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 1 – Space Frame Chassis

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

however towards the end of the 1950’s 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 chassis’s 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 1950’s 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; it weighed less, was stiffer with a smaller frontal area.

Figure 2 – Lotus 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 1970’s 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 effect put a greater emphasis on a strong and safe chassis. In the

early 1980’s, the chassis designers were finding that the aerodynamic loads were flexing

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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). The performance 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 3 – The 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

As Newton’s 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 follows that 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 of the 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 from the suspension components (springs, dampers and

tyres). If the 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 is of paramount importance to ensure that

the car remains balanced under all conditions. 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 assume a 50:50 weight distribution front to rear and the same front

and rear centre of gravity heights.

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 10

Front roll stiffness as % of total roll stiffness

Fron

t loa

d tr

ansf

er a

s % o

f tot

al lo

ad tr

ansf

er

0

Chassis stiffness 100 Nm/degChassis stiffness 300 Nm/degChassis stiffness 600 Nm/degChassis stiffness 1000 Nm/degChassis stiffness 2000 Nm/degChassis stiffness 4000 Nm/degChassis stiffness 8000 Nm/degChassis stiffness 16000 Nm/deg

Figure 4 - Lateral Load Transfer from a 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 load transfer changes for different values of front:rear roll

stiffness. In a rigid chassis, a change in relative roll stiffness of 10% 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 of chassis 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 same rate, 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-speed impact, it is important that the chassis

absorb as much of the energy as possible and reduce the acceleration on the driver’s

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:

• 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 exceed

80kN 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 5 – Ferrari Carbon Fibre Chassis after the FIA Side Impact Test

3.0 Carbon Fibre vs. Other Materials

The main attraction of carbon fibre for Formula 1 chassis’s 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 of the material for every unit of it’s

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.

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Stiffness Per Unit Weight

0

20

40

60

80

100

120

Steel CarbonFibre (HighStrength)

Carbon(High

Stiffnes)

Kevlar Aluminum Glass

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

The same analysis with strength shows that carbon fibre has a specific strength over ten

times that of basic steel.

Tensile Strength Density Specific Strength Steel 1.30 7.90 0.17

Carbon Fibre 3.50 1.75 2.00 Table 1 – Specific Strength [9]

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 1950’s. (Please note, values are only approximate)

Chassis Type Stiffness (Nm/deg) Weight (kg) Space Frame 1100-1400

Lotus 25 ≈ 3250 Aluminium Honeycomb 6100-8100

MP4/1 ≈ 20000 Modern F1 Chassis 29000-35500 35-40

Table 2 – Stiffness and Mass for Different Chassis Types [7] & [8]

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While quantifying the ability of a material is hard 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/cm whereas some forms 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 7 – Different forms of Carbon Fibre and Carbon-Matrix Composites [1]

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 8 – Monomers Copolymerised with PAN Carbon Fibres [2]

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 form by 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 fibres to 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.

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Figure 9 – Melt-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 uniform cross

section.

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-

280°C) at tension from anywhere between 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 10 – The Heat Treatment Process 2]

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The principal reactions that occur during this step (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 10 – The 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 an interlinked turbostratic layers of carbon

with an interlayer spacing considerable larger than that of graphite. They show a low

degree of graphitisation, and the turbostratic layers are not highly orientated with the

fibre axis.” [3] Figure 11 below shows two models of PAN microstructure developed

from the studies into the effect of microstructure on mechanical properties.

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

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After the production process, the carbon fibres can be used to form different weaves. As

F1 chassis are predominantly laminated, only the will be discussed. However, it should

be noted that other forms of carbon 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 them ideal for areas of an F1 chassis that have complex, aerodynamically driven

shapes.

Figure 12 – Weaves 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 in that 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.

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5.0 Building the Chassis

A modem day 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.

Figure 13 – Honeycomb Cells

The first step in constructing the chassis involves an extremely accurate female mould

of the outer shape of chassis (accurate to 50 microns) being produced by a large multi-

axis CNC (Computer Numerical Control) machine.

From the 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 same fashion. The

surface finish from a 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 14 – Renault Autoclave [11]

Figure 15 – The Renault R24 Underside [10]

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6.0 References

[1] Buckley, J. D., “Carbon-Carbon Overview” in Carbon-carbon Materials and

Composites (ed. J Buckley, Noyes Publication, USA) pp. 1-15 (1993).

[2] Edie, D. D., and Diefendoft, R. J., “Carbon Fiber Manufacturing” in Carbon-carbon

Materials and Composites (ed. J Buckley, Noyes Publication, USA) pp. 19-39 (1993).

[3] Edie, D. D., and Stoner, E. G., “Effect of Microstructure and Shape on Carbon Fiber

Properties” in Carbon-carbon Materials and Composites (ed. J Buckley, Noyes

Publication, USA) pp. 41-69 (1993).

[4] Burns, R. L., “Manufacturing and Design of Carbon-Carbon Composites” in

Carbon-carbon Materials and Composites (ed. J Buckley, Noyes Publication, USA) pp.

197-222 (1993).

[5] Deakin, A., Crolla, D., Ramirez, J. P. and Hanley, R., “The Effect of Chassis

Stiffness on Race Car Handling and Balance”, SAE Technical Paper, 00MSV-5 (2000).

[6] “2004 Formula One Technical Regulations”, Federation Internationale de I

‘Automobile (2004).

[7] Wright, P., “Formula 1 Technology” (Society of Automobile Engineers), (2001).

[8] Matchett, S., “Chariot Makers: Assembling the Perfect Formula 1 Car” (Orion),

(2004)

[9] “Carbon Fibre”, http://www.f1technical.net/article3.html (accessed on November 1st

2004).

[10] Scarborough, C., http://scarbsf1.com/chassisconstruction.html (accessed on

November 1st 2004).

[11] “Producing the Composites – R24”, http://www.f1technical.net/feature682.html

(accessed on November 1st 2004).

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[12] “Grand Prix Cars – Lotus-Climax 25”,

http://www.ddavid.com/formula1/lotus25.htm (accessed on November 1st 2004).

[13] http://www.f1mech.co.uk/manufacture.htm (accessed 1st November 2004).

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