One of the most neglected items today on most bikes is the forks. Most people never have done any maintenance on them. They have never changed the fork oil unless one of the fork seals has leaked fluid on the tube. Then after getting the seals changed out most ll it with the wrong weight of oil or don’t put in the proper amount of fork oil. I remember bikes back from the late 60’s and early 70’s that when changing the fork fluid on the forks made a huge difference in performance. Why? Most forks fluids on Asian forks then came with sh oil in them. Wow, sometimes when changing this oil it smelled up the garage like rotten sh. Back then on all of my race bikes I immediately change the fork oil as soon as I got them home. By changing the fork oil and trying different fork weights it always made a huge difference in how the forks performed. The choices for different weight fork oils in those days were somewhat limited.
Nowadays it is easy to come up with different fork weight oils. Fork oils ranging from 2.5wt up to 30wt is available from your local shop. You can even mix different weight oils to come up with oil between the company weights.
First, it is extremely important to flush out the old fluid that came in the forks before adding any new fluids. This is easily done when the forks are taken apart when changing out the seals. Otherwise, if you are just replacing the fluid you must drain the forks and then they can be flush out with ATF fluid. Drain your forks per your bike manual. What, no bike manual for your bike? Go out and buy one for your make, model and year. The procedure for flushing. Drain out old fork fluid, put ATF in, pump the forks up and down and the drain out the ATF fluid. Repeat this procedure until you see the ATF come out without any crud in it. Some people use kerosene but I always have had good luck with the ATF fluid.
If you were happy with the dampening that the original fork fluids gave use the same weight fluid that is recommended. Bikes that are ridden by people that weigh on the light side may want to try a fluid a little thinner. For instance, 5 weight oil is thinner than a 10 weight, 10 weight is heavier than 5 weight. Let’s say you are a big guy, you may want to try heavier weight fork oil. Maybe you ride the majority of time with a passenger, you may want to try heavier fork oil. Now, let’s say you are a BIG guy with a big passenger or have a lot of cargo weight you will need to try a heavier weight. Sometimes it takes a few changes to nd out what weight fork oil works best for your bike. Fork oils are antifoaming, inhibit corrosion and usually have additives for the care of the fork seals.
Some forks will have an ounce or cc fluid quantity for replacement. Even a better method is to measure the fluid height. With either method it is important to pump the forks up and down when putting the fluid in to purge out the air. The measure the fluid height method is when the forks are fully depressed and the springs are out. Measure the distance from the top of the fork tube to the top of the fluid level. Why did I say this is a better method? As the fork is depressed the fluid is forced up the fork tube. The air in this chamber becomes compressed and acts like a stiffer spring. This keeps the fork from diving. If the fluid levels are not exactly the same from one fork leg to the other leg, one fork will be stiffer than the other which can make the fork work harder and not work as smoothly. Also by raising or lowering the fluid height will change how progressive this air spring action will be due to the air being compressed in this chamber. Want the forks to be a little stiffer? Raise the fluid level slightly. A good start is 1⁄4”-1/2”. Want it a little softer? Lower it 1⁄4”-1/2”.
As you can see by doing the maintenance on your fork not only helps the fork seals but will fight any the wear of the internal parts of the forks. By changing the fluid you can easily change the dampening and the stiffness of the fork. You will be pleased on how much better your bike will now ride and perform. Also causing less wear and tear on the fork will make the bike more trouble free for years to come.