Cycle News - Archive Issues - 1980's

Spencer's championship w inning NX250 V -twin sits in a twin -s par frame; Spencer tested the bike and asked for a 500 from the same mo ld. 14 Fr eddie Spencer a t Surfers Pa rad ise i n December 1984. H e tri ed a n o ldstyle fou r w ith wei ghts added so that it mi gh t ha nd le th e wa y he wa n ted it to , someth ing that it had not d one all yea r as it push ed th e front end into a d an g erou s sl id e at th e slight est provocation. Spen cer and his font -oftechnical -wisdom. Erv Kanernoto, had a very good id ea o f what need ed c ha ngi ng and th e tests proved them ri ght. . At the same test. Spen cer ro de th e new 250 V-twin for o n ly th e seco nd time after seeing it in J apan , H e was so impressed with it th at he asked H onda to build him a 500cc from th e sa me m old. It had a twin -spar frame a nd was j ust as neat a nd co m pact as a ny 250cc imag in able. T he bik e needed littl e a lte ration befo re it was raced a t Davt ona a nd th en th e " 'orld Champ io ns hi p . Th e engine sh ows it s parentag e cl early a nd is half a V-Iour sp lit ver rica ll y down th e middl e o f it s si ng le cra n k. Like th e four. th e 250cc ha s varia ble volum e exhaust th anks to th e ATAC system. me ch anicall y-contr oll ed off th e rig ht -ha nd end of t he cr ank . Als o on th e r ig h t is the prim a ry drive a n d the clutch , The ge arbo x i nterna ls can be cha nged by rem o ving th e cl u tch a nd th e co ver behin d it. T his co ver is deeply recessed j ust i n from its edge. a nd th e cl utch sits in as tight as poss ib le. keep in g en g ine width to an a bso lu te mi nimum . The NS250 ridden by Spencer shares no co m mon p ans wi th th e o ther Iact orv-issued 250s raced bv T on i Mangand Faus to Ricci - they ha ve cra n kcases fro m th e 250cc twin -cylinder ro adster a nd a re much m ore bu Ik v. It is a g rea t tribute to Se p p Sc h log l that T oni Mang' s tuner m anag ed to m ake th e RS250 as fast as S pe ncer's a t least o n top en d by th e last q uarter of th e season. Th e RS is a p iston -port e ngine with a ca rb a ttached to each cy l inder a nd thi s m ak es th e co n trol o f induct ion ai r much m ore diffic ult . Spen cer's 250cc has th e sa me system o f segregated inlet a ir as hi s 500cc a n d th e seat base form s th e top o f a n a irbox o u t of which the sing le-cas tin g , twin -ch ok e ca rb br eathes. The hol es seen in the sea t side panel s a llow access to the fuel taps in th e bas e of th e la nk and a re not air intak es, T hev are more likelv to let a ir o ut a nd preve nt p ressure b ui ldi ng u p in th e box . In every respect, the 250cc a n d 500cc are close ly rel at ed a nd the m ost o bvi ous sim ila rity -is th e twi n- spar frame m ade from leng th s of extruded a lu m in u m tu be. T h e rect angu lar secti on is no t enrirelv h ollow a nd th e two ribs ru nning 'the lengt h o f th e tube point to th e p o sition s o f cross ribs th a t split th e rec ta ngu lar section into three squares, thu s hel pi n g its to rs io na l sti ffness . T he 500cc h as its rea r sw ingarm m ad e fro m th e sa me m a teri al. Bo th th e 500cc and th e 250cc hav e a l u m in u m pi eces (mach ine d from so lid) welded to th e rear o f th e twin spa rs to ta ke th e rear engine m ounts a nd sw ingarm pivot s, T hey are works o f an. a s are the rear sus pe ns io n link ages. T h e rocker a rm p ivots in th e sw inga rm with o ne en d co n nected to th e bo tto m of th e unit. T he o ther en d o f the ro cker is linked to th e fram e by a n a lu m in u m wi sh bone. Th e Yamaha 500cc is q ui te d ifferem to the H onda in its fra m e cons truction as its twin spars a re fabri ca ted fro m a lu mi n u m sheet fo rm ed a nd