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Because of the scheduled production volume, the resultingneed to
increase the level of automation and the simpler
body concept, the number of component parts comparedwith the A8
has been significantly reduced. For example,on this
second-generation ASF the castings no longersimply have the
function of mere connecting nodes; theyhave to all intents and
purposes become multi-functionallarge components. The percentage of
straight extrudedsections has been boosted significantly, to avoid
complex2D/3D bending processes as far as possible. The ASFstructure
now incorporates only six bent extruded sections(Fig. 1).
The techniques of punch-riveting and MIG welding thatwere
applied successfully on the A8 remain in use, thoughthe proportion
of MIG-welded seams has beensignificantly reduced (Fig. 2).
Spot-welding, clinching andstructural bonding have been completely
eliminated.Representing a world first in the automotive industry,
laserwelding is now used to produce approx. 30 metres of weldseams
on the aluminium body sections. The MIG-weldedT-fillet joins on
extruded sections in the floor assembly arealso new.
1
1st
generation ASF
2nd
generation ASF
Fig. 2: Comparison of joining techniques,
1st-generation/2nd-generation ASF
Both the new technology of laser welding and theestablished
principle of MIG welding, for which a newarea of application was
created in the floor assembly's T-
joins for the refined body concept, necessitate a highdegree of
component precision. In order to achieve therequired component
dimensional tolerances of 0.2 mm,most of the extruded sections for
the floor structure must
be calibrated by means of hydroforming.
FRONT END
The front end of the Audi A2, as was already the case onthe Audi
A8, consists of a bolted longitudinal-memberstructure, which
permits a low insurance typeclassification. The front longitudinal
member comprises anextruded section which is stamped in the area
where it
joins onto the impact-absorbing elements, to permitdefined
collapsing in order to absorb impact energy.
On the Audi A2, the connecting longitudinal member forthe first
time consists of two vacuum diecast elements,which are designed to
absorb some of the impact energy.Both split shells have a varying
wall thickness and a ribbedstructure defined according to
structural calculations, to
produce defined deformation behaviour. The bolt-onpoints of the
front axle on the lower shells have beendesigned to transmit the
deformation energy into thelongitudinal member, and not into the
rigid subframe. Byintegrating the transmission and
engine-mountconnections, the subframe connections, the mounting
pointfor the slot-in car jack and the suspension frame mounting
points, these two cast split shells constitute a
multi-functional large component.
CENTRE FLOOR
The H-frame of the floor structure consists of straightextruded
sections joined by means of MIG fillet welding.The cast nodes still
used on the Audi A8 for connecting
purposes are no longer required.
Fig. 3: Floor structure
The high position of the longitudinal and cross-members,
on account of the concept, permits a level floor structure
MIG 20 m
Punch-riveting 1800
Laser 30 m
MIG 70 m
Punch-riveting 1100Spotwelds 500
Clinching 179
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with a raised seating position for the occupants. The resultis a
lightweight, high-strength member structure whichsatisfies very
high standards of rigidity and crash safety(Fig. 3).
The arrangement of the cross-members in relation to thesills
looks geometrically straightforward, but very highstandards of
component precision and welding technology
are required to weld these components.
Burr-free, level trimmed ends of the cross-members
areparticularly important to a stable welding process. Theseare
achieved by means of a special sawing technique. The
profile T-joins were investigated as a welded assembly asearly
as the concept phase to establish the weldingsequence and
direction, and determine the clamping andfixing techniques, so as
to minimize thermal distortionduring welding of the complete
structure. This plays amajor role in ensuring that the entire body
of the Audi A2can be built within a very close tolerance range.
The power flow in the forward structure is transmitted tothe
centre floor by additional diagonal longitudinal
members and a central longitudinal structure.
