THINGS YOU NEED TO KNOW…

Motorcycle News (UK) - - Garage -

Steel

Orig­i­nally evolv­ing from bi­cy­cles, mo­tor­cy­cle frames sim­ply used big­ger and stronger tubes as en­gine weight and power in­creased. For ease of man­u­fac­tur­ing, com­plex el­e­ments such as head­stocks, swingarm piv­ots and other frame el­e­ments were made from weighty steel cast­ings, braised onto the frame tubes. As weld­ing and pro­duc­tion tech­nol­ogy evolved, man­u­fac­tur­ers were able to re­place the heavy cast sec­tions with lighter com­po­nents, typ­i­cally fab­ri­cated from steel press­ings and more so­phis­ti­cated cast and ma­chined pieces. Steel qual­ity also im­proved over time, pro­duc­ing stronger ma­te­ri­als en­abling the use of thin­ner, lighter tub­ing with­out com­pro­mis­ing stiff­ness.

Alu­minium

Be­cause of the rel­a­tively high cost of alu­minium, it is of­ten re­garded as be­ing rather ex­otic, but it ac­tu­ally makes up around 8% of the earth’s crust. Un­for­tu­nately, it doesn’t turn up in handy lumps but in var­i­ous min­eral com­pounds and it is the re­fin­ing process, re­quir­ing huge quan­ti­ties of elec­tri­cal en­ergy, which ramps up the cost.

By the 80s, the need for stiffer frames, the higher value of pre­mium mo­tor­cy­cles and im­proved ma­te­rial tech­nol­ogy paved the way for alu­minium frame con­struc­tion.

In gen­eral terms, alu­minium is ap­prox­i­mately one third of the weight of steel by vol­ume and about one third as strong. As a tube’s di­am­e­ter in­creases it gains stiff­ness, so alu­minium can be used in big­ger, stiffer sec­tions than steel with­out any weight penalty - hence the pro­lif­er­a­tion of beam frames. The rel­a­tive ease with which alu­minium can be formed, or ma­chined, into com­plex shapes also trumps steel.

Car­bon fi­bre

Car­bon fi­bre com­pos­ites get their strength from long, pre­cisely aligned fi­bres glued to­gether by a poly­mer that is cured at high tem­per­a­tures and pres­sures.

Its fan­tas­tic strength to weight ra­tio does give it po­ten­tial to be the per­fect frame ma­te­rial how­ever high costs mean that car­bon fi­bre is un­likely to be­come main­stream any time soon.

Pick’n’mix

Given the par­tic­u­lar ben­e­fits of each ma­te­rial, it makes sense to op­ti­mise their use. Bi­mota were early adopters, us­ing alu­minium for the swingarm pivot and bolt­ing it to a tubu­lar steel frame. Bi­mota de­signer Mas­simo Tam­burini car­ried this idea for­ward to the en­tire gen­er­a­tion of mod­ern MV Agus­tas.

Steel framed bikes reg­u­larly fea­ture alu­minium sub-frames and car­bon fi­bre body­work is a sure fire way to re­duce weight on race bikes.

SFA

Ul­ti­mately, no frame at all would be the ideal. Mount­ing of the swingarm pivot through the gear­box cast­ing is not un­com­mon and with the Pani­gale’s mono­coque frame in­cor­po­rat­ing the head­stock into what is ef­fec­tively the air­box, you could ar­gue that Du­cati achieved the ideal. Not many en­gine con­fig­u­ra­tions lend them­selves to these min­i­mal­ist de­signs and there will al­ways need to be some­where to sit!

Du­cati have used steel trel­lis frames for years

Alu­minium is ideal for beam frame con­struc­tion

Du­cati’s near-frame­less de­sign is very light­weight

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