DE VELO PMENT CHA S SIS
50 YEARS OF THE PORSCHE 911 OPTIMISATION OF CHASSIS CONTROL AND BRAKE SYSTEMS The Porsche 911 is celebrating its 50 th birthday this year. From 1963 to the present day in 2013, this sports car has been synonymous with racing success and driving pleasure combined with a high level of everyday practicality. Continuous technical improvement of the Porsche 911 over seven generations and five decades, always with the aim of achieving perfection, are certainly good reasons why the Porsche 911 is seen as the benchmark for all other sports cars. This report describes its pioneering role in chassis technology, with its innovative chassis control and brake systems. These include such features as ABS, PSM and rear-axle steering, as well as carbon composite brake discs.
14
AUTHORS
DR. MANFRED HARRER is Director Chassis, Vehicle Dynamics and Performance at the Dr. Ing. h. c. F. Porsche AG in Weissach (Germany).
DR.-ING. HANS-JÜRGEN GÖRICH is Manager of Chassis Development – Strategic Projects at the Dr. Ing. h. c. F. Porsche AG in Weissach (Germany).
DIPL.-ING. UWE REUTER is Technical Manager in Advance Development Chassis at the Dr. Ing. h. c. F. Porsche AG in Weissach (Germany).
MILESTONES IN CHASSIS DEVELOPMENT
As examples of the high level of technical development of the Porsche 911 sports car over seven generations since 1963 [1], the chassis control systems and brakes are examined in detail in this report. The seven generations range from the 911 original model and the 911 G series followed by the 964, 993, 996 and 997 though to the current 991, ➊. Indeed, the evolution of this iconic sports car also reflects the milestones in chassis development over the past 50 years [2]. CHASSIS CONTROL SYSTEMS FOR SEVEN GENERATIONS OF THE 911
The introduction of chassis control systems opens up completely new opportunities for influencing vehicle dynamics and the ride comfort of a vehicle. In the 911, chassis control systems are used to improve dynamics and safety without making compromises in driving enjoyment. The basis for these chassis control systems are mechatronic components controlled by an ECU. These were not ready for series production until the end of the 1980s, which meant that it was only in the second half of its fi ftyyear history that the chassis of the 911 was characterised by chassis control systems.
TRACTION CONTROL SYSTEMS
Simultaneously with the introduction of an electronically controlled four-wheel drive system in the 964 Carrera 4 in 1989, the 911 was equipped for the first time with an anti-locking brake system (ABS), ➋. This was a second-generation three-channel ABS from Bosch with an electronic control unit of the Bayreuth series. The input signals come from four wheel speed sensors. Since 1989, the hybrid-design ABS control unit has been directly flanged to the hydraulic unit. This eliminates the need for complex connecting lines, and the weight of this generation (ABS 2E) has been reduced compared to earlier versions. The traction control system in the 911 has been continuously updated to keep it at the cutting edge of technology. The 993 model from 1993 was equipped with generation 5.0 of the proven Bosch ABS with solenoid valves. This is a four-channel ABS with individual control of each wheel. Following this, Bosch ABS 5.3 was launched in series production in the fifth 911 generation (996). The numerous integrated control functions are summarised under the term Porsche Stability Management (PSM). The 996 Carrera 4 was equipped for the first time with a chassis control function that was tuned for a sporty driving style. Porsche waited for the availability of a vehicle stabilisation function (ESP) in the 911 until an application was found that also satisfied
DIPL.-ING. GEORG WAHL is Director Development Chassis Axle Systems at the Dr. Ing. h. c. F. Porsche AG in Weissach (Germany).
