EPS (Electric) vs. HPS (Hydraulic) Power Steering

[MexicoCityM3]

I was checking out this month's C&D and they did an interesting test of EPS vs HPS using two similar BMWs with different systems. A 528i with EPS and a 535xi with HPS. It was a blind test with their editors and, to their own surprise (after all their bitchin' about EPS for years), they actually preferred the feel and response of the EPS system.

Beyond this being a sort of vindication for BMW's efforts, it made me reflect on my own experience which is: frankly I can't tell the difference between systems. I have driven several EPS cars (new 5 er, old & new Z4, new 3er, new 1er) and have never found the steering wanting. I mean, I can sense that some cars have more feel than others (say, a 5er vs a 1er) but not a difference between systems (old 3er vs F30 3er).

Do you notice the difference? Any real preference? Non-issue?

And Raza, remember your car was one of the EPS pioneers and if your comments are anything to go by, you love it (only the Z4M had hydraulic steering).

[SVT666]

I honestly can't tell the difference.  The EPS Mustangs have the best steering of any Mustang in history, but I like whatever feels best and that all depends on how the car is setup and pretty much has nothing to do with type of system.  The feel is created through the steering linkage and design, not what is doing the boosting.  EPS at least frees up a couple horsepower.

[2o6]

This is more of a car to car thing rather than a technological thing.


My car has electric PS, and it has more substance than my brothers Corolla.

[SVT666]

This is more of a car to car thing rather than a technological thing.

Wrong.  Completely opposite.


Just fucking with you.

[Raza]

The Z4's steering is great.  But I feel like I've had great steering and shitty steering from both setups. 

[MrH]

It's not "more of a car thing".  Vast majority of EPS systems suck.  But I prefer them when they are actually done right, but there were years of EPS systems done horribly wrong.

A properly done EPS system is my preference, but that's pretty rare.

[SVT666]

It's not "more of a car thing".  Vast majority of EPS systems suck.  But I prefer them when they are actually done right, but there were years of EPS systems done horribly wrong.

A properly done EPS system is my preference, but that's pretty rare.

The vast majority of Hydraulic systems suck too.  The source of the boost isn't what affects steering feel.  It's the steering linkage and design that does that.

[GoCougs]

Yeah, that's not right. Boost definitely matters on feel. Hydraulic systems have inconsistencies (non linearities) in pressure and thus motion/assist. Hoses expand with a change in pressure, pump pressure changes a bit based on RPM, pressure regulators have a tolerance, etc., etc. Electromechanical (i.e., servo motors) don't have that.

[MrH]

But servo motors are much much more sensitive and need to be "tuned".  If the controls are out of whack in the slightest, you'll get something over boosted, or with no feedback, or unnatural in feeling.

[MX793]

Every HPS system I've ever driven has been consistent, though not all of them have been good.  Some have been numb or overboosted, but the resistance was pretty consistent as you turned the wheel.  The EPS in the Mustang is decent.  It's a bit numb for my tastes (although the old HPS in previous versions was also on the numb side), but the steering effort is consistent.  I drove a Chevy Cobalt maybe a year ago and the EPS was "lumpy".  It would create resistance when the wheel was returning to center after making a turn, and when turning it might start out with higher effort and then go easy, or vice versa.

[Onslaught]

I really don't give a shit. I am taking the PS out of the MX-5 however.

[r0tor]

I read the c&d article and laughed a bit when the final tally shows the editors picked EPS despite the fact they have cursed at the technology ever since it was invented.

[MX793]

I read the c&d article and laughed a bit when the final tally shows the editors picked EPS despite the fact they have cursed at the technology ever since it was invented.

I don't ever recall them cursing the NSX or the S2000's steering.

