-for-the-road imparted by hydraulic systems. This made it hard to sense when a vehicle's tires were running out grip and starting to slip.

Although driving enthusiasts like us were, not surprisingly, up in arms about these negative developments, they actually affected all drivers-and still do. There's a real-world need for vivid feeling through the steering wheel when a vehicle is approaching its limits-say, when it's about to skid on a surface that's slick from rain, snow, or ice. A vehicle with more communicative steering makes for a better-informed, safer, and more confident driver in all situations. However, the good news is that engineers have spent much time and effort through the years evolving electric power steering and creating sophisticated algorithms that faithfully recreate the steering sensations lost after the switch from hydraulic units. Today, the latest EPS systems-particularly those from Porsche, Mazda, and GM (on the Chevy Corvette and Camaro, and the Cadillac cars) now feel intuitive. They let you know what the front tires are doing just as faithfully as the old hydraulic steering systems did-which is a very positive development for both cars and their drivers.

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