By Bones Bacon
I got a call from a customer not too long ago who said he had been very happy with the suspension on his 2017 RM-Z450 but decided he wanted to play around with the adjusters and air pressures to get himself familiar with how his Showa TAC forks worked. He had been happy with how they worked before, but now, even though he put them back to the original base settings, they didn’t feel that good anymore. This guy thought he had lost his mind, because now his suspension didn’t feel the way he remembered it.
It’s not a bad idea to mess with your adjusters or air pressure to get familiar with what they do. After all, when you get to a race and have a problem, it comes in handy to know what to adjust to make it better. Still, in the back of his mind, this rider couldn’t help but think his suspension handled better before he started experimenting with it. He remembered that his dad had told him, “If it ain’t broke, don’t fix it.”
He asked me, “What did I do wrong?” Air forks are still a relatively new concept, even though they are actually simple push/pull systems. The problem is that over a period of time and after several small adjustments, the pull can overcome the push. In a spring fork, the fork spring has a set length to it. You can measure it with a tape measure. When your fork extends and the spring reaches its maximum length, it stops pushing at almost the same time as the fork reaches its full mechanical extension. Air doesn’t have a set length, and even though the pressure decreases as the fork extends, when the forks reach their full mechanical extension, there is still a lot of air pressure inside trying to extend them even more. To combat this, air-fork designers include either a balance chamber of air or a balance coil spring. The balance spring exerts a negative pressure on the forks that pulls the fork back down into its stroke. How much balance air pressure there is or how stiff the balance spring is determines how the forks work in both directions. The main air pressure pushes the fork downward, while the balance pressure counters this force. So, depending on the type of air forks you have, there is either a balance (negative) chamber of air or a balance spring that is pulling the forks in the opposite direction of the main spring force. How much balance pressure or how stiff the balance spring is determines the actual ride height of the front end of your bike. Think of it this way: if you put more air pressure in the balance chamber, the forks will have a lower ride height because the balance pressure will offset the main air pressure. Conversely, less air in the balance chamber than in the main chamber will make the forks ride higher. Depending on the pressure differential between the two, the fork will actually get longer or shorter.
If you understand this principle, we can get back to solving the RM-Z450 rider’s assertion that his forks don’t work like they did before. His problem is common to all Showa SFF TAC forks found on the CRF250, RM-Z450 or KXF450. It is possible, after experimenting with the air pressures in all three chambers, to have the forks pulled down farther in their stroke than normal, even though all the air pressures are the same in each chamber as before he started messing with anything. I have had a set of TAC forks be pulled down into the stroke as much as 30mm when changing the air pressures so much that the push/pull balance goes out of whack.
The solution? Hit the reset button. The goal is to get the forks back in symbiotic balance. Here is what he needs to do to fix it.
Step one: Let all the pressure out of the balance chamber. This will make the forks extend until they are completely topped out (theoretically reaching full-maximum mechanical extension).
Step two: Over-pressurize the inner and outer air chambers by 4 psi.
Step three: Pressurize the balance chamber until the fork legs get noticeably shorter. At this point, he needs to set his balance pressure back to the original setting that he liked so much before he started playing with the air pressures.
Step four: Go back up to the inner and outer air pressure Schrader valves and bleed them down to the original air pressure. Through this process, the rider will notice his forks being pushed up and then pulled back down to their happy spot. This returns his forks back to their correct ride height, just as a fork spring would have with a little bit of preload on the fork springs. If he follows these steps in this exact order, his forks will be back in happy-land.
I explained it all to him over the phone, reminding him that he doesn’t need to reset his forks all the time and that he should only check his fork’s pressures once a week at the most. Additionally, I told him that he could adjust them a little here and there with no problem; but, every once in a while, it’s not a bad idea to bleed them and hit the reset button. Modern air forks are really simple—if you understand them, especially compared to the mechanics of the engine or the mapping of the ignition.
Jim “Bones” Bacon has tuned the suspension of the biggest names in motocross, including Jeremy McGrath, Ricky Carmichael, Ryan Villopoto and Adam Cianciarulo. If you have a suspension question, send it to email@example.com.