[Rich]

Or RX-8

[SVT666]

Every HPS system I've ever driven has been consistent, though not all of them have been good.  Some have been numb or overboosted, but the resistance was pretty consistent as you turned the wheel.  The EPS in the Mustang is decent.  It's a bit numb for my tastes (although the old HPS in previous versions was also on the numb side), but the steering effort is consistent.  I drove a Chevy Cobalt maybe a year ago and the EPS was "lumpy".  It would create resistance when the wheel was returning to center after making a turn, and when turning it might start out with higher effort and then go easy, or vice versa.

The old Mustang had shitty steering feel.  The steering feel in the new Mustang is a little on the light side and a little numb but it's still the best steering any Mustang has ever had.

[GoCougs]

But servo motors are much much more sensitive and need to be "tuned".  If the controls are out of whack in the slightest, you'll get something over boosted, or with no feedback, or unnatural in feeling.

Why would they be out of whack?

That a servo system is a high gain system is precisely why it is better than a hydraulic system (better that is for response, and thus better for design the desired response).

[MX793]

Or RX-8

RX-8 had hydraulic assist driven by an electric pump, it wasn't pure electric.

[MrH]

Why would they be out of whack?

That a servo system is a high gain system is precisely why it is better than a hydraulic system (better that is for response, and thus better for design the desired response).

The damping controls sometimes aren't designed right from the beginning.  You said it yourself, it's a high gain system.  You run the risk of overshooting and increasing your settling time.  Critically damping an EPS system for every possible scenario can be harder than it sounds.  I used to work for an electric motor company that was making some of the first electric power steering units for ATVs.  Pretty funny to watch the things go nuts when the controls weren't optimized yet

[sportyaccordy]

It's not "more of a car thing".  Vast majority of EPS systems suck.  But I prefer them when they are actually done right, but there were years of EPS systems done horribly wrong.

A properly done EPS system is my preference, but that's pretty rare.

HPS has had a pretty big head start

I am pro-EPS. Its teething right now but it has way more potential than HPS.

[sportyaccordy]

I don't ever recall them cursing the NSX or the S2000's steering.

O yea that too.

[r0tor]

RX-8 had hydraulic assist driven by an electric pump, it wasn't pure electric.

I often wonder why that setup never caught on... removed the load from the engine while keeping the hydraulic feel

[Eye of the Tiger]

I really don't give a shit. I am taking the PS out of the MX-5 however.

That may be a mistake once you have 300 HP under the hood. Your arms won't be able to keep up with the extreme drift angles.

[Onslaught]

That may be a mistake once you have 300 HP under the hood. Your arms won't be able to keep up with the extreme drift angles.

If I ever get near 300 HP in an MX-5 then I won't be driving it anymore. I don't want to die. Not yet anyway.

[Eye of the Tiger]

If I ever get near 300 HP in an MX-5 then I won't be driving it anymore. I don't want to die. Not yet anyway.

Don't worry, I'll drive it for you.

[MX793]

I often wonder why that setup never caught on... removed the load from the engine while keeping the hydraulic feel

I suspect that full electric is just more efficient than electro-hydraulic.  Probably lighter as well.

[Payman]

I'd like to see a system where the EPS diminishes at higher speed. It wouldn't be hard to implement a sports car with a system with 100% EPS at 0-10 mph, then progressively fade until you have zero power steering at, say, 60 mph.

[Soup DeVille]

I'd like to see a system where the EPS diminishes at higher speed. It wouldn't be hard to implement a sports car with a system with 100% EPS at 0-10 mph, then progressively fade until you have zero power steering at, say, 60 mph.

I believe that most of them do that at least to some extent.

[Soup DeVille]

RX-8 had hydraulic assist driven by an electric pump, it wasn't pure electric.

As did the Subaru XT6- they called it "Cybrid."

[2o6]

I'd like to see a system where the EPS diminishes at higher speed. It wouldn't be hard to implement a sports car with a system with 100% EPS at 0-10 mph, then progressively fade until you have zero power steering at, say, 60 mph.

Mine loses assist at high speed.

[Payman]

Wife's G5 is numb at all speeds. You actually feel out of control sometimes in a high speed sweeper.