Mysteries of 991 GT3 PASM?
10-13-2014, 06:47 PM
#16
Nordschleife Master
For those interested, here is a description in general of how PASM works:
Unlike conventional dampers, the PASM map-controlled damper also has an electrically actuated hydraulic bypass valve. The damping effect is provided by the oil in the damper
flowing through a piston valve when the vehicle moves. The smaller the valve cross section, the harder the damping.
With the PASM damper, the oil can flow through a bypass valve, as well as
through a fixed valve in the main piston. The flow can be increased or reduced by opening and closing the valve via a slide,
producing continuous adjustment of the damper force. In the event of a system failure, the valve automatically closes. PASM is then in the hardest damper setting
and thus the safest mode from a driving dynamics point of view (fail-safe principle).
Lane-change module
The damper forces at both axles are immediately increased in response to
rapid steering movements, for example sudden evasive manoeuvres.
This reduces body tilt and instability, thereby significantly improving vehicle
control even in extreme situations.
Vertical-control module
In the Normal programme, the damper force is increased as soon as the
vertical movement of the body, for example when driving over uneven surfaces, rises over a specific threshold value.
This prevents body instability and therefore woolly driving behaviour.
In the Sport programme, the damping is slightly reduced automatically to
improve contact between the road and the wheels as body movements
increase. This also results in a noticeable increase in comfort.
Lateral-acceleration module
If specific, speed-dependent thresholds for lateral acceleration are exceeded
when cornering in the Normal programme, the damper force is increased by
different, defined amounts for each side of the vehicle. This prevents vehicle
instability and significantly increases driving precision. In the event of large vertical movements and high lateral acceleration coinciding,
the higher of the vertical-control and lateral-acceleration damping values is set. This happens if, for example, the damping in the
Sport programme was previously decreased by the vertical-control module.
Brake module
PASM switches to harder damping at the start of a braking operation to reduce vehicle nose-dive when braking. This way, higher brake forces can be
transmitted to the road faster. It switches back to a softer setting (this setting is different for the front and rear axle) after a specific amount of time.
The result is improved road contact, and thus a shorter braking distance, particularly when braking on uneven surfaces.
Load-change module
The damper characteristics for the front and rear axle are individually switched when accelerating heavily, releasing the throttle or changing lanes. In Normal mode,
the dampers are briefly switched to a harder damping setting in these driving conditions. This avoids excessive lifting or diving at the front of the vehicle (“pitching”).
In Sport mode, a softer damper characteristic is briefly selected if necessary to improve traction when accelerating, particularly on uneven surfaces.
Components
The PASM system comprises the following components:
• Four dampers with continuously adjustable damping force (each with one
bypass valve)
• PASM control module
• Two acceleration sensors for detecting vertical movement of the body
(one at the damper dome at the front right and one at the rear left).
Further signals such as lateral acceleration, steering angle, travel speed,
brake pressure, engine torque, etc. are read in via the CAN bus.
The system measures body movements via one acceleration sensor on each
axle (front and rear). Values such as lateral acceleration, steering wheel angle, vehicle speed and information on possible braking operations are provided by the PSM via the CAN bus.
Engine rpm and torque values are provided by the Motronic via the CAN bus.
Unlike conventional dampers, the PASM map-controlled damper also has an electrically actuated hydraulic bypass valve. The damping effect is provided by the oil in the damper
flowing through a piston valve when the vehicle moves. The smaller the valve cross section, the harder the damping.
With the PASM damper, the oil can flow through a bypass valve, as well as
through a fixed valve in the main piston. The flow can be increased or reduced by opening and closing the valve via a slide,
producing continuous adjustment of the damper force. In the event of a system failure, the valve automatically closes. PASM is then in the hardest damper setting
and thus the safest mode from a driving dynamics point of view (fail-safe principle).
Lane-change module
The damper forces at both axles are immediately increased in response to
rapid steering movements, for example sudden evasive manoeuvres.
This reduces body tilt and instability, thereby significantly improving vehicle
control even in extreme situations.
Vertical-control module
In the Normal programme, the damper force is increased as soon as the
vertical movement of the body, for example when driving over uneven surfaces, rises over a specific threshold value.
This prevents body instability and therefore woolly driving behaviour.
In the Sport programme, the damping is slightly reduced automatically to
improve contact between the road and the wheels as body movements
increase. This also results in a noticeable increase in comfort.
