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Porsche Active Suspension Management (PASM)
For the first time in the 911 series, the new 911 Carrera S is offered with a
chassis that has actively adjustable dampers. The PASM is standard equipment
for the 911 Carrera S. In comparison with the standard chassis, the vehicle with
the PASM chassis lies 10 mm lower.
The newly developed, variable damping system 'Porsche Active Suspension
Management" helps to do justice to the demands for a modern chassis.
The aim was to maintain the high comfort level of the standard chassis while
increasing performance at the same time.
This technical data is merely intended to provide an overview. As it is subject
to change during the course of the model year, the data published in the
Technical Manual always remains authoritative.
The driver can choose between two programmes: 'Normal" and 'Sport", both
are selected via a button on the centre console.
The function light in the button comes on with the Sport setting. At the same
time a damper icon appears in the instrument cluster display together with the
text 'PASM Sport". When the normal programme is activated, the damper icon
appears accompanied by the text 'PASM Normal". The text and icon remain
visible for 4 seconds and then disappear.
PASM combines two chassis rolled into one: One with equal measures of
sportiness and comfort and one entirely dedicated to sportiness and the race
circuit.
Normal mode (the basic setting when the vehicle starts) on the one hand
provides the same excellent comfort as the standard chassis, on the other
hand, the chassis approximates the sportiness of the sports chassis when
regulative interventions are made with active PASM. In conjunction with specially
developed software modules, the PASM ensures excellent performance and
even greater driving safety in extreme situations.
Operating principles of PASM
PASM selects the required damper hardness for each individual wheel from a
precisely co-ordinated map in both the Normal and the Sport programme.
The possible damper settings range from comfortable to decidedly sporty.
Both programmes, which overlap slightly in some areas, are additionally
superimposed with five special software modules to provide the optimum
damper settings for every driving condition.
The system automatically selects the appropriate damper hardness based on
the PASM programme selected and the driving condition identified.
The Normal programme offers comfortable settings with low damper forces.
Special control algorithms in the PASM software modules enable the chassis to
offer greater active driving safety in extreme driving situations, even with the
Normal programme. To increase driving safety at higher speeds, the dampers
are automatically switched to a harder damper setting as speed increases.
The dampers switch to a hard characteristic when Sport mode is activated.
This offers superior agility and excellent steering precision on uneven surfaces.
If the system detects an uneven driving surface in Sport mode, it immediately
switches to a softer characteristic to improve contact with the road surface.
PASM selects the optimum damper setting for this softer characteristic from
the Sport map.
Since extremely hard damping is not always the ideal solution in every driving
situation (depending on the driving surface, the vehicle may start to bounce or
shift), the intentional overlap between the Normal and Sport maps allows a
noticeably soft setting to be selected if necessary. The customer gets an
'active sports chassis" which automatically responds to the actual road surface
and switches from a hard, sporty damping setting to a comfortable range as
necessary. PASM switches back to the original characteristic as soon as the
road surface is smooth enough.
The following is a detailed description of the five software modules overlapping
Normal and Sport mode.
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.
- One button for selecting the programme (Normal or Sport)
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.
Acceleration sensor
The acceleration sensors are attached to the front right and rear left spring
strut domes, respectively.
The arrangement allows lifting, pitching and rolling movements of the body to
be measured.
PASM control module
In left-hand drive vehicles, the PASM control module is installed on the front wall
(footwell passenger's side) at the right. In right-hand drive vehicles it is installed
on the front wall (footwell passenger's side) at the left.
Shock absorbers
Unlike conventional dampers, the PASM map-controlled damper also has an
electrically actuated hydraulic bypass valve (arrow).
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).
