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

BIKE (UK) - - THE JOY OF FRAME TECH - By Mike Ar­mitage

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’

This is a steel-framed Yamaha TRX850 from the mid ’90s. Look at all the gaps be­tween the tubes – short tri­an­gu­lated sec­tions give high rigid­ity, with the added bene t of lots of holes to route pipes and ca­bles and stu­. Steel doesn’t mean heavy ei­ther....

Du­cati go the whole hog with the Su­per­leg­gera with frame (what there is of it), swingarm, wheels and body­work all in car­bon. It’s the perfect demon­stra­tion of the ma­te­rial’s ex­cit­ing po­ten­tial – the 215bhp su­per­bike weighs less fully fuelled and ready...

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