Chassis Structure

Dr. Mark Battley & Tony Gray          

                                                   

The main vehicle structure is critical to numerous aspects of the design and performance. It needs to hold all the major components; engine, rockets, suspension, cockpit etc., as well as transmitting and reacting all the loads generated in vehicle operation. These include pressure loads, acceleration and braking (including the chutes) and the inevitable bumps in the surface being operated on. It must also be rigid enough provide the desired dynamic responses allowing accurate vehicle control. The aerodynamic pressures are very high, so it is crucial that the structure is able to maintain the correct aerodynamic shape.
This requires both strength (we don’t want anything to break), and stiffness (we don’t want anything to deform too much and make the car hard to control, or upset the aerodynamics).  The role of the structural engineering team is to ensure that these requirements are met while minimising the weight of the car.                                                            

Traditionally, the most commonly used material for this type of car is aluminium alloy, just as has been used in aircraft and spacecraft for many years. This is reasonably light, fairly stiff and strong, and relatively cheap. However it is difficult to form complex shapes (which we need to get the body shape right for the complex aerodynamic requirements), and requires lots of time-consuming joining. Steel is used for some local fittings such as suspension and steering components, but is too heavy to be used for the main structure. 
There are better options available that are stronger, stiffer and lighter – particularly carbon-fibre reinforced composites.  These are the materials of choice for weight critical structures, whether they are cars, aircraft, racing yachts, bicycles or tennis rackets.  Carbon-fibre composite materials provide excellent strength and stiffness at very low weight. One of their great advantages is that the designer can choose to only put fibres in the directions that are needed for the main loads – this is called “tailoring”, meaning that we can have material only where we really need it.

Much of the car will be made as a sandwich structure – no you can’t eat it! What this means is that there are three main layers to the bodywork. A very light foam “core” material is surrounded by outer and inner ”skin” layers of the carbon fibre composite. The job of the foam is keep the skins apart, which make the overall sandwich very strong and stiff in bending. This is particularly important for resisting the aerodynamic pressure.                                        

Why carbon fibre….isn’t that hard to work with and make changes to in a tough environment…?Actually no – a significant advantage of composite materials is that they can be formed into complex shapes very easily (far more easily than aluminium alloys), and also easily modified if needed. The resin that is mixed with the fibres to create the composite is basically very similar chemistry to an adhesive, so it is easy to bond more composite materials to it. America’s Cup sailing yachts are a good example – in many cases they are substantially modified or repaired overnight ready the racing the next day.  The involvement of Paul Hakes Marine in the project means that we have access to very high levels of composite manufacturing expertise.                                                          

Much of the design is driven by the aerodynamic and packaging requirements of the major components - wheels/suspension and engine/rockets. Topology optimisation with OptiStruct was used to determine the general layout of the major structural members by considering the available package space and the major loads. This method essentially ties together the major load paths with the most efficient material use. A preliminary engineered layout was then determined with consideration of manufacturing and operational practicalities. This was then modelled in detail with initial composite layups which can then be refined and optimised dependent on the responses that are calculated. In addition to the engineering performance requirements we also need to ensure that the design is cost effective and practical to build.                                                         

 

 

 

 

 

 

Up-front optimisation enables engineers and designers rapidly develop lightweight, structurally efficient designs by creating innovative concepts. As the design process advances, shape and size optimisation capabilities can be applied to further improve design performance. Using highly advanced optimisation algorithms and the high performance computing facilities now readily available, complex optimisation problems with thousands of design variables can be solved in a short period of time.