No springs! How KYB’s Pneumatic Spring Fork works

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The difference between a traditional front fork (left) and the new KYB PSF (right) is immediately obvious: no springs. Shifting both spring duties to compressed air allows a radical simplification of the fork mechanism, reducing parts count and, as a result weight. It also allows an unprecedented level of adjustability; altering the “spring rate” is as simple as breaking out your foot pump.

These air forks, a production first, are fitted to the 2013 Honda CRF450R. There, they reduce unsprung weight by nearly two pounds, meaning suspension movements will be burdened by that much less inertia, allowing the tire to better maintain contact with the ground. Ride quality and steering response will also benefit from the reduced weight.

Exclusive to the CRF, the compression adjuster screw now sits on a raised mount, angled back towards the rider. The air valve, the same design used on tires, is also mounted up top. To check pressures, you just use your tire gauge. To adjust them, you either let some air out or pump some in. Stock, pressure is set at 33psi, a range that shouldn’t necessitate excessive amounts of pumping to reach.

In this video, created by VitalMX, KYB’s Dan Worley talks about the new forks.

The key to the concept’s successful execution lies in the significantly increased piston size, increased from 21 to 32mm. The oil movement is then spread across a larger surface area, enabling more precise control. This is the resolution Dan talks about above. Basically, thanks to technology like that, we’re now able to realize a concept first attempted with the 1976 YZ125.

I’m warned that I shouldn’t push variable “spring” rates as the big deal here. While it is adjustable, that’s only within a fairly limited range. Think of dropping or upping pressures as more akin to adjusting preload. Just now, you can do that with a bicycle pump. The big deal here is reduced weight.

“Adjusting preload on an mx fork is a bitch and the new fork makes that as easy as setting your tire pressure,” says former MX race bike builder Sean. “Ditching the steel springs may mean you don’t have quite as many setup options, but you also don’t have the weight of those springs or the hassle of changing them out. And, you’ll definitely know if you blow a fork seal.”

  • Glenngineer

    Mountain bikes have been using air springs, front and rear, for almost 20 years. They’re lighter but they tend to feel like shit relative to conventional forks, mostly due to stiction. Downhill and freeride bikes have tended to avoid them for the potential for catastrophic blowout, heat related changes in spring rate, and the physical realities of compression the the volume of air required to accommodate 7 or 8 inches of travel.

    Motorcycle can probably afford to mitigate a lot of these issues by just being bigger…all the extra mass of a motorcycle is going to make stiction less apparent, and a weight less critical.

    • Paul

      As a bicycle mechanic and mtn biker, I agree with Glenngineer. All things equal, air forks are much lighter and can be adjusted more quickly, but feel awful. Most mtn bikers get away with air suspension because we use suspension like mx riders use their tires: absorbing small bumps. Anyone who sees more significant impacts (DH or Freeride) usually sticks to traditionally sprung suspension.

      It will be interesting to see if the mx engineers can improve the breed.

  • Sean Smith

    No oil you say? Thas cray cray.

    • doublet

      No springs… Pretty sure there is still oil. Something has to dampen the movement.

      “The key to the concept’s successful execution lies in the significantly increased piston size, increased from 21 to 32mm. The oil movement is then spread across a larger surface area, enabling more precise control.”

      Is this actaully going to work? Sometimes I get the feeling we are reliving the 80′s… Hydraulic anti-dive coupled to the brake system.. Air over oil forks. My ’83 KZ550 has air adjustable preload. It just has a spring as well, which can be changed.

      My big question is this: What happens when you blow a seal and you’ve suddenly lost your ‘spring’! I guess the seals will be much more complex / expensive to thwart this.

      • Sean Smith

        In the original version of the story, Wes said that the forks were all air, which is of course cray cray.

        And actually, the seals aren’t any different than they were last year. Hell, even last year they were using the air inside the fork as a bottoming spring. Basically all they did was get rid of the steel spring, add some valves and throw in some compression adjusters that don’t suck. It might seem like the 80s, but this time around they’re not rushing to market with new tech hoping it’ll work.

