The Yamaha RD 350 is among Yamaha's legendary twin two-stroke street bikes. Manufactured from 1973 to 1975, the lightweight and nimble RD had a reputation as not only a "giant killer," out-accelerating much larger bikes, but also as a "wheelie machine" because it was prone to loft its front wheel. This was an attribute that made the front suspension of the RD that much more important.
How Forks Work
Like most motorcycles of the era, the RD 350 used telescopic forks for its front suspension. There is an alloy lower that houses the front axle. A chrome upper tube slides snugly into the lower, sealed with a gasket. Fork oil is contained in the lower component of the cartridge. Inside the cartridge, springs and oil are used for damping and rebound. The oil handles the damping (or absorption of bumps), and the springs handle rebound.
Forks have a plunger-like base with "valves," which are essentially holes. When forks are compressed, viscous oil is pressed through these holes. The viscosity of the fork oil has a major effect on the damping characteristics of the forks.
RD 350 Fork Oil
Fork cartridges use motor oil. They are not filled right to the cap. If they were, the fork would have no room to compress. So factory specifications indicate both the amount and the viscosity of fork oil used. Yamaha indicates 140ml of oil in each cartridge with a viscosity ranging from 10 to 30. Unlike motor oil, more experimenting can be done with the type of oil used in forks. But experimentation with viscosity should be done with caution, as extreme variances from factory specs may cause the front suspension to "bottom out" or cause front wheel to slide if the oil is too heavy.