Generation
Year
Type
Wheelbase [mm]
7th
2011-present 991
2450
6th
2005-2011
997
-
5th
1997-2006
996
2350
4th
1993-1998
993
2272
3rd
1988-1994
964
2271
2nd
1973-1989
911 G
2268
1st
1963-1973
911
2211
➊ Production periods of the seven generations (left in the table) and the development of the wheelbase in the seven generations of the Porsche 911 (right, shown are only the models highlighted in red) 12I2013
Volume 115
15
DE VELO PMENT CHA S SIS
ACTIVE DAMPING SYSTEM
In 2004, the 997 was equipped for the first time in the history of the 911 with an active damper control system called Porsche Active Suspension Management (PASM), ②. This system combines both sporty and comfort-oriented damper characteristics. These can either be preselected by the driver or the damper forces automatically adapt to the driving situation. The features of PASM on the
16
Rear-Axle Steering
HAL
EPS
Electric Power Steering Porsche Dynamic Chassis Control
PDCC
PTV Porsche Torque Vectoring PASM Porsche Active Suspension Management RDK Tyre Pressure Monitoring ABS
Anti-Locking Brake System
TC
Traction Control
PSM Porsche Stability Management (from 996 Carrera 4) ABD Active Brake Diff. Transfer case All-wheel drive system Controlled rear differential only on Carrera 4
964 1989
PTM Porsche Traction Managem. / Hang-on for FA
Four-wheel drive system with a viscous coupling
993 1993
PTV+ Electronically controlled
Mechanically controlled rear differential
996 1997
997 2004
rear differential
997 2nd gen. 2008
991 2011
2013
➋ Development of chassis control systems from ABS in 1989 to HAL in 2013
10
8
Agility index
sporty drivers. The intervention thresholds of the ESP functions were chosen in such a way as to improve driving safety in normal operation, for example on public roads, without impairing driving enjoyment. When the 911 is being driven on the race track or in special situations (for example, driving with snow chains), ESP can be deactivated. From 2004, Porsche fitted all 997 Carrera 2 and Carrera 4 with PSM as standard equipment. On the function side, there was the addition of a sports set-up coupled with the ‘Sport’ mode button, a prefill function for the brake system when the accelerator pedal is released quickly and connections to the all-wheel control of the 997 Carrera 4. The second generation of the 997 saw the series introduction of a brake assist and hillstart assist function, which maintained the pressure in the brake lines for a limited period as required. The further development of the traction control system had the objective of expanding the range of functions while at the same time reducing system weight. For example, the pre-charger pump in the 997 was eliminated. This provided a weight reduction of around 3 kg. The Bosch ABS 8.0 system used in the 997, for example, weighed 4.7 kg less than the ABS 2 fitted to the 964. The current seventh generation of the 911 (type 991) uses the further optimised and more compact ESP9 hardware from Bosch. On this platform, driver assistance functions such as Adaptive Cruise Control (ACC) in combination with a radar sensor are available in a 911 for the first time. A further step in supporting the agile chassis set-up of the 991 is Porsche Torque Vectoring (PTV). Targeted braking interventions on the inner wheel on a bend increase the vehicle’s yaw agility depending on the driving situation, which is reflected in the agility index, ➌, for the 991.
6
4
2
0 1963 911
1973 911 G series
1988 964
1993 993
1997 996
2004 997
2011 991
➌ Development of the agility index of the seven generations of the 911
997 are shown in ➍. The input signals used for the PASM control unit are longitudinal and lateral acceleration, the steering angle, driving speed, brake pressure and engine torque. REAR-A XLE STEERING
The current Porsche 911 Turbo and GT3 (type 991) are fitted for the first time with electromechanical rear-axle steering (HAL) as standard equipment [3], ④. The rear wheels are steered by two independently operating electromechanical actuators integrated into the multi-link rear axle. Due to the package arrangement specific to the 911, with the engine and transmission in the rear of the vehicle, it was not possible to use a central steering actuator (for example, similar to frontwheel steering) with a rigid connection between the left and right rear wheel. The
ECU integrated into the actuators calculates an adjustment variable for the respective rear-wheel steering angle on the basis of various signals, such as the steering angle, vehicle speed and lateral acceleration. This wheel steering angle is controlled by the lifting motion of the actuator – consisting of an electric motor, a control unit and a spindle drive – and a corresponding control arm connection to the wheel carrier. The clear functional objective of rear-axle steering was to further improve the lateral dynamic handling properties of the vehicle. Rear-axle steering on the Porsche 911 fundamentally enables two principles to act on the steering direction of the front wheels. The first principle is that, at driving speeds below 50 km/h, the rear wheels are steered in the opposite direction to that of the front wheels. This moves the instantaneous centre of rota-
tion forwards, which has the virtual effect of shortening the wheelbase, ①, and reducing the turning circle. As a result, the 911 seems even more agile and easier to drive at low speeds. At driving speeds above 50 km/h, the system steers the rear wheels in the same direction as that of the front wheels. The instantaneous centre of rotation is moved to the rear, which has the virtual effect of extending the wheelbase. The stability properties of the Porsche 911, particularly during sporty or dynamic driving manoeuvres, are further enhanced by the faster build-up of lateral forces at the wheels and minimisation of the car’s yaw reaction. ACTIVE ANTI-ROLL SYSTEM
Porsche Dynamic Chassis Control (PDCC) is an active anti-roll system that is being introduced on the 991 for the first time, ➎. Anti-roll bars with hydraulic adjustments keep the body horizontal when cornering, as well as influencing self-steering behaviour in the limit range
and suspension comfort when the wheels are loaded on one side only. The performance-oriented set-up offers a lap time potential of approximately 5 s on the north loop of the Nürburgring. BRAKE SYSTEMS FOR SEVEN GENERATIONS OF THE 911
The Porsche 911 has been able to defend and further consolidate its status in the field of brakes as the “braking world champion” over decades as a result of the transfer of technologies from motor racing, ➏. Long-distance races in particular, including regular participation in the classic “24 Hours of Le Mans”, provided expertise that significantly improved the endurance and braking performance of the series-production vehicle. THE FIRST GENERATION – 911 ORIGINAL MODEL
The first 911 already featured disc brakes on all four wheels as standard equipment.
