Overview: Fiat Group’s ‘Compact’ platform
Alfa Romeo’s new Giulietta sees the introduction of an all-new platform, known as the ‘Compact’. It is aimed, not surprisingly, at satisfying drivers in the areas of road-holding, agility and safety, but the Company is also very keen to stress that refinement was a high priority during the platform’s development.
The Giulietta is a completely new car. Alfa Romeo’s preferred description of it — and we assume this is an expression of the Company’s priorities in designing it — is ‘a sports car capable of expressing both great agility over the most demanding routes and providing comfort on everyday roads.’ Very clear here is an intention to sell the Giulietta to a broad audience.
The new Compact platform continues Fiat Group’s platform rationalisation programme. The ‘Mini’ platform for segment A and its ‘Small’ companion for segments B and L0 have already appeared. Fiat Group sees the new platform as a point of departure from its predecessor, chiefly from the standpoint of ambitious quality and performance goals.
The new platform’s underbody represents a generational leap in terms of the materials used. The entire structure is now significantly stronger and provides better performance than its forebear, without any increase in weight. This has been achieved by the wider use of high-strength materials, which now make up more than 90 per cent. of the total mass of the system. Some structural parts, designed to provide high strength and the ability to absorb impact with minimal deformation, have been made from hot-pressed tempered steel.
Lightweight materials have been used for several structural elements that are bolted to the main body structure. For example, the front beam that connects the front struts with the third load path is made from extruded aluminium. The rear bumper beam and the third load path energy absorption elements are made from Xenoy, a thermoplastic with excellent energy absorption properties in relation to its weight. The third load path energy absorption elements absorb the energy of low-speed impacts with obstacles and support the bumper in impacts with pedestrians. Using weight-saving techniques has delivered a weight saving of 8.5kg at the front and 4kg at the rear compared with conventional steel structures, without compromising safety performance.
Another new feature of the underbody is what Fiat calls ‘modularity’, making the platform more adaptable for use on a family of vehicles with different wheelbases. This modularity is provided by several central floor panels and the side members and does not affect any part of the front and rear structures. This approach also reduces tooling outlay.
Structurally, the new third load path increases the energy absorption potential of the car’s front end. It reduces both inertial forces and passenger compartment intrusion at higher impact speeds.
Many of the systems and components that make up the Compact platform are re-engineered versions of existing equipment. As ever, improved performance and reduced weight were targets. Both the framework for the rear seats and the magnesium alloy crossmember lost 35 per cent. in weight compared with earlier incarnations.
Alfa Romeo is very confident about the ride-handling balance it has achieved in the Giulietta. The McPherson strut front suspension may not be exotic, but it can be made to work pretty well; wheel location at the rear is by means of a new multi-link arrangement.
Weight has been reduced by switching to aluminium wishbones, which are 4kg lighter than cast iron ones. The multi-link rear suspension uses aluminium wishbones to cut weight: 10kg is saved compared with a cast iron set-up.
The Giulietta’s electric power steering system is new, set up with an emphasis on steering feel. Dynamic Steering Torque (DST) provides the driver with feed-back torque at the steering wheel that helps him to perform critical manoeuvres correctly, before the Vehicle Dynamic Control system (VDC) intervenes to prevent loss of control.
For detailed graphics, see bottom of page.
Alfa Romeo Giulietta: NCAP safety performance
Alfa Romeo’s Giulietta has been awarded five stars for an 87 per cent. score in Euro NCAP safety tests. The makes it, at the time of writing, the safest compact car yet tested.
In 2009, the Euro NCAP régime changed. It now comprises four areas of assessment: Adult Occupant, Child Occupant, Pedestrian Protection, and a new category: Safety Assist. Injury avoidance and mitigation functions, such as seat belt reminder, ESP and speed limiter, are also rated. Achieving a five-star rating will become increasingly more difficult in three stages — 2009, 2010-2011, and 2012. Despite this, the Giulietta’s results are still good enough for a five-star rating under the 2012 criteria.
So far as electronic systems are concerned, the Giulietta deploys all of the usual suspects. The car’s Vehicle Dynamic Control system manages the key functions: the hill holder, traction control and assisted panic braking; the MSR system, which prevents a wheel losing traction when the throttle is released sharply; the Dynamic Steering Torque system; electronic Q2, which electronically simulates a limited-slip differential; and the braking prefill system, which pressurises the braking circuits when the accelerator pedal is released, making the braking system more responsive.
VDC monitors longitudinal and lateral tyre grip, the car’s yaw angle, and the angle of the steering wheel. These values are compared with the system’s ‘map’ to decide whether driving conditions are safe.
An integral part of VDC is Anti-Slip Regulation (ASR). This uses the ABS sensors to measure slip. If excessive power is causing both front wheels to slip, the system reduces engine power; if only one wheel is slipping, it is braked.
The Giulietta’s VDC controls a system called Dynamic Steering Torque (DST). This provides active (artificial) feedback torque through the steering wheel to help the driver respond to critical situations before more serious electronic intervention is needed. It has a split-mju control function for when one side of the car has significantly more traction than the other.
The Hydraulic Brake Assistance system increases brake circuit pressure under emergency braking situations to shorten stopping distances. The car’s hill holder function is part of HBA.
Fiat Group boasts that, because of the increasing use of virtual simulations in car development, only four prototypes were actually built. The virtual design was materially put to the test on the cars made at the Cassino plant by running two hundred tests on components and subsystems, more than a hundred Hyge slide shock test simulations and more than eighty crash tests — frontal impact, side impact, roll-over and shunting, taking into account different speeds and types of obstacles, and the need to protect occupants physically very different from one another.
A Giulietta is fitted with six airbags as standard, two of which are multi-stage devices, deploying to different degrees according to the severity of the impact. Double pretensioners and load limiters are fitted to the seatbelts. Self-aligning head restraints move closer to the occupants’ heads in the event of a crash to lessen the effects of whiplash. Both the pedal unit and steering column are collapsible.
The Giulietta’s front structure uses three load-paths to make the distribution of crash forces as broad as possible. For example, in an offset impact — where only part of the front of the Alfa collides with an object — the collision forces might be expected to pass almost entirely down one side of the car, with a large amount of energy absorbed by a relatively small amount of the car’s structure. Increasing the number of load paths allows impact forces to be directed across a much greater part of the structure, so each structural element runs a much lower risk of being compromised.
The Giulietta’s third load-path serves not only to help in distributing impact forces, but also to absorb energy in low-speed impacts.
Another result of this approach to energy absorption is that ‘contributory safety’ is improved: the front of the Alfa represents a (relatively) less damaging structure in an impact with another vehicle. The crash energy is distributed more evenly not only through the structure of the Giulietta but also across the area of impact with the other car.
The Giulietta’s NCAP results were:
- Adult Occupant: 97%
- Child Occupant: 85%
- Pedestrian Protection: 63%
- Safety Assist: 86%
The last category relates to on-board electronic systems that assist the driver in avoiding trouble or mitigating the consequences of mistakes.