2 - ALU-WEB.DE

12 AUTOMOTIVE 4 | 12-2009 APT ALUMINIUM NEWS
Lightweight design is an Audi core competence
Lightweight design enjoys
a special status among all
the technologies that Audi
is constantly advancing.
It is one of the carmaker’s
most important core competences.
As the inventor
of the self-supporting aluminium
body, Audi is the
worldwide leader in lightweight
design. “One of our
most enduring aims for
the future is to reverse the
weight spiral. Lightweight
design is the foundation
of our entire approach to
improving efficiency”, says
Michael Dick, member
of the Audi management
board and responsible for
technical development.
Lightweight design is a
strategic responsibility at
Audi. It makes a significant
contribution to sportiness
and, even more important, to
efficiency, thus it helps to conserve
resources and reduce
operating costs. The electric
drives of the future will add
additional weight to the car
and will initially only offer
a limited range, making systematic
lightweight design all
the more important. When it
comes to the body, the development
engineers at Audi get
a lot of ideas from the outside.
The aerospace industry and
motor sports provide important
inspiration.
The best examples of
lightweight design are provided
by nature, however.
In nature, only the amount
of material required in the
respective context is used. In
the field of bionics, solutions
to technical solutions are specifically
sought in biology.
“Many of the extruded sections
we use in the ASF design,
such as the sills of the TT, follow
bionic principles in their
topology. They are hollow, but
heavily ribbed on the inside,
reminiscent of the bones in
the skeleton of a human or
a bird”, says Heinrich Timm,
head of the Aluminium and
Lightweight Design Centre
Neckarsulm.
The architecture of Audi’s
Space Frame (ASF) bodies
differs widely between the
individual model series. The
superstructure of the R8
high-performance sports
car makes extensive use of
extruded sections, which
make up 70% of the wrought
components, in other words
the starting components. In
the TT Coupé, metal panels
account for 45% and thus the
largest fraction of the aluminium,
whereas large, multifunctional
castings play a decisive
role in the structure of the A8,
with 29 of them accounting
for 34% of the weight.
Integrated approach
to reverse the weight
spiral
Heinz Hollerweger, head
of Total Vehicle Development,
emphasizes that Audi
considers lightweight design
to be not simply a collection
of individual components,
but rather a complete, highly
integrated project. “An alu-
Body-in-white featuring composite construction
Frame-and-body construction of the Audi TT Coupé
minium tailgate permits a
lighter gas strut. Axle components
made of aluminium
transmit lower forces to the
body than do steel suspension
links, thus the superstructure
can be lighter, which in turn
allows for more compact
brakes, a smaller engine and a
correspondingly streamlined
exhaust system. This reverses
the weight spiral while actually
improving efficiency and
driving dynamics.”
The engine itself also holds
significant untapped potential.
A reduction of the conrod
masses results in a reduced
load on the crankshaft, which
in turn allows for a lighter
crankshaft design. The resulting
reduction of the rotating
masses with their moments
of inertia has a very strong,
positive effect on acceleration
and fuel consumption far
beyond the simple reduction
in weight.
Lightweight design is the
top priority for every Audi
vehicle development project,
and this applies to every step
of the process through the
construction of the prototype.
Each individual component
is assessed with respect
to its weight and the effect
on the total vehicle. Regular
weight balance calculations
help drive further continuous
optimisations.
Lightweight design is a
benefit in all respects, including
driving dynamics and
passive safety. The lighter a
car is, the less kinetic energy
it develops and the less of this
energy needs to be converted
into deformation in the event
of a crash. Protection for other
road users is also improved
since a lightweight car places
less of a load with which it
collides.
Acceleration behaviour
plays a major role in the field
of driving dynamics. A car
weighing 1,200 kg reaches
the 100 km/h mark from a
standing start twelve metres
sooner than a rival weighing
1,400 kg. The reduction
of so-called rotating masses
with their moments of inertia
has a particularly strong effect
on acceleration. Reducing the
weight of a car’s flywheel by
one kilogram has the same
effect as a 16 kg reduction in
the weight of the translational
masses, such as the body.
A lower vehicle mass also
has a positive effect on braking
– and that in a number of
regards. Overall stopping distance
is shorter, brake pressure
develops more quickly
and the discs do not get as
hot, which reduces the risk of
fading during a long mountain
descent, for example. A
lightweight car gets by with
smaller and lighter brakes.
This reduction of the unsprung
masses at the wheels
brings numerous advantages.
For example, a less stiff suspension
setup can be used,
thus improving vibrational
comfort. A 10 kg reduction in
the unsprung masses reduces
the load on the suspension
strut by 4%. One would have
to reduce the weight of the
sprung masses by nearly 50 kg
to achieve the same effect.
