MXA TECH SPEC: FORK-OIL HEIGHT IS THE MAGIC BULLET

 

 

 

MXA TECH SPEC: FORK-OIL HEIGHT IS THE MAGIC BULLET

Now that the motorcycle manufacturers have come to their senses and, for the most part, returned to coil-spring forks, it’s time for some of the old fork-tuning tricks to resurface. KTM, GasGas and Husqvarna are the only brands sticking with air forks, but even WP Xact air forks can benefit from the sage advice that their predecessors relied on. There are lots of things a racer can do to make his forks work better. The MXA wrecking crew would rather try to fix our forks ourselves than spend hard-earned cash on a re-valve. Once you have clicked the clickers in every imaginable way, slid the forks up and down in the triple clamps, and taken advice from strangers, it is time to apply the magic bullet.

What is the magic bullet? Fork-oil height. Adjusting the fork-oil height is the trick that every suspension tuner keeps hidden up his sleeve. It works. It is free. And, it doesn’t require taking the forks apart. What does it do? By adding or subtracting oil in your forks, you change the air volume. Since air is compressible, a smaller air space is harder to compress than a larger air space (and vice versa), which results in a significant change in how a fork feels.

Stiff or soft. The compression of the fork’s air space is gradual. Lessening the air space by adding oil makes the forks stiffer from mid-stroke to the point of bottoming. Taking oil out of your forks makes them softer from mid-stroke on. It has a negligible effect on the first half of the travel.

Measurement. On old-school 1970 forks, the oil height was measured with a tape measure in inches from the top of the fork tube (with the springs removed and the forks collapsed). Modern cartridge-style forks have their oil volume measured in cc (not by inches from the top of the tube). 

Adding oil. The simplest way to add oil to a fork is with a graduated syringe that has a tip on it small enough to fit into the air-bleed screw on the fork cap. Insert the syringe tip into the air-bleed hole so that it has a good seal (if it doesn’t seal well, oil will leak out without going into the fork). We often remove the O-ring from the air-bleed screw and put it back in the hole to seal the syringe better. Slowly squirt the oil into the forks.

Burp the fork. Remember that as the 10cc of oil goes in, 10cc of air must come out. Bleeding the air while inserting the oil is tricky. The easiest way is to push down on the syringe to squirt in about 1cc of oil, then pull back on the syringe plunger to allow air bubbles to escape back into the syringe. Repeat until you have 10cc in the fork. 

Subtracting oil. To lower the oil height in a fork, you have to remove the fork from the bike. With the fork off the bike, take the air-bleed screw out of the fork cap, turn the fork upside down and let oil drip out of the air-bleed screw hole into a graduated beaker or tube. If the fork oil drips out too slowly, gently pulse the fork leg to get more oil to flow. Be careful not to overdo it. 

Rule of thumb. As a rule of thumb, 10cc is the best amount of oil to add or subtract in conventional forks (WP AER air fork owners should only add or subtract oil from the damping leg). And don’t be afraid to remove more than 10cc. On current-model forks, you can safely take out up to 30cc.

Strategy. If your forks are too stiff or too harsh, lowering the oil height is a simple method of making them feel softer. Conversely, adding oil will make the forks feel stiffer. More oil works best when a rider has a bottoming problem; less oil works best when a rider has a mid-stroke harshness problem. Often the MXA test crew will remove 20cc of oil at the start of a test day and add it back in 5cc increments until we find the perfect setting. Adjusting your oil height is a valid tuning process and, best of all, it is free.

 

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