Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/127251
inte nded for the Suzu ki RM series two stroke mo tocrossers, America n tuners have fallen in love with them for their performance (o nce modified) on big four -strokes. Lights, camera, action! Although the bike had proven itself in 1988, its true coming of age was Daytona '89 whe n Lewis swapped the lead repeatedly with Quarterley, and finished second by a hair. The bi ke proved that it could ru n wi th the best in the class, convinci ng Adams to run the machine in the remaining ro unds of the Pro Twins series . Adams en listed former AMA Formula One and T wo Ch ampion Randy Ren frow to handle the rid in g cho res, and Renfrow 's first race resu lted in a win at Road Atlanta. A mid-sea son DNF by th e FBF Ducat i (caused by rod failure) coupled with strong fini sh es for the Honda kept Renfrow's cha mp io nship hopes alive. At th e last race of the Season, Quarterl ey's Ducat i enjoyed o nly th e slimmest of points lead s and had to fini sh ah ead of the Honda to reta in the number o ne p late. Alas, Quarterley cras hed o n the last lap while tryi ng to avo id rear-ending Renfrow, leaving the door open for a Renf row /Hon da victory - and cha m pions hi p. The plot develops In its first inca rnation, the Co m- (Left) The Ho~da RS750 engine as developed for di n track racing utilized a four-speed gearbox; for Pro Twins road racing it required the installation of a five-speed. Note lengthy shift linkage. (Below) The dry-sump motor originally carried its oil supply in th e frame backbone, but when that led to an engine failure at Daytona '88 Ra y Plumb fabricated th is oil tank. monwealth Honda pro ved to be a bit rear-h eavy, a nd also lacked cornering clearance. Pl umb blamed part of ,th is o n th e swi ngarm ang le (or lack th ereo f). Ideall y, th ere sho uld be abou t a n in e to 10-degree droo p , a nd th e Co mmo nwealt h bike's swingann lay pa rallel to th e ground. In an effort to lighten th e rear, the swingarm was cha ng ed from th e origi na l '86 NS500 unit to a sho rte r '84-85 RS500 p iece, complete with RS risin g ra te lin kage and works Showa shock. Even with th e bracing Pl umb added to the swingarm (simi lar to what was do ne to Freddie Spe ncer's G P racer), th e new piece is lighter. The NS500 rear brake was swapped for a lighte r NS250 assembly, sheddi ng even more weigh t. Up front, the bike now wears Honda superbi ke bits: Magnesium legs slide up 43mm chro me moly tubes pinch ed by th e team 's choice o f three diff erently-offset tri p le cla mps. Bill et ca lipe rs put , th e sq ueeze o n 320mm meh onite ro tors. Wh eel sizes are a bit of 'a story : Alt hough the machine was bu ilt for an 18-in ch rear and 17-inch front, Lewis rod e it at Daytona '89 with 17inchers at both ends. It seems that Lewis is a Mich elin man , and the Fren ch tir e company didn 't mak e a prope r 18-in ch rea r. With Renfrow aboard, th e bik e switched back to an 18-in ch er in th e back , and stayed that way. Dunlop 's Jim Allen did suc h a brilliant jo b with the tir es that Ada ms n ever q ues tio ne d th e choices: h e simply took the tires he was give n a nd ins ta lled them o n the bike. With the 24.5-degree steer ing head angle and shorter swi nga r m, th e w hee l base ra nged from 56 to 56.5 inches, depending on which triple cla mps were used . This ma kes for a q u ick steer ing machine, so a works magnesiu m steeri ng da mper was added to slo w things down a bit. At Daytona '89, the bike tipped the scale s a t 342 po unds dry, with the weigh t biased toward the rear. After Daytona, Kerk er fabricated a set of th in wa ll, sta in less-stee l head er pipes and joined th em to ca rbo n -f ibe r mufflers to repl ace the o rigina l system. Becau se of th e engine's layo ut , only a tw o-in to-two ex ha ust sys te m is. practica l. Add it ionally, every nut and bolt that could be replaced wa s - with a titanium on e. In its curre nt incarnation, the bike rests 53% of its 326 pounds on the front wh eel , making it a bit eas ier to ride than wh en it was rear-heavy. The 80.5 x 75.5mm1768cc din tra ck moto rs used N ikasi l liners, but th is bik e's 12.6:I Venolia pistons run in cas t-iro n line rs, than ks to a bor e and sleeve job. At Dayton a , the 85mm bor e/ 857cc mi ll was shy of the winn in g Ducati's 888cc displacement , bu t by Lo udo n in J un e, the bor e had grown anot her Imrn, yieldi ng 877cc. Alo ng Con tinued on page 55.