Press Release
Body and Roof
Open-Air 911 with Outstanding Stiffness and Stability
The four-wheel-drive Cabriolet develops its specific fortes and qualities particularly on winding roads and serpentine routes. The name of the game in such situations is superior body stiffness minimising the rattling noise and trembling effect so typical of an open-air car on bad roads and surfaces. In the interest of extra stiffness and stability, the body-in-white of the 911 Carrera 4 Cabriolet comes with additional sill reinforcements and components with twice the usual panel thickness. These components are primarily reinforcement plates integrated in the sills as well as three-dimensional junction plates added at the connection points leading from the sills and the door hinge column/A-pillar to the rest of the car’s body.
The car’s high level of structural body stiffness is ensured in particular by spot welding and bonding connecting the side panels and the floor assembly. In other words, the components are not just welded to one another at a large number of points, but are additionally bonded together in order to significantly strengthen the connections from one component to another.
The longitudinal arms or "chassis legs", to use the colloquial term, are a particularly important feature: To acheive the same standard of passive safety as in a Coupé while at the same time maintaining the lightweight structure so typical of a Porsche, the front and rear longitudinal arms are tailored blanks, fine panels made of high-strength and ultra-high-strength steel of varying thickness welded together by laser technology. This combines supreme strength and stability with minimum overall weight.
As a result, the body of the new Cabriolet not only fulfils the significant demands made by Porsche of a sports car in terms of stability, but rather outperforms all safety requirements in both Europe and the USA: The new 911 Carrera 4 Cabriolet easily passes the offset crash test at an impact speed of 64 km/h or 40 mph in Europe, just as it complies with the US 100 per cent overlap impact test at a speed of 35 mph or 22 km/h.
With the body of the 911 being adapted for four-wheel drive, the load-bearing and support structure at the bottom of the luggage compartment responsible for crash safety had to be modified accordingly: Instead of the horseshoe-shaped carrier plate featured on the rear-wheel-drive models, Porsche’s engineers have given the four-wheel-drive models a self-contained, triangular carrier structure made up of aluminium profiles establishing an additional load path in the middle. In the event of a crash, therefore, forces generated at the front of the car are fed back like in the 911 Carrera 4 Coupé both to the wheel arches at the sides and to the front axle final drive.
Wide rear end also on the Cabriolets
The body of the new four-wheel-drive Cabriolet offers an extremely attractive combination of Cabriolet and four-wheel-drive design features. The folding roof and the particular contours of the engine compartment lid typical of the Cabriolet, for example, further accentuate the characteristically wide track of Porsche’s four-wheel-drive models: A total of 44 millimetres or 1.73" wider at the rear than the rear-wheel-drive Cabriolet, the new open-air 911 offers ample space for the extra-large and voluminous wheels and tyres.
The body structure is in principle the same as on the two-wheel-drive Cabriolets. Significant features and components at the front end of the car have however been re-conceived in order to accommodate the final drive at the front as well as the larger tank: Apart from specific modifications of the body-in-white around the front-end final drive, the bulkhead made up of several components has been moved further to the front. The complex shape and design of the fuel tank, in turn, is characterised by the split tank sections to the right and left of the final drive unit. Fuel is pumped out of these two recesses by an intake extraction pump on either side. Useful tank volume is 67 litres or 14.7 Imp gals, that is three litres more than on the 911 Cabriolet with conventional drive.
Offering capacity of 105 litres or 3.68 cu ft, the luggage compartment is able to accommodate two suitcases. Instead of a spare or emergency wheel, the new generation of the 911 comes with a tyre repair system comprising a tyre pressure compressor, a tyre sealant, a towing hook, the on-board toolkit, and wheel bolt adapters. This Mobility Kit is housed conveniently in the floor of the luggage compartment in between the body carrier structure.
The big advantage of the new luggage compartment design is however not just the extra volume of five litres compared with the former four-wheel-drive Carreras, but also the extra loading depth serving to conveniently accommodate the usual crates for drinks and beverages.
Aerodynamically styled roof
The roof of the open-air Carrera 4 and Carrera 4S comes in the same proven shape and technology as the roof on the Carrera Cabriolets with conventional drive. The roof opens and closes fully automatically at the touch of a button, folding into the roof compartment in Z-configuration, with the outer side of the roof facing up when open to protect the heated rear window made of glass. The entire roof mechanism can be operated at a speed of up to 50 km/h or 31 mph, the roof opening or closing together with the windows in a mere 20 seconds.
