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AUDI A8

Body

The ASF Body of the

Audi A8

The Authors

Dipl.-Ing. Hubert Mayer

ist Leiter der Entwick-

lung Karosserie der A8-

Modellreihe im Alu-

minium-Zentrum bei

der AUDI AG

in Neckarsulm.

Dipl.-Ing. Frank Venier

ist Leiter der Entwick-

lung Aluminium-Tech-

nologie im Aluminium-

Zentrum bei der AUDI

AG in Neckarsulm.

Dr.-Ing. Klaus Koglin ist

Leiter der Fetigung-

stechnik-Aluminium bei

der AUDI AG in

Neckarsulm.

The body of the new Audi A8 is intended to re-establish the

benchmark within the luxury class (referred to as the D seg-

ment), thanks in particular to its refined Audi Space Frame.

1 Introduction

2 The Audi Space Frame of the

Audi A8

In 1994, the Audi A8 became the first car to

appear on the market with an Audi Space

Frame (ASF) body. The new Audi A8 is the

outcome of the systematic refinement of

this lightweight concept. It thus satisfies

very rigorous standards of comfort and

safety.

The new Audi A8 serves as the benchmark

in its class for lightweight design in con-

junction with outstanding stability. This is

achieved through the use of innovative Au-

di Space Frame technology for the design of

the A8 body. The technical progress repre-

sented by this unique body concept results

Audi A8

from the systematic way in which our ex-

perience with the aluminium-bodied A8

and A2 has been applied. On the basis of the

findings obtained, the number of body

parts has been further reduced and the de-

gree of automation in the production

process significantly boosted compared

with the previous A8.

2.1 The Body Concept

The specifications for the new A8 laid down

that this car was, among other things, to

provide greater comfort and satisfy higher

safety standards than its predecessor.

A body that is capable of meeting tough

requirements in respect of static and dy-

namic rigidity is needed as the basis for

greater comfort. Local rigidity is moreover

an important consideration. Little vibration

should be felt by occupants wherever they

are contact with the vehicle, such as on the

seats and touching the steering wheel.

The safety requirements have been

made much tougher by new legislation in-

troduced in Europe and the USA (such as

FMVSS 301). These requirements need to be

fulfilled first and foremost by the body.

Other requirements included attaining a

greater level of automated production and

optimising costs.

All the aforementioned requirements

for the body concept were fulfilled by sys-

tematically refining the tried-and-tested

Audi Space Frame technology used for the

body.

The Audi Space Frame was first adopted

on a production vehicle in 1994, for the cur-

rent A8. This body concept was then re-

fined on the A2, which appeared on the

market in 1999, and adapted for use on a

compact car. The collective experience ob-

tained from both these projects was de-

ployed for the new A8. A special feature of

the ASF is the combination of semi-finished

aluminium elements used, comprising

castings, profiles and panels. These individ-

ual components form a self-supporting

frame structure into which each planar

component is integrated as part of the

structure. The ASF achieves maximum sta-

bility with minimum mass. It has been di-

mensioned to satisfy maximum rigidity,

comfort and safety requirements. Whereas

the familiar production techniques from

the A2 such as hydroforming are used for

the profiles, green sand casting is used for

the first time for the castings as well as the

familiar Vacural die-casting process. In ad-

dition to the familiar alloys used on the A8

and A2, new alloys are used for the panels,

as a separate hot exposure process is no

longer used for this body. Instead, the heat

from the drying process after cataphoretic

dip priming is used to harden the alumini-

um.

Other design features include multi-

functional large castings, long continuous

profiles and a high proportion of straight

extruded sections. Curved profiles are used

only at those points where the outer pan-

elling of the A8 (for instance the side of the

roof frame) necessitates this.

In contrast to its predecessor, the new

A8 has a continuous space frame which in-

cludes the rear structure.

The number of individual parts has been

significantly reduced through the afore-

mentioned structural measures. This on the

one hand improves comfort (fewer con-

necting points between the components)

and on the other hand has a beneficial ef-

fect on the costs and production processes,

Figure 1.

The joining techniques used first for the

current A8 and then for the A2 have gradu-

ally been refined and optimised. The famil-

iar joining techniques for the A8, such as

MIG welding and riveting, together with

the technique of laser-welding introduced

for the A2, have been further optimised for

the A8. Laser hybrid welding is now being

used for the first time. It exploits the ad-

vantages of both thermal joining technolo-

gies (MIG and laser welding), combining

the joining forms of MIG welding (overlap

seam, fillet weld) and the strength of the

joining zone with the lower heat input of

laser welding, while simultaneously mak-

ing higher processing speeds possible.

The combined effect of all these struc-

tural and technological improvements is to

satisfy the targets laid down in the techni-

cal specifications.

