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in the 992 4s coupe test drive which btw drove great, there was good amount of body lean in long sweepers compared to my 718 GTS. The 992 was composed and surefooted though and I think in those situations PDCC would make it lean a lot less
I find it strange that RAS and PDCC are referred to as somewhat similar and making 911 into a mini panamera. I could buy it for PDCC but RAS is completely the opposite. It allows for much more precise suspension behavior, and easier parking is just a side-benefit. RAS is great because it allows for less give in the rear suspension. All rear suspensions steer to some extent, but passively - the car leans in turn and that causes suspension deflection built in via bushings and other mechanical tricks. The problem with this is that it's always playing catch up to front wheels - front turns, car leans, rear turns. With electric RAS, both axles can turn simultaneously - no lag. Another benefit is that suspension can be less sloppy because is does not need the bushings and other flexible stuff that would allow for the passive wheel deflection. Also, 911 benefits from RAS more than most other cars because it RAS virtually extends wheelbase to behind the engine, which is particularly beneficial for a rear weight biased car. Unless I'm buying some classic car, I'd want RAS. RAS makes the car feel crisper, and there is nothing unnatural about it - the non RAS car does the same rear-wheel wiggle just in a slightly sloppier and laggier way.
PDCC is a different story. For most people, the biggest benefit of it will be more comfort on rough roads. Less lean is not that noticeable in a 911, with its low center of gravity and wide track. Also, clever shocks control side to side transitions (like in S-curves) well enough so lean is not an issue there too, although I noticed some conditions where PDCC-equipped Turbo S was more stable than non-PDCC GT3. PDCC also makes it harder to feel when the car "takes a set" in a turn - you still can feel it, it's just harder to sense. So yeah, saying that PDCC makes 911 a bit more Panamera-like makes sense. I'd still skip PDCC on a sporty car but get it on a luxury GT or cruiser.
After all this time, hasn't anyone does a real comparison test so we can look at objective results? The Sport Auto article referenced by the OP is as close as anything I have seen, but still falls short.. How about two similarly equipped 911's, one with RAS and one without, same track, same pro driver? Then we could review the lap times and hear the driver's explanation of the differences, if any. Some data logging might be nice, also.
Apologies if this info is out there, please send a link as I have done a fair amount of looking.
I find it strange that RAS and PDCC are referred to as somewhat similar and making 911 into a mini panamera. I could buy it for PDCC but RAS is completely the opposite. It allows for much more precise and suspension behavior, and easier parking is just a side-benefit. RAS is great because it allows for less give in the rear suspension. All rear suspensions steer to some extent, but passively - the car leans in turn and that causes suspension deflection built in via bushings and other mechanical tricks. The problem with this is that it's always playing catch up to front wheels - front turns, car leans, rear turns. With electric RAS, both axles can turn simultaneously - no lag. Another benefit is that suspension can be less sloppy because is does not need the bushings and other flexible stuff that would allow for the passive wheel deflection. Also, 911 benefits from RAS more than most other cars because it RAS virtually extends wheelbase to behind the engine, which is particularly beneficial for a rear weight biased car. Unless I'm buying some classic car, I'd want RAS. RAS makes the car feel crisper, and there is nothing unnatural about it - the non RAS car does the same rear-wheel wiggle just in a slightly sloppier and laggier way.
PDCC is a different story. For most people, the biggest benefit of it will be more comfort on rough roads. Less lean is not that noticeable in a 911, with its low center of gravity and wide track. Also, clever shocks control side to side transitions (like in S-curves) well enough so lean is not an issue there too, although I noticed some conditions where PDCC-equipped Turbo S was more stable than non-PDCC GT3. PDCC also makes it harder to feel when the car "takes a set" in a turn - you still can feel it, it's just harder to sense. So yeah, saying that PDCC makes 911 a bit more Panamera-like makes sense. I'd still skip PDCC on a sporty car but get it on a luxury GT or cruiser.
That's a great point. Never thought of that. The 993 has a kinematic toe feature that tries to accomplish the same effect, mechanically (at least the behavior of RAS at speeds >51mph).
^ Yes, that's what I was referring to, thanks for digging it up. Newer 911 models and many other cars have it too - it was just called out explicitly when it was new and a differentiator of sorts. You can see that the inherent response lag is described there (deflection after leaning rather than simultaneously with steering input) and the effect is accomplished with flexible elements in suspension, which will flex for many other reasons besides the intended application, creating less controlled suspension behavior. RAS addresses these limitations of the purely mechanical design and has a bonus of easier parking. As long as it's well-calibrated and does not break, it's a great idea.
^ Yes, that's what I was referring to, thanks for digging it up. Newer 911 models and many other cars have it too - it was just called out explicitly when it was new and a differentiator of sorts. You can see that the inherent response lag is described there (deflection after leaning rather than simultaneously with steering input) and the effect is accomplished with flexible elements in suspension, which will flex for many other reasons besides the intended application, creating less controlled suspension behavior. RAS addresses these limitations of the purely mechanical design and has a bonus of easier parking. As long as it's well-calibrated and does not break, it's a great idea.