        • doublet

          I still wanna know what happens when the seal goes! I know, if the other one is good, it’s just going to sag lopsidedly. But say (just say) a jump is landed and dirt has been chewing at the seals. Now they both decide to leak a little.. the leak some or all of the air, and your front end just goes flat, partially of fully compressed.

          I’m not saying it won’t work or they haven’t tested it. As he said, they have tested it. It just stands to reason that at some point, somebody is going to have some fully compressed forks with no spring because it leaked out.

          Aircraft struts are this way. Just hydraulic fluid and nitrogen. The seal blows out and they just go flat. Of course they’re only designed coarsely for large hits anyway.

          • Sean Smith

            I bet that when the seal does start to go, that it’s a gradual leak rather than a sudden blow out. You’d feel that something wasn’t right, probably that the front end felt mushy or low, and you’d pull off the track to check it out.

  • HammSammich

    Wouldn’t changes in Altitude and temperature significantly affect handling? I suppose for an off-road focused dual sport, this might not be a big deal, but it would seemingly present a barrier to widespread adoption on street bikes. When I go out on decent rides (beyond my daily commuting) I frequently head toward mountain passes in the Cascades and can span 3,000-4,000 ft. of Elevation and a 60F temp split in a day.

    ***Disregard. I saw, CRF450R and thought “CRF250L.” I’m sure these make perfect sense for Dirt bikes.***

    • Wes Siler

      Sure, and you just connect a pump and alter the pressures. Most MX tracks don’t vary in elevation by thousands of feet, at least not in my limited experience.

      • HammSammich

        Yeah, I misread the bike we were talking about (see edit) and also can’t see the video at work so I missed that we were talking specifically about Dirtbikes…

        MX tracks with thousands of feet of elevation change!? Now there’s an idea… ;)

    • doublet

      If your carburetor keeps it’s jetting I’m sure the small amount of air in a sealed chamber is going to be ok.. I don’t think it would affect air in the fork any more than it would nitrogen in a shock, or the air in your tires for that matter.

  • Josh

    The great enemy of air shocks in the mtb world has always been heat. The hotter the air ‘spring’ gets, the stiffer the spring rate gets. That’s been conquered, even for downhill bikes, for the most part. They still require more maintenance, but mx bikes already require a ridiculous amount of time turning wrenches…

  • dux [87 CBR600, 95 XR600R]

    I’m curious how the spring rate changes as the forks heat up. Cool design.

    • doublet

      Hot air is less dense, IE it expands. If it is still at a constant volume, it will increase in pressure.

      Same reason your tires will read a higher PSI at temp than cold.

  • rohorn

    Changing pressure affects preload, not rate.

    To change rate, change the oil level (which tuners already do.

    The whole concept is funny to me due to my memory of the hype surrounding air forks the first time around, followed by aftermarket fittings on steel sprung forks for dumping air pressure to reduce fork seal stiction.

  • dan

    I think my 1984 MG LeMans 3 came with air forks which never worked.

    • ontheroad

      Honda and Yamaha both had air forks on dual-sport models in the 80′s: they were kinda shitty as well. Pretty sure they were manufactured by KYB, too. Thankfully, I’ve never had to open up said forks, so I don’t know if they had some manner of springs in them, but I do remember the schrader valve on the fork cap and the hopelessly mushy retro-bike feel.

      It’s a neat idea, and since it looks like Kawasaki is adopting it for their MX bikes as well, I’m sure it’s been tested extensively. Like the MTB guys, I’m a little skeptical, but curious to see how they fare this time around.

  • Campisi

    “Costco says I should fill my suspension with nitrogen instead of air, but they charge more for it. Seems like a scam to me…”

    … Yeah, I’ve worked at a tire shop. What of it?

    • Zirq

      Wouldn’t filling them with helium give you more unsprung weight?


      • doublet

        Actually, yes. Everything is a state of matter, and the lighter the mater per volume… But I don’t really think by much, especially in contrast to the problems it would introduce.

        Helium molecules are tiny compared to air or nitrogen and it leaks out too easily to be useful. Temperature changes affect it much more severely

        There is no water in nitrogen, this is why it’s better.

      • okto

        just fill them with awesome