One of the particular fortes of the soft roof on the Porsche 911 is its outstanding aerodynamic quality. While the tissue structure of the soft roof has a slight influence on the car’s drag coefficient, the actual measurements are exactly the same as on the Coupé: As a result, both Carrera 4 Cabriolets pass the wind tunnel test with a drag coefficient (Cd) of 0.30 (Carrera 4) and, respectively, 0.29 (Carrera 4S).
To use the flow of air rushing past the car for optimum downforces, the spoiler moves up 20 millimetres or almost 0.8 inches higher than on the fixed roof Carrera. This it does automatically at a speed of 120 km/h or 75 mph. And since aerodynamic forces are less significant at low speeds, the spoiler moves down again once the speed of the car drops below 80 km/h or 50 mph.
Low centre of gravity thanks to the soft roof
The outer skin of the roof is made of top-quality textile tissue protected inter alia from UV light in order to avoid even the slightest bleaching effect. The front section of the roof between the windscreen and the first tightening bracket running parallel to the B-pillars is tightened by a light but stable cast magnesium panel. A plastic panel fitted between the outer fabric and the inner roof lining, in turn, serves not only to hold the roof fabric in place, but also to keep out noise and ensure adequate thermal protection.
The roof structure is made up of 10 major components, with a share of 26 per cent aluminium and 20 per cent magnesium. The other materials featured in the roof include steel forgings, the fabric and textile itself, the intermediate roof layer, the roof liner, and various seals.
Including the hydraulic opening and closing system at the rear, the soft roof weighs just 42 kg or 93 lb overall. As a result, the soft roof on Porsche’s Carrera Cabriolets weighs only half as much as a comparable retractable steel roof, with a correspondingly positive effect on the car’s driving characteristics: The less weight there is in the upper section of a car, the lower the centre of gravity. And this, obviously, is essential for a sports car with a high standard of lateral dynamic performance.
Hardtop as an option
Weighing only 33 kilos or 73 lb thanks to its aluminium structure, the hardtop comes as an option. Porsche is thus accommodating the wishes of many customers who prefer to drive their Carrera Cabriolets with a soft roof also in winter – especially as the car has all the qualities for leaving on the soft top during the cold season.
To optimise soundproofing, the hardtop is now fastened at the rear by two bolted connections, and no longer by bayonet catches. Fitting the hardtop in position is very easy thanks to its low weight, as on the previous model: All you do is place the hardtop on the car, the two connections points at the rear automatically establishing an electrical connection for the rear window heating, without requiring any additional cables. And like the soft roof, the hardtop now comes with new seals for perfect drainage of rainwater.
Four-Wheel Drive and Chassis/Suspension
Supremacy at All Speeds
The supremacy so characteristic of the body structure of the new open-air 911 on every road and surface is reflected by the same superiority on the drivetrain: Benefiting from four-wheel drive, the Carrera 4 Cabriolet offers an even higher standard of roadholding and even better grip even in difficult situations than its rear-wheel-drive counterpart.
Thanks to four-wheel-drive technology and the viscous multiple-plate clutch, the front wheels constantly convey a minimum of 5 and, whenever necessary, a maximum of up to 40 per cent of the engine’s drive power to the road. The focus, however, is not so much on pure traction, but rather on outstanding driving qualities and motoring safety at all times.
The multiple-plate viscous clutch is housed within the front axle final drive conveying drive forces to the front axle and setting off any difference in speed between the two axles. Like a multiple-plate lock, the clutch is made up of inner plates connected to the hub and outer plates connected to the housing. Silicon fluid comes in between in a fully sealed-off cavity, power being fed to the front axle as a function of differences in speed via sheer forces between the silicon fluid and the clutch plates.
Four-wheel drive for even faster acceleration
One of the advantages of this four-wheel-drive technology is that even in a fast bend the front wheels will not "push" hard to the outside, the car thus remaining neutral in its handling qualities and behaviour even in extreme bends and on very winding roads. And while the improvement of traction made possible by four-wheel drive is not the most essential point, it is nevertheless noticeable, the viscous clutch responding to even the smallest differences in engine speed.
Should the driver floor the gas pedal while driving dead slow, say, on a snowbound road, with the rear axle being able to convey only some of the engine power into traction, up to 40 per cent of the engine’s torque goes to the front axle without the slightest jolt or unsmoothness.
