oFFROAD SPIRIT AND MECHANICAL SYSTEMS
 

Stylish good looks and the mechanical configuration of a true off-roader. This, to sum up, is the spirit of the new model that builds on the extraordinary success of the first Panda 4x4 while also setting out to become the benchmark in this particular market band due to its sophisticated style, carefully honed details and wealth of features. The car therefore offers a four wheel drive system that engages automatically when the front wheels lose grip. The Panda Cross is therefore an extraordinary explorer that is entirely at ease over unsurfaced, difficult terrain, as evidenced by the ramp angles in standard A. First and foremost, the 'incidence' angle, which measures the maximum front angle that may be tackled by an off-road vehicle without the body touching the ground. The second value is the 'rear' incidence angle, i.e. the maximum slope that can be taken in reverse starting from a horizontal surface. The last one is the slope angle, i.e. the maximum possible angle at the base of the triangular slope (isosceles) that the car can be driven up without its bottom touching the ground. The values for the SUV specification of the new model are as follows: 24° (front), 42° (rear) and 24° (slope). The maximum climbable gradient is more than 50%.

The Panda 4x4 also offers a host of dynamic and comfort features that allow it to tackle long and demanding routes in comfort, due in particular to its suspension layout and special tyres. The Panda Cross is fitted with tyres (175/65 R15) that achieve the best balance between road performance (handling land comfort) and off-road performance (over unsurfaced roads) and winter performance (over snowy surfaces). The 175/65R15 tyre cannot be fitted with snow chains while alloy wheels come as standard.

Four wheel drive

Four wheel drive (4WD) means that all four wheels are driven; offering the benefit that twice the torque of a two wheel drive (2WD) can be discharged to the ground, particularly on slippery surfaces. In other words, the 4x4 transmission doubles the drive force discharged to the ground and means that the vehicle can overcome considerable differences in level or steep and difficult terrain while also propelling the vehicle forward if two drive wheels are not enough, e.g. over an icy or snow-covered road surface.

The configuration chosen for the Panda 4x4 is a four wheel drive transmission with two differentials and a viscous coupling. With this system, the four wheel drive is engaged automatically (i.e. without requiring any engagement by the driver) and allows outstanding drive torque distribution over the front and rear axle according to the road surface and vehicle application requirements. For example, on asphalt roads with normal grip, 98% of the drive torque is directed to the front wheels so that the car can behave in a similar way to a front wheel drive vehicle with equally satisfying handling. It also provides excessive fuel and tyre consumption due to small differences in rotating speed between the front and rear wheels. The Fiat Panda 4x4 is also equipped with an antislip system that operates during deceleration by exploiting the potential of the ABS system (i.e. making use of the sensors that record wheel rpm) and the electronic engine control system to prevent the engine braking torque and the viscous coupling from causing the rear end to lock during sudden decelerations on downhill slopes or on bends. Together with the four wheel drive system, this ensures great driving safety over slippery surfaces.

Lastly, the Panda 4x4 drive system makes full use of the viscous coupling that forms an integral part of the rear differential unit. The advantage of this configuration is that the drive system is engaged in a relatively immediate, automatic fashion to offer traction and effort when difficult road surface conditions make this necessary. Now it is time to take a closer look at the way a viscous coupling works. The devices contain a set of perforated or slightly finned discs submerged in a viscous fluid with very specific properties: as the temperature rises, fluid viscosity increases. When the two driven axles slip, they stir up this fluid and cause its temperature to increase. As soon as the temperature rises, the fluid becomes more viscous and tends to drag the discs together and transmit drive torque to the rear axle. This is a striking and clever piece of engineering because the oil acts as a sensor, i.e. it detects the presence of slip between the axles by its temperature - and also as an actuator because it tends to counteract the slipping effect by increasing its viscosity.

Locking Differential function

For the first time in the Panda 4x4 range, the SUV version is fitted as standard with the LD (Locking Differential) function that acts to brake slipping wheels and transfers the torque to the other wheels on the same axis: for example, during take-off over snowy, icy or muddy ground. This function can also be activated manually and acts below a threshold of 30 km/h (it disengages automatically over this speed). Details of its operation are described below.

