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MX EDUCATION: YOU CAN FIX YOUR OWN FORKS WITH SOME BASIC FORK OIL HEIGHT TUNING TRICKS

October 22, 2013
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The MXA wrecking crew does everything in its power to avoid sending the forks on our test bikes out to be revalved. It’s not that we don’t believe the suspension shops can improve the performance of the forks; it’s just that we don’t want them to. We are stubborn. We think we can find a fork fix without having to resort to spending hard-earned cash. We click the clickers in every imaginable way, and when push comes to shove, we change the 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.

It should be noted that if all else fails, we have our favorite suspension shop’s phone number on speed dial.

What is the effect of changing oil height?
Adding oil to your forks reduces the air volume. Since air is compressible, a smaller airspace is harder to compress than a larger airspace, which results in a stiffer fork. Since the compression of the airspace is gradual, lessening the airspace will be felt by the rider from the middle of the fork’s stroke to the point of bottoming. In essence, adding oil to your forks makes them stiffer from the midpoint on. The obvious corollary is that when you take oil out of your forks, you make them softer from the midstroke on. Adding or subtracting oil has a negligible effect on the first four inches of travel.

How do you add 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 in the air-bleed screw on the fork cap. Use the graduated scale to siphon 10cc of fork oil into the syringe. Noleen sells an affordable syringe for $6.95. Call them at (760) 948-1677. Insert the syringe tip into the air-bleed hole so that it has a good seal; if it doesn’t, oil will leak out without going into the fork. Remove the O-ring from the air-bleed screw and put it back in the hole to seal the syringe better. Slowly squirt 10cc of oil into the forks. It is important to note that as the 10cc of oil goes in, 10cc of air must come out. Bleeding the air while adding 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. This back-and-forth motion is tedious, but it is easier than pulling the forks apart. The whole procedure can be done with the forks on the bike.

How do you lower the oil height? First, you have to remove the forks from the bike. With the forks off the bike, take the air-bleed screw out of the fork cap. Turn the forks upside down and place a graduated cup (the syringe with the plunger removed works great) directly under the air-bleed screw hole. Gently compress the forks by hand to force oil out of the fork. Don’t worry about the oil spraying; it doesn’t want to come out. Once the graduated cup says you have removed 10cc, stop and do the other fork leg.

What can be achieved? Taking 10cc of oil out will make the fork feel softer from the midstroke on. It is the simplest and quickest way to take midstroke harshness out of stock forks. If your forks are too stiff, too harsh or don’t get full travel at your track, lowering the oil height will make them feel suppler. Conversely, adding oil will make the forks feel stiffer. More oil works best when a rider has a bottoming problem.

Don’t be afraid to experiment. When MXA was testing the 2013 TM 450MX, we thought the forks were too harsh, so we took 10cc of oil out of each leg. They were still harsh, so we removed another 10cc. Eventually, we ended up running the TM forks with 40cc less oil in each fork leg. Surprise, they worked better and still weren’t bottoming. It wouldn’t have mattered if we had taken too much out, because then we would have learned how far we could go. Check your owner’s manual for the maximum and minimum oil heights allowed for your forks. Adjusting your oil height is a valid tuning process and, best of all, it is free.

 

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