Light fantastic
BMW and Ducati are making production bikes with carbon frames. Here’s why they look like they do, and why it’s a great thing for motorcycling
AMOTORCYCLE FRAME CONNECTS the wheels together, holds the engine and provides somewhere for your bum. Simple enough. There are rather large forces it contends with, however. Your potent 180bhp sportsbike can readily apply over 1000lb of force to try and compress the wheelbase if you’re greedy with the throttle. The front fork tries to snap off the steering head under heavy braking, and during cornering the gyroscopic forces attempt to prise the rear wheel out of the swingarm. Having a strong frame is therefore important, but isn’t the key factor. Strength is how much force a material can take before it breaks, and is essential in engine parts, for example. Stiffness, however, is how much (or preferably how little) a material deflects when loaded, and your bike frame needs to resist the forces thrown at it, not carry them. So stiff is good; it allows wheels to stay in line, steering to behave and other agreeable things. And getting the ideal stiffness is all about the material used, and the pretty shapes you make with it...
Keeping it rigid
Conveniently for engineers there are handy formulas when considering deflection. They’re very complicated. Thankfully the general rules are easy to grasp: if the length of a part increases so does the amount of deflection; if the material’s modulus of elasticity (its ability to return to form after being deformed) isn’t very high then deflection decreases; and if there is simply more material filling a space the deflection is smaller again. Short sections of material are therefore preferable to long wibbly ones. It’s why you can bend a twelve-inch steel rule but a six-inch version is more resistant to your grunting. Triangulation helps too. Dig your Meccano set out the shed, construct a square out of four strips and it’ll squash no matter how many other strips you attach inside it unless one creates a triangle. This is why Ducati still enjoy a trellis (the steel used has the added benefit of resisting fatigue, helpful with a big thumping V-twin). Improvements to elasticity and the section’s inertia (affected by the amount of material in the space) depend on the type of material and its shape. Take hold of your long bendy rule again, but turn it edge-on: it won’t give at all. Increasing the quantity of material reaps further benefits, which is why aluminium twinspar (or beam) frames are the go-to solution for stiffness, especially on sportsbikes. They look strong too, which always helps. Aluminium (or an aluminium alloy) is typically only a third of the weight of steel but only has around a third of the strength. However, for a given weight or strength there will be a lot more aluminium. Steel box-section, 20mm square with a wall thickness of 3mm, will be around the same weight as aluminium alloy measuring 32mm square and with 6mm walls. The difference is the alloy will be some two and a half times stiffer. It’s a huge stride. Now imagine replacing these metals with an engineered material that’s strong yet super-light, allowing less weight or even greater stiffness. That’s carbon fibre, that is.
Man-made magic
Carbon fibre reinforced polymer (CFRP) is just like glass fibre really, except strong and exotic. It takes carbon fibres, made from thousands of stuck-together carbon atoms in a long and very thin string, and holds them in the required form by setting them in a polymer – usually a thermoset resin like epoxy. Metals can have their grain structure and properties altered by processes such as heating or smacking with a massive hammer, but composites like carbon can be engineered exactly for specific applications and properties. The result is material that’s only fairly elastic, extremely stiff, has high tensile strength and weighs naff-all. That huge stride we made from steel to aluminium is made all over again. Making carbon parts is more involved than welding-up metal tubes. You need to decide on things like fibre direction for a start. The diagonal fibres commonly seen in accessory parts arise from a double strand weave. This is a common weave for curved shapes as it’s ‘loose’. But for, say, a fancy pushbike fibres are unidirectional, straight and not in a weave, but layered to run in different directions. You need vacuums to suck material into moulds, massive ovens and precise temperatures, finishing processes… and knowledgable, experienced people to do it all. So it’s the opposite of cheap. Carbon will still need bonded-in metal sections at load-bearing points – like the swingarm pivot. And it’s brittle. Whack a crossbar of an aluminium pushbike frame with a hammer and it’ll dent; do the same to one of the carbon versions filling roads every Sunday and it’ll crack, or even shatter. Carbon can exhibit curious fatigue properties, too. Fatigue is when a material gives up after fighting small, fluctuating, repeating loads – think bending a strip of metal many times to snap it, only we’re talking millions of small inputs. Carbon’s fatigue resistance is good, but it can then fail with minimal visible warning. In specific situations it can behave like timber: it’s either strong enough or it breaks, with little between.