The lightweight aluminium
bodies from Audi also
have a very positive ecological
effect. They spare the environment
large quantities of CO 2
emissions – through their low
weight and the overall energy
balance. At the end of the
vehicle’s life, the material can
be melted down and reused
over and over again without
any loss in quality. Although
more energy is required to
produce primary aluminium
instead of steel, the effect of
recycling makes the overall
balance positive compared
with steel.
Framework of
aluminium –
the ASF body
The basic structure of the
Audi Space Frame resembles
the framework of a timbered
framework. Its skeleton comprises
extruded sections and
pressure diecast parts of aluminium.
The aluminium
panels – the skin of the roof,
the floor or the side panels –
are integrated into this frame
by means of a frictional connection
so as to be semi-supporting.
The components of
the ASF have very different
shapes and cross-sections
depending on their function.
The great strength of
extruded profiles lies in their
design flexibility. The side sills
of the TT Coupé and the TT
Roadster, for example, appear
identical from the outside,
but on the inside they have
been topologically optimized
according to bionic principles.
all Photos: Audi
This means that the geometry
of a part at a given load is optimised
to save as much weight
as possible. The differences in
their ribbing determines their
rigidity, which is higher in the
Roadster than in the Coupé
to compensate for the loss of
the roof. The sections used in
the TT are made of advanced
alloys developed by Audi for
greater strength and a further
reduction in weight.
The profile and cross-section
of each extruded section
has been optimised for its
respective use. In the Audi R8,
for example, the roof arch is
shaped by means of hydroforming
– the section is shaped
by a liquid forced into it at
high pressure. This high-tech
production process enables
complex shapes that eliminate
the need for a number of
parts and ensures maximum
precision. It also keeps the
A-pillar narrow, thus reducing
the obstruction of vision
when looking forward at an
angle.
The extremely durable
vacuum diecast components
are used wherever high forces
are induced locally and where
there is a need for versatility
and design freedom. A classic
example is the A-pillar
node in the TT, which reinforces
the lower section of
the A-pillar. This multifunc-
tional component connects
the longitudinal member, the
sill, the pillar, the windshield
cross-member, the roof frame
and the strut mount with one
another.
Like all aluminium castings,
the cast nodes are characterised
by precise geometry
and optimal utilisation of
space. Such complex shapes
are only possible with the use
of intelligent design and computation
programs. Vacuum
diecasting means maximum
precision in fabrication. Casting
under reduced pressure
also improves component
quality.
The latest version of the
ASF principle is a hybrid
construction of aluminium
and steel such as that used by
Audi in the TT Coupé and
Roadster. The front end, the
floor and the superstructure
of the compact sports car
are fabricated of aluminium,
with deep drawn steel being
used for the doors and the
trunk lid. The rear section
of the floor assembly, the tail
panel and the bulkhead of the
Roadster are made of highstrength
steel. The distribution
of the materials provides
for an optimal distribution
of axle loads and thus for
dynamic handling.
The material mix is dominated
by aluminium, which
accounts for 68% of the
Coupé’s weight and 58% of the
Roadster’s. The body of the TT
Coupé weighs 206 kg, which
breaks down to 140 kg of aluminium
and 66 kg of steel. It
would be 48% or nearly 100
kg heavier if made entirely of
steel. The aluminium fraction
comprises 63 kg of panels, 45
kg of castings and 32 kg of
extruded sections.
The low weight of the
body is a key factor for the
highly dynamic road behaviour
and exemplary efficiency.
Empty, an Audi TT
1.8 TFSI weighs only 1,240
kg. The TT 2.0 TDI quattro,
which weighs 1,370 kg and is
equipped with a 125 kW (170
hp) engine consumes on average
only 5.3 litres of diesel
fuel per 100 kilometres. The
values are similarly low for the
Roadster, which weighs only
45 kg more than the Coupé.
The ASF body of the
Audi TT is in all regards the
An Audi employee creates a computer simulation of an A8
space frame using CAE (Computer Aided Engineering)
ideal solution for a sports car.
Compared to the preceding
model, static torsional rigidity
increased by roughly 50%
in the Coupé and 100% in the
Roadster. This is the foundation
for precise, dynamic
handling. The rigid ASF construction
is also responsible
for the high vibrational comfort
inside the car.
The TT makes no compromises
when it comes to
crash safety. The longitudinal
members in the front end
comprise extruded sections
and highly durable castings
at the transition to the passenger
cell. In the back, largevolume
members protect the
passenger cell. High-strength
aluminium profiles in the
doors provide protection in
the event of a side-impact collision.
Transverse extruded
profiles reinforce the floor of
the passenger cell.
A strong roof frame provides
protection in the event