FORGOTTEN MOTOCROSS TECH: WHAT WERE SPEEDO & TACH FORKS?
Motocross history is filled with examples of creative ideas that were heralded as groundbreaking, but, because of the rapid rate of change in development, many sank into the swamp of forgotten technology. Although some are best left abandoned, others were truly innovative (if not ultimately successful). MXA reveals motocross’ tech trivia. Do you remember this idea? Yamaha’s 1976 Speedo & Tach air forks.
Air forks are nothing new in motocross, as the 1976 Yamaha’s 35mm Kayaba air forks prove. The dual air canisters resembled the instrument cluster of a street bike, leading to the forks being called “Speedo & Tach forks.” The design was incredibly creative. There were two Schrader valves on each canister—one below a floating piston and one above it. The two separate air pressures served as a dual-rate fork spring.
The each canister had a floating piston inside that was supported by the low air pressure below it and made stiffer by the high air pressure above it. For 1976 the Kayaba design was very inventive. Jeff Blix demonstrates.
The trick was to put just enough air pressure in the lower air valve to hold the forks at the proper ride height (they would stiffen as the air was compressed by upward movement). The upper air valve fed the high-pressure side and, as the fork moved through its stroke, the air pressure in the lower chamber pushed the floating piston upward against the higher pressure inside the canister. The goal was to have plush forks for small to mid-sized bumps and additional bottoming resistance.
The 1976 YZ125 (above), YZ250 and YZ400 all came with Speedo & Tach forks.
MXA ran 27 psi in the low-pressure side and 50 psi in the high-pressure side. While the idea was innovative, the forks were doomed to failure because their valving was atrocious and they tended to top out on the rebound stroke. Kayaba’s air forks were dropped after one year, only to come back 40 years later.