THIS MUCH WE KNOW! MASTERING THE ART OF LIVING WITH KAYABA PSF AIR FORKS
Honda angled the compression clicker, but put the air cap in straight. We’d like them to both be angled.
The big news in the suspension world back in 2013 wass all about Kayaba’s PSF (Pneumatic Spring Fork) forks. They were and still are standard equipment on the Kawasaki KX450F and Honda CRF450. To rider new to the sport the idea of air springs seems revolutionary, but this isn’t the first time that air springs have been tried on production bikes. As far back as 37 years ago, Yamaha was offering YZ125, YZ250 and YZ400 models with air forks?the so-called “speedo and tach” forks. They were not a success, but mostly because of poor damping?not just the limitations of 1976 air technology.
Kayaba’s goals with their new air forks were to improve performance, lessen complexity, lower the weight and save production costs by using an air spring instead of steel coil fork springs. The manufacturers’ interest in specing air forks was threefold:
(1) They saved almost 2 pounds, and in the era of porker motocross bikes, that is a big deal.
(2) They cost less to produce, and that savings is not passed on to the consumer, so they are good for the bottom line.
(3) The factory R&D departments felt that performance was on par with what they had offered with coil springs, and they could be modified by the consumer to fit a wider range of rider weights.
THE SCIENTIFIC RULES OF BOYLE’S LAW
Air forks are bound by the scientific rules of Boyle’s law. Boyle’s law describes how the pressure of a confined gas is inversely proportional to its volume. As a fork travels halfway through its stroke, the air volume is cut in half, which makes the pressure inside double. At three-quarters of the way through the stroke, the pressure is doubled again. The air pressure ramps up very quickly.
To reduce the ramp-up effect and achieve more linear pressure, Kayaba’s engineers added a balance spring in each fork to act directly against the air spring for the first half of the fork’s stroke. The balance spring starts out fully compressed, but as the fork compresses, the balance spring extends. At 140mm (full travel is 300mm), the balance spring is fully extended. This firms up the initial part of the stroke. Balance springs can be changed to stiffer or softer rates to help fine-tune the performance of the fork. Old school air forks would top out when the load was reduced on them because they didn’t have a balance system.
Kawasaki’s air cap is under the brass screw and requires a special adaptor.
THE NITROGEN CONUNDRUM
One common misconception about air forks is that nitrogen would offer a significant improvement in performance. Not completely true for the average rider. First and foremost, nitrogen is bound by Boyle’s law just like any other compressible gas. Second, since air is 78 percent nitrogen, it expands at the same basic rate and has almost the same density as nitrogen. On the plus, nitrogen has a larger molecule than air, which slows leakage through rubber tires or rubber shock bladders by approximately 30 percent. Since air in a PSF fork is not in constant contact with rubber seals, that advantage is mitigated. Both Kayaba and Showa originally tested with nitrogen in their prototype air forks, but found that the difference was not worth the trouble. The one caveat; air contains water vapor, and it is the water vapor content that causes the difference in expansion rate over nitrogen.
From a technical standpoint, Kawasaki’s system is better than Honda’s. It protects the air valve under a brass screw and uses a special O-ring-equipped adaptor that ensures that no air leaks when removing the pump. But, it is more time consuming to work with.
Another myth is that the oil seals on an air fork are different from the oil seals on a coil spring-equipped fork. They aren’t. The seals on a PSF forks are subject to the same risk of leaking from wear, nicks or grit as any other fork. What is different is that the air pressure inside the PSF fork presses on the oil at the seal lip with more force than in a coil-spring fork. This means that a small nick, which would only weep on a conventional fork, will gush oil on an air fork. The caveat on air forks is that you cannot use a feeler gauge, film strip or Risk Racing oil seal tool to clean the fork seals. If you do, air will leak out when you insert the tool under the seal’s lip. The MXA wrecking crew runs SKF seals in both our KX450F and CRF450F (and have been testing a new breed of special SKF air fork seals). We also run Neoprene fork gaiters over the seals to cut down on grit and dings that are a bigger problem for air forks. We suffer twice as many blown fork seals with air forks as we do with coil spring forks.
GETTING PAID BY THE HOUR
When forks came with coil springs, the springs came with the bike. Now that they have air springs, Honda charges you to buy a pump so you can use the forks. It could be said that they provide the first air volume for free?you gotta get any extra air for yourself.
Air forks are much more labor-intensive than coil-spring forks. The air pressure must be checked before every day of riding. Why? Because air pressure fluctuates with the ambient temperature and the heat of friction during riding. The forks will have more air pressure in the heat of the day than in the cool of the morning. Plus, PSF forks will gain approximately 4 psi during a long moto. The best strategy for racing is to set the forks to your selected air pressure before practice (stock pressure is approximately 35 psi), and then add or subtract air pressure based on how the fork performs on the track. As a rule of thumb, it takes 2 psi to equal one spring-rate change on a coil spring?33 psi is a 0.47 kg/mm spring and 35 psi is a 0.48 kg/mm.
It is important to remember that once you choose your favorite air pressure, you should stick with it. Do not use air pressure to try to fix damping problems?that is what the clickers are for. You never came in from a moto on your coil spring forks and tried to change springs between motos to increase or lessen the compression. The air pressure just replaces the coil springs?not the valving.
The SplitStream cross-over tube makes setting air pressure easier?and since you have to set it every time you ride, easy is a big plus.
Most MXA test riders run SplitStream or Works Connection air valves. They make setting the air pressure easier. Back in the old says, many riders uses cross-over tubes on their forks and the failure rate almost nil?and that was with 35-year-old technology.