Porsche Active Suspension Management (PASM) : Four map-controlled dampers with gate-controlled electromagnetic damping force valve : : : :
Rear-Axle Steering by Porsche (HAL) : Individual electromechanical actuators instead of track rods
Control unit and selection of Sport mode (damper force mode) via buttons on the centre console Acceleration sensor in each spring strut dome Rear-axle damper with aluminium m oute outer tube Optimised rear-axle support bearing aring
Active suspension managementt comp components on the 996
Rear-axle steering actuator on the 991
➍ Active damper control and rear-axle steering
Porsche Dynamic Chassis Control (PDCC)
From the start of series production, Porsche equipped the 911 with fixed-caliper brakes with a high caliper stiffness and low inherent weight. The rear-axle disc brake included a duo-servo drum brake in the disc head for the parking brake function. This drum-in-head concept is still applied today and is a design feature of the 911 models. Whereas the brake system of the basic models was fitted with two-piston grey cast iron fixed calipers, the S model from 1969 was equipped with more weight-efficient, ventilated aluminium fixed calipers. The brake disc diameters were 282 mm at the front axle (FA) and 290 mm at the rear axle (RA). In 1967, a dual-circuit brake system and ventilated brake discs were introduced into the L and S versions of the 911. All other detailed optimisation measures for the first 911 generation were aimed at increasing the stiffness of the brake calipers while maintaining the lowest possible weight, thus adapting them to the higher performance resulting from the increased displacement of the boxer engines. The brake calipers of the series-production vehicles in the 1960s and 1970s were still provided from a supplier’s range and were optimised for use in the 911. Porsche initiated new developments for the S models, for example aluminium brake calipers. Porsche was already using its own brake designs in motor racing. The beginning of Porsche’s own brake development was in 1962, ⑥. The Porsche 804 Formula 1 racing car used internally gripped aluminium fixed-caliper brakes of Porsche’s own design for motor racing. The principle of an internally gripped brake disc did not find its way into the series-production vehicle as insufficient disc cooling was available. The example shows, however, that the company was certainly prepared to take unconventional approaches in the search for a high-performance brake system.
Concept Anti-roll bar adjustment by means of : hydraulic linear cylinders (two per axle) : hydraulic pressurisation (radial piston pump)
THE SECOND GENERATION – 911 G SERIES
Customer Benefits :
Improved comfort through minimisation of the roll angle and roll copying
:
Reduced steering angle requirement
:
Increased performance through variable roll moment distribution and minimisation of the camber angle on bends
Influence on the Yaw Moment with roll stabilisation at the front axle of the 991
➎ Concept and customer benefits (left) of Porsche Dynamic Chassis Control (PDCC) as an active anti-roll system and yaw moment control at the front axle 12I2013
Volume 115
The second generation of the 911 was characterised by carefully selected adaptation developments aimed primarily at guaranteeing high braking performance as well as sensitive brake response. For example, the models up to the middle of the 1970s still had no brake servo, before increasing
17
DE VELO PMENT CHA S SIS
Racing Cars
1962
2005 1982
1976
1973
: Al monobloc fixed caliper : One-piece Al fixed caliper : Double caliper : Beginning of Porsche : Four-piece Al fixed caliper : Brake pads supported brake development : Cross-drilled brake disc by bolts : Al fixed-caliper brake : Involute cooling ducts : Internally gripped 956/962 brake disc
: PCCB series Porsche Supercup
997 Cup
935 Group 5
917/30 Formel 1 Type 804
2011 2000
: Six-piston Al monobloc : PCCB Al monobloc fixed caliper fixed caliper : Al monobloc fixed : Cross-drilled brake caliper disc : Cross-drilled brake 991 Turbo : Involute cooling disc ducts : Involute cooling : Brake pads supported ducts by bolts : Brake pads supported by bolts ➏ Timeline of the Porsche brake system pedigree – from 996 GT2 motor racing into road cars 1996
1977
Road Cars 1969
: Four-piece Al fixed caliper : S models: : Cross-drilled Al two-piston fixed brake disc calipers for FA : Involute cooling ducts
1963 : Two-piston ton cast iron fixed caliper : Disc brakes on all wheels
911 G series Type 930
vehicle weights led to the introduction of a 7" and later an 8" brake unit. As a rule, the most powerful S and Carrera models were equipped with two-piston aluminium fixed calipers at the front axle. On the 911 Turbo 3.3 (type 930), an increase in engine output from 260 to 300 PS in 1978 resulted in the adoption of the brakes from the 917 long-distance racing car, ⑥. This was a four-piston aluminium fixed-caliper brake with a crossdrilled brake disc with involute ventilation ducts. This brake caliper is characterised by its axial screwing with four screws and side ribs for caliper cooling. The crossdrilling of the ventilated brake discs has the purpose of reducing the disc temperature and improving wet response behaviour, and has become the identifying feature of Porsche high-performance brakes. The brake system was reworked within the framework of the possibilities offered by the design concept with the introduction of the 3.2 l Carrera engine from 1984. New features were, for example, a brake pressure limiter at the rear axle and a mechanical vacuum pump for brake boosting. THE THIRD GENERATION – 964
