We briefly review a method that improves rigidity, helps achieve the goal of a lighter car and also simplifies production. Which car have you sat in that uses this method?
The car body must meet two contradictory requirements: lightness and strength. Lightness abets performance and improves agility: less car to turn. It also usually helps keep the cost down. At the same time, a car must not fall apart while standing still or while in motion. And if the car should hit something it needs to protect the occupants. Usually you may have lightness or robustness but not both.
For a long time cars’ bodies have been made by assembling parts – usually metal – in such a way as to balance the conflicting needs of lightness or strength. Some makers have tended towards lightness. Colin Chapman famously wanted to add lightness with the result that his cars managed creditable performance and excellent road manner at the expense of falling apart with lamentable readiness. Lotus, as has been said, stands for lots of trouble, usually serious. The French marques, Renault and Citroen more than Peugeot, have preferred lightness and many of their cars have been very pleasing to drive. Alas, they aren’t incredibly durable. At the other extreme, Rolls Royce bodies are usually very robust (though older ones are rust prone). This means one can
enjoy impressive insulation from noise and vibration but also enjoy visiting petrol stations with considerable frequency. Mercedes and Volvo also went down the path of strength over lightness. A Volvo 240 is a truly strong and long-lived car at the cost of driver enjoyment. One can enjoy driving a Volvo but it is an intellectual pleasure rather than a sensory one.
What can one do? One can use lighter materials which can add expense: special alloys and aluminium have been tried along with carbon fibre. More recently a process innovation, hydroforming has been tried. This uses pressurised liquids to bend a tubular form into the required shape. For small volumes it has also been used on sheet metal (Aston Martin have used it sparingly). A nice benefit is better quality on the a-surface since it doesn’t come into contact with the die.
Hydroforming comes with various advantages over welding pressed steel parts together. The process can reduce the part cost by reducing the handling time, reduce weight (15%) tool cost and increasing rigidity. A typical chassis component might require pressing six sections and welding them can be hydroformed as one part.
The process is not confined to the high-end models: the Opel Vectra and Ford Mondeo, the Chevrolet Silvarado has hydroformed parts in it front end. The 2003-2006 Chevrolet SSR used hydroforming for the side rails. At Ford the Fusion (2013) and Silverado (2015) made use of it too. Jeep used the process for the upper cross member and this meant a better fit to the bonnet and headlights. Mercedes 2014 C-class is an example you might see more frequently and the process helped add lightness to the tune of about a 100 kilos over the outgoing model.
At the moment, hydroforming is mostly restricted to non-visible body parts. According to Maki (2013) automotive designers have been hampered in using aluminium because of its lower formability compared to similar steel grades. However, using hydroforming these problems might be overcome for deep draw aluminium sheets.