Historical development and requirements for a modern convertible top


The automobile is no longer used exclusively for transport or locomotion, but is nowadays associated with the concepts of romance, sportiness, driving pleasure and prestige. These are mainly associated with convertible vehicles. The trend towards convertibles is unbroken in the European markets and in the USA, and many people's hearts beat faster with the sight of a beautiful convertible. High-quality convertibles in particular, such as the new SL-Class from Daimler-Chrysler and Lexus from Toyota, are enjoying increasing popularity. All renowned automobile manufacturers meet the market requirements and offer a wide variety of convertible models in various classes and versions (Fig. 1).

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Already at the beginning of the automobile industry, towards the end of the 19th century, the first automobiles were open vehicles that developed from coachbuilding (Fig. 2). Much has changed since then in the development of convertible tops, but the high demands on the optimal function and storage of the convertible top package have still not been sufficiently fulfilled. Great efforts are being made by the automotive industry to meet customer requirements.

In a survey of Cabriolet drivers and dealers regarding an optimal convertible system, the following wishes were filtered out:

  • Fully retractable side windows,
  • Suitable for use in car washes,
  • Suitable for winter,
  • low noise level,
  • fully retractable hood,
  • good thermal insulation,
  • large, heatable rear window
  • electric soft top actuation,
  • Damage resistance.

In addition to this customer requirement specification, the automotive industry must now increasingly incorporate the state's environmental and safety requirements into the design phase.

Basic principles of the cover mechanisms


Four-part hood mechanism: a) Gear sketch b) kinematic diagram


kinematic scheme (Audi TT convertible)

Since the beginning of automotive history, different roofs with different structures and designs have emerged? 1-part, 2-part, 3-part, folding top, hardtop - for covering the passenger compartment (Fig. 3). The length of the passenger compartment to be covered and the available installation space for the combined canopy (packaging) are decisive for the type of canopy mechanism.

The larger the number of roof elements, the more complex and complex the required cover mechanism becomes.

The hood mechanism of a two-piece can be quite simple as shown in figure 4. This is a four-unit swivel joint transmission in which the swing arm 1 represents the rear element and the coupling 2 the roof cover.

A possible approach to the structural determination of hood mechanisms of more than two parts is the consideration of the overall transmission in partial gears.

The subdivision is carried out in a four-unit basic gearbox (Fig. 2) and a multi-unit "auxiliary gearbox" (roof-top gearbox) mounted on the coupling (roof cap) of the basic gearbox. Audi TT is an example of such a composition (Fig. 5 and Fig. 6).


kinematic scheme (Audi TT Cabrio)


Gear unit display in Kintop

Our activities

While the kinematic dimensions of the four-unit basic gear unit are relatively easy to determine due to design constraints, the selection of the structure and dimensions of the secondary gear unit is a very demanding and complex design task.

Deviations in the nominal dimensions of the links of a mechanism due to manufacturing, assembly or operation naturally lead to deviations in its kinematic and dynamic nominal behaviour. The "control" of tolerances is crucial for optimum function and cost-effective assembly. There is therefore an urgent and special need in the roof construction industry to get the tolerances and thus also the costs under control. A meaningful tolerance study is essential. For this reason, the Institute for Transmission Technology and Machine Dynamics at RWTH Aachen University is pursuing the development of convertible tops. With the knowledge of gearing technology and the appropriate programs, various soft top mechanisms have been investigated with regard to their kinematics and tolerances. In particular, the tolerance analysis of complex mechanisms is very complex.

Our fields of competence in this project:

  • Kinematic analysis
  • Kinematic synthesis (structural and dimensional synthesis)
  • Tolerance analysis
  • Test benches (We build and design a canopy test bench together with you)



Institute of Mechanism Theory, Machine Dynamics and Robotics

RWTH Aachen University

Eilfschornsteinstraße 18

52062 Aachen



Phone: +49 241 80 95546

Fax:      +49 241 80 92263


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