A further milestone was the axially screwed aluminium fixed caliper, which had become the standard brake caliper in the 964 and 993 series-production models. An increase in the size of the brake system and therefore an improvement in braking performance was made possible
18
996
on the 964 by the use of 17" wheels. At the beginning of the 964 series, the aluminium fixed calipers at the front axle were fitted with four pistons and those at the rear axle with two pistons, although these were also converted to four-piston calipers from model year 1992. In the design of the brake system, the typical axle load distribution of the 911 models is taken into consideration by applying a high proportion of braking force at the rear axle. An external indication of this is the almost identical brake disc diameters (298/299 mm) of the basic models 964 Carrera 2 and 964 Carrera 4. As the Bosch ABS 2 anti-locking brake system was not yet able to perform electronic brake force distribution at this time, a brake pressure limiter valve was used at the rear axle. Cross-drilled brake discs with diameters of 322 mm at the front and 299 mm at the rear transferred Porsche’s expertise in weight reduction and wet response behaviour from motor racing into series production in the 964 Turbo 3.6 model. Its high-performance brake system reduced the braking distance from 100 km/h to between 35 and 36 m, a remarkable figure for series-production vehicles at this time. A particular feature of the brake system of the 964 Carrera 4 four-wheel drive vehicles is hydraulic brake boosting, which was combined with the hydraulic energy supply of the four-wheel control system. A hydraulic servo pump supplies a diaphragm accumulator which not only pressurises the hydraulic brake
booster of the ABS-controlled brake system but also supplies brake fluid to the actuator for the longitudinal and rearaxle differential lock. Brake boosting in the 964 Carrera 2 models was performed by a conventional 8" vacuum booster. THE FOURTH GENERATION – 993
For the 993, the diameter of the frontaxle brake discs on the basic models was increased to 304 mm (993 Turbo: 322 mm). The discs at the rear axle, as on the 964, already had a diameter of 299 mm. The maximum full deceleration provided by the brake system already exceeded the figure of 1.2 g at this time. The electrohydraulic power supply for the brake system of the 993 Carrera 4 was now also used on the 993 Turbo models. A power supply for the four-wheel drive system was no longer required due to the conversion to a four-wheel drive system with a viscous coupling. THE FIF TH GENERATION – 996
Porsche’s patented aluminium monobloc fixed-caliper technology, ⑥, which was first fitted on the 996, is characteristic of high-performance Porsche brakes that offer the ultimate in caliper stiffness combined with minimum weight, low volume, rapid response and maximum modulation. Initially developed for the Porsche 935, monobloc technology was transferred from motor racing into the
series-production vehicles. Radial screwing to the wheel carrier and support for the brake pads by means of axially arranged bolts are further design features of this brake caliper generation. The piston diameters of the four-piston calipers are graduated in order to prevent tangential brake pad wear. The front axle has piston diameters of 36 and 44 mm, while the rear axle calipers have piston diameters of 28 and 30 mm. In order to cool the brake fluid, the connection lines of the pressure chambers are mounted externally on the caliper. The diameter of the front brake discs increased to 318 mm compared to the predecessor model, while the diameter of the rear axle brake discs remained at 299 mm. The discs were still cross-drilled and characterised by involute cooling ducts (at the front axle). The wear limit of the brake pads was monitored individually at each pad. The brake servo followed the trend of reduced pedal forces and was supported by a 10" vacuum brake booster with an ejector pump at the engine. For effective brake cooling, the 996 series featured air ducts at the rear. The brake caliper located behind the centre of the wheel allows cooling air to flow to the brake disc without hindrance. In 2000, the Porsche Carbon Composite Brake (PCCB) was introduced as standard equipment on the 996 GT2, and was available as optional equipment for the Carrera models. These brake discs with a diameter of 355 mm are fitted with six-piston monobloc fixed calipers made of aluminium. The PCCB achieved a reduction in unsprung mass of 15 kg per vehicle.
