Car
The evolution and construction of monocoque chassis in motorsport
by Steve Rendle
3min read
A monocoque chassis forms the backbone of most modern single-seater and sports competition cars. With a monocoque, the entire homogenous chassis structure is stressed, directly carrying all the loads applied to it.
The Lotus 25 is the first Formula 1 car to use a monocoque chassis
In 1962, the Lotus 25 became the first F1 car to feature a fully stressed aluminium monocoque, rather than a spaceframe chassis, which had previously been de rigueur. This early monocoque carried all the suspension loads, with the rear suspension attached directly to the monocoque, and the front suspension and steering assemblies attached via a subframe.
The engine was bolted directly to the rear of the monocoque.
The advantages and roots of monocoques in F1
A monocoque has several key advantages over alternative chassis types, providing a lighter structure with significantly improved torsional rigidity, facilitating a more compact unit, which benefits packaging and aerodynamics.
Following the pioneering Lotus 25, rear-engined car designs soon evolved to use the engine/gearbox assembly as a fully stressed component, with the rear suspension and rear-wing assembly mounted directly to the engine/gearbox, in turn bolted directly to the monocoque.
In addition to forming the core of the car, the monocoque also acts as a safety cell for the driver, and usually also houses the fuel cell.
Although aluminium-honeycomb monocoque chassis are still in use, the majority of monocoques today are constructed from carbon-fibre – technology pioneered by McLaren’s John Barnard in 1981 for the McLaren MP4/1 F1 car. The carbon-fibre monocoque brought a significant weight saving and much-improved torsional stiffness in comparison to aluminium, along with increased strength.
The monocoque is a critical factor in car performance. Strength and stiffness are key, as the monocoque must maintain its structural integrity and provide consistent performance when handling the significant and varying mechanical loads from the suspension, steering, engine and transmission, plus aerodynamic loads, which can equal or exceed the mechanical loads.
Today, aerodynamic performance is a key parameter – often the primary factor – taken into account during the car design process, and so aerodynamicists are usually responsible for determining the optimum monocoque shape to suit the overall concept of the car.
Composites and chassis engineers then take the aerodynamic cues, designing the monocoque to accommodate the required components, while providing the best possible compromise for the car’s overall packaging.
The monocoque is essential to the driver’s ‘feel’ for the car, as all the forces acting on the car are communicated to the driver via the monocoque, to which he or she is securely strapped. Therefore, any damage or flaws in the structure can compromise the driver’s ability to get the most out of the car, in addition to affecting its mechanical and aerodynamic performance.
An example of a monocoque in another motorsport. The Audi R18 was campaigned by Audi in the LMP1 class of the World Endurance Championship until 2016. The engine is bolted directly to the monocoque here.
Computer Aided Design (CAD) is used in the production of an F1 chassis
How carbon monocoques are designed and produced
A carbon chassis today is designed using CAD (Computer Aided Design) and Finite Element Analysis (FEA – a process using a software package to predict and analyse structural loads). The use of CAD and CNC (Computer Numerically Controlled) tooling during manufacture enables accurate repetition of the processes to ensure consistent dimensional accuracy and stiffness, and therefore consistent performance, between multiple monocoques of the same design.
The first step in the manufacturing process is to produce a pattern from which moulds can be made. The pattern must be highly accurate, as any imperfections will be reproduced on every monocoque manufactured.
The pattern is used to make ‘female’ moulds from carbon-fibre, the configuration of the moulds varying depending on the manufacturer – upper and lower half-moulds may be used, or several moulds may be used to enable the monocoque to be built in several sections.
The moulds are carefully machined to remove imperfections, and are then used to manufacture the required production run for the monocoque.
The actual monocoques are manufactured using pre-preg carbon-fibre (matting impregnated with resin), and several hundred individual machine-cut pieces of pre-preg material are used for each chassis. Each piece, or ‘ply’, of carbon-fibre must be laid in the mould to run in a specific direction, to provide the required stiffness properties, and to cope with the directional loading to which each specific area of the monocoque will be subjected. The orientation and exact positioning of each ply is determined by the designers.
After a number of plies have been laid up, in many cases, aluminium or Nomex honeycomb material is placed on top of the plies to increase rigidity, and improve impact resistance, with no significant weight increase. If honeycomb is used, further layers of pre-preg material are then laid-up on top, providing a sandwich construction.
A close-up look at carbon-fibre, showing a ‘weave’ pattern following a certain calculated direction for rigidity
Inside an autoclave - an ‘oven’ that bonds together layers of carbon fibre
During the lay-up process, allowance is made for the integration of components such as metal inserts and studs to act as mountings for components that will later be attached to the monocoque.
Once the lay-up process is complete, the monocoque components (still inside the moulds) are placed in vacuum bags, which have all the air extracted to squeeze the plies together, and the components are then placed in an autoclave (a large oven with precise control of temperature and pressure) to begin the curing process. Curing takes place in several stages, tailored to the monocoque design, and actually comprises two processes – debulking and curing itself. Debulking involves compressing the carbon plies, and curing involves transforming the pre-preg resin from a fluid to a solid.
Once the autoclave processes are complete, the components are removed from the moulds and can then be bonded together, using accurate jigs, to form the complete monocoque. At this point, items such as bulkheads may be bonded in place to provide mounting points for various components.
The completed monocoque is machined and trimmed to provide any final detailing, along with mounting points for suspension and other components.
In many cases, a sample monocoque must be subjected to a set of homologation tests. These are primarily structural tests to ensure the structural integrity of the survival cell in the event of a serious accident.
Finally, the finished monocoque normally undergoes a rigorous inspection and Non-Destructive Testing (NDT) process, prior to being released for use as the basis of a car.
The end result is a solid, lightweight carbon-fibre monocoque that forms the central part of the F1 car - protecting the driver and providing the foundation to add performance-enhancing aerodynamic components.