The Black Stuff: a History of Carbon-Fibre use in Motorcycles
Part 1: Carbon Fibre – the Science
‘Composite Material’ to many people will mean glass- or carbon-fibre reinforced epoxy resin, but a true definition of a composite is any two materials combined together to make a new material with unique properties. These materials can be anything from grass to various plastics and metals in metal matrices – in fact anything where two substances are joined to provide new structural properties. It is a mechanical definition rather than chemical.
Carbon-Fibre Reinforced Plastic (CFRP) has become the more widely known composite thanks to its use in aircraft, racing cars and as cosmetic lightweight pieces on numerous products. Its key qualities over and above all metals (even titanium) are ultra strong and ultra lightweight. (Actually, as with Stell Alloys it can have variable strength and weight properties depending on the specific application requirements). It remains relatively expensive because of its complex chemical production and labour intensive manufacture. Carbon Fibre was invented by English chemists - notably Lesley Phillips at what was RAE Farnborough - shortly after WWII, but was not widely considered until the Standard Oil company perfected the Sohio process of amonoxidation of propene (or propylene) and ammonia. Previous production had involved hydrogen cyanide with obvious problems and not particularly good yields. This acrylonitrile production is then polymerised with the strands stretched and further oxidised before baking at 2500 degrees C. in a nitrogen rich atmosphere to convert the plastic to carbon or graphite. These strands of material are then weaved together to form the carbon-fibre mat or spread out to form strips of parallel fibres known as UD (Uni-Directional). These are the basis for making components when the fibres are combined with thermoset resin and cured. Resins are primarily epoxy since the properties better match the performance of the strands although polyester and vinylester are sometimes used.
There are a number of variables that come into play when considering carbon-fibre based structures. Firstly there are three main types of curing the two components:
Wet-lay-up: which is the same as basic fibre-glass production where dry cloth or fibres are laid in a mould and resin mixed with liquid hardener or catalyst is added manually by brush , roller or spray, or in better processes, mechanically injected or infused. Vacuum bags or closed moulds can be used to improve the consolidation and accuracy of this process.
Pre-Preg: in which the cloth or UD comes pre-impregnated with the optimum amount of resin: this must then be heat cured in a vacuum bag either in an oven i.e. with atmospheric pressure only, or, to obtain even better results, with extra pressure and heat in an Autoclave, which is how the majority of applications of c-f now use.
Resin-Transfer Moulding: this third version allows for optimum resin usage by injecting this into pre-laid-up (with matt) and vacuumed moulds. A further variant of this is ‘forged’ carbon where a c-f ‘paste’ or chopped strand are put in a compression mould which is then injected with resin.
The key for lightweight carbon-fibre (reinforced) components is to not have excessive resin and the third process of RTM optimises such.
Different types of fibres are available ranging from stiff but more brittle high-modulus strands through extremely strong, but lower stiffness, intermediate modulus and down to the more common and cheaper, relatively low strength and stiffness, low-modulus fibres e.g. used in mass-produced ‘carbon’ fishing rods, tennis rackets etc.. Different weaves of mat are available – e.g. plain, twill, satin, etc. - in addition to UD and sometimes including other materials such as glass- or aramid-fibres or cosmetic enhancements. The cloth and UD can be laid directionally to give specific strength characteristics anisotropically (depending on direction) for certain areas, and can be combined in different layers to give compound properties. In complex structures such as Formula 1 car tubs mixtures of all of these are used along with the core structural honeycomb sandwich elements designed using Finite Element Analysis to show where the directional properties are required.
For main structural applications, layers of specific moduli weaves are combined with ‘core’ material. This is usually honeycomb material such as aluminium or Nomex aramid formed into a mat of hexagonal tubes, the latter coated with heat-resistant phenolic resin, but the ‘core’ can also be fillers as simple as plywood or balsa. These sandwiched centres add thickness therefore increase the bending stiffness and buckling resistance of the thin carbon-fibre skins and allow the material to be used in a variety of fully weight-bearing areas such as aircraft tail panels, Formula One monocoques or motorcycle seats or headstocks. There are dozens of different resin systems that can also be tailored to individual applications so that an almost infinite variety of composites can be prescribed for specific structural and cosmetic applications. The whole raison d’être for all this variety of composite is of course the materials properties which offer considerable advantages in lightness and strength over the various steel alloys and aluminium. Typical values appear below.
