Front suspension performance relies greatly on fork oil quality, (is it old and dirty?), viscosity (is it too heavy or too light for outside ambient temperatures?) and amount of oil in each leg, (is your fork bottoming or feel too firm?). We'll focus on the effect the amount of oil in each fork leg has on performance and why controlling the air volume (known as "air chamber") in each fork leg, is very important for tuning consistency and performance.
You might ask "Why can't I just pour in a certain amount of fork oil?" You can, if you already know how much fork oil will give you an air chamber size that works for you. If a ballpark setting is good enough for you, drain the oil and refill the same amount. However, just like changing engine oil, you can pour in the recommended amount but you're never sure if it's too much or too little until you check the dipstick or remove the oil level screw.
The real reason for setting the fork's oil level is to achieve a certain air chamber size and to maintain tuning consistency, but most of all because of the way a sealed fork system works. If the oil level is set correctly, after about 1/3 of a fork's total suspension stroke (or travel) there should be more air pressure inside the fork than there is fork spring force (or spring tension). In other words, you could remove the springs after 1/3 of the fork's stroke and you wouldn't miss them. At this point, the fork relies on air pressure to control the action and to resist bottoming.
Exactly control of the air chamber volume in each fork leg is important. Equal air pressure in both legs achieves full benefit from the system. As little as 5mm (1/4 inch) change in fork oil level (or air chamber size) will affect the way your fork feels, controls action or resists bottoming. It can be that critical.
If after about 1/3 compression your fork feels soft, bottoms hard or too frequently (bottoming once or twice on most MX tracks is okay), goes very deep into the stroke easily, feels loose or rebounds too slowly, then raise the fork oil level. This decreases air chamber volume, increases air pressure and firms the action. Don't let anyone talk you into heavier fork springs until you've maximized this air chamber effect. In the past, we used air forks on KTM's (no fork springs!) and used very small air chambers, (around 40-45mm) with about 30 psi of static air pressure. Believe it or not, these 'no spring' forks could take the biggest hits and jumps without any problems.
If your fork feels too firm overall, won't dive deep during hard cornering, doesn't use most of the available stroke or rebounds too quickly when compressed, then lower the oil level to decrease air chamber size, thus lowering the fork air pressure. HINT: Measure fork travel by snuging a "zip tie" around the chrome fork tube before your test ride.
There are a few ways to set oil levels, some easier than others, and some more expensive than others. The least expensive way requires a measuring tape and a little time. Fill the fork leg with fork oil or suspension fluid, properly bleed the system per manufacturer instructions. With the fork tubes (or fork cartridge if applicable) collapsed and fork spring removed, measure from the top of the highest fork tube down to the fork oil level. Adjust to manufacture specs or your own desired setting. For all KTM models oil level specs are listed in your owner's manual and in the "chassis specification" section of your chassis/frame parts book.
There are a number of siphoning tools available from aftermarket manufacturers and suspension tuning services. If you do a lot of suspension work, these are worth the money. They vary in design and cost but they all achieve the same end result of setting the proper oil level.
- A clean empty plastic bottle. Most fork oil bottles work well.
- A piece of vent or fuel line with an ID of 1/4" (6mm). Clear works best.
- Straight metal tubing 8-10" (250mm) long with a 1/4 inch (6mm) OD and an ID as large as possible. Most hardware stores have copper tubing that works.
- A drill bit that's 1/32" larger than the OD of the vent hose you're using.
- A small plastic tie-wrap.
- A measuring tape.
With the fork tubes (or fork cartridge if applicable) collapsed and the fork spring removed, insert the metal tube into the fork leg with the plastic strap parallel to the top edge of the highest fork tube. If the oil level is lower than the tube's end pour in some oil to cover it. Squeeze the air out of the plastic bottle and release, the expanding plastic bottle should create enough vacuum to siphon out the fork oil to the tube's end. You're all set at that point; cheap, quick and easy.