A motorcycle's suspension is a thankless job as it attempts to isolate riders from the irregularity of the road. With every fork compression and expansion your motorcycle's fork oil gets hotter and ever loses viscosity. As the oil loses viscosity your forks compress easier and easier, and since there is no more added rebound your suspension wallows and the bumps in the road become bumps in your handlebars and seat. Keep your forks still, strong and cool by spending a half hour and $ 15 installing some fresh fork oil. You will need a liter of fork oil ($ 10), a Ratio Rite ($ 4) to measure the proper amount of oil, a car jack, a funnel ($ 1) and some basic hand tools.
Look in your motorcycle's workshop manual to figure out the amount and viscosity of the oil currently in your forks. If you would like a stiffer suspension purchase thicker weight fork oil and purchase thinner oil if you prefer a softer suspension. Raise the front of your motorcycle off the ground using a car jack under the engine block. Remove the front wheel and front fender to allow you access to the triple trees. Measure the amount your fork stanchions stick out above your triple trees and loosen the caps on the left and right fork stanchions. Loosening the upper and lower triple trees and remove both fork stanchions. Unscrew the fork caps but be careful as the caps have a tendency to fly off as a spring is pushing against them. With the caps off, turn the stanchions upside down, remove the fork spring and let the oil drain out. Pay attention to see if the oil is dark, thick or filled with bits of fiber or metal. Pump the suspension up and down to fully evict all of the used oil.
Once the old oil has been removed we can start filling the fresh oil. Since the amount of oil affects how the forks will compress, think about if you liked the handling of your suspension or would prefer a stiffer or softer set up. If you liked the old set up, fill the Ratio Rite with the amount of oil specified in your workshop manual and fill a fork leg. Pump the fork stanchion up and down to eliminate all of the air in the system. This typically involves pumping the fork for two minutes. Once you are done move on to the other leg and repeat the process. When you are finished compress both forks and measure how far the oil is from the top of the forks. The number should be the same for both forks. If not keep pumping or add some oil to the fork with the lower level of oil. Insert the fork springs, compress the fork, install and tighten the fork cap. Insert the fork stanchions through the triple trees up to their previous height above the top triple tree. Reassemble all of the parts, making sure to torque the axle, axle pinch bolts, fork caps, upper and lower triple trees to factory recommended settings. Say thanks to your suspension and your suspension will thank you by running cooler and better absorbing every bump you hit.