02 November 2010

Space Frame or Monocoque Structure for Cars

The most popular approach to designing the structural framework for an automobile to date is to use a steel monocoque design. The monocoque is a series of pressed panels which are typically spot welded together to form a self supporting steel framework which is strong enough from which to mount all other component parts of the vehicle e.g. the power train, suspension, doors, bonnet etc.

An alternative to the monocoque is a space frame which is a is a structure of thin walled closed extrusions, typically produced in aluminium, which is bent and twisted to the desired configuration and has thin walled ductile die castings which can be used as attachment nodes or at locations requiring complex changes in section. Sheet aluminium is added as needed for closure of space and shear diaphragms.

It is quoted that this use of the aluminium space frame can result in a 40% weight reduction and an increase in energy absorbing capability compared to a steel monocoque counterpart. The car company Audi believed that this could have been improved to a 67% weight reduction but this would have been at the cost of a reduction in crash safety.
Logic Underlying the Space Frame
  • Closed section aluminium extrusions used instead of fabricated box sections.
    • provides safety in a crash
    • provides frame stiffness
    • provides strength
    • provides durability
    • reduction in number of parts required
Ref. Ashley S. (1994)

Honda attempted to combine the best features of the monocoque design with the best features of the space frame design and produced what they called an aluminium hybrid body structure.

Honda's opinion is that aluminium has major manufacturing restrictions due to inferior press moulding characteristics and difficulties with respect to welding. This leads in turn to the following problems: -
  • An increase in production time
  • An increase in the number of parts used
  • An increase in the number of welding points
  • A large investment in dies to produce the components
For Honda, these considerations form the reasoning behind not simply taking a monocoque design and replacing the steel materials with aluminium. The bottom line is that the problems identified all lead to increased production costs.

If the space frame approach is taken, as it was by Audi in the development of the ASF (Audi Space Frame), it enables the characteristics of aluminium to be utilized more freely, reduces the number of pressed parts required and provides a more rigid structure as previously discussed.

But in Honda's opinion there are drawbacks with simply adopting a space frame design in that there is a requirement for fastening parts/fittings for mounting outer panels and other functional parts. Honda considers that this makes it difficult to meet cost targets.

By optimally combining the 3 moulding methods, Press moulding, Extrusion moulding and Casting,

Honda believed they could gain the correct balance between achieving the necessary rigidity and safety performance in relation to cost savings and productivity.

Honda’s general approach consisted of 3 factors: -

  1. Increased application of extruded materials that could be moulded with complex cross sectional shapes to achieve strength with minimum weight.
  2. Use cast parts to improve coupling rigidity and reduce number of parts.
  3. Reduce number of pressed parts.

Honda claim that they achieved a 15% reduction in the number of parts with 24% reduction in welding points and a weight reduction of 47% for the body in white structure.

Ref. M Saito et al. (2000)

It would appear that the use of the space frame is the predominant approach to car manufacture when trying to utilise aluminium as the primary material. The approaches taken by Audi and Honda in their development activities tend to support this assumption, the underlying logic for both companies are based on very similar perceptions of the problems faced.

Honda may be calling their design an aluminium hybrid body structure but it seems clear that the features are much more closely aligned with those of the space frame than those of the monocoque.

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