F1 chassis

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1.0 Design History and Usage

The past half-century has seen a progressive development of the chassis styles used in top line

Motorsport. Formula 1 represents the peak of both human skill and technological advancement as this is

echoed by the chassis and material development as much as engine and computational advancement.

When Formula 1 began in post-war Europe, the mostly German and Italian teams used basic space

frame chassis, which comprised of a series of beams that formed the shape of the car and contained the

engine, driver, suspension and other car sub-systems.

Figure 1 below shows a typical space frame chassis.

Figure 1Space Frame Chassis

During the early 1950s, these types of chassis were suitable for Formula 1 racing, however towards the

end of the 1950s the engine power had increased as had cornering speeds, both of which necessitated

an increase in both the strength of the chassis and an increase in safety as speeds increased.

An increase in the torsional rigidity was also required as suspension development increased. Torsional

rigidity is the chassiss resistance to torsional deflection under loading. As will be showed later, chassis

stiffness is a very important factor in the handling and performance of a top level racing car.

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In the late 1950s Colin Chapman, the chief designer of Lotus, introduced the monocoque chassis to

Formula 1 by placing thin plates around the bars of the space frame chassis, which acted as shear panels

in effect. This increased stiffness without increasing mass.

Soon after in 1961, Chapman used a complete tub in the design of the Lotus 49 (See Figure 2). The tub

was constructed entirely from aluminium sheeting and it marked an evolution of the space-frame; itweighed less, was stiffer with a smaller frontal area.

Figure 2Lotus 49 [12]

In 1978 Lotus created an aluminium honeycomb chassis, which, like the 49, was a fully enclosed

monocoque. However, instead of just using aluminium panels, Lotus used sheets of aluminium

honeycomb with consisted a hexagonal honeycomb with skins of thin sheet aluminium,which gave a

very good increase in torsional stiffness without increasing the weight of the chassis.

The 1970s also heralded the introduction of the aero revolution into Formula 1. The use of wings and

ground effects dramatically increased the loadings through the chassis and the speeds, which as en


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effect put a greater emphasis on a strong and safe chassis. In the early 1980s, the chassis designers

were finding that the aerodynamic loads were flexing - 2 - the chassis, which not only reduces the

effectiveness of such devices, but also accelerated fatigue failure.

In 1981, John Barnad from the McLaren, together with the American company Hercules Aerospace,

designed and constructed the first carbon fibre Formula 1 chassis, the MP4/1 (See Figure 3). Theperformance gain over the conventional aluminium honeycomb chassis was amazing, and subsequent

developments lead to the MP4/4 winning 15 out of 16 races in 1988 in the hands of Alain Prost and

Ayrton Senna.

Figure 3The McLaren MP4/1

2.0 Mechanical and Physical Requirements

As pointed out briefly in section 1, there are three basic requirements for the chassis of a Formula 1 car

that make Carbon Fibre the ideal material for the design and construction.

2.1 Weight


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As Newtons second law of motion dictates, the force required to accelerate an object is directly

proportional to its mass. Since motor racing comes down to maximising the acceleration in the required

direction, it followsthat reducing mass is one of the best ways of improving acceleration. This not only

applied to straight-line acceleration, the cornering performance of a racing vehicle is a function ofthe

weight transfer; the less weight that can be transferred, the better the allowable traction on corner exit.

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2.2 Torsional Stiffness

As discussed earlier, the torsional stiffness (or rigidity) of a vehicle affects its handling. Suspension

systems are designed with the assumption that the chassis is rigid and that any motion under loading is

fromthe suspension components (springs, dampers and tyres). Ifthe chassis is not sufficiently stiff,it will

deflect and render the suspension design flawed.

The vehicle can only accelerate as quickly as the wheel (or wheels) with the lowest available traction will

allow it. Therefore it isof paramount importance to ensure that the car remains balanced under allconditions. One of the biggest factors governing this is the ability to control weight transfer. The ability

to control the lateral weight transfer distribution comes only if the chassis is stiff enough to transmit the

torques [5].

Deakin et al. performed a study to show the effect of chassis stiffness on the handling and balance of a

car. Figure 4 shows the difference in front to rear lateral load transfer distribution for a range of chassis

stiffness with the overall roll stiffness of 5000 Nm/deg. The results assumea 50:50 weight distribution

front to rear and the samefront and rear centre of gravity heights.