|
|
TENSILE STRENGTH |
DENSITY |
SPECIFIC STRENGTH |
|
CARBON FIBRE |
3.50 |
1.75 |
2.00 |
|
STEEL |
1.30 |
7.90 |
0.17 |
As can be seen, carbon-fibre has a tensile strength almost 3 times greater than that of steel yet is 4.5 times less dense (lighter!). When combined with crushable cores and designed-load-deformation, carbon-fibre has proved invaluable as an impact absorbing structure even for small areas such as nose-cones. It’s value in lightness has seen it’s adoption in major aircraft structures such as Boeing’s new Dreamliner 787 and the Airbus 380 to the extent that causes shortages of the mat and despite the constraints, and cost, of production.
Part 2: Carbon-Fibre in Motorcycles
Many people at the 1990 Czechoslovakian Motorcycle Grand Prix were intrigued to see the debut of a unique new engineering concept sitting in the Italian Cagiva team’s garage.
The bike’s colourful yet experienced rider Randy Mamola would debut the C590 two-stroke 500 as the first GrandPrix bike featuring an all carbon-fibre chassis. Instead of the standard fabricated aluminium beams holding everything together the bike featured a glossy black hoop of the exotic material that had revolutionised the car-racing world in the 1980’s. While carbon-fibre had been readily adopted to replace fibre-glass as the material of choice for bodywork fairings it was only now being introduced as a fully structural element.
The bike would also race with the new chassis at the following Hungarian GP but problems adapting to the new material meant it was subsequently shelved. In the heat of Grand Prix racing with the bike giving unusual handling characteristics the small team did not have the time or budget to persevere with the new chassis and reverted back to the metal frame.
Whilst carbon-fibre had been around in Formula One motor-racing for nearly ten years very few motorcycles had featured it – there was therefore little practical data for Cagiva to draw upon. However a few years before GP bikes were cottoning-on to the material, a number of small specialist fabricators had realised the major benefits carbon-fibre might offer. Perhaps the most high profile of these was New Zealand architect John Britten’s home grown V-twin racer. Evolved over a number of different iterations the bike featured fully structural carbon-fibre front forks, swing-arm and seat from 1987 onward and culminated in the final 1991 version which won at Daytona eight times and the 1995 BEARS World Championship as well as being a star exhibit in the Guggenheims “ The Art of Motorcycling”. From his second prototype (“aero d-one”) Britten had made use of carbon-fibre to both cradle the engine and join the front and rear suspension systems. By 1992 the bike had been refined to a half-faired minimum featuring a self-designed and built engine, carbon-fibre swingarm and carbon-fibre front fork in place of conventional telescopics, all in the interest of maximising strength and minimising weight.
Racing against the Britten in the World BEARS - British, European and American Racing Series - was the Tayormade/Saxon Triumph which, although featuring a tubular aluminium frame made extensive use of carbon-fibre in it’s enveloping bodywork which ducted air to the rear-mounted radiator.
This was designed by John McQuilliam who progressed his knowledge of carbon-fibre to become chief designer at Jordan Grand Prix (which has now morphed into Aston Martin F1 via Force India). John was also involved at the time (1993) with another carbon-fibre prototype by British frame specialists Hejira that was a copy of their steel alloy beam frame. Like the Cagiva this was tried and shelved. The Triumph also ran modular carbon-fibre-rim/aluminium centre wheels. The Britten also ran on fully composite hoops not dissimilar to those now offered by UK company Dymag and South Africa’s Blackstone Tek. The benefit of such wheels are the considerable handling advantages of less inertia and much lighter un-spring mass with greater strength than equivalent cast or forged magnesium race wheels.
