Forgive my ignorance but is it true that whilst journos are complaining about the transition from HPAS to EPAS, there is no longer any mechanical connection between the steering rack and the front wheels?

The reason i ask is 2-fold. I can't seem to find any articles that highlight the difference between the porsche/zf system and that of other manufacturers beyond electric assist tuning.

Am I correct in stating that it is not just the assist that has changed from hydraulic but that the connection between the steering and the wheels turning is simply a computer interface that decides the the degree of turning involved and the amount of feedback to the wheel needed?

I have just test driven the new cayman and the slight loss in feel doesn't bother me at all. In fact, I'd be hard-pressed to know about it if I wasn't trying that hard to detect the difference. But I'd still like to clarify the new system is not just a difference in the type of assist but also the entire connection.

thanks

 

There remains a proper mechanical connection between your steering wheel and the front tires, if the electrons go out in a whimper you can still stear the car (but without power assist).
Electrical power assist consists of a single DC motor connected to the steering rack via a helical race containing lots of ball bearings.
As the DC motor spins it transfers mechancial energy via the ball bearings contact patch friction and the accute angle of the helical race.
Pretty simple.
Its big potential I believe has not yet been fully developed but it's pretty close in the 991 - with steering forces being applied by the computer when braking hard on a compromised, uneven surface for example (using the yaw gyro as reference to keep going in the direction demanded by the driver). Using the quick reacting DC motor a precise amount of force can be injected into the rack as needed - the faster the motor spins the more force gets injected and the direction of the motor of course determines the direction of the force.

The very same can be done with a hydraulic system when electronically controlled valves are used to direct the fluid but it tends to get complex. The method employed in the 991 is nicer and avoids the need for a hydraulic pump and a lot of plumbing altogether. Probably cheaper too.

Would think that electrical systems like this will start replacing the old ways across the board in years to come. Somewhat like clever automatic boxes replacing MT. And then these getting replaced by electrical systems (probably).

My next daily driver will be a plug in hybrid. Just waiting for the right one to be made...

Rainier

 

thanks. It makes sense that there exists a proper mechanical linkage rather than a "drive by wire" system in case of failure either of assist or of steering.

little wonder that the steering felt quite good. I think it only a matter of a few more years for porsche before it is as "lively" as a completely manual rack.

 

Way I see it, the current system could be tweaked any way you want - but mechanically only. Want more road feel ? Remove a couple of ball bearings.
Want less - add some.

The current system acts like a low pass filter - low frequency forces are attenuated much less than high frequency forces. This is perhaps why many feel the steering is not lively. It does give really good feedback though - the "weighting" you feel when driving fast through a corner - that is what is left after the low pass filtering and it is just as good as it has always been. This is really what matters. You can feel instantly if you are starting to loose traction - the steering goes light (or threatens to do so). The high frequency forces are noise, mostly. They tell you if the surface is smooth. So does the roar of the tyres of course - just that now there is not a lot of that getting through to your hands. It still does - but your hands now have to "listen" for it. It is quite muted. But its there. It's very noticable with the non-PDCC SPASM, but the wheel never twitches - unless of course you do loose traction in the front. Then it does. A lot.

Rainier

 

Rainier, thank you for a wonderful couple of replies; a really good explanation (and fox-shooting) of some stuff which was the obsession of 997 owners for at least a year!

 

It has been very long time since Porsche has a "completely manual rack". Power steering - via a hydraulic pump- has been standard since the late 1980s. If the web had existed I am sure purists would have posted complaints!

 

From what I got told one of the design goals of the electric steering was to make it more livable at high speeds when driving normal roads. Normal roads in this case means autobahns and speeds would be in the 180-250km/h range (if traffic allows).
The previous steering, quite rightly, was not optimal for a Carrera as it would fatique the driver quickly.
The electric steering we have now fixes this bang on. However, there are countries that have draconian speed rules and enforcement. If you are forced to drive your new Carrera at 50mph, well, yes - the steering certainly does not even wake up. One could argue that it should not wake up at these kind of speeds as there is no point. However, it does take away from the driving experience when forced to commute at low speeds. It gets difficult to pretend that you are driving a sports car when it feels much like a C class Merc.

Driving at speed though (say 120Km/h upwards - about where the spoiler pops up) and the steering is really great in my opinion. Definitely a lot of feel and even at 250Km/h it is not overdone giving a good sense of stability.

Would be real great if some time in the future another little button finds its way into the center console with a steering wheel on it. Then you could have it either way as the mood takes you.
Until then, if steering feel is high on your list and you want a Carrera, try and bum a ride in a non-PDCC SPASM. It's pretty good IMHO, I can't really tell much of a difference between that and a 997.2 except that the 991 feels more precise. That may have more to do with suspension and geometry through.

Rainier