If you buy a
car today, it will likely have a major difference in the power steering than cars from 10 or even just 5 years ago: The steering system will rely on an electric motor instead of a hydraulic piston for power boost. The majority of new cars sold today use electric power steering.
I’ve seen — and felt — this change in cars ranging from Toyotas to Porsches over the years. My own car, a 1999 BMW, is firmly in the hydraulic boost camp. But I’ve grown to appreciate the precise response and linear boost in modern sport cars, which has improved dramatically as engineers learn how to program these steering systems.
Not everyone feels this way. The switch to electric power-assisted steering (EPAS) met its detractors among driving enthusiasts, often citing a lack of road-feel in newer cars. Jeremy Clarkson of BBC’s “Top Gear” said, in a review of the Ford Focus ST, that cars with electric power steering tended to understeer, a claim that doesn’t make much sense when you compare the architecture of the competing systems.
Given the backlash, why have automakers adopted electric power steering almost across the board?
Fuel economy has been one of the major drivers of the switch to electric power steering systems. In its press materials, German auto parts maker ZF Lenksysteme notes that its electric power steering system uses 90 percent less energy than hydraulic power steering. TRW, another auto parts manufacturer, points out that its electric power steering systems result in a 4 percent fuel savings in cars.
Pressure or electricity
To understand why electric power steering offers better fuel economy, we have to look at how these systems work. Hydraulic power steering, used on the majority of cars from the last century, relies on pistons in the steering rack with pressurized fluid. A pump, turned by the car’s engine, maintains hydraulic fluid pressure.
EPAS does away with the hydraulic pistons and pump, instead using a simple motor to help push the steering rack as you turn the steering wheel. Some systems have a column-mounted motor, while others use a motor on the rack itself, enhancing your own effort when turning the steering wheel.
The problem with a hydraulic system is that the pump is always sapping energy from the engine, whether you are turning the wheel or not. EPAS uses electricity generated by the engine, but it only needs that energy when you are turning the wheel.
Some cars use a hybrid of both hydraulic and electric power steering. These cars still have hydraulic pistons in the steering rack, but maintain pressure using an electric motor rather than a pump attached to the engine.
The advantage of using an electric pump solves one problem of purely hydraulic systems: uneven pressure. The engine speed, which varies from 1,200 to 6,500rpm in a typical car, affects the hydraulic pump speed. A car driven at low speed that’s put through many turning maneuvers, such as in a parking lot, can lose boost pressure, making the wheel difficult to turn. An electric pump won’t vary its pressure based on engine speed.
As for reliability, the complexity of hydraulic steering makes it more prone to failure than EPAS. Hoses and belts need replacement, while seals in the pistons and pump will eventually age and leak. The electric motor and chip controlling an EPAS system is going to be much more age tolerant.
Matt List, Vehicle Dynamics supervisor for Ford, told me in an interview that equipping cars with EPAS systems required upgrading the alternators and electrical systems, which seems a small price to pay for getting rid of the hydraulic plumbing.
List has many reasons to appreciate EPAS. He said that, tuning hydraulic steering for a new car, his team required only a few tries to get it right. Adjusting the steering feel for a new car required varying the fluid flow selectively by adjusting valves. Tuning EPAS-equipped cars merely requires changing parameters in a digital file. The Dynamics team can quickly adjust these parameters, then try the car out on a test track.
List pointed out that, with hydraulic systems, they would sometimes have to use a different size pinion gear for the same car depending on the tire size. With EPAS, he can use a different program depending on the tires and wheels fitted to the car.
How different steering systems feel to drivers is a thorny and subjective issue. I’ve driven plenty of cars where the EPAS system was obvious through over-boosting, with no heft to the steering wheel and an accompanying whirring noise from the motor that I could hear in the cabin. Cars such as these have given EPAS a reputation for numb steering feel.
However, I also used to have a 1969 Dodge Coronet with heavily boosted hydraulic power steering. There wasn’t much heft to the wheel or road feel from that car, either.
Many of the complaints about EPAS systems come down to programming, as there isn’t a very significant difference in the architecture of the rack and pinion between these two type of systems. In fact, you could argue that removing the hydraulic pistons allows a more direct mechanical coupling between the rack and wheels. The minor differences between these systems certainly argues against claims of increased understeer.
One of the most extraordinary cars using EPAS is the new Corvette Stingray. The steering in this car couldn’t feel more natural. Chevy engineers tuned the system to allow precise control and just enough boost so the driver still feels heft when turning the steering wheel. I haven’t read a single complaint about numb steering in the Stingray.
Another car to use EPAS is the Bugatti Veyron. When I had the opportunity to drive one, I asked American LeMans race driver Butch Leitzinger, my appointed codriver, what he thought about EPAS versus hydraulic power steering. He was surprised to hear there was any controversy at all, concluding that road feel should be the same.
Leitzinger pointed out that the cars he races use EPAS, because hydraulics just don’t react fast enough for the number of quick steering adjustments made during high-speed cornering.
The future of steering
Given the advantages of EPAS, I expect more cars to convert to this system as they receive model updates. At the same time, engineers are going to get better at tuning these systems, figuring out which parameters to program to satisfy both casual drivers and enthusiasts.
But another technology waits in the wings, which would make for an even more radical change in how we control our cars. Drive-by-wire steering systems are currently undergoing development by automakers, and Infiniti has made a system based on this technology available in the 2014 Q50.
Drive-by-wire means removing the mechanical linkage between steering wheel and a car’s front wheels. A computer rates the amount of turn input from the driver, then sends instructions to actuators at either a steering rack or the front wheel control rods.
Infiniti calls its system Direct Adaptive Steering and notes that it transmits driver inputs to the wheels faster than possible with a mechanical system. However, Infiniti includes a backup mechanical system, which takes over if the electronics fail. The proliferation of drive-by-wire steering is likely to stem directly from the success of the Q50’s system.
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