Lateral-acceleration module
If specific, speed-dependent thresholds for lateral acceleration are exceeded
when cornering in the Normal programme, the damper force is increased by
different, defined amounts for each side of the vehicle. This prevents vehicle
instability and significantly increases driving precision. In the event of large vertical movements and high lateral acceleration coinciding,
the higher of the vertical-control and lateral-acceleration damping values is set. This happens if, for example, the damping in the
Sport programme was previously decreased by the vertical-control module.
Brake module
PASM switches to harder damping at the start of a braking operation to reduce vehicle nose-dive when braking. This way, higher brake forces can be
transmitted to the road faster. It switches back to a softer setting (this setting is different for the front and rear axle) after a specific amount of time.
The result is improved road contact, and thus a shorter braking distance, particularly when braking on uneven surfaces.
Load-change module
The damper characteristics for the front and rear axle are individually switched when accelerating heavily, releasing the throttle or changing lanes. In Normal mode,
the dampers are briefly switched to a harder damping setting in these driving conditions. This avoids excessive lifting or diving at the front of the vehicle (“pitching”).
In Sport mode, a softer damper characteristic is briefly selected if necessary to improve traction when accelerating, particularly on uneven surfaces.
Components
The PASM system comprises the following components:
• Four dampers with continuously adjustable damping force (each with one
bypass valve)
• PASM control module
• Two acceleration sensors for detecting vertical movement of the body
(one at the damper dome at the front right and one at the rear left).
Further signals such as lateral acceleration, steering angle, travel speed,
brake pressure, engine torque, etc. are read in via the CAN bus.
The system measures body movements via one acceleration sensor on each
axle (front and rear). Values such as lateral acceleration, steering wheel angle, vehicle speed and information on possible braking operations are provided by the PSM via the CAN bus.
Engine rpm and torque values are provided by the Motronic via the CAN bus.
10-14-2014, 06:36 PM
#18
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Mike, thanks for posting that info, it's more detailed than what I've read previously.
Sounds like PASM is fairly sophisticated, with ability to adjust the damping continuously (I don't know how frequently and rapidly), and separately at each wheel based on input from multiple sensors.
And I suspect that the PASM in the 991 GT3 takes all of this to another level, in terms of the inputs it uses, sophistication of the algorithms, and real-time adjustability of damping. I'd wager that the PASM a big factor in the performance of this car - more than many people realize.
Sounds like PASM is fairly sophisticated, with ability to adjust the damping continuously (I don't know how frequently and rapidly), and separately at each wheel based on input from multiple sensors.
And I suspect that the PASM in the 991 GT3 takes all of this to another level, in terms of the inputs it uses, sophistication of the algorithms, and real-time adjustability of damping. I'd wager that the PASM a big factor in the performance of this car - more than many people realize.
10-14-2014, 06:49 PM
#19
Nordschleife Master
And I suspect that the PASM in the 991 GT3 takes all of this to another level, in terms of the inputs it uses, sophistication of the algorithms, and real-time adjustability of damping. I'd wager that the PASM a big factor in the performance of this car - more than many people realize.
The electronically controlled system minimizes the oscillations and vibrations of the entire drivetrain, especially the engine, and combines the benefits of a hard or soft engine mounting arrangement.
A hard engine mounting delivers optimum dynamic performance because it offers the highest degree of handling precision possible. Soft engine mounts, on the other hand, minimize oscillations and vibrations. While comfort is improved on uneven road surfaces, this comes at the expense of dynamic performance.
Our engineers have solved this problem by enabling the engine mounts to adapt their stiffness and damping performance to changes in driving style and road surface conditions. This has been achieved by use of a fluid with magnetic properties in interaction with an electrically generated magnetic field.
Handling is perceptibly more stable under load change conditions and in fast corners. The dynamic engine mount system also reduces the vertical oscillations of the engine when accelerating under full load. The results are greater and more uniform drive force at the rear axle, increased traction and better acceleration. Whenever a less assertive driving style is adopted, the dynamic engine mounts automatically soften to provide a greater level of comfort.
A hard engine mounting delivers optimum dynamic performance because it offers the highest degree of handling precision possible. Soft engine mounts, on the other hand, minimize oscillations and vibrations. While comfort is improved on uneven road surfaces, this comes at the expense of dynamic performance.
Our engineers have solved this problem by enabling the engine mounts to adapt their stiffness and damping performance to changes in driving style and road surface conditions. This has been achieved by use of a fluid with magnetic properties in interaction with an electrically generated magnetic field.
Handling is perceptibly more stable under load change conditions and in fast corners. The dynamic engine mount system also reduces the vertical oscillations of the engine when accelerating under full load. The results are greater and more uniform drive force at the rear axle, increased traction and better acceleration. Whenever a less assertive driving style is adopted, the dynamic engine mounts automatically soften to provide a greater level of comfort.
