By Bones Bacon
Air Forks. That’s all anybody talks about anymore. The big debate is air versus spring, KYB versus Showa and, pretty soon, WP versus everybody. Like it or not, air forks are here to stay, so let’s calm down and make the best out of what could very well be a good thing. I want to strip air forks down to their nuts and bolts and get on with making your bike have the best suspension possible.
I can’t emphasize enough to keep it simple, stupid (KISS). Don’t overcomplicate what is actually a very simple fork. The Japanese think of air as a spring, so let’s do the same. Selecting air pressures is all about picking a spring rate—just like you used to do when you had coil-spring forks. Spring rate is selected based on your body weight, size of bike and riding style. There are several steps in choosing the proper air pressure: (1) Let your owner’s manual guide you to a starting point. (2) Choose a good quality air gauge and pump and stick with that exact system to insure consistency in your readings. (3) Decide whether you are going to use KPA, psi or BAR as a pressure measurement. For this example, let’s use psi to analyze the Showa SFF-TAC forks that come on the 2015-16 KX450F, RM-Z450 and CRF250. I selected these forks because they have lots of setup options, and the number of choices seems to confuse people. Showa SFF-TAC forks have three chambers in the air side fork. It sounds complicated, but don’t panic.
The inner chamber is the workhorse of the fork. It represents the fork’s actual spring rate. The outer chamber is there to assist if needed, but we don’t want to rely on it too much because it pressurizes the fork’s main seal (and if you blow a main seal, you may lose pressure). If you feel like you need a little extra bottoming control, you can add a little air to the outer chamber (which makes the combined pressure of the inner and outer chambers your actual spring rate.
“A COIL FORK SPRING HAS A LENGTH, AND ITS LENGTH CAN BE DETERMINED WITH A TAPE MEASURE; HOWEVER, IT IS HARD TO MEASURE AIR WITH A TAPE MEASURE. THIS IS WHERE THE BALANCE CHAMBER COMES INTO PLAY.”
Now for the mystery chamber. The third chamber (called the balance chamber) is the least understood. Let’s start with the basics. A coil fork spring has a length and its length can be determined with a tape measure. However, it is hard to measure air with a tape measure. This is where the balance chamber comes into play. Its job is control fork length and hold the fork in position. Normal air forks, those with a single chamber, like Kayaba PSF forks, have a coil steel balance spring inside them to do this job. Showa SFF-TAC forks eliminate the coil spring and use air in its place. When you add air to the balance chamber it pulls the fork down (by over-riding the spring pressure in the main chamber), shortening its length and simulating less preload on the fork’s inner chamber.
Conversely, if you want to go a little stiffer on the spring rate on your TAC forks, then just run less pressure in the balance chamber. This will allow the main chamber’s air pressure to win the tug-of-war with the balance chamber. In fact, the fork actually changes its length based on how much air is in both chambers.
In practice it works like this, say we started with 145 psi in the inner chamber, 3.6 psi in the outer chamber and 145 psi in the balance chamber. If this setup seems a little undersprung (soft or dives too much when letting off the throttle), we increase the inner-chamber pressure rate a little and leave the balance chamber alone. The numbers would now be 152 psi in the inner chamber, 3.6 psi in the outer chamber and 145 psi in the balance chamber. Just by changing the pressure in the main chamber, so that it can over-ride the balance chamber’s pressure, results in a fork that is stiffer initially and rides a little higher. Get the idea, sounds simple doesn’t it. Now keep it simple and before you know it you’ll have your spring rate picked and your preload set.
Now comes the hard part. Once you set your three air pressures to their best settings, leave them alone! When you had a bike with coil spring forks, you weren’t changing forks spring between motos. The same holds true for air forks. Use the clickers, found on the opposite fork leg from the air side, to make the fork feel stiffer, softer, faster or slower. For now, concentrate on mastering your air pressures.
Jim “Bones” Bacon has tuned the suspension of the biggest names in motocross, including Jeremy McGrath, Ricky Carmichael and Ryan Villopoto. Bones has a rehular column in MXA every month—where he hands out valuable advice. If you have a suspension question, send it to email@example.com.