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/126345
; • • Trans-AmA Bike Test ~ t 00 t" 0') ~ / • Q(') ~ l-< V ..c 8 v u v Cl , • 20 Bab Hannah-'s faltary OW-39B-BID Yamaha By Jim Gianatsis _ his was it. THE BIKE. The machine which dominated motocross during the 1978 ..racing season. In the hands of T Heikki Mikkola it took the Flying Finn to his second consecutive 500cc World Championship. In America, it gave Rick Burgett and Rex Staten the top two positions in the 500cc National Championship. And in the hands of Bob "Hurricane" Hannah it brought the Trans-AMA Series championship home to stay, with Rocket Rex once again playing backup. No machine has been more successful and so totally dominant in one season of racing than Yamaha's exotic 425cc works motocrosser, Bob Hannah's winning OW-39B-810 factory Yamaha. My thanks go to Bob, mechanic Keith, and Yamaha International competition director Dave Tucker for allowing me to put thfoi r multi-thousand dollar weapon to the ultimate test - fifteen minutes of thrashing by tBe Greek on ' the Lake Whitney Cycle Ranch Trans-AMA course, just minutes after Bob had used the bike to clinch his title. The OW-39B is quite similar to the OW-38B, Bob's 250cc National and Supercross bike I tested for Cycle News this past summer. The chassis and the suspension, .except for the amount of travel, are identical between the two. The OW-38/39 was a complete redesign to a more conventional all-tube unit, welded up of 4130 chrome-moly steel. The first OWs featured a nearly complete stamped-steel monocoque frame which housed eveything from the monoshock to the airfilter, Only the front frame down tubes which cradled the engine were of actual seamless tubing, Over a period of four years , though, sections' of the monocoque were gradually replaced by more tubing, until .1977, when only the backbone section which enclosed the monoshockwas left. Reasons for reverting back to a tube frame of the old YZ days were numerous. Though a monocoque chassis has an extremely high strength to weight ratio compared to a tube frame, the fact that aluminum can't be used 'a s the monocoque material as in race 'cars where the chassis loading is less and there- is more area to distribute forces, meant tlie steel monocoque frame of the Yamaha had to be somewhat heavier than a conventional tube type frame. Monocoque frames have too much torsional rigidity · as well, and in motocross design it has been proven that a frame which' flexes actually handles better and is less prone to breaking than a totally rigid design . What finally brought an' end to the monocoque frame was the design of a new, much shorter monoshock by Yamaha. 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 increase each year, With their new monoshock design, Yamaha was able to ' correct two problems at once, increasing .travel and keeping the shock cool. Handmachined out of aluminum, the monoshock is a conventional-deCarbon design utilizing a frame-mounted reservior where the floating piston is located. Past problems with cooling 'and reliability are now totally solved. 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 Bob selects the damping characteristics he likes for his bike at the beginning of a series like the Trans-AMA. Springing on the monoshock is' handled by a progressive coil-over sprin~. It is a true progressive rate design, acid