Classic Dirt Bike — Autumn 2017

Inthe 1972 GPseries,thisisthe

viewmostotherriderswould

haveofdeCosterandhisfactory

Suzuki,thoughIdoubtthey’dbe

admiringtheslimlinesofthebike.

Withmagnesiumhubsandnon-valanced

rimsoneitherend,notjusttherear,

unsprungweightisaslowaspossible.

H

ow lightistoo lightand what is

neededto createsuch alight bike?

Thefirstquestionisadifficult thing

to quantify,asgeneralthinking in the

classicperiodhad it that bikeswerebecoming

toolight,yet weighinginatnearly300lb –

136kggiveortakeagram. Once two-strokes

hitthe scene, weightsdropped dramatically

though.Mostriderswill realisethat thekey

to successinoff-road sportisagood rideron

alight bike andusclubmenalways knew the

worksbikes weregoingtobeway lighterthan

ones wecouldbuy.

Take Jeff Smith’schampionship BSAs for

instance, allmadewithvarying degrees of

titanium, magnesiumand othersuchexotica

unobtainable to mere mortals. Then,when

theJapanesecameonthe sceneand money

wasstartingtobepouredintooff-roadsport,

or at least theMXsideofit, goodnessknows

what sortofmetalswould be used when the

factorymentalitywas ‘a bike had to last two

40-minute plus twolap motos’,sothe factory

couldbeseentobewinning.

Foryearsitwas assumed theonlyway

to buildsuchlight machines wastoemploy

exoticabut,asinsomanyfields,the truth

is alittle differentfrom thefolklore.Yes it is

possible to usespace-ageand aeronautical

metals to make lightmachines,but it is also

possible to useordinarymaterials andgood

practice to do thejob.Thiswas theway Suzuki

chosetogo.

Yes, youcould arguetitaniumfasteners

andmagnesiumcastingsare exotic but

they wereavailable even in theThirties and

accepted by all. Seamless mild steeltubing

certainly wasn’t exoticand this isthe material

Suzuki chosefor theframe of their 1972 RN.

Usingthin gaugesteel,the framewas lighter

andlower than previous incarnations andit

housedanenginebothnarrower andshorter

withcasingscastfrom magnesiumhelping to

achieve an allupweightof180lb.

This featherweightprojectilecould be the

It isthetaskoffeaturewriterstoattemptanexplanationofcomplexphysicsand

automotivedynamicsinasfewwordsaspossible.Whileweightreductionisadesirable

aimforacompetitionbikedesignertherearedifferenttypesofweight,thesebeing‘all-up’

‘sprung’and‘unsprung.’ Attentiontoeachareacanalterthewayabikefeelsandit’seasy

togetit wrongandalterthebalanceofabike.Briefly–andthissubjectisaninvolvedone

–aloweroverallweightisgenerallygoodbutcarvinglbsorkgsoffoneendofthebikeand

nottheotherwilllowertheweightbutalterthebalancepoint...thenthere’ssprungweight

whichisthebulkofthebikeabovethesuspension.Unsprungweightisthatmasswhich

moveswhenthebikeisbeingridden,effectivelythewheelsandhalfofthesuspensionunits.

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