What’s it like to drive a pickup with electric power steering? We drove an EPS-equipped Chevy Silverado at Nexteer headquarters in Saginaw, Mich., and we found that, despite the huge technological change, the effects on driving were minimal, but the potential is nearly limitless.
Several weeks ago, we reported on Nexteer's efforts to replace conventional 12-volt hydraulic rack-and-pinion steering systems in half-ton pickups with all-new electric power steering systems in the near future. In today's fast-changing and environmentally conscious times, EPS has several benefits. It offers up to a four percent improvement in fuel economy by getting rid of the losses that come from operating a hydraulic pump, a nearly infinite tuning of steering feel and the elimination of hydraulic steering fluid and hoses.
Tony Dodak, Nexteer's manager of rack-based EPS and premium manual gears, provided us with two late-model Chevrolet Silverado 1500 pickups: one with standard hydraulic steering and the other an EPS engineering demonstrator (not from GM).
After driving both trucks, it’s clear that the changes that drivers notice when EPS finally makes it to their driveways won't be near as dramatic as the technology switch itself. In fact, drivers probably won't notice much difference at all. And, in some ways, it's better than hydraulic steering could ever hope to be.
Probably the easiest way to tell the difference between old and new steering systems was when the truck was parked or moving at low speeds. In a hydraulically steered pickup, when you crank the wheel all the way to one side, you can feel and hear the steering pump strain as it hits the boundaries of the mechanical stops. In the EPS-equipped truck we drove, rather than let us steer all the way to stops and get a clunk, the system gently reduced assistance to zero up to right before the stop. There was no motor noise because there was no assist.
This diagram shows an enlarged cutaway of the EPS motor assembly (bottom middle) attached to the steering system on the passenger side of the vehicle. The ball screw portion visible (top left) under the driven (large) pulley and extending out is the rack. The small pulley is attached to the driveshaft of the electric motor. A belt wraps around both pulleys and acts as the first stage of a two-stage gear reduction that makes EPS possible in a light duty pickup. The second stage is between the ball nut and ball screw.
Running on Nexteer's track, we could tell the difference between the hydraulic and EPS steering systems because of tuning differences, not because of component changes. To prove that was the case, Chris Fabien, a Nexteer EPS system engineer -- who's never worked on hydraulic steering systems -- changed steering characteristics as we drove. At the start of our lap, the EPS Silverado's steering felt slightly firmer than when we turned the hand wheel of the hydraulic Silverado. Fabien said it was based on another Nexteer engineer's personal driving preferences.
"Using my laptop” -- connected to the truck's steering system -- “I'm able to change [software] knobs and settings that we use to change how the steering feels," Fabien said. Fabien also showed us graphed curves that represented how much electric steering assist was provided, based on the truck’s speed and steering input. In real-time, the curves changed, depending on how fast the truck was traveling. What we felt with our hands, we could see on his laptop.
"There's a large table data the system looks up again," Fabien said. "It uses that to create the graphs, which represent feel."
That wasn't the most remarkable item. With a click of the mouse, Fabien changed the truck's steering feel as we drove. One moment it felt firm but responsive, then, the next moment, it felt like we were driving a Class 7 utility truck as Fabien switched to a setting that offered much less assist throughout the steering range.
In other words: Need better steering feel? There's an app for that.
Rack-mounted EPS system. It's arranged opposite from the illustrated EPS diagram. The ball screw rack (shown in front) sits inside the assembly.
The demonstrations hammered home the advantage that EPS can give to truck manufacturers, who often spend months agonizing over the steering feel of a new pickup.
With hydraulically steered trucks, engineers may have to drive a truck for days or weeks to get a feel for its turning characteristics. When a change is needed, a valve that controls the flow of hydraulic fluid has to be machined. It's an approach that's both labor- and time-intensive.
In contrast, EPS allows engineers to tune steering feel at the moment that they determine a change is needed.
"We have a crosshair on the computer that we use to follow steering performance over curves as we drive," Fabien said. "It's really great for tuning because when we find a maneuver that feels too light or too heavy, we can immediately drag that point [on the graph] to change the steering feel. It can save weeks of development time."
Dodak compares EPS tuning to an eyesight test. "Is A clearer than B? Is B better than C? We go back and forth quickly to find just the right feel," he said.
Pickup trucks manage much heavier front-end loads than the small cars, where EPS is already used. Can a 12-volt EPS system can handle big weights over the front axle? Nexteer showed as a full-size Cadillac Escalade SUV with about 1,000 pounds of sand sitting over its front end. At rest, without the benefit of forward momentum to help reduce steering effort, we were easily able to turn the wheels in both directions without any sign of strain.
That doesn't mean we could tell the system’s durability from our brief test, but without hydraulic fluid and hoses, there won't be any messy leaks as the system ages. And Dodak said that Nexteer has been testing EPS in pickups for 97th percentile truck owners for up to five vehicle lifetimes or about 1.2 million miles.
Nexteer's Chevrolet Silverado 1500 demonstrator equipped with a 12-volt rack EPS.
Nexteer says that EPS can help improve vehicle safety and stability.
Today’s stability control systems reduce throttle or apply the brakes without the driver intervening to help recover a vehicle when there's a near or full loss of control. EPS can add a third control feature by taking inputs from the stability control system to provide wheel torque and assist to help the driver regain control quicker than they would have with the brakes or engine alone.
There's also the possibility of creating special EPS driving modes to help trucks that are towing or fighting crosswinds. Parallel parking and low-speed maneuvering can be tuned with EPS, and certain rough road conditions can be mitigated before you feel them in your hands.
We're impressed with Nexteer's EPS system and think it's time to say farewell to another legacy technology in trucks. How soon can you expect to drive a light-duty pickup with EPS? One full-size truck manufacturer has plans to offer it on their half-tons for 2011 models and at least three are expected to offer it within three years.