20th November 2013 by Royston Jones
This post has been contributed by my colleague, Lars Fredriksson, Director at Altair ProductDesign, Germany
Minimizing the weight of the primary vehicle structure (e.g. Body in White, closures, etc.) is an increasingly important part of many OEM’s design objectives as they look to reduce the environmental impact of their products. When we think about redesigning a component to use less material through the use of optimization techniques or by using advanced materials, we focus on the mass savings achieved on primary structure.
However, if we can reduce primary weight, this has a positive effect on the weight requirement of secondary (e.g. HVAC, brakes, etc.), often non-structural components. A reduction in secondary weight leads to a reduction in primary and consequentially a virtuous circle is formed.
“Secondary” mass saving describes the potential additional weight reduction that can be achieved as a direct result of taking weight out of a primary structure. For example, reducing the weight of a car engine block means that the supporting structure around the engine does not need to carry as much weight. The supporting structure can therefore be redesigned to withstand a new set of forces and additional weight savings can be achieved. Maybe we could go as far as removing some of the supporting structure all together, saving additional weight but also reducing manufacturing complexity in the process. After all, you can’t get a much better weight saving than removing a component all together!
Back in 2007, the Auto/Steel Partnership issued a report that went as far as suggesting that a 1.0 kg saving in primary mass can lead to a 1.5kg saving in secondary mass if utilized correctly and all subsystems are available for mass reduction. The table below which was produced at the time of the report demonstrates this relationship.
As often when it comes to maximizing the effect of optimization, the earlier, the better also applies for the utilization of secondary mass effects. In order to draw on the primary mass savings later on, for instance a lighter engine or gearbox, the reduced mass should be known when starting the BiW development. Especially if different departments are handling the different systems, communication is key in this case. If the same department is responsible iterative or semi-automatic iterative methods can be used. A weight reduction of chassis parts directly leads to a set of reduced loads. This can potentially lead to further weight reductions. In this case, the optimization can be put into a loop with calculating new forces followed by performing a new optimization iteration.
While our experience suggests that the relationship is a little more like 1 kg of primary weight leads to an additional 1 kg of secondary weight saving, the findings are nevertheless an indicator of the impact that saving primary weight can have a knock-on effect on secondary weight reduction.
When looking for potential weight savings for your next product, don’t forget that any change in design can have a further positive effect on none structural elements around it. For companies wanting to minimize mass, these additional savings can add up to something truly optimized.