The system compares the speeds of the drive wheels on the same axis and detects slip when the difference between both wheels exceeds a threshold level. The speed is controlled by applying braking torque to the slipping wheel. This is achieved by increasing the pressure in the corresponding branch of the brake circuit. This allows the differential to transfer torque to the wheel with the best grip to get the vehicle out of trouble.

The braking pressure on the individual wheel is increased by activating the TC ISO and TC Supply valves in the hydraulic circuit branch for the wheel to which the brake torque is to be supplied, the pump motor and the ISO valve in the other wheel on the same branch to which brake torque is not to be applied.

The Locking Differential function can be engaged only at speeds less than 30 km/h. If the vehicle exceeds this speed, the function disengages automatically. The function remains disengaged until it is next activated manually. Above this speed, the Locking Differential does not intervene in order to safeguard the braking system and viscous coupling against overheating. The system disengages automatically even when the control unit calculates that the brake temperature has exceeded a given safety threshold and then cuts back in when the brakes reach a safety temperature.

Front and rear suspension

The suspension system of a four wheel drive vehicle must meet two main requirements. Firstly, it must ensure passengers all the handling, roadholding and safety you would expect from 2WD vehicles, while secondly it must ensure greater pulling power over any route, particularly off-road routes. To achieve both these aims, the engineers adjusted these parameters to offer absolutely outstanding performance with particular attention to off-road comfort and performance. Hence the choice of two efficient tried and tested layouts that work very effectively due to the attributes of the chassis.

On the front, an independent MacPherson configuration as adopted on the 2WD version with certain changes. For example, the dimensions of the shock absorbers are increased (bush rod 22mm instead of 20mm) to offer reliability and practicality even in the most demanding off-road missions.
 

Compared to the Panda 2WD, the four wheel drive version offers special springs that add to the greater ground clearance so that the vehicle can ride easily over any route and ground surface. The technical specifications of the Panda 2WD front suspension have been retained. The system therefore offers a torsion bar connected to the shock absorber by means of bars (a solution used to date only on higher segment cars). In this case, the suspension ensures a greater ability to filter out vibrations due to driving over rough ground and has been improved by the adoption of:

• new design coil springs that increase the absorption of minor surface roughness;

• wishbones with rubber and metal front bushes and new upright rear bushes in rubber and metal; the latter contain a higher volume of rubber and ensure an excellent compromise between vibrational comfort and handling on corners by means of differential radial stiffness;

• mechanical beam designed to accept the new wishbones and fitted with body attachments without the interposition of flexible elements;

• double-acting telescopic hydraulic shock absorbers (perform differently during compression and stretching) located beneath the floor to improve comfort without detracting from boot space.

Moving on to the rear, the solution selected for the Panda 4x4 differs significantly from that used on the 2WD model. Instead of an interconnected wheel (semi-independent) suspension with a torsion beam, the four wheel drive vehicle adopts a configuration with longitudinal tie rods with independent wheels. This layout ensures the support of the rear differential (secured to the chassis with the interposition of flexible elements), is designed to accept rear transmission components (drive shafts) and ensures the rear wheels the independence necessary to travel over any routes or roughness.

In detail, a rear frame fastened rigidly to the body at four points acts as a load sparing element for the rear mechanical components (transmission and suspension). The tie rods are also fastened to the frame with the interposition of flexible elements (2 rubber/metal bushes per arm). The axis of rotation of the arm (a virtual line that joins the centre of both bushes) is inclined in space in order to stabilise the wheel for safe roadholding under critical stability conditions.

A torsion bar is fitted and connected to the tie rods by means of ball joints that give the suspension extra rigidity and stabilise it during rolling for safer performance on bends.
Compared to the Panda 2WD, the size of the springs on the four wheel drive version has been selected to give the vehicle a raised ride (ground clearance) and thus allow it to tackle any route and road surface. The position of the shock absorbers, angled under the floorpan as on the two wheel drive version, has been optimised to reduce hysteresis during shocks and thus to increase the system filtering capacity over ground roughness.