Figure 4 - Lateral Load Transfer froma racing car with Roll Stiffness of 5000 Nm/deg for 50:50 Weight

Distribution [5]


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Figure 4 shows how the front loadtransfer changes for different values of front:rear roll stiffness. In a

rigid chassis, a change in relative roll stiffness of10% will give a 10% change in the total load transfer

characteristics. Therefore, the relationship will be purely linear for a completely rigid chassis. As this

never occurs the figure reflects the relationship for different values ofchassis stiffness and shows how it

will become linearised as the stiffness increases.

The stiffer the chassis, the more predictable the car becomes and the quicker it can be driven. As the

weight transfer increases into a corner, the driver expects the car to roll at the samerate, however a car

without adequate stiffness will be very unpredictable and therefore much harder to drive.

2.3 Safety

Given the extremely high speeds (the highest speed recorded in 2004 was 369.7 km/h) the sports

governing body, the FIA, set regulations governing prerequisite chassis strengths and absorption ability.

In a high-speedimpact, it is important that the chassis absorb as much of the energy as possible and

reduce the acceleration on the drivers body.

The main impacts that the FIA test are side, front, rear and top impact and for each test, certain results

must be achieved for the chassis to be allowed to race. The side impact test (16.3, FIA 2004 Formula 1

Regulations) for example, involves the following [6]:

The resistance of the test structure must be such that during the impact:


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The average deceleration of the object, measured in the direction of impact, does not exceed 20g.

The force applied to any one of the four impactor segments does not exceed80kN for more than a

cumulative 3ms.

The energy absorbed by each of the four impactor segments must be between 15% and 35% of the

total energy absorption.

Furthermore, all structural damage must be contained within the impact absorbing structure.

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Figure 5 below shows the four measurement points for the side impact test on a development Ferrari

chassis that weighs just 31.6 kg.

Figure 5Ferrari Carbon Fibre Chassis after the FIA Side Impact Test

3.0 Carbon Fibre vs. Other Materials


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The main attraction of carbon fibre for Formula 1 chassiss is the amazing strength and stiffness for its

weight. No other (appropriate) material comes close to carbon fibre in terms of specific weight and

stiffness.

Figure 6 shows the specific stiffness (the rigidity ofthe material for every unit ofits weight) as a function

of the specific stiffness of high stiffness carbon fibre.

As can be clearly seen, carbon fibre has a specific stiffness in the order of 2-3 times that of conventional

metals such as steel and aluminium.

Figure 6Stiffness per unit Weight for Common Engineering Materials [13]

The sameanalysis with strength shows thatcarbon fibre has a specific strength over ten times that of

basic steel.

Table 1Specific Strength [9]


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Combining the increased strength and stiffness properties of carbon fibre compared to conventional

metals and alloys, it is by far the best material for the construction of Formula 1 chassis, not to mention

other critical components of the car.

The following table shows the increase in stiffness through chassis development since the 1950s.

(Please note, values are only approximate)

Table 2Stiffness and Mass for DifferentChassis Types [7] &[8]

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While quantifying the ability ofa material ishard to quantify, the Charpy and Izod tests estimate the

amount of energy per unit of distance. Most Mild Steels can absorb around 1.5 J/cmwhereas someforms

of carbon fibre have been shown to absorb around 6.09 J/cm.

4.0 Carbon Fibre Manufacturing

Carbon fibre belongs to the carbon-carbon (CC) group of materials, which is a generic class of

composites similar to the graphite/epoxy family of polymers.

Carbon fibre can be made by a number of methods in many different forms and as a result, their

mechanical properties can be tailored to suit the required application.

Figure 7 below shows the multiformity of carbon fibre and carbon-matrix composites.

Figure 7Different forms of Carbon Fibre and Carbon-Matrix Composites [1]


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Generally, all carbon fibres are manufactured by the thermal decomposition of different organic fibre

precursors. The most commonly used precursors are fibres of:

Polyacrylonitrile (PAN)

Cellulose (Rayon)

Pitch Fibres

Carbon Fibre used in F1 is generally PAN based and as such, Rayon and Pitch-based carbon fibre will not

be discussed here.

Normally, PAN is copolymerised with a small amount of another monomer in order to lower its glass

transition temperature and increase control of its oxidation.

Figure 8 below lists the monomers copolymerised with PAN and their structures.

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Figure 8Monomers Copolymerised with PAN Carbon Fibres [2]


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A typical precursor fibre will be 93-95% acrylonitrile and the rest a combination of one more of the

above monomers. PAN decomposes below its melting point, and as a result it is normally extruded into

filament formby a spinning process.

Solution Spinning was the first process used, but it required a great deal of solvent and as a result a new

method was required.

