Hot tubs
Racecar Engineering|Design of a Racecar
Carbon fibre monocoques lie at the heart of most top-level racecars, and they’re a complex piece of engineering in their own right. Racecar investigates
Lawrence Butcher
Under the skin of any modern racecar, at least of the Sports Prototype or formula variety, is the tub, or monocoque. A multi-functional component that must fulfil many design requirements, some of which are diametrically opposed.

Its primary role is to fill the void between the front suspension and the engine, providing sufficient structural stiffness for said suspension to operate correctly. At the same time, it must safely house the driver (to varying degrees of comfort, depending on the application), the fuel cell and, as is increasingly the case, an energy storage system (ESS) for either an electric or hybrid drivetrain.

Additionally, the tub must be as light as possible, while also having as minimal effect on the overall footprint of the car as is feasible. Aero is a predominant concern and, as such, the monocoque, just as with the powertrain, cooling system and any other ancillary components, plays second fiddle to aerodynamicists’ desires when it comes to real estate.

Think of the shape of a modern Formula 1 tub. It is not through choice that the driver sits with their legs up in the air, with the chassis cut away sharply below. The form is driven by the need to channel as much air as possible under the nose of the car.

All of these competing demands need to be balanced out. Ultimately there will be compromises, but with contemporary design, simulation, manufacturing methods and materials, they can be reduced.

Of course, racecars have not always been built this way. The spaceframe chassis ruled supreme until the early 1970s when aluminium monocoques started to become commonplace, and remained in use, including in Formula 1 and Sportscars, until the late 1980s, when carbon fibre took over. Though designers had dabbled with the material in the 1970s, cars such as John Barnard’s McLaren MP4/1 revolutionised racecar chassis construction, first in Formula 1 and then other series. Relying on a combination of uni-directional carbon fibres and aluminium honeycomb, those cars laid the groundwork for much of what still exists today.

Design implications

As noted, the design of a car’s tub will be a delicate balance of trade-offs. The smallest detail needs to be considered, especially at the highest levels of racing where tiny percentages can be a significant performance differentiator. Take the example of the then Force India team in 2015. Driver, Nico Hulkenberg, was 8kg heavier than team mate Sergio Perez. To account for this, the team built him a lightweight chassis to maintain leeway for ballasting the car. The compromise with the lighter chassis was it was less stiff, which was detrimental to suspension performance, but not to the extent that it offset the gain from the greater freedom to place ballast.

This is just one example, there are many others such as the relatively regular occurrence of a team needing to switch engine or power unit supplier late in a car’s development cycle. Invariably, this leads to compromises as tubs are inherently long lead time parts, so it is impossible to tweak the design to perfectly match a new engine’s specifics.

Architecture

At a basic level, there are a few common features of monocoques across car types. The rear bulkhead will have mountings for the engine, the exact layout determined by whether this is a stressed or semi-stressed part. These will be in the form of inserts bonded into the carbon, which have to withstand both the forces generated by the powertrain, and those of the suspension and aero.

The rear of the tub will feature a rollover structure that must tie into the rest of the monocoque securely enough to resist extreme crash forces. Most rule sets also require the tub accommodate a car’s fuel cell, which is positioned as centrally as possible. This is not simply a safety consideration, it also helps to maintain balance regardless of fuel load.

Moving to the centre of the chassis, regulations usually dictate the minimum volume for the driver’s cockpit, as well as elements such as compulsory side impact structures. From these fixed points, the rest of the tub’s geometry can be calculated.

The front of the chassis can be something of a packaging nightmare. Frontal area will be kept to an absolute minimum for aero purposes, while the driver’s feet, pedals and various suspension elements are all competing for space. At the very least, there will be two dampers and a third element, with either coil springs or torsion bars, the latter also requiring mountings incorporated into the tub.

Then there are components such as the steering box and mounting points for the wishbones. Again, aero rather than packaging concerns tend to dictate their location, often leaving structural engineers with a headache. Finally, there will be the mountings for the nose, which incorporates the front crash structure. These have to be substantial enough to both survive an impact, as well as carry the aero loads from the front wing.

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