Four-wheel drive offers the same smooth response under negative acceleration: Even when applying the brakes all-out, there is hardly any distortion or build-up of adverse forces in the drivetrain, the PSM and ABS functions therefore remaining unchanged.
Yet a further advantage, particularly in a sports car, is the low weight of the system, all the specific features and components of four-wheel drive in a Porsche Carrera Cabriolet adding only an extra 50 kg or 110 lb. This makes four-wheel drive from Porsche one of the lightest systems of its kind in the market.
Ideal set-up of both the conventional and active suspension
A crucial prerequisite for converting the inherent benefits of four-wheel drive into superior dynamic driving qualities on the road is excellent suspension. This applies both to the Carrera 4 Cabriolet with conventional suspension and to the Carrera 4S Cabriolet with its active PASM suspension system featured as standard.
The features and qualities of the suspension on both open-air models are the same as on their respective Coupé counterparts. Which means that the springs on the Cabriolet, to take one example, have the same rating, but are slightly longer. The result is greater pre-tension with the springs fitted in position, serving to set off the slightly higher weight of the open-air model. A further modification is the reduction of the spring rate by 10 per cent on the front axle of the Cabriolet with Porsche Active Suspension Management (PASM) (with the exception of the 3.6-litre Carrera featuring Tiptronic transmission), giving the driver the same feeling of comfort as in the Coupé thanks to the modified set-up in conjunction with the higher weight of the car.
A further specific modification involves the stiffer support mounts on the rear axle: The support function is now provided by Cellasto elements connecting the upper spring and damper mounts with the support plate on the body of the car. With these elastomers having a reduced damping effect, the dampers respond to weaker forces and movements than under normal circumstances.
In conjunction with the pre-tension specific to the Cabriolet, this allows Porsche’s suspension engineers to offer two effects in one: First, roll movement of the Cabriolet is more comfortable; second, frequencies potentially making the body of the car tremble are eliminated right from the start.
Wider track on the rear axle for higher lateral acceleration
Up front, the four-wheel-drive cars have the same wheel and tyre dimensions as their rear-wheel-drive counterparts: 8J x 18 wheels running on 235/40 ZR 18 tyres on the open-air 911 Carrera 4 and, respectively, 8J x 19 wheels with 235/35 ZR 19 tyres on the Carrera 4S Cabriolet. 19-inch Carrera Classic, SportDesign, and Carrera Sport wheels are available as an option.
As with the four-wheel-drive Coupés, the Cabriolets clearly focus on the rear axle when it comes to the tyres and wheels: The open-air Carrera 4 runs on 295/35 ZR 18 tyres, the rear tyres on the Carrera 4S measure 305/30 ZR 19. Since these wider tyres are combined with rims with reduced hump depth, rear track measures 1548 millimetres or 60.9". This is attributable above all to driving dynamics, the tyre/road contact points moved further out providing better body support with a smaller roll angle and, as a result, enhanced lateral acceleration. The bottom line is an even higher standard of driving behaviour for even faster speeds in bends.
PASM suspension with two damper programs
Launched together with the new Carrera model series, Porsche’s PASM suspension is naturally one of the highlights on the new four-wheel-drive Cabriolets. Lowered by 10 millimetres or 0.39", this active suspension is standard on the Carerra 4S and comes as an option on the Carrera 4.
PASM offers the driver two different programs: PASM Normal or PASM Sports. The normal set-up provides a more comfortable basic suspension with the dampers switching over to a more sporting mode as the driver’s style of motoring becomes more dynamic. The advantage for the car’s occupants is a significantly higher standard of motoring comfort particularly on long trips on the Autobahn, PASM quite simply absorbing minor and medium bumps on the road even better than the standard suspension. The Sports Program, in turn, activates a harder damper control map for a very agile and dynamic style of motoring quite comparable to the features of an all-out sports suspension.
Porsche’s active PASM suspension is made up of adaptive dampers with continuous adjustment of damper forces, two accelerometers determining vertical movement of the body, and the PASM control unit relating the signals from the two accelerometers fitted on the damper domes front right and rear left to the car’s lateral acceleration, steering angle, road speed, brake pressure, and engine torque. Taking this data provided by the CAN-bus, the system then determines the optimum damper control line and sets the appropriate level of damper control individually on each wheel.
