991.2 GTS no rear axle steering and standard PASM
#31
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Still, both a nice-to-have for all but competitive driving. the 911 is very agile as it is. If you can afford it, and you'd regret it down the road, it would make my options list.
I like the look of SPASM but some days regret the rougher ride and yes I scrape - I live in a hilly area. When I was shopping GTSs I would have bought one new if they didn't all have SPASM!
I like the look of SPASM but some days regret the rougher ride and yes I scrape - I live in a hilly area. When I was shopping GTSs I would have bought one new if they didn't all have SPASM!
#32
Instructor
Still, both a nice-to-have for all but competitive driving. the 911 is very agile as it is. If you can afford it, and you'd regret it down the road, it would make my options list.
I like the look of SPASM but some days regret the rougher ride and yes I scrape - I live in a hilly area. When I was shopping GTSs I would have bought one new if they didn't all have SPASM!
I like the look of SPASM but some days regret the rougher ride and yes I scrape - I live in a hilly area. When I was shopping GTSs I would have bought one new if they didn't all have SPASM!
If you're not apposed to that kind of thing in a sports car
Might as well get RAS while you're at it lol.
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Back to RAS: Once you have working actuators, the only mechanical property that really matters from a control perspective is response speed. Once you have reliable inputs (sensors) and actuators, everything else is software. A slowly-responding single-variable (i.e. road speed only) control system is far, far easier to field than the high-speed multi-input system that would be required for dynamic (e.g. through a corner, etc.) control.
It’s all possible if not cheap. I’m just skeptical that the RAS system is a truly-dynamic multivariate system when a simple speed dependent (and maybe steering angle) can provide all of the benefits that have been described in Porsche’s press materials.
Last edited by worf928; 01-13-2018 at 03:31 PM.
#34
Great link. Do the videos impart additional technical info? (I can’t summon the patience to watch 10s-of-minutes of video when a page of technical documentation can be absorbed in a minute or two.)
Every thing I have come across suggests that the current RAS system is speed dependent. While it is certainly possible to incorporate additional sensor inputs (e.g. accelerometers, steering wheel angle, etc.) to enable truly dynamic control as you described above, do you/we/anyone know if the current public road-going version of RAS does so?
This above all else. But it isn’t simply tuning. Control system design, software implementation and validation is the lion’s share of the non-mechanical development. Tuning is important, but much of that effort can, and should be done with simulations.
Back to RAS: Once you have working actuators, the only mechanical property that really matters ers from a control perspective is response speed. Once you have reliable inputs (sensors) and actuators, everything else is software. A slowly-responding single-variable (i.e. road speed only) control system is far, far easier to field than the high-speed multi-input system that would be required for dynamic (e.g. through a corner, etc.) control.
It’s all possible if not cheap. I’m just skeptical that the RAS system is a truly-dynamic multivariate system when a simple speed dependent (and maybe steering angle) can provide all of the benefits that have been described in Porsche’s press materials.
And you'd be surprised how much chassis calibration is still done in-vehicle by highly skilled engineers. If anything, the tuning is harder than designing the hardware and control system. THAT can be done in simulation.
#35
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Every thing I have come across suggests that the current RAS system is speed dependent. While it is certainly possible to incorporate additional sensor inputs (e.g. accelerometers, steering wheel angle, etc.) to enable truly dynamic control as you described above, do you/we/anyone know if the current public road-going version of RAS does so?
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I doubt it. A RAS system as sophisticated as you and Racer20 postulate would almost certainly be classified as an active suspension system. I think even the simplest RAS system (which is what I postulate *we* are driving around with) would also be classified as an active suspension system.
Active control of the suspension (along with the differential, brake bias, aero, and probably other things) has been banned, as far as I know, in all forms of organized motorsports for decades. This is certainly true of F1 as of 1995.
If true - that RAS is, or would be, banned, in motorsports - which I'm willing to bet lunch it is, Porsche has no motorsports-based reason to develop a highly-sophisticated RAS system. The current PTV/PTV+ system which mimics (arguably poorly) a computer-controlled dynamically adjustable differential is - as far as I know - also not legal in organized motorsports. This, though, serves as a counter-example: Porsche might spend money on 'active' technologies with no motorsports-based motivation. The follow-on question is: how sophisticated do they need to be?
