Pickup trucks are rapidly changing to become more fuel efficient. But it’s not just new powertrains and aerodynamics that are pushing the limits of frugality. Even the way you steer your truck is about to change.
Executives from Nexteer Automotive, a driveline manufacturing spinoff from General Motors and Delphi, say conventional 12-volt hydraulic steering systems are about to be replaced in half-ton pickups with all-new electric power steering systems.
"We expect that all three domestic [truck manufacturers] will converge to a 12-volt rack EPS architecture within the next three years or so," said Mike Richardson, Nexteer’s vice president of engineering and global steering business line.
Until recently, EPS systems were available only in smaller cars and crossovers because the components weren't strong or efficient enough to meet the higher front axle steering loads of pickup trucks. Only GM's 2009-10 Chevrolet Silverado and GMC Sierra Two-Mode Hybrid pickups offered EPS because their powerful (and unique) 42-volt electrical architectures provided enough energy to support the feature.
Whereas a hydraulic power steering system uses an engine-driven pump and pressurized hydraulic fluid to help turn a truck's wheels and reduce driver effort, EPS replaces the hydraulic pump and fluid with an electromechanical motor-driven connection to the steering system.
And whereas a hydraulic steering pump always requires power, even when the truck is traveling in a straight line, EPS requires power only during steering maneuvers, which helps make the vehicle more fuel efficient.
"In general, there's about a 4 percent improvement in fuel economy," said Tony Dodak, Nexteer's manager of rack-based EPS and premium manual gears. "[In a pickup] that equates to around 0.5 to 1 mpg depending on the vehicle and the mileage of the powertrain."
Several innovations have made 12-volt EPS in a pickup possible. The first was the conversion of half-ton pickups from traditional recirculating ball steering linkages with greater leverage capabilities to handle high steering loads (they're still used in almost every heavy-duty pickup) to rack-and-pinion systems, which offer better feedback to the driver.
Nexteer's EPS setup uses a so-called belt and ball-screw system, where the electrical motor turns a belt that runs to a ball screw that provides direct assist to the rack-and-pinion gear set.
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.
The second innovation was the development of a two-stage gear reduction system — which eases the load on the electrical motor while still delivering high enough torque to help with steering duties in heavy-vehicle applications — coupled with high-efficiency brushless electric motors.
But perhaps most important innovation was measuring and metricizing the feel of hydraulic steering systems so that a similar feel could be replicated and potentially improved with EPS. Early EPS-equipped vehicles have been criticized for feeling numb and disconnected in drivers' hands.
"We've spent the last 12 to 15 years converting our competencies from hydraulic-based steering to electric," Richardson said. "Now, OEMs can’t tell the difference whether they’re driving an electric-powered steering system or hydraulic. Some refuse to do blind rides. That's really a milestone. The industry has gotten to the point where you can’t tell the difference."
It's important to note that EPS is not drive-by-wire, which removes direct mechanical linkages from the steering wheel to the drive wheels. EPS provides power assistance while the truck is still controlled from the steering column to an intermediate shaft to the rack-and-pinion gears and to the tie rods at the wheels and tires. If you lose power assist, you can still manually turn the wheels.
In some cases, EPS systems can go one better than conventional hydraulic systems by increasing or decreasing power assistance and steering feel based on specific speed and driving conditions. In high-wind situations, extra torque could be automatically applied to help keep a vehicle straight on the road, a feature that could benefit pickups towing trailers. It could also help half-ton pickups parallel park themselves with minimal assistance from the driver.
But EPS also has a few disadvantages in its current state of development.
Nexteer depends on software to provide just the right level of power assistance and feedback that's tailored and calibrated to the model and powertrain available in a pickup. A buyer customizing an EPS-equipped pickup with new wheels, tires or suspension modifications could find the truck behaving or feeling differently from what’s expected. It's just one of several factors we've previously cited that could limit how much you can modify your pickup in the future.
Future EPS with more intelligent software could be the answer, but it won't be available in first-generation EPS systems in half-ton pickups.
"Right now, the system has a lot of margin for change at the vehicle level. It can allow for different adaptations and still be OK for performance," Dodak said. "Eventually, manufacturers may add custom features that we could apply as an aftermarket tool. We could provide interfaces in the future for highway tuning versus sporty city tuning."
EPS also won't work in HD pickups with recirculating ball power-steering systems. The parasitic load is too high for a 12-volt electrical architecture to provide enough torque. One possible solution to this issue is to apply some of the lessons learned from EPS to provide more intelligent hydraulic power assist that can vary boost levels based on driving conditions.
It's not often that we see this significant of a mechanical change in pickups. It could be compared to the switch from carburetors to fuel injection. But one thing is certain: EPS is coming to half-ton pickups and, ready or not, you're going to get your turn to steer your truck with this technology.