The single-section floor pan in conjunction with a raisedfloor
panel at the front, in the vicinity of the driver's andfront
passenger's seats, created additional storage space forvarious
auxiliaries and control units. The rear passengers'legroom and the
ergonomic position of the seats wereappreciably enhanced by this
low-level floor pan. The sizeand complexity of the floor pan and
the walls thickness,which is relatively slight for reasons of
strength, couldonly be achieved by simulating the deep-drawing
processin parallel with development.
REAR END
The rear end, which is again of a relatively simplestructure,
and its structure of longitudinal and cross-members, are connected
to the centre floor by means of afurther multi-functional large
casting. This "longitudinal-member/sill connecting element"
contains the rear-axlemounts, the spring cap supports, the mounting
for the slot-in car jack and the suspension frame mounting
points.
The rear floor panels do not merely permit a level load-area
floor; their design means that they are ideal for laser-welding by
means of pressure roller.
The rear impact-absorbing elements permit screw-typetowing
eyes.
SIDE-WALL FRAME
This assembly basically comprises the side section, theside of
the roof frame and the castings for the A + B-postsand tailgate lid
hinge mounting. The one-piece deep-drawnside section forms the
outer shell of the side structure.Manufacturing this deep-drawn
element of aluminium
presented the planning and toolmaking specialists with a
major challenge. The design and deep-drawing simulation
processes were conducted iteratively. The advantage ofthis
one-piece component is its dimensional accuracy andthe absence of
joins which would exhibit locally differentstrength characteristics
if thermal joining techniques wereused on them, and would require
comprehensive finishing
because they are in a visible zone. Both the A and B-postsare
vacuum diecast components into which the mounting
points for hinges and door arresters, seat belts and otherscrew
connections are integrated.
In the case of the B-post, there is one casting on the A2
asopposed to a welded structure comprising 8 individual
parts on the A8.
The component's ribbing was defined in close conjunctionwith the
rigidity and crash strength calculations, anddesigned substantially
facing the outside in order to use theribs to absorb pressure in
the event of a lateral collisionand to increase the moment of
resistance to bending.
In the upper section, the B-post is fixed to the roof frameby
laser welding, for which process very close tolerancesare required
in view of the rigid components and the
minimal welding gap required.
ROOF STRUCTURE
There are two body variants for the roof structure area:
Full-panel roof
Double-spoiler glass roof.
The roof structure for the full-panel roof consists of
roofcross-members at the front and rear with a sheet-metal
shell, and the sheet-metal roof.
On the version for the double-spoiler glass roof, there is
nosheet-metal roof panel. The rigidity loss due to the absenceof
shear strength in the roof is compensated for byreinforced roof
cross-members. The double-spoiler roof is
bolted to the structure as a complete module, after thebody has
been painted.
BODY CALCULATION
Computer simulation, with the aid of which vehiclefunctions such
as crash safety, rigidity and strength can be
optimized well before actual prototypes exist, is a key toolin
the development process. Front-loading, i.e. the use ofCAE tools in
the early phase of the product development
process, is of critical importance to the efficient use
ofsimulation tools. In the concept and early prototype phasein
particular, body calculation is the principal means ofmaking
reliable concept-definition decisions within thedevelopment
process. The degree to which simulationtools are used depends
decisively on the degree to whichthe calculation process is
interlinked with other areas
participating in development work, such as Design,Testing,
Planning and Quality Assurance. To makeoptimum use of the available
synergy benefits, Audi hasset up the Aluminium Centre at in
Neckarsulm, where all
partners involved in the product development process are
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located closely together, with the result that the flow
ofinformation can be largely optimized.
RIGIDITY
The Audi A2's static torsional rigidity was configured
andoptimized by means of calculation. The objective here was
to obtain a standard of lightweight body design that
issignificantly better than the norm on this class of vehicle.
The dynamic properties of the body were investigated bothon the
body-in-white and on the trimmed body. In thelatter method, all
mass-affected components which arerigidly connected to the body are
represented in the finite-element model. By means of synthetic
excitation at typicalforce application points, responses which
permitimprovements to all the relevant components in order
toenhance driving comfort are obtained at customer-relevantcontact
points, such as the foot contact point, steeringwheel or seat rail
connection.
