Taylormade Racing Inc.

JOHN KEOGH                 www.racetaylormade.com


Black Stuff: a History of Carbon-Fibre use in Motorcycles

Side –Bar: Carbon Fibre Manufacturer:

"Composite Material" to many engineers will mean glass- or carbon-fibre reinforced epoxy resin, but a true definition of 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) is becoming the more widely known composite thanks to its use in aircraft, racing cars and as cosmetic lightweight pieces on numerous products. It remains relatively expensive because of its complex chemical production and labour intensive manufacture. Carbon Fibre was invented by English chemists 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 that is the basis for making components when the mat is combined with epoxy resin and cured.

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 mat is laid in a mould and an excess of resin mixed with liquid hardener or catalyst painted over.

Pre-Preg: in which the mat comes pre-impregnated with the correct ratio of resin: this can then be cured either in a vacuum bag i.e. with pressure (or lack of!) or to obtain even better results with both pressure and heat in an Autoclave.

Different types of mat are available ranging from stiff but more brittle high modulus fibres to lower modulus, higher strength weaves (e.g. used in Japanese fishing rods!) The mat can be laid directionally to give specific strength characteristics in certain areas and can be combined

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, the latter coated with heat-resistant phenolic resin, but can also be fillers as simple as plywood. These 'cores' add both thickness and a magnitude of rigidity and again can be used in a variety of fully weight-bearing areas such as aircraft tail panels or Formula One monocoques. There are at least thirty different epoxy 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 dՐtre 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. 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.

History

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 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.

Whilst CF had been around in motor-racing for nearly ten years very few motorcycles had featured it. However a few years before GP bikes were cottoning on to the material a number of small specialist fabricators had realised the major benefits the material 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 a fully structural CF monocoque from 1985 onward and culminated in the final 1991 (although continuously up-dated) 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, 1987) Britten made use of a fully structural CF monocoque to both cradle the engine and join the front and rear suspension systems. The bike was refined to a half-faired minimum featuring a self-designed and built engine, CF swingarm and CF front fork in place of conventional telescopics.  

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 CF in it's enveloping bodywork which ducted air to the rear-mounted radiator. This was designed by John McQuilliam who has progressed his knowledge of CF to become chief designer at Jordan Grand Prix (now Spyker F1 Team). John was also involved at the time (1993) with another CF prototype by British frame specialists Hejira that was a copy of their steel alloy beam frame. The Saxon also ran modular CR-rim/Aluminium centre wheels. The Britten ran on fully composite hoops not dissimilar to those now offered by UK company Dymag and South Africa's Blackstone Tek. These offering the considerable handling advantages of less inertia and much lighter un-spring mass.

The use of structural CF was limited to these small independent specialists. Very few production bikes have featured CF because of the sheer expense of the material and it's 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 a Bimota's YB8 Furano - cosmetic panels - and Mantra - dashboard surround (actual dash being of wood!). Bimota have also produced mudguards in carbon-fibre as have Ducati. No production road bikes have used CF 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 which has an aluminium frame bolted to CF 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.

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 CF bodywork bearing CFRP decals. This weight saving was somewhat lost in the overall package since the bike still weighed more than 220kg.

A number of after-market suppliers offer non-structural pieces such as covers and mudguards as the material became more widely known and available and obviously inspired by racing use. In fact CF became so revered in the late 90s that many production bikes would feature mass-produced plastic trim pieces decorated with cheap looking fake CF 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 CF at the highest level of racing and non of the current 800cc motoGP bikes use carbon-fibre for either of their two main structures of chassis or swingarm. Why not? To answer this we need to go back to that first Cagiva GP bike – Suzuki also tried a cut-and shut CF 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 CF sheet but McLarenÕs 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 actual fact CF is used in small areas on gloves, boots and for body-armour. The primary qualities of high strength and light weight is attractive but while offering less weight CF can too easily be too strong for a given application. When CF first crossed over to motorcycles the chassis trend was to increasing stiffness in the aluminium frames. 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 didnÕt prevent Cagiva persevering with CF 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 90s have used a good percentage of CF in other areas. In addition to the bodywork fairings made as thin and light as possible yet still able to withstand a 200 mph gust a multitude of brackets, mudguards and engine covers feature the material. CF has replace the separate 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 CF 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 didnÕt 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 80s 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). GP bikes often run with carbon-fibre shrouds to retain heat and if a wet race is declared dispense with them altogether in favour of steel discs and appropriate pads. However US company Starfire Systems currently offer the STARblade carbon – ceramic discs manufactured by polymer infiltration and pyrolysis (PIP) of carbon fibre and silicon carbide to produce a braking surface usable in both wet and cooler conditions yet giving full braking performance and with a third of the weight of standard austenitic steel fitments. While not necessarily offering improved braking performance, which is already superb, they do offer, as with CF wheels, reductions in rotational inertia, gyroscopic forces and un-sprung weight benefits. 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 CF. 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 CF'sÕ unique characteristics. Not-with-standing the superfluous crash-protection requirement, the materials strength-to-weight ration can still offer advantages if used appropriately. The time is ripe for motorcycling to re-evaluate CF. The legacy of Cagiva and also Suzuki's early attempts at a chassis in CF has been for motorcycling to abandon CF. 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. The problems with these experiments have meant a reversion to the extremely well developed technology of telescopic front forks and controlled-flex aluminium beam chassis. Better understanding of CF 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.

This resonance of structures, separate from the bump-absorbtion springs has proved a critical subject in recent years. World Champion Valentino Rossi's 2006 Yamaha took a step backward in the early part of the year beset with chatter. This is an un-damped low-frequency vibration caused by incompatibility between tyre, suspension and frame and it was no coincidence that along with a revised frame the Yamaha was using new grippier Michelin tyres – the combination of the two throwing up an unexpected problem 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.

CF is ideally suited to addressing such issues and symbiotic constructions. And it still looks really cool!

John  Keogh,  August  2007