Suspension Service Frequency Guidelines
Just like the oil in your cars engine, the fluid in your suspension must also be changed regularly.
Burnt up and contaminated suspension fluid cause the following:
Oil change frequency in your car is determined by mileage; oil change frequency in your bike is determined by time.
Shock Oil Change Frequency Analysis:
To control your suspension movements, a piston plunges through oil creating resistance (damping) and generating heat. By the end of a moto your shock fluid can reach up to 300°F. This extreme temperature is achieved because the shock is relatively small and has very little surface area to dissipate the heat. Additionally the shock is shielded from cooling air by the air box.
A good rule of thumb is that for every 10° F rise in temperature, the life of suspension fluid is cut in half. Under 200° suspension fluid will last a long time. Once suspension fluid reaches about 200°, the polymer chains begin to burn up and the viscosity stabilizers break down. So to determine oil change intervals we must look at the time the oil spends above 200°. We will call this "Time At Elevated Temperature". High performance suspension fluids last about 20 hours at elevated temperature before suffering significant viscosity breakdown and loss of lubrication properties.
To determine oil change frequency, the rider must determine when his shock has experienced "20 Hours at Elevated Temperature". A good rule of thumb is to subtract 5 minutes from every ride since it takes about 5 minutes to get the shock over 200°.
If a MX bike races a 20 minute moto, it counts as 15 minutes at temperature. Thus if you ride 4, 20 minute sessions, you put 1 hour on your shock fluid. So 20 days of riding use up your 20 hours of oil life. If you ride once a week, your oil is cooked in about 5 months. Obviously if you ride twice a week, you should change your oil in half that time.
If a desert bike races a 2 hour and 35 minute race, it counts as "2 1/2 hours at temperature". Thus if you race or ride for 8 extended sessions you use up your 20 hours of oil life. At 2 races or extended riding sessions a month, your oil is cooked in 4 months. And remember a 1 hour spirited ride with your friends before the race adds another "55 minutes at temperature" and shortens your oil life further.
Fork Oil Change Frequency Analysis:
Fork oil does not get very hot because forks do less work than the shock, there are two forks full of almost a quart of oil, and there is allot of surface area to dissipate the heat. Thus from a temperature standpoint, fork oil should last a long time. However, fork oil suffers extreme contamination from the sliding action inside the forks and the spring twisting and creating chips. These small metal particles become suspended in the oil and act as sandpaper in the oil.
Because of all the contamination, the fork oil should be changed often. To make your life simple, I recommend changing the fork oil when ever the rear shock is serviced. This ensures that both front and back are maintained on a regular basis and will minimize wear to your valuable components.
Rick Johnson