TECH: VARIORAM, PADM, PTV PLUS
The 991’s controversial electrically assisted power steering system overshadowed the arrival of PTV Plus and PADM, but all three technologies are intimately related…
As the 911 neared its fiftieth anniversary, Porsche launched the 991, effectively a clean-sheet design, ditching 996-based 997 architecture in a bid to start over with an optimum basis for future development. This new beginning allowed Porsche’s engineers to alter the proportions of the 911 while focusing on improving overall efficiency and reducing weight. It also enabled the introduction of a raft of new technologies. As demonstrated by reaction to the arrival of electrically assisted power steering, not every 991 development was universally welcomed, but other features, including Porsche Torque Vectoring Plus (PTV Plus) and Porsche Active Drivetrain Mounts (PADM), have seriously advanced the 911 driving experience. In this article, we'll explain how it all works.
PADM is a relatively simple idea implemented in characteristically clever Porsche fashion. In a road car, the engine mounts are always compromised. On one hand, you want compliance in the mount, which reduces the transmission of noise, vibration and harshness (known in the industry as NVH) from the engine to the car body. A soft mount, however, can introduce movement between the engine/transmission and the body, which can affect the feel of the host vehicle, particularly when being driven enthusiastically on a winding road or race circuit, as many Porsches are.
Picture a 911 being pushed hard through a series of S-bends demanding quick changes of direction. Now picture standard rubber-based engine mounts and what happens to them during this manoeuvring: as the driver turns the steering wheel into the first bend, the car follows suit, but due to compliance in the mounts, the engine takes a fraction longer. This causes a secondary weight transfer, which might not be obvious to the driver, but it’s a real effect. Leftright-left-right cornering could even set up a slight shimmy of the engine on its mounts, destabilising the car, even if only by a tiny amount. The same theory applies to acceleration and braking, where the engine moves on its mounts a fraction later than the rest of the car. It’s
the reason race car engines are solidly bolted to their chassis, though this approach would drastically reduce the level of refinement enjoyed in a Porsche road car.
HOLD FIRM
The solution is an engine mount adapting to the conditions, offering the best of both worlds. Enter PADM. These mounts are filled with a magnetorheological fluid, mostly made up of oil (similar to that found in regular dampers), but also featuring tiny particles that can be magnetised. It’s possible to alter the viscosity of this fluid by simply applying a current to surrounding electromagnets, thereby creating an electromagnetic field. PADM mounts are at their softest by default and, when required, all it takes is an electrical signal to quickly stiffen them up. The characteristics of both settings can be fine-tuned by the interior geometry of the mounts to suit different applications, while the switchover point can be varied electronically. In fairness,
PORSCHE RECKONS, ON AVERAGE, A STEERING WHEEL IS AT THE STRAIGHT-AHEAD POSITION SOME NINETY PERCENT OF THE TIME
PADM was used first on the 997 GT3 and GT2, but it was improved for the 991. Indeed, the non-gt 991 was the first ‘regular’ 911 to receive PADM, which was offered as part of the optional-butdesirable Sport Chrono package. So equipped, two PADM mounts are found at the back of the engine.
Two different versions of PTV were available in the 991. The system was a standard feature of cars loaded with a manual gearbox, while PTV Plus was present if a PDK transmission was fitted. The only difference between PTV and PTV Plus (though it is, admittedly, a significant difference) is the latter making use of an infinitely variable electronically controlled limited-slip differential, while plain ol’ PTV makes do with a mechanical limited-slipper. In case you were wondering, torque vectoring is simply the control of how much torque goes to a given wheel. In the Porsche system, the brakes can be applied to the rear wheels individually, automatically. For example, as you begin to turn into a left-hand bend, the PTV system gently applies the brakes to the left-hand rear wheel. This makes the car feel more agile, a result of inducing a yawing movement rotating the car into the corner.
Meanwhile, limited-slip differentials help with traction while exiting corners, though the electronic version is far more sophisticated, allowing adjustment of the differential’s characteristics over a wide range of conditions. PTV Plus is particularly good at helping to maximise traction on slippery surfaces, meaning it is a safety device as much as a tool for performance. Interestingly, the then new electric power steering system fitted to the 991 also came with safety features nigh-on impossible to achieve with hydraulic power assistance. For instance, when braking with different levels
of grip under the left and right tyres, signals from the 991’s anti-lock braking system inform the steering controller, encouraging small automatic steering movements implemented to help the driver maintain stability. Of course, the main reason for adopting electrically assisted power was efficiency — with hydraulic power assistance, a power steering pump is driven by the engine at all times, sapping power and using fuel. An electric system is far more efficient, not least because it makes use of electrical energy only when the steering wheel is actually turned. And though you might assume you’re almost always giving the steering wheel input, Porsche reckons, on average, a steering wheel is at the straight-ahead position some ninety percent of the time. That’s quite a saving of energy!