My Porsche habit has been paid for by about 25 years of experience in developing, fielding, and supporting software-based control systems in mission- and life-critical environments. There's at least an order of magnitude difference in the expense of developing and fielding between a simple RAS system and one that can dynamically adjust toe through the entry and exit of a corner.
It is not, at all, impossible. Most (maybe even all) of the sensors you would need are - as you point out - already on the car. The processing power is also, I suspect, already on the car. An F1 team with experience from 'back in the day' could knock-it-out in a few months. But, in F1 expense is rarely a limitation if it will buy you several tenths and lawyers are further and fewer between. (And there are other simplifying factors.)
In the real world expense is a limitation. I'm skeptical that Porsche (via ZF) has gone to this expense since there are no motorsports reasons and the street-marketable benefits (that we are aware of) can be achieved without the sophistication you two speculate might exist.
On the subject of Porsche's acumen viz-a-viz embedded control software: have you two read the thread(s) on 991s going bonkers on the banking at Daytona? (https://rennlist.com/forums/991-turb...t-daytona.html) ... Doesn't leave me with the impression that Porsche's got its $h1+ together on the subject and strengthens my skepticism (perhaps fear) of a highly-sophisticated RAS implementation.
Without some actual technical data on what's going on inside the RAS code, further speculation is, while perhaps fun and interesting, not going to do anything but expend time.
If you've got it post it. For me, it would be more fun and enlightening (and possibly scary given the above "Daytona Fail") to be proven wrong.
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On this I would bet an important body part: it is ALL in software.
#38
In the real world expense is a limitation. I'm skeptical that Porsche (via ZF) has gone to this expense since there are no motorsports reasons and the street-marketable benefits (that we are aware of) can be achieved without the sophistication you two speculate might exist.
On the subject of Porsche's acumen viz-a-viz embedded control software: have you two read the thread(s) on 991s going bonkers on the banking at Daytona? (https://rennlist.com/forums/991-turb...t-daytona.html) ... Doesn't leave me with the impression that Porsche's got its $h1+ together on the subject and strengthens my skepticism (perhaps fear) of a highly-sophisticated RAS implementation.
On the subject of Porsche's acumen viz-a-viz embedded control software: have you two read the thread(s) on 991s going bonkers on the banking at Daytona? (https://rennlist.com/forums/991-turb...t-daytona.html) ... Doesn't leave me with the impression that Porsche's got its $h1+ together on the subject and strengthens my skepticism (perhaps fear) of a highly-sophisticated RAS implementation.
Last edited by Racer20; 01-13-2018 at 08:07 PM.
#39
Pro
Sorry Racer20 but you are in fantasy land. Porsche claims the slight angle changes at < 31 mph and >50 mph , opposite and same as front wheels respectively. Nothing more sophisticated, no camber adjustments.
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Yes. I was responding to 'digits' not you.
I've rarely been accused of over-estimating LOL. I may however, as you point out below, not be counting-up the cumulative investment that has occurred over the past decades.
You're so sure of this? For what Porsche tells us RAS does? Why?
We? Do you work in the industry? On these systems? Or with folks that are and do? Tell us.
Agreed. You have, however, missed my primary point: What non-race regime marketable or objective benefit (the two are not always the same) would derive from a RAS control system with a sophistication and response period fast enough to dynamically change toe through a corner? Certainly it would manifest on the Northloop, but what's the ROI versus areo, power, tires, etc. If it did what you say it does, wouldn't Porsche's Marketing Machine be all over it?
Let's cut the speculation. My knowledge of RAS is based upon what Porsche says it does and what my butt-o-meter says it does. The rest is somewhat-informed speculation based upon non-automotive (but physical) control systems. Regardless of how many sensor inputs are needed to make RAS do what Porsche Marketing says it does (and as far as I've seen they do not claim that RAS changes toe dynamically through a corner) do you have any knowledge of RAS's actual requirements or design? If you do then let us know and tell us what's really going on in there.