The use of structural carbon-fibre was limited to these small independent specialists. Very few production bikes have featured carbon-fibre at all let alone structurally because of the sheer expense of the material and its aversion to automated production manufacturing. The most consistent use of the material has been in race fairings, which have also featured on a few road bikes such as Bimotas as cosmetic panels (i.e. non structural). Bimota have also produced mudguards in carbon-fibre as have Ducati. No production road bikes have used carbon-fibre structurally except Ducati for the airboxes on their limited run 916 Superbikes to add rigidity to the frame and the limited-production Bimota SB8-R/K first introduced in 1997, which has an aluminium frame bolted to carbon-fibre swingarm plates. However the latter also falls into the trap of mimicking a different material in having facets and indents as if machined from billet which leads to suspect that the plates’ core was solid aluminium rather than honeycomb in which case this was a bit of a con.
Honda made a big splash in 1992 with its limited edition NR750 which, in addition to it’s unique oval-piston engine, featured all-enveloping carbon-fibre bodywork bearing prominent “CFRP” decals. This weight saving was somewhat lost in the overall package since the bike still weighed more than 220kg.
From the mid ‘90’s a number of after-market suppliers offered non-structural pieces such as covers and mudguards as the material became more widely known and available and obviously inspired by its racing use. In fact carbon-fibre became so revered in the late ‘90’s that many production bikes would feature mass-produced plastic trim pieces decorated with cheap looking fake carbon-fibre pattern decals. By then many race teams were also making use of the material for more than just the bodywork and brakes: significantly in structural applications for the tail units and for covers and camshaft boxes. Equally significant was the fact that most had shied away from the main components of chassis and swingarm despite the success of one-offs such as the Britten - although this had not raced at GP level. So 17 years after the original debut of carbon-fibre at the highest level of racing and only one Grand Prix bike – Casey Stoner’s 2009 Ducati - used carbon-fibre for both main structures of chassis and swingarm. This bike featured a carbon airbox combined chassis that supported the headstock directly from the V4 engine. Subsequently the Ducati GP bikes reverted back to metal structures but fast-forward to 2023 and we find Austrian manufacturer KTM sneaking in a new carbon-fibre frame in an effort to win the Championship.
But why have the other teams at the cutting edge of motorcycle design seen fit to investigate the material once again? To answer this we need to go back to that first Cagiva GP bike – Suzuki also tried a cut-and shut carbon-fibre chassis at the time, again abandoned – and look closer at the parallel histories of GP motorcycle design and racing car design.
Cagiva had close ties with Ferrari and this connection had encouraged the cross-over of engineering, the logic being: “Carbon-fibre is very expensive but works brilliantly as the main structure for Formula cars – surely it can do the same for GP motorcycles?” Formula One was first introduced to the material in 1981 with both Lotus and McLaren debuting carbon-fibre chassis cars. The Lotus was somewhat crude in replicating the previous aluminium sheet monocoque with cut and folded carbon-fibre sheet but McLaren’s MP4/1 showed the future in a more sophisticated moulded tub by US aerospace company Hercules. The advantages of a structure that was lighter, stronger and lent itself to smoother aerodynamic profiling were immediately apparent. Two crucial benefits in its universal adoption for top-level racing are its rigidity and potential for incorporating controlled-deformation crash structures. That this was in a sport where cost was not necessarily a problem was also significant.
However these two key structural factors are not required in the same way with motorcycles: Protective structures are not applicable to an open two-wheeled racer - the rider is not tethered to the vehicle and is left to his own devices in the event of a crash. In fact CF is used in small areas on gloves, boots and for body-armour for this reason – and again for its fashionability! Protective helmets have also come in for the carbon-fibre treatment with the best of the latest generation of head protection featuring carbon-fibre for the main shell.
The primary qualities of high strength and light weight are attractive but while offering less weight carbon-fibre can too easily be too strong for a given application if just mirroring metal structures. When CF first crossed over to Grand Prix motorcycles the trend was to increasing stiffness in the aluminium frames to cope with the contemporary rubber and suspension forces. The jump Cagiva made was a logical next step in construction especially in the light of Formula One’s use of the material. But hindsight shows that as bike racing progressed through the next few years with matching increases in tyre performance, very stiff chassis were actually counter-productive, particularly when banked over where suspension travel is severely compromised. Under extreme lean angles the front and rear springs and dampers are almost completely ineffective in controlling bump absorption although can still react to fore and aft weight transfer. Under such conditions what is needed is a measure of flex both in the suspension and the chassis itself.
