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Pneumatic Spring Forks (PSF)

Pneumatic Spring Fork (PSF)

PSF Forks

For 2013, both Honda and Kawasaki will be using the new Kayaba pneumatic spring forks on the 450s. In corporate nerd-dom terminology, "PSF" is the technical sounding acronym everyone will be using when referring to these forks. So, what are they, actually?

In simple terms, they aren't any different than the previous twin chamber sealed cartridge forks, but the steel springs have been removed. The result is significant weight reduction in the forks, nearly a couple pounds I think. I suppose an added benefit would be there is no longer a steel spring clanking around in the tube potentially resulting in mechanical wear. Of course, good maintenance practices minimize such. The really cool aspect of these forks is you can adjust your spring rate easily without disassembly. How sweet is that?

As nifty as this technology is, my skeptic light bulb is turning on. Granted, I have never ridden a motocross bike with these forks. I suspect that Kawasaki and Honda tested these forks thoroughly, I hope. Maybe thay are the next best thing. These forks are no doubt new, but the concept is nothing new at all. For example, I ditched the pneumatic spring forks on my mountain bike about 15 years ago for reliability reasons. I am not trying to make a comparison here, just saying the concept has been around for a long time. It wouldn't be fair for me to criticize these forks, but I do have some concerns about them.

While I totally dig the idea of being able to adjust the spring rate on the fly, I think some shortcomings will become evident with these forks.

  • Spring rate adjustment

    If I recall correctly, a standard spring rate setting for the Honda 450 would be 33 psi and a 1 psi change in pressure would be significant (similar to going to the next stiffer spring). Proper tuning then, though simple and convenient, requires careful attention to what the pressure is. Perhaps this is no big deal, so long as riders use the appropriate measuring tools (a good pressure gauge) and use them correctly. Setting the pressure exactly on a small volume pneumatic cylinder can be troublesome because you can lose desired pressure simply by removing the pump or gauge.

  • Maintaining spring rate while riding

    Most (if not all) cartridge forks up to this point in time have a plug atop each fork tube. The purpose of this plug is to facilitate venting of the fork due to increased pressure (or decreased pressure) as a result of heat. As the forks heat up while riding, the air in the tubes expands and the pressure rises. This increase in pressure alters the performance of the suspension, possibly making the suspension harsh. In fact, this is why after market companies such as Motion Pro offer fork bleeder valves.
    Motion Pro Micro Fork Bleeders - Silver The push button style bleeder valves allow quick venting without the need for a tool. If we know that temperature change has this kind of effect on the air within the fork tubes, then I would suspect the same characteristic with the pneumatic spring fork. Will riders need to constantly adjust the air pressure in the PSF forks to keep them performing consistently? My guess is "yes".

  • Catastrophic loss of air pressure

    Can there be catastrophic loss of air pressure in the PSF forks? I can't really say. They're probably built well enough that it isn't likely to happen. However, my experience as an engineer in industry tells me that logical possibilities, no matter how unlikely, can (and do) happen. So what if this does happen? What if the valve core blows out? If it does happen, in a matter of a moment the spring rate will decrease to nothing as if there was no spring at all. Not good as you can imagine.

    A more likely scenario relating to pressure loss would be due to component wear resulting in leakage. In what ever manner leakage would occur, inattention to the pressure setting would result in a decreasing spring rate over time. The severity of the leak would determine how fast the rate would decrease. Could the PSF forks develop leaks sufficient to impact performance during a moto? Maybe. But if so, you could start a moto with good suspension and then struggle at the end with an excessively soft front end.

  • Tuning characteristics

    Compared to the typical sealed cartridge forks with mechanical springs, the PSF forks are indeed different. Whether the difference is good or bad will become evident as time goes on. Of course, the bike builders will tell you the new forks are great. And they may be, but riders will decide. Here I simply want to point out the difference with regard to the spring rate. As you already know, the PSF forks have no springs. The springs for the PSF forks are the pressurized air. The difference is the PSF forks have a progressive spring rate while forks with springs have a linear spring rate in addition to a progressive component. The following two charts show the difference between fork springs and pneumatic springs with respect to force exerted as the forks are compressed.
    chart: spring force in mechanically sprung forks chart: pneumatic spring force in mechanically sprung forks

    With mechanically sprung forks, the rider has the advantage of tuning the forks in more ways than the PSF forks. With both styles of forks, hydraulic damping (clickers and valving) can be adjusted. Also, the progressive action in both can be adjusted. With the mechanically sprung forks, the oil height can be changed to accomplish a change in the progressive component of the forks. This change in oil height reduces the air volume within the forks, so the air compresses more quickly as the forks compress, providing more bottoming resistance. The oil height change behaves very much the same (in character) as the increasing of air pressure in the PSF forks. Because the PSF forks have no springs, the ability to adjust the linear part of the spring rate is removed.

    In conclusion, I have no intent to speak negatively about the PSF forks as I have not used them. I simply want to share my skepticism about them. I think my concerns are legitimate though, and my decision to by a 2013 450 will be influenced by these forks. At this point, I would have to stick with springs for the reasons I have discussed.

    By: MotoObsession admin

View of the PSF forks shrader valve
View of the PSF forks with pump attached to schrader valve

All that is needed to adjust the spring rate of the pneumatic spring forks is a simple hand pump.

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