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
dipped at each end to decrease diameter and allow variable compression rates from soft to firm. Other than bikes using an .air shock. Yamaha has the only true variably progressive rear suspension . Adjustments are available for ride height with a threaded spring seat on the shock body, and shaft travel may be adjusted as well to offer different amounts of rear wheel travel. Up from are KYB built works air / spring forks . These have hefty 38mm tubes. The ·top triple clamp on the fork is magnesium. with the handlebar mounts located in rubber damping. The bottom clamp is aluminum and it has an aluminum steering stem through the frame head with Timken roller bearings. I had only two complaints with Hannah's 250cc bike when I rode it this past summer. One we'll get to in a moment; the other concerned the amount of suspension travel. He was running only 10.5 inches of travel on his OW at the time, and while the components themselves performed exceptionally, well, the travel seemed too short. and I mentioned it needed an additional inch to cope with really rough sections. With his Trans-AMA bike Bob has gone to 11.5 inches of travel at each end, similar to what Heikki Mikkola and the other Team Yamaha riders are using. With this one additional inch' of travel. the OW·39B seems to be at the head of its class in the suspension department. The front forks were equal to the KYB units of nearly similar design on the factory Kawasaki and Suzuki . but (at the rear of the bike ) the Yamaha monoshock made all the difference when compared to the other factory bikes __ The progressive rate suspension worked smoothly - you could feel it working following every contour on the smaller shudder bumps instead of skipping across . While on the bigger fifth-gear whoops, the bike could be slammed across them wide o~ without any fear of the rear end kicking back...just sucking itself into the Lake Whitney bomb craters like it was Tony D's oatmeal. Without a doubt, Bob's bike had the best suspension -of all the Trans-AMA bikes tested. The OW-39B also has a new engine design over the previous season. Capacity is increased from 400 to 425cc inside the lightweight chrome plated bore/aluminum cylinder. The magnesium sand cast engine cases are more compact and lighter, with the countershaft of. the transmission relocated closer to the rear of the cases, and hence, the swingarm pivot . Where the 250cc works bike needed a 6-speed close . ratio transmission , the 425cc bike gets - by quite nicely with what felt like a.more widely spaced out 5-speed . . . . Carburetion is a 38mm Mikuni ' pumping its way through Yamaha's normal torque induction. Though actual port timing specifications aren't the same as the production YZ-400, the general porting design and number of transfer ports is similar to the production bikes: ' One innovation which hasn't . ap' peared on the works bikes yet". but I've been hearing hints of, is some type of exhaust port valve. Such a device is reportedly already ' in use on the fac tory 750cc road rac ers and could find its way to the motoc ross bikes . perhaps begi nning on th e Open class bikes th is 21