Cagiva found this out in the space of those two races back in 1990 with Mamola and second rider Ron Haslam finding the suspension set-up and feedback from the carbon frame so fundamentally different that their normal bike adjustments and suspension settings no longer worked and that the bike raised more questions than it answered. Although they swiftly abandoned the chassis this didn’t prevent Cagiva persevering with carbon-fibre swingarms - allowing un-sprung weight reduction - until the withdrawal of the team from GP’s in 1995.
Despite this hiccup most race-bikes since the mid ‘90’s have used a good percentage of carbon-fibre in other areas. In addition to the bodywork fairings made as thin and light as possible yet still able to withstand a 220 mph gust a multitude of brackets, mudguards and engine covers feature the material. Carbon-fibre has replace the separate aluminium rear subframe supporting rider and tail bodywork by combining them into one self-supporting whole as well as being used to strengthen airboxes, fuel tanks and exhausts. Both Aprilia and Ducati motoGP teams have tried carbon-fibre 42mm diameter front-fork outers although reasons for not consistently using these are probably down to problems caused by different stiffness ratios or simply that they didn’t see an overall improvement over the latest spec. and larger diameter (up to 50mm) aluminium sleeves. The most mature use of carbon technology has been in race braking systems derived in the mid ‘80’s from commercial aircraft systems and the obligatory F1 usage. However these carbon-carbon discs are completely unsuitable for road-use because of the operating temperature requirements (300 – 600 degrees C). They are also partciularly expensive because of the lengthy production process – 3-6 months of Carbon Vapour Infiltration baking – with the cost per disc running at nearly $5000. GP bikes often run with carbon-fibre shrouds to retain heat in the carbon disc surfaces and if a wet race is declared dispense with them altogether in favour of steel discs and appropriate pads. Carbon wheels are banned in GP racing but for road applications they offer reductions in rotational inertia, gyroscopic forces and un-sprung weight. What we are seeing in these composite uses is a more specific targeting of material properties for a given engineering requirement.
The mistake Cagiva and others would make when originally considering composite use was to replicate metal fabrications in carbon-fibre. The new material was entirely different in it’s physical properties to aluminium so why duplicate the metal parts physical layout? Admittedly this is what a number of the car teams initially did but they quickly matured the technological use to better exploit carbon-fibre's unique characteristics. Not-with-standing the superfluous crash-protection requirement, the materials strength-to-weight ratio can still offer advantages if used appropriately. As Ducati have realised with the help of ex-F1 designer Alan Jenkins (Arrows, Stewart) the time is ripe for motorcycling to re-evaluate carbon-fibre. The legacy of Cagiva (and also Suzuki)’s early attempts at a chassis in carbon-fibre up until now had been for motorcycling to abandon carbon-fibre. This has been somewhat mirrored in experiments with alternative front suspension systems with pioneering attempts such as the Elf series racers also being found wanting. Ironically that standout bike, the Britten, combined both newer technologies. The problems with these experiments have meant a reversion to the extremely well developed technologies of telescopic front forks and controlled-flex aluminium beam chassis. Better understanding of carbon-fibre material and more sophisticated engineering particularly in the application of directional weaves, varying moduli and core structures mean that the design should be reviewed. Its use should not be to simply replace metallic structures but to be applied appropriately. The key issue of controlled chassis and swingarm flex is now addressable with directional lay-up and the possibilities of self-damping within the structure. Some resin groups currently offer hysteresis qualities but developments could see further progress on ‘chemical damping’. It is these mixtures that KTM will be perfecting on their new bike.
This resonance of structures, as separate from the bump-absorption springs’ own frequencies and damping, has proved a critical subject in recent years, with GP riders complaining of ‘chatter’. This is an un-damped low-frequency vibration caused by incompatibility between tyre, suspension and frame and often rears itself when new tyre compounds are introduced giving a discrepancy between the vibrational modes of tyre, suspension and chassis causing instability much as the Cagiva’s suddenly much stiffer chassis had back in 1990. As recent developments have shown the bikes overall design should not be seen as separate engineering components but as an organic whole with varying degrees of flexibility, resonance and damping.
Carbon-fibre composite is ideally suited to addressing such issues and symbiotic constructions. And it still looks really cool!
Written by John Keogh Design.