And tell us when Porsche's going to fix the control software for Daytona? (No comment on that?)
Otherwise it's just us speculating on what might or might not be true, necessary or expensive. I think it is certainly possible to dynamically control toe through a corner with the hardware that's on the car. I just don't think they actually did it.
I think you're overestimating the cost and complexity of this;
... not to mention that using only wheel speed and steering angle to control the RAS behavior wouldn't work ...
We are ...
... generally past developing the basic functionality of these systems now, and the industry is working on making them better, faster, more integrated, and cheaper.
Let's cut the speculation. My knowledge of RAS is based upon what Porsche says it does and what my butt-o-meter says it does. The rest is somewhat-informed speculation based upon non-automotive (but physical) control systems. Regardless of how many sensor inputs are needed to make RAS do what Porsche Marketing says it does (and as far as I've seen they do not claim that RAS changes toe dynamically through a corner) do you have any knowledge of RAS's actual requirements or design? If you do then let us know and tell us what's really going on in there.
And tell us when Porsche's going to fix the control software for Daytona? (No comment on that?)
Otherwise it's just us speculating on what might or might not be true, necessary or expensive. I think it is certainly possible to dynamically control toe through a corner with the hardware that's on the car. I just don't think they actually did it.
#41
If brake pressure > 0
rear steer rate = x
else y
x and y would be different look up tables or gain ratios where steering angle is determined by vehicle speed and steering wheel angle. There would be a similar one for acceleration, and probably one for when stability control is activated.
Also, keep in mind that the system would have to respond quickly enough to deal with an emergency avoidance maneuver, otherwise the rear wheels would be left pointing the wrong way on the correction steer and create a yaw moment rather than prevent one. The system has to respond at least as fast as a driver can work the steering wheel. All the ZF literature mentions integration with stability control and the ability to adapt to "driving situations" besides just speed and steering angle.
I work in controlled chassis systems. My group doesn't do RAS specifically, but my company does something similar. I don't work on Porsche, but PASM, PDCC, RAS, and similar systems throughout the industry share similar architectures and functionality. I didn't come here to prove anything or to argue, just to share some interesting information with my fellow Porsche enthusiasts. Cheers . . .
Last edited by Racer20; 01-14-2018 at 09:35 AM.
#42
I work in controlled chassis systems. My group doesn't do RAS specifically, but my company does something similar. I don't work on Porsche, but PASM, PDCC, RAS, and similar systems throughout the industry share similar architectures and functionality. I didn't come here to prove anything or to argue, just to share some interesting information with my fellow Porsche enthusiasts. Cheers . . .
All of this extensive fine tuning between different 991 versions would be rather pointless IMO if the RAS merely steered opposite to the front wheels up to 30mph and in the same direction above 50mph..I partly base this assumption on owning both 991.2 GT3 and a 991.2 GTS with RAS..
#43
Pro
Hard to believe Porsche would not make more claims about RAS in the 991.2 if it did more than this... and charge more for it LOL.
#44
My general advice to you after going through a recent build and now having put a few thousand miles on the clock - don’t kill yourself vacillating on this or that option. You’re going to really love your GTS no matter how you spec it. Just be true to what you want and don’t let the dealer or anyone on this forum push your decisions. With that said, if it’s not your daily consider the manual gearbox (will save you a few bucks even after spec’ing rear steer) for its engagement factor. Also, I’d say for you to consider the annoyingly expensive interior GTS package. It provides a jewel-box like quality to the interior, glad I sprang for that as well.
But to answer your question - yes, you will be missing out on performance but there’s a 99% chance you won’t know the difference unless you back to back it with a GTS that has rear steer and spasm.
Good luck with all the late night spec builds at Porsche.com!! You will love it like a family member when it finally arrives!
Last edited by BeepBeep88; 01-14-2018 at 06:32 PM.
#45
One of the early track reviews had some comments from the reviewer who drove cars with/without RWS back to back on track, including through a coned chicane and he said the difference was very noticeable and that the RWS car was much more nimble.