10-14-2014, 07:10 PM
#20
The 991 GT3 chassis stiffness must in part be due to this chameleon feat of being brilliant on road and track. I dont have prior ownership experience of 997 gen GT3 but compared to my old 996 GT3 this is one very stiff chassis indeed (lifts a wheel every time I exit my gas station drive way apron at an angle!).
10-14-2014, 07:57 PM
#21
Rennlist Member
Manifold. I suspect there is some truth in what you say, although the 997 GT3 also ran PASM, however like all things technology has no doubt improved since the previous generation. It would certainly explain how the new GT3 has been pretty much set up to juts jump into and drive without any need to make multiple settings changes. Apart from hitting the sport auto button the car is frankly perfectively effective at our local tracks with the standard suspension settings (our tracks have huge elevation and very tight corners). That said on a nice warm day where traction is plenty and the track is dry the firm setting does have some small benefits. The fact PTV and electronic engine mounts are incorporated may in fact have as much a noticeable effect as the the improved PSAM although its impossible to isolate their effectiveness and contribution alone.
Yes my 993 track car lifts a wheel too (it has a half cage and three point front brace which probably makes it twice as stiff as the original factory chassis which wasnt very stiff at all!). However the difference with the 991 GT3 is not only stiffness but how little travel the wheel has when suspended. I had to get out of the car to see it to believe it when one of my track mates pointed it out but of the right wheel for example encounters a drop in elevation by say 80-100mm from the other three wheels it will just basically "hang in space". I had not experienced this before. Essentially there is very little spring here Im guessing this means the strut is definitely part of the magic! There is a picture of this in EVO magazine in the article Car of the Year 2013 and they make mention of it also surprising them. A screen shot of it is below along with a picture of my car on the hoist so you can see "full droop" on front and rear wheels (not well I confess due to the angel of the pitcure!). Comparing it to a 997.2 GT3 on the same hoist next to mine when the engine swap was being done it appeared there was at least another couple of inches travel in the 7.2 GT3 front suspension but that could have been optical illusion I guess...the techs however did comment the wheels hardly droop...
Yes my 993 track car lifts a wheel too (it has a half cage and three point front brace which probably makes it twice as stiff as the original factory chassis which wasnt very stiff at all!). However the difference with the 991 GT3 is not only stiffness but how little travel the wheel has when suspended. I had to get out of the car to see it to believe it when one of my track mates pointed it out but of the right wheel for example encounters a drop in elevation by say 80-100mm from the other three wheels it will just basically "hang in space". I had not experienced this before. Essentially there is very little spring here Im guessing this means the strut is definitely part of the magic! There is a picture of this in EVO magazine in the article Car of the Year 2013 and they make mention of it also surprising them. A screen shot of it is below along with a picture of my car on the hoist so you can see "full droop" on front and rear wheels (not well I confess due to the angel of the pitcure!). Comparing it to a 997.2 GT3 on the same hoist next to mine when the engine swap was being done it appeared there was at least another couple of inches travel in the 7.2 GT3 front suspension but that could have been optical illusion I guess...the techs however did comment the wheels hardly droop...
10-14-2014, 08:56 PM
#22
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^ Hmm, the lack of droop on the lift makes me think the springs are actually fairly stiff (?), since dampers should have no effect in that static situation. So to still get a comfy ride on the road, the damping should be soft. But that would normally result in a floaty car. Yet, the car isn't floaty. So maybe the damping is being adjusted very actively, and/or there's more than one-way control of the damping (eg, multiple bypass valves)?
In other words, I'm (merely) speculating that the ability of the suspension to work so well in such diverse situations is a result of control of damping which is a quantum level more sophisticated than the other 991s such as the 991 C2S. And technologically, there's really nothing preventing such sophistication, it's just a matter of cost.
Hope guys better versed in suspensions will chime in ...
BTW, car looks badass on the lift.
White is still my default color, couple weeks to change my mind before order locks.
In other words, I'm (merely) speculating that the ability of the suspension to work so well in such diverse situations is a result of control of damping which is a quantum level more sophisticated than the other 991s such as the 991 C2S. And technologically, there's really nothing preventing such sophistication, it's just a matter of cost.
Hope guys better versed in suspensions will chime in ...
BTW, car looks badass on the lift.
White is still my default color, couple weeks to change my mind before order locks.
10-14-2014, 09:09 PM
#23
Rennlist Member
Manifold. I think you may be right. Chick Morelands (Elephant Racings) excellent photo analysis of the GT3 suspension on his own car shows a single spring of reasonable thickness with no keeper, but gives few other clues as to what is going on here....