THE SIXTH GENERATION – 997
The 997 also adopted the dimensioning of the monobloc fixed-caliper brakes. For the newly introduced S engine, a brake system with the dimensions of the predecessor model, the 996 Turbo, was used. The brake discs at the front and rear axle had a diameter of 330 mm. The S brake calipers can be recognised by their red paintwork, while the basic brake calipers are finished in black and the PCCB brake calipers in yellow. Brake boosting on the 997 was improved by the use of a 9/9" tandem brake unit with a motor-driven vacuum pump. The PCCB was available as an option on the 997 GT3. For the GT and Turbo versions, the disc diameters were 380 mm at the front axle and 350 mm at the rear, while the Carrera and Carrera S had 350 mm discs at both the front and rear. The brake disc on the 997 is connected to a weight-reduced aluminium brake-disc chamber. From 2005, the PCCB was further developed under motor racing conditions in the Porsche Supercup.
A new feature of the S and Turbo models is a weight-optimised six-piston monobloc fixed caliper made of aluminium, ⑥. For the parking brake, the drum-in-head concept, which has proven itself since the launch of the 911, is actuated electromechanically with the aid of a spindle actuator. DEVELOPMENT OF THE BRAKE PEDAL FORCE
An interesting side aspect is the development of the brake pedal force in the 911. The introduction of a 7" vacuum brake servo with a moderate boost effect on the 911 SC made it possible to reduce brake pedal forces for the first time. However, one can recognise the early end point of the boosting effect by the slight bend in the force-deceleration characteristic curve, ➐. Only with the introduction of the 996 and subsequent generations did Porsche follow the trend of reducing brake pedal forces to a level that was sufficient for a sports car but was adapted to customers’ preferences at the time.
THE SEVENTH GENERATION – 991
SUMMARY AND CONCLUSION
The new brake generation for the 991 is characterised by using common parts in both the Carrera and Boxster series. The dimensions of the wheel brakes were largely adapted by varying the disc diameters and thicknesses. At the front axle, a brake disc with a diameter of 340 mm (basic model 330 mm) is used. The rear axles in the basic and S models are fitted with discs with a diameter of 330 mm.
The Porsche 911 has been continuously further developed with every model series – but without sacrificing its character or its authenticity. Over the past 50 years and with every generation, Porsche’s engineers have succeeded in improving the car’s handling while ensuring the ultimate in driving stability and precision. Consistent lightweight design in the chassis combined with innovative manufacturing processes have enabled the lightweight index to be raised to a top level over seven generations. An increasing number of chassis control systems focused consistently on optimising vehicle dynamics have further sharpened the character of the 911 models. REFERENCES [1] Krehl, D.: 50 Jahre Porsche 911. In: ATZ 115 (2013), No. 9, pp. 657-662 [2] Harrer,M.; Görich, H.-J.;Reuter, U.; Wahl, G.: 50 Jahre 911 – Perfektionierung des Fahrwerks. Teil 1 bis 3. In: www.springerprofessional.de, September to October 2013 [3] Lunkeit, D.; Weichert, J.: Performance-oriented Realization of a Rear Wheel Steering System for the Porsche 911 Turbo. 4. Internationales Münchner Fahrwerk Symposium, 2013, München
➐ Reduction in brake pedal force while improving deceleration, illustrated by the Porsche brake from 1968 to 2011 12I2013
Volume 115
19