sea m- welded. The swi ngarm p ivot a nd sus p ens io n p ivo ts are machi ned o u t o f so li d a nd welded to the sheet a l u mi n u m sec t io ns. Th e e ng i ne m ounts o n th e 500cc Yamaha did ca use a number o f problems thro ugh th e year as th ey cracked, bu t fo n una tely it o n ly affected th e m ount s them selves. not th e m a in frame m em ber s. which is what happen ed to H onda 's tw in -sp a re end ura nce racin g frames. T he si ngle Oh lins shoc k is wo rk ed fro m und er the sw ingarm rath er th an a bove as o n th e H onda . O n the Yama ha . the roc ker pi vots on th e bac k o f the frame a nd th e li nkag e run s to th e sw in ga rm . T he spring prel o ad o n th e un it ca n be adj us ted by a specia l a rra ngem ent d evelo ped by Oh lin s th at gi ves th e rid er a knurled kn ob bo lt ed to th e to p o f th e left fork leg . Lawson not o n ly u ses th is to set th e bike up du rin g practice hut to a lte r th e sus pe ns io n d urin g the race to co m pensate for cha nges in machi ne wei ght (a s gas is used up) a nd in varyin g tir e co ndi tions, U n like the H onda wi th its T R AC, th e Yamaha u ses no a n ti -dive svs tem, bu t ha s adj usta ble dampi ng in th e fro nt forks. The brakin g systems are si m ila r. th ough ; th e H onda ha s tw in d iscs a nd four-pist on N issin ca l ipers wh ereas th e Yamaha emp loys si m i la r Brem bo uni ts. Th e most o bvious a lte ra tio n made to th e 1984 Champi onsh ip-w inning Yam aha eng ine was to th e prim ary drive o n th e right-h and side o f the powerp lan t. The 1984 engi ne had th e cra n ks geared direct ly to th e la rg e p rimary gea r o n th e back of th e cl u tch. with the res ult th a t bot h cra n ks tu rned in th e sa m e d ir ect ion . Ya m a ha de cided to a lt er th e balan ce o f th e engi ne for the 1985 season by runni n g the cra n ks in o p posite directi ons. A cl ose exa m ina tio n of th e ri ghthand cra n kca se co ver indica tes tha t the tw o la rg e p rimary gea rs o n th e e nd o f th e cra n k run tog eth er. Th e ex tra bu lge covering th e end of th e (A bove) The NSR500 V-four Honda. A twin-spar frame like the NS250 and just as successful; both are works of art. The Yamaha OW81 uses flat-slide Mikuni carburetors for a shorter inlet tr act. The carbs are made in two twin-choke casting s. The Honda NSR500 uses cylindrical-slide Keihins that breathe from an airbox suppl ied through a channel leading from a plast ic scoop . lo wer c ra n k indicates the positi on of th e clutch-driving primary gea r. The reas on s for running th e two cr a n ks in opposed d irections a re not o b vio us b ut a re probabl y co n nected to th e a n g le of th e Vee (or W. if you want to differen tia te it fro m the true, single -cra n k- V H onda ). Th e facto ri es like to keep precise cyli nder a ng les a sec re t, but yo u wo uld not be far from th e truth if vo u sa id th at the orig ina l rot a ry-valv e a nd reed- va lve Yamah as had th eir cyl inde rs sp rea d a t 45 °. Th e low er cv Ii rid er s wer e m ount ed hori zom all v' on th e frame, but th is year's engine has it> lo wer cylinde rs tipped down towa rds th e g round and, th ough th e upper cy lin der is obscured in side shots by th e frame, it seems th at th e cy lind er a n g le ha s been opened . T h e a ngle be tw een clu tch a nd cra n k ce n ters loo ks like 60° a nd th at m igh t well be th e cy linder a n g le a s we ll. Yamah a made ot her cha nges, incl uding swa p p ing th e elec trica l ge nera to r a n d ig n itio n tri gg ers 'ro u n d so tha t the ge nera to r is now o n th e right-hand end o f th e lower cra n k a n d th e tr igger s o n th e upper. More (Con tin ued 10 page 30)

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