Build a strong frame for a vehicle and hang the body panels from it, and you have a space frame structure. Audi did this in 1994 with the aluminium A8 and is now working on a better, lighter version for its new small car. Fiat has used the same principle, but in steel, for its boldly styled Multipla six-seater. Other car makers are working on it. But it is a challenging route to take, both in design and manufacturing technology.
A space frame structure is defined as concentrating all the load-bearing responsibility onto a frame or skeleton, onto which non-load bearing panels are hung. In a monocoque, the structural responsibility is more widely distributed, with the panels taking the strain too.
For car makers, the attraction of space frame structures is that they reduce the need for pressings, because panel design is much simpler. This can create big savings in press tool investment – often a limiting factor in vehicle development, and one which inhibits design changes during the life of a vehicle.
A space frame structure also lends itself to low-cost weight-saving technologies such as hydroforming. This uses just a few steps to produce a complex hollow-section frame, which would otherwise be made of numerous welded pressings.
When Audi developed an all-aluminium A8 car in the 1980s, it found that a space frame structure was best for eliminating steel materials. The A8 gave a 38% weight saving compared to a similar-sized monocoque body-in-white structure made from steel, claim Audi engineers. Audi’s new small car, to be revealed this year, will also be an all-aluminium space frame vehicle. Refined technology will enable the weight saving, compared to a monocoque body, to exceed 40%.
Changes to the original Audi space frame idea have increased weight saving and allowed larger-volume manufacturing – Audi plans to produce 60,000 units per year.
In the first generation, the `skeleton’ was based on extruded sections, joined with aluminium castings. With the new car, these node castings have been almost eliminated, except for some large components.
Instead, the extruded sections are welded directly together. This is only possible with high component accuracy and, as a result, Audi has introduced hydroforming.
`This was the biggest challenge of the second generation concept,’ says Karl Heinz von Zengen, manager of manufacturing technology at Audi’s aluminium centre in Neckarsulm, near Stuttgart. `We don’t yet know a better way of getting extrusions of a high enough precision.’
In the new A12, Audi will laser-weld panels to the frame. `It’s six or seven times faster than MIG welding and, because the heat is very localised, distortion of the material, which is one of the problems with aluminium, is practically eliminated,’ von Zengen explains.
Laser welding is easily performed by robot. He says this will help Audi achieve 80% automation on the body fabrication of the A12, compared to 20% on the A8.
The future for space frame structures in mass produced cars remains hard to judge. The success or failure of Fiat’s steel space frame will affect how widely the technology is used for higher-volume models.
The challenge to most car makers is the change it implies for their manufacturing systems. `When Audi moved to an aluminium space frame, it had the good fortune to start with a new factory and a new product,’ says Johnny Larsson, senior engineer in Volvo’s advanced body engineering department.
`If you are changing over to a space frame, you have to use different joining and fastening methods and change the loading sequencing. You really have to start with a new plant. That is the factor which is working against space frame introduction today.’
Steel finds its space
Audi is not the only car maker looking at space frame technology, nor are others confining themselves to aluminium.
Fiat has developed a steel space frame concept for the Multipla mini-van. It plans to extend the concept to mainstream vehicles like replacements for the Brava/ Bravo, making it easier to produce derivative vehicles, Fiat says.
Ease of model variation is also the main reason why General Motors is considering space frame vehicles, adds Bill Surber, at its research centre in Warren, Michigan. By incorporating hydroforming technology with a steel space frame structure, he believes it is possible to reduce by up to 90% the number of tooling sets needed to make a primary structure. Aluminium is a low priority, while US gasoline ischeap.
Volvo is looking at steel (for cost reasons) and aluminium for its space frame concept for low-volume models to be launched in 2006. Insiders say the Volvo concept will be a simplified variation of Audi’s version.