SCALE BACK
Development of the 991 also focused on weight reduction. You’d have thought the deletion of the hydraulic power steering pump, power steering fluid reservoir and associated pipework would have helped, but the electrically assisted steering system was, in fact, no lighter overall. It was, however, easier to package, freeing up valuable space in the engine bay.
The base hardware was provided by ZF, under its Servolectric brand. The electric motor — its housing integrated along the axis of the steering rack — is slightly offset from the centreline of the car and moves the steering rack via a reduction gear and toothed belt. 911 & Porsche World contributor, Karl Ludvigsen, details the development of the 991’s electric power steering system in his Porsche bible, Excellence Was Expected, indicating how, early on in the model’s design, Porsche intended to use this technology from the start of production. There was a problem, though: the company’s engineers were unimpressed with the lack of control and minimal feedback from the existing systems they sampled.
Coupled with the new 911’s high performance demands, it was clear a Porsche-produced variant would be required. August Achleitner was instrumental in the development of the system for production. Achleitner started his career at Porsche in 1983, working in the field of chassis development. From 1989 to 2000, he led the Technical Product Development, Vehicle Concepts and Package department, before taking over responsibility for the 911 model series in 2001. He’s quoted in
Ludvigsen’s book as saying the motor in the electric power steering system had to be “small enough, light enough and powerful enough to accommodate forces acting on the front wheels of the 991 GT3 RSR when it bumps kerbs,” meaning the road car’s steering system would be seriously over-engineered. Indeed, though most electric power steering systems are current based, this presented a problem for Porsche because current can fluctuate in accordance with external influences, such as temperature, encouraging nonlinear outputs. In contrast, the resultant Porsche system is torque-based and controlled by a predetermined ECU map. Incidentally, that’s torque input at the steering wheel, generated by the driver.
RÖHRL WITH IT
To investigate whether this technique would actually work, Porsche and ZF conducted a detailed study at Munich University. Their research was a resounding success. Two-time World Rally Championship victor and Porsche test driver, Walter Röhrl, was promptly drafted in to ensure that the system operated correctly in myriad conditions, from fast driving on ice and snow to relentless pounding around the torturous Nürburgring Nordschleife. The finished system takes the driver’s input as the starting point, measuring the force and speed of steering wheel movement. Using this data, along with information received from a vast number of vehicle sensor inputs, the level of electric steering assistance is determined. Porsche promoted all the usual marketing information about road surface and available grip being transmitted to the driver through sublime steering, but one of the biggest advantages is that the system filters out unwanted movement of the steering wheel, which might occur, for example, over unanticipated bumps or particularly rough surfaces. Seemingly, one of the intentions of using electric power steering was to enhance the feeling of stability at autobahn speeds, eliminating the need for unnecessary steering input at such pace. Interestingly, at all speeds, the electric power steering system actively self-centres, while the steering rack itself is a variable ratio item, producing more direct steering the more you turn. The optional Power Steering
Plus add-on, meanwhile, offered speedsensitive assistance, notably to make parking and manoeuvring at low speeds much easier.
Looking back at the first media drives of the 991 following model launch a decade ago, most correspondents disapproved of the then new 911’s electric power steering, though it’s important to note it went on to become the default system across the entire Porsche product line-up. Sure, the brand may have learned how to improve upon 991 technology since its introduction all those years ago, but we can’t help wonder if all the other changes (longer wheelbase, wider track, lower weight, lower centre of gravity and redesigned suspension, among others) introduced to the 997’s successor might have had just as much influence on reviewers’ opinions as the new steering system. In other words, the 991 represented such a radical redevelopment of the 911, we’re not convinced it would have been possible to isolate the steering as cause for complaint. Nevertheless, there’s no doubt the 991 was a major milestone in the development of the 911 — we’d expect nothing less to mark the evergreen flagship’s five-decade anniversary.
THE 991 REPRESENTED SUCH A RADICAL REDEVELOPMENT OF THE 911, WE’RE NOT CONVINCED IT WAS POSSIBLE TO ISOLATE STEERING AS CAUSE FOR COMPLAINT
l