The revolutionary 70 bhp 1.3 Multijet

The Fiat Panda 4x4 range would not be complete without the 1.3 16v Multijet, the smallest, most advanced of the second-generation Common Rail direct injection diesels. The 1.3 Multijet 16v is a 1248 cc 4 cylinder in line power unit with a bore of just 69.6 mm and a 82 mm 'long' stroke. The four valves per cylinder are driven directly by a twin overhead camshaft. Maximum power output is 51 kW at 4000 rpm (70 bhp) and the torque delivered is 145 Nm (14.8 kgm) at just 1500 rpm.

The 1.3 Multijet 16v is a true masterpiece of miniaturised technology: when clad with all its accessories, it weighs in at just 130 kg. Its size is small at less than 50 centimetres in length and 65 in height. The component layout has been designed to ensure it takes up the smallest possible space. Yet it guarantees the same advantages as bigger engines because it has not been reduced but miniaturised. The power unit has thus been built to ensure the greatest rationalism, efficiency and reliability and is the smallest Common Rail four-cylinder diesel on the market. The only one able to enclose no fewer than six normal-sized components into a space of less that 70 millimetres: four valves, one injector and one spark plug.

The new engine also adds another record to this major feat of miniaturisation: it is the most powerful of its type. Despite a truly miniscule cylinder capacity of 1248 cc, the pocket Multijet came out top when compared with all the small diesels with fixed geometry turbines currently present on the market. Even the most vaunted. Suffice it to say that it offers the best specific performance of any diesel engine with 800 to 1500 cc of cylinder capacity: power output of 51 kW/l and torque of 145 Nm/l.

This compact, technologically sophisticated new engine also offers outstanding efficiency and is practically guaranteed for life. The 1.3 16v Multijet is in fact designed to last 250,000 km instead of the usual 150,000. During this long lifetime, it does not require any maintenance to mechanical parts (even the normally essential fan belt change at 80,000 km is not required). The oil change intervals have also been increased from 20 to 30,000 km. The oil is naturally low viscosity (i.e. designed for fuel economy) and environmentally friendly.

And more. The 1.3 Multijet 16v is environmentally friendly because it already meets Euro 4 emission limits not due to come into force until 2006. It is also one of the very few cars in the world that has been able to achieve this result without the need for a sophisticated exhaust post-treatment device such as a particulate trap. All in all, an intrinsically clean car: the particulate emission level (responsible for dust and fine dust) - for example - is even lower than that established by the forthcoming Euro 4 standard. The 1.3 Multijet 16v therefore represents a true technological leap that translates into a reduction in fuel consumption and emissions for the customer. All this comes with:

• lower noise levels (if we imagine the cylinder combustion to be like striking a drum: striking a small drum three times is less noisy than striking a large drum once);

• greater comfort: fewer alternating masses means less vibration;

• smooth, gentle drive due to outstanding torque progression (in turn guaranteed by the possibility of greater control of combustion, moment by moment);

• the flexibility and prompt responses of a diesel engine that resembles petrol engines more and more due to its wide rpm range (e.g. you no longer feel the fuel cut-off at just over 4000 rpm);

• environmentally-friendly features that allow this engine to improve on the greenest feature of a diesel (fuel consumption) by minimising its main defect (particulate emissions).

The 70 bhp 1.3 16v Multijet engine is combined with a five-speed manual gearbox able to transmit a torque of up to 14.8 kgm that takes up little space and offers excellent selection lever manoeuvrability. This has been achieved by reducing clutch driven plate inertia and introducing new gaskets. The gearbox configuration is transverse with two shafts in a cascade, while the gear mechanism is internal over four selection planes. The exterior mechanism is twin cable type to filter out engine operating noise and vibrations transmitted from engine to gear lever. The gear teeth offer high meshing capacity while the fifth speed and final drive gear sets are machined to a superfinish after heat treatment (for lower noise levels). The gear casings are light and very effective at absorbing noise. The gear cases are developed using finite element structural computing methods. The Panda Cross manual gearbox is also equipped with a syringe mechanism to prevent involuntary reverse engagement.