Yep - not to mention the nightmare of trying to get a proper alignment. Kinematic toe alignment is the bane of 993 ownership, requiring special tools and experience to do properly. It's often screwed up. I agree - RAS should be much more consistent and predictable in a variety of conditions.
I've gone back and forth on the issue of RWS myself over the years. Currently for the type of driving experience I really enjoy I'd say I'm mildly against, but it really depends on both the car and the mission.
I'm surprised to see Sport Auto's test suggesting without RWS could be slightly faster. I'm sure Porsche's internal numbers say otherwise, at least in most situations.
At higher speeds where spirited driving occurs RWS is designed mainly to save time in transitions by limiting the "slip angle" the car adopts. Tires 101: no tire generating G force is moving exactly with the road. Instead it's "slipping" slightly, often imperceptibly, so that it's either rotating at a different speed or in a different direction than the road it's rolling on. That's "slip angle", and in a car corning hard it can be in the range of a few degrees with modern low profile tires...
What does this mean? For the front if the tire's got a large slip-angle in a corner we call it understeer: we're turning the wheel but the car's not fully listening. In the rear we might call slip angle oversteer, though only if the rear's slipping more than the front (forcing us to unwind the wheel to prevent swapping ends). Just because the rear's not oversteering, however, doesn't mean the rear isn't slipping by some amount, in fact at speed it needs to slip for the car to turn.
How does a rear tire "slip" (ie point in a different direction than the road) if it's attached to the car? Simple: slip the entire car. And that's what you're doing when you go around a corner quickly even if you're unaware- the entire car has adopted a slight angle relative to the road. Picture a dirt-track oval racer being far less enthusiastic:
This is all well and good... at least until you need to change direction and go the other way. At that point the car's slip angle needs to switch from perhaps 4 degrees in on one direction to the identical amount in the other. And as you might expect with moving any 1.7 ton object this takes both time and care.
There is however a way to short-circuit this process of changing direction. Don't point the car in a different direction, point just the rear wheels. Pointing wheels takes virtually no time at all and accomplishes the same thing because the tires get the slip angle they need to achieve grip. Thus it's typically a good thing, enough so that Porsche has been doing it since the '60s when it first introduced the 911: they have carefully engineered a "toe-curve" into their cars so that as it rolls into a corner the rears steer inwards. In the late 80s they added the "Weissach axle" to the 964 to augment this with extra passive steering as the car leaned into the corner. Finally recently they introduced full active (powered) rear wheel steering on top of the other systems to point the wheels that much further towards the apex. Keep in mind that all of this wheel pointing only serve to reduce the drift angle the chassis itself adopts: less dirt track racer, more roller coaster on rails.
Therein lies the rub. If you've never raced on dirt I can highly recommend: it's a lot of fun. It's much like managing the very high slip-angles generated by early cars with skinny tires. The enjoyment comes not simply from the slip-angle itself but the process of controlling it: managing under-steer on turn-in then counter-steering to catch the rear on the knife's edge before the back comes around. Getting that rear drift angle just right becomes both instinctive and addicting; from 60s 911 through my 991 GT3 it's one of the things I like most about 911s. It's also exactly what rear wheel steering gives you less of. Even worse it interferes with those slip receptors deep in the seat of your pants that are listening to understand what the car's doing. If you're used to rear wheel drive cars and especially 911s they are expecting the rear to slip in quick corners. And they can get awfully confused/ disappointed when it doesn't.
I get why people like rear wheel steering. I'm certain it's faster, especially in the wet. In fact autocrossing my 991.2 GT3 with RWS I can tell you all the places my car outperforms my buddy's 997.2 GT3 RS. There's also no question it's easier... But take away the clock and there's also no question which I find more challenging, fun and rewarding: it's not the car with RWS. Would I feel comfortable pushing to the point where I could make the same easy distinction on public roads? Generally not in those cars- they are both awfully fast for that. Thus if I wasn't planning for the car to make its living near the limit the extra performance and margin of the RWS would certainly appeal- I'd certainly spec something like the Turbo in a street car role with RWS (if it wasn't standard). For a "fun" car, however, my gut tendency would be to spec without. That said wheelbase, tire type, tuning of each setup, etc make a big difference to this conclusion so I'd like to test the 992 specifically. And I haven't.
I'm not able to track my car and part of the coldness I feel to modern cars is simply how competent and fast they are. It's impossible to unsettle them on public roads. It's practically a video game. Turn the wheel and the car goes.
Sometimes I think the older cars are fun because of their faults and limitations, as much as anything else.
This is the biggest question I have about RAS and I'm unable to answer without a back to back. I could not care less about faster lap times - does it make the car *more* like a video game and *less* analog? If the answer is yes, then I'll take the old mechanical toe correction and call it a day.