‘A material that’s fairly elastic, extremely stiff and weighs naff all’
Carbon has become a proven material, however. Knowledge is increasing all the time, and things like helicopter rotors, tennis racquets, large parts of aeroplanes, fancy cars and those pushbikes demonstrate it’s safe, effective application. Better still, carbon isn’t as expensive as it was. We’re not talking cardboard-cheap, but a process called resin transfer moulding (RTM) allows quicker, cheaper manufacture. Its suitability for large components has allowed BMW to become a trendsetter in the automotive world – their quirky-looking electric car and luxury saloons feature significant carbon sections.
From race to road
The potential of this super-stiff, superlight material has long been attractive to bike builders, not just for accessory trinkets. No sooner did Mclaren make the first carbon racing car than Honda used it on versions of their ill-fated NR500. Niall Mackenzie won titles on carbon Armstrongs in the ’80s, and Skoal Bandit Suzuki RG500S featured carbon frames. Cagiva tried a carbon beam for their 500cc Grand Prix bike in 1990, made by Ferrari. Getting to the 130kg dry weight limit wasn’t too taxing with regular aluminium beams, so effectively replicating the metal with carbon gave something ridiculously rigid. Feel is everything in bike racing, however, and the Cagiva was declared ‘too stiff’ by rider Randy Mamola (when Eddie Lawson took over they wanted big changes so reverted to aluminium – it’s easier, quicker and cheaper to alter). While race teams have continued to dabble with composites, this year is the most exciting yet for carbon bikes. It’s the first that you or I can buy carbon-framed production bikes. BMW’S HP4 Race (p40) is effectively a carbon S1000RR. With some TT racers making similar noises to Mamola about the RR in Superbike guise (we’ve heard about drilling holes in swingarms and removing engine bolts, to increase flex – scary stuff, but it creates feel), the HP4’S frame replicates the stiffness of the RR’S alloy beams. But it’s much lighter. It’s made as a single part using knowledge from their cars, in an automated process – there’s no laying-up by hand any more, and BMW can make one in a couple of hours. And it saves a vast 4kg. With carbon’s advantages ‘fully exhausted’ on the wheels, plus self-supported bodywork, the HP4 Race is 146kg dry – the 999cc inline-four is only two-and-a-half stone heavier than Cagiva’s delicate 500 GP bike. Fuelled and ready it’s 171.4kg, only a tad over the World Superbike minimum and 36.6kg lighter than an S1000RR. With a tuned-up 215bhp, power-to-weight is increased by over 30%. No wonder it costs £68,000. BMW use aluminium for the HP’S swingarm (I assume for feel), but Ducati go all-out on their road-legal Superleggera. Airbox-cum-frame, wheels, swingarm and bodywork are all carbon, for huge weight loss: the monocoque is 38% less than on a 1299 Panigale, the swingarm loses 900g and wheels save 1.4kg (slashing rolling inertia by 26% at the front, 58% at the rear). The Superleggera’s wet weight is just 167kg and with 215bhp its power-to-weight is 1.287bhp/kg – that’s double that of, for example, a Triumph Speed Triple R. While both these bikes are hideously expensive, they’re paving the way. Carbon bicycles have gone from being posh to readily available – they start at £650, which is the pushbike equivalent of a Yamaha MT-09. The potential for affordable, super-light carbon motorbikes is mouth-watering, as less weight means lighter handling, more acceleration and greater economy. ‘Lightness defines the performance of a motorcycle,’ say Ducati. So true.
‘This year is the first year you can buy carbonframed production bikes’