STRENGTH
On the A2, abuse load scenarios such as driving overground ramps
were evaluated with the aid of simulationand the structure
optimized as a result. For this loadscenario, the forces that are
introduced into the structureare calculated by means of MKS
simulation. These forceswere then applied to the body through a
finite-elementcalculation, and the distribution of stresses and
extensionsin the structure analyzed and optimized.
PASSIVE SAFETY
Thanks to its vehicle concept, the Audi A2 has a soundbasis for
an optimum flow of power under the influence ofthe loads occurring
in crash load scenarios. For example,the relatively slight
difference in height between the frontand rear longitudinal member
and the supporting structurein the passenger cell zone permits a
weight-optimizedstructure, with precise control of the deformation
whichoccurs in a frontal and rear-end collision. The sill, which
isrelatively high up compared with familiar vehicleconcepts,
likewise contributes to the flow of power and tothe absorption of
energy in a side collision.
In the lateral impact to Euro-NCAP at 50 km/h, the vehicleis
struck at right-angles by a trolley carrying a barrier, thefront
side of which represents a deformable impactelement. Here again,
the rigid cabin constitutes thesurvival space for the occupants. In
this test, the overlap
between the doors and the posts and sill prevents the doorsfrom
being displaced into the interior. The doors moreoverincorporate
large-area impact-absorbing elements whichtransfer the forces
occurring into the cabin structure.
The B-post of the A2 for the first time consists of a
diecastelement. With the aid of calculation, this component has
been designed to adapt ideally to the loads which
occur,permitting precise control of its deformation behaviour;this
contributes significantly to the car's high safetystandards in the
Euro-NCAP lateral collision.
The rear-end collision to ECE R32 involves a rigid barrierwith
an overlap of 100 percent striking a stationary,unbraked vehicle.
In this case, the structure in thesimulation process was designed
such that only the rearsection of the vehicle becomes deformed. The
passengercell and doors remain largely free from deformation.
To this end, there is a stable matrix of longitudinal and
lateral members in the floor area, and the forces in the
roofzone are supported via the casting for the tailgate-lidmounting
and the side roof frame. The zone at the rearlongitudinal members
has been designed such that atargeted folding-dent process occurs
in the extrudedsection, whereas the dimensions of the casting in
front of itare such that it sustains only slight deformation.
Thisallows the cabin to provide maximum protection for theoccupants
even in this load scenario.
The frontal crash at 64 km/h against the deforming barrierwith
an offset of 40 percent imposes a very high load onthe structure.
The objective of the layout was to makedeformation take place in a
controlled manner in theforward structure in the event of a
collusion, leaving the
cabin intact. To this end, the front longitudinal members ofthe
space-frame structure absorb the bulk of thedeformation energy,
whereas the beam structures behind itconstitute the passenger cell
and are scarcely deformed.The doors can still be opened easily
after the crash (Fig.4).
Fig. 4: Structural deformation in front crash to Euro-
NCAP at 64 km/h
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SUMMARY
The development of the Audi A2 demonstrates that it isalready
possible to use an aluminium body for a compact-class vehicle.
Technological advances in aluminiumvehicle design were
systematically implemented on thiscar, both as a means of reducing
costs compared with the1st-generation Audi Space Frame and to
permit its volume
production. Audi has now succeeded in realizing apioneering
lightweight design concept that sets newstandards.
REFERENCES
[1] D. Engelhart, W. Leitermann et al: Der neue Audi A2
(The new Audi A2), ATZ/MTZ-Sonderheft 03-2000
[2] J. Christlein, H. Behrens: Aluminium als Karosserie-
Leichtbauwerkstoff fr Personenkraftwagen
(Aluminium as a lightweight material for cars). Bnder
Bleche Rohre 10-1996: 16-24.