The result is a product in which con-

struction, calculation, vehicle safety and

2.1 The Body Concept

2.2 The Structure of the Body

Figure 1: Number of parts and

breakdown by weight

Figure 2: The Audi Space Frame for the A8 and its sheet metal components

Special Edition ATZ/MTZ

AUDI A8

Body

2.2 The Structure of the Body

vehicle strength are neatly dovetailed. As

part of a simultaneous engineering ap-

proach spanning all divisions, such as De-

velopment, Planning, Production and Qual-

ity Assurance, a further objective in the

technical specifications for optimised pro-

duction and optimised costs was achieved:

the creation of four body versions of the ve-

hicle, excluding rear lid. These are the ver-

sions for the short and long-wheelbase A8,

in each case with and without sliding roof.

All legal requirements worldwide are thus

satisfied by a single body version.

2.2 The Structure of the Body

The structure,

Figure 2,

starts with the for-

ward and rear structures. These two assem-

blies are joined together with the under-

body to form a space frame. The single-sec-

tion side and the roof are then joined to the

structure, to which the wings, doors and

lids are added; the structure is then fin-

ished, and transferred to the painting

process.

The forward structure contains the large

cast radiator tank as its central component,

Figure 3.

This component takes the place of

several separate components on the prede-

cessor; it supports the air conditioning sys-

tem, the pedal mount and the windscreen

cross-member, and connects the left and

right A-posts. The A-post likewise consists

of two large cast shell halves,

Figure 4.

These are clasped together beneath the sill

and, at the top, through the side of the con-

tinuous roof frame. Both extruded sections,

together with the tunnel structure, are of

central importance to the body’s torsional

rigidity.

The concept of the remainder of the for-

ward structure resembles that of the cur-

rent A8. The longitudinal members are di-

vided into one front and one rear section,

joined together by a green-sand casting.

Thanks to the considerable design freedom,

the casting makes it possible to combine a

variety of functions, such as serving as the

mounting for the subframe and engine

cross-member, and supporting the fender

bracket mounting and the suspension

struts. The front longitudinal member is

bolted on, as on the predecessor, for ease of

repair. Apart from its modified length, this

component is identical to that used on the

predecessor model.

The rear structure has been developed

entirely from scratch. The air suspension

and the US legislation (FMVSS 301 –

80 km/h rear-end collision) necessitated a

departure from the predecessor’s concept

for this zone. The two central large castings

are the connecting element between the

sill/longitudinal member and connecting

element between the C and D-post. The

Figure 3: The multifunctional large

cast radiator tank with its various add-on

components

Figure 4: A-post with

sill and side of roof

frame

Figure 5: Sill/longi-

tudinal member con-

necting element

Audi A8

sill/longitudinal member connecting ele-

ment is the largest casting,

Figure 5.

It sup-

ports the entire rear subframe and connects

the longitudinal member with the sill at

the rear. Its high rigidity protects the tank

in between in the event of a rear-end colli-

sion. The connecting element for the C and

D-post serves as the mounting for the top

end of the air suspension and has the belt

reel mounted on its front side. It also forms

the lateral limit of the roof frame. These

two castings are connected transversely

with the rear shelf via two straight extrud-

ed sections, simultaneously providing a

rigid basis for the tweeter in the rear shelf.

The upper and lower planes are likewise

supported vertically by two straight ex-

truded sections on either side. This arrange-

ment forms the portal for the air suspen-

sion's strut mount,

Figure 6.

The forward structure and the rear

structure are joined together with the side

of the roof frame, the sill, the seat cross-

members, the B-post and the floor panels to

form a continuous space frame. The side of

the roof frame is a hydroformed extruded

section which is much larger than the roof

2.2 The Structure of the Body

Figure 6: Portal for

the air suspension

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Special Edition ATZ/MTZ

AUDI A8

Body

3.1.1 Panels and Alloys

Figure 7: Sheet metal processing chain in body production

Table 1: Materials in the

A8 Audi Space Frame

frame familiar from the A2. Its cross-section

varies as necessary. To allow this consider-

able variation in cross-sections, no flange

has been used. The flange for the door seal

is formed by a separate sheet metal ele-

ment, which also serves as a connecting

piece for interior equipment components.

This ensures that the roof frame permits a

high degree of shaping and requires only

little machining, as this would be very elab-

orate to perform by hydroforming in view

of its wall thickness of 4 mm. The B-post is

a large multifunctional casting, which

must fulfil the exacting lateral collision re-

quirements as well as serving as the hinge

mount for the rear door and the striker

mount for the front door. The B-post is in

addition of central relevance to comfort.

The quality of its join to the roof frame and

the sill has a major influence on this prop-

erty of the body.

The single-section side and the roof are

added last. In contrast to its predecessor,

the A8 has a single-section side which, like

the roof, is joined to the structure by laser

welding.

3 Materials and Production

Technologies

so necessitates the use of production tech-

nologies for joining together the body that

differ from the classic shell principle. The

semi-finished articles and production

methods used are explained in greater de-

tail below.

3.1 Panels and Methods

3.1.1 Panels and Alloys

Because of the materials involved, the Audi

Space Frame body construction principle al-

Both the number and percentage content of

panels are lower in the D3 A8 than on the

D2 A8. Only thermally hardened alloys (of

the type AlMgSi) are now used.

Thermally hardened alloys are used pre-

dominantly for the outer skin in view of the

fact that the surface is free from flow lines.

Audi A8

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