I'm not able to track my car and part of the coldness I feel to modern cars is simply how competent and fast they are. It's impossible to unsettle them on public roads. It's practically a video game. Turn the wheel and the car goes.
Sometimes I think the older cars are fun because of their faults and limitations, as much as anything else.
You’re not the only one. I’m pretty passionate about older cars that are fun at legal speeds. If they approached modern safety standards I’d use mine more often. Skinny tires and slip angle are a big part of the attraction.
Originally Posted by rk-d
This is the biggest question I have about RAS and I'm unable to answer without a back to back. I could not care less about faster lap times - does it make the car *more* like a video game and *less* analog? If the answer is yes, then I'll take the old mechanical toe correction and call it a day.
In my experience (not on the 992) I would say RWS reduces the sensation of the car moving around under you and makes it track more precisely- a bit more like it’s on rails. In a video game you get none of those sensations of movement, with RWS you get less. So I’d say that (to me) yes- RWS makes the car feel less analog and more like a video game. It’s largely personal preference, but that’s likely why you’ll see some (even within the factory) advocate foregoing a technology that makes the car objectively “better”. Big asterisk, however: each chassis is different and a 992 is not a 991 or a 997, so I’d really want to do as you’ve suggested and try them back to back.
That's a great point. Never thought of that. The 993 has a kinematic toe feature that tries to accomplish the same effect, mechanically (at least the behavior of RAS at speeds >51mph).
993:
991/992:
In the (German) literature for RAS on the GT3 when it came out, the primary benefit listed was active kinematic toe, not reduced turning radius. I also came from 993 land, and when I was research my Carrera T order, this was one of the things I found. As I have posted before in the 991 forum, RAS does much more than just change turning radius. It is an "active" kinematic toe adjustment - which is why it feels so stable at speed. The hardware is ZF and you can read all about it here:
The 991/992 implementation is the dual-actuator version. Elsewhere on the interwebs an engineer for Ford claimed the ZF hardware is good for 500K miles under normal use (having trouble finding the link again)...but in any case it should be durable.
I could feel rws being more noticeable in 991 over 992, in the 992, i like the 4 over rwd but in 991 version I found rwd to be sweeter and even better with rws
I think the fact that it comes standard on both the GT3 and the GT3 RS speaks volumes of what the engineers in the GT division think of the option. If they were lukewarm on its performance benefits, they would throw it in the trash bin for the sake of weight savings.
there is a video interview with AP, discussing the new at the time 991.1 GT3, and stated that they experimented with RAS during 991 GT3 development and he originally really didn’t want it on the GT cars because it adds 14 pounds, but they tested the 991.1 GT3 extensively on the track with and without it and RAS made the car much faster on the track and also more agile in driving feel, so they made RAS standard on all 991 GT cars.
clearly the head of the Porsche GT car division knows things about optimal 911 performance that sports auto does not....
In the (German) literature for RAS on the GT3 when it came out, the primary benefit listed was active kinematic toe, not reduced turning radius. I also came from 993 land, and when I was research my Carrera T order, this was one of the things I found. As I have posted before in the 991 forum, RAS does much more than just change turning radius. It is an "active" kinematic toe adjustment - which is why it feels so stable at speed.
RAS is really two completely separate features that just happen to use the same hardware. From Porsche [emphasis mine]:
Rear-axle steering
Rear-axle steering enhances performance and suitability for everyday use in equal measure. For particularly nimble handling combined with a significant increase in driving stability.
The advantage for day-to-day driving: at low speeds, the system steers the rear wheels in the opposite direction to that of the front wheels. This leads to a virtual shortening of the wheelbase. The turning circle is reduced, cornering steering response becomes considerably more dynamic and parking is noticeably easier.
The advantage for sporty driving: at high speeds, the system steers the rear wheels in the same direction as that of the front wheels. Driving stability is increased by the virtual extension of the wheelbase and agility is enhanced by the simultaneous steering of the front and rear axles.
The latter of these two separate functions is "active kinematic toe" and the former is "reduced turning radius". These two functions don't have anything to do with each other except that ... they happen to use the same hardware, and they've been grouped together into a compound feature called Rear-axle Steering.
the 992 is essentially mid-engine at this point....
It's not. The engine is entirely behind the rear axle, in the same position as it was in the 991. Only the position of the engine mounts changed to the sides. The weight balance may have shifted to the front regardless due to the larger track up front.
there is a video interview with AP, discussing the new at the time 991.1 GT3, and stated that they experimented with RAS during 991 GT3 development and he originally really didn’t want it on the GT cars because it adds 14 pounds, but they tested the 991.1 GT3 extensively on the track with and without it and RAS made the car much faster on the track and also more agile in driving feel, so they made RAS standard on all 991 GT cars.
clearly the head of the Porsche GT car division knows things about optimal 911 performance that sports auto does not....
not to mention low speed maneuvers are that much easier... turning circle reduced... etc..
01-24-2020 | 02:59 PM
