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/126318
petltlve with high hors ep ower bikes like th e RC a nd CR Hondas on ce you ge t off th e lin e a nd thru the first tu m . In fac t. th e latest works RH Suzu ki bikes ha ve nearly simila r low RPM powerbands to the OW Yamaha. A powerband nea rly op posi te from th at on their production RM·250C which does n't pull until nearl y 5,000 rpm a nd then it com es on like a light swit· ch. T he reason for this is based on on e point.i .rhe public wants to buy a b ike that feels fast . One tha t snaps th eir a rms out of th eir sock ets no matt er how much a handful such a bike is to ride around a real motocross course . That's one reason Suzuki's RM·250C is so much more popular than Yamaha's YZ·250E and its less .explosive and powerful , but more easily to manage powerband. . Also receiving a major redesign is the OW-38 's frame which now is a more conventional a ll- tube unit welded up of 4130 chrome-moly steel. The first OW·25 featured a nearly complete stamped-steel monocoque frame (like the old Honda 50 step' thru) that housed everything from the monoshock to the air filter. Only the lower engine cr a d le consisted of tubing. Over a period of four yea rs , though, sections of th e monocoque were gradually replaced by tubing. until last year when only the backbone section was left which enclosed the monoshock. The reasons for reverting back to a tube frame are probably numerous. Though a monocoque chassis has an extremely high strength to weight ratio over a tube frame, the fact that aluminum can't ' be us ed as the monocoque material as in race cars where the chassis loading is less, meant the steel monocoque frame of the Yamaha would be somewhat heavier than a conventional tube type frame . Monocoque frames have too much torsional rigidity as well . and in motocross it has been proven that a frame which flexes actually handles better and is less prone to breaking than a totally rigi d design. The change which fmally did in the monocoque frame was the design of a new , much shorter monoshock unit. By making the monoshock shorter the frame backbone didn't have to enclose the shock and this meant the shock could run much cooler. Keeping the monoshock cool to prevent fade is one problem Yamaha has been fighting as rear wheel travel continues to get longer each yea r. With their new monoshock design Yamah seems to have taken care of two problems at once, increasing travel and keeping the shock cool. Hand-machined out of aluminum, the monoshock is a conventional deCarbon design using a frame mounted reservoir where the floating piston is located. Past problems with cooling and reliability seem to be totally elim inated. The Yamaha monoshock is not adjustable for damping as the entire shock has to be replaced to effect a damping change. But this usually isn' t necessary once the proper shock has been selected. Springing is handled b y a progressive coil -over spring. Adjustments are available in ride height with a threaded spring seat on the shock. and in travel by changing th e top-out bump stops. Actual rear wheel travel can presently be adjusted to between 10 and II inches. Presently it's set at 10.5 inches on Bob's bike. The sho ck spring is a true progressive rate design , acid dipped a t ea ch end to decrease wire diameter and allow variable com p ression ra te from soft to firm . Other than b ikes using an ai r shock, Yamaha has th e only true variably progressive rear suspension with their OW. • Up front a re ' KYB built works IAbove leftl KYB air forks. magnesium/aluminum triple clamps and other exotica.IAbove right) Weight saving items include a lightened countershaft sprocket. (Below) Monoshock cooling is via a frame-mounted reservoir. air/spring forks . These hav e he ft y 38mm upper tu bes. T ra vel matches the back at 10 .5 inc hes . Pierre Ka rsm akers has been riding with longer travel suspensio n . but H a n nah see ms sa tisfied ,w ith what he h as at p resent. T he top tri p le cla m p on the forks is magnesiu m with the han d lebars located in rubber mou nts. The bottom clamp is alum in u m and it has an aluminum steering ste m thru the frame head with T imk en r oll e r bearings. W heel assem b lies are made up of Sun n m s and use magnesium hubs which are smaller than a YZ-250E. T hey are sim ila r in size to th e hubs on a YZ-125. but of course lighter. -. For tires Hanna h likes a 3..00x2 I Metzeler MX End u ro up front . At the (Contin ued to page 47) 17