Melt-assisted spinning is the best method for converting PAN fibresto carbon fibre with excellent

mechanical properties. BAST Structural Materials, INC., developed this method by there the acrylonitrile

copolymer is polymerised in an aqueous suspension.

Then, after polymerisation, the copolymer is purified and dewatered before extrusion.

Extrusion involves the solution being pumped through a spinnerette containing a large number of small

(approximately 100 microns) capillary holes.

Figure 9 shows a basic schematic of the melt-assisted spinning process.

Figure 9Melt-Assisted Spinning Process [2]

The PAN polymer is extruded into a steam-pressures solidification zone. After passing through, the fibre

is stretched and fried. His process has an advantage over non-melting processes as it reduces solvent

use, wastewater and creates a more uniformcross section.


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After the forming process the PAN precursor fibres are heat treated to secure their final properties. This

is accomplished by heating the PAN precursor fibres (usually to 220-280C) at tension fromanywhere

etween 30 minutes and 7 hours. The time taken and temperature is dependant on the composition and

the size of the fibre.

Figure 10 below shows a schematic of the heat treatment process.

Figure 10The Heat Treatment Process 2]

The principal reactions that occur during thisstep (generally referred to as oxidation) are cyclization of

the nitrile groups, dehydration of the saturated carbon-carbon bonds and, of course, oxidation. Many

smaller structures result, but overall, the final structure of the PAN based carbon fibre is shown in Figure

10.

Figure 10The Structure of PAN-based Carbon Fibre [2]

Many studies have been performed to find the relationship between grain size and the subsequent

mechanical properties. These studies have shown that the PAN based carbon fibres appear to have

extensively folded aninterlinked turbostratic layers of carbon with an interlayer spacing considerable

largerthan that of graphite. They show a low degree of graphitisation, and the turbostratic layers are not


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highly orientated with the fibre axis. *3+ Figure 11 below shows two models of PAN microstructure

developed fromthe studies into the effect of microstructure on mechanical properties.

Figure 11Microstructure of a PAN based Carbon Fibre [3]

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After the production process, the carbon fibres canbe used to formdifferent weaves. As F1 chassis are

predominantly laminated, only the will be discussed. However, it should be noted that other forms ofcarbon fibre weaves to exist, as shown in Figure 7.

There are two principal weaves used in laminated. Plain weave (Figure 12a) uses basic cross-hatching

and is commonly used for plat surfaces (i.e.panels). Satin weaves (Figure 12b) produce smooth fabrics

that have good drape and are ideal for use when there are depressions and bends in the macroscopic

shape of the end product. This makes themideal for areas of an F1 chassis that have complex,

aerodynamically driven shapes.


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Figure 12Weaves used for Laminates [4]

The laminate structure has an obvious inherent drawback in the sense that its strength is unidirectional.

The strength of the laminate in any given direction is a function of the yarn strength and the volume

fraction of yarn inthat direction. In general, the properties off-axis are hard to predict. As one would

expect, this produces a laminate with low out of plane tensile strength. As the strength lies is mostly in

tension/compression, most of the resistance to deformation lies alone these planes also.

5.0 Building the Chassis

A modemday Formula 1 chassis is comprised of three individual layers; the outer skin, the core and the

inner skin. The inner and outer skins are multiple layers of carbon fibre laminates as discussed in Section

4.

The core is a made of honeycomb, which constructed of very thin material divided into cells as shown

in Figure 13. It has excellent shear resistance and an extremely high energy absorption that dramatically

improves the crash safety of a Formula 1 car.


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Figure 13Honeycomb Cells

The first step in constructing the chassis involves an extremely accurate female mould of the outershape of chassis (accurate to 50 microns) being produced by a large multiaxis CNC (Computer Numerical

Control) machine.

Fromthe female, the carbon fibre laminate sheets (which are pre-impregnated with an epoxy resin) are

layed up on top of each other, with careful observation to the direction of the weave. The direction of

the weave is dictated by the direction of the particular loads in that area. Depending on the region of

the chassis and the strength required, the number of layers can vary significantly.

Once the outer skin is completed, it is placed in a large autoclave and cooked under pressure to squeeze

the layers together, which is known as de-bulking. *11+ Figure 14

over the page shows the Renault F1 autoclave. After the inner skin is finished, the layer of honeycomb is

added, and then the outer skin is layed up in the samefashion. The surface finish froma carbon fibre

chassis is amazingly smooth, as required to reduce aerodynamic drag. Figure 15 shows the surface finish

of a base Renault R24 chassis.


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Figure 14Renault Autoclave [11]

Figure 15The Renault R24 Underside [10]

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