ASK THE MXPERTS: HELP! MY WP FORKS COLLAPSED WHILE SITTING

On Apr 6, 2023 Last updated Apr 6, 2023

Dear MXperts,
I bought a new KTM 350SXF, and two days later, I went out to the garage and the forks were collapsed. I pumped them back up, but it didn’t help. Is there a simple fix for this problem?

Yes, there is, and we will tell you what it is after we explain how and why your air forks could collapse without you riding the bike.

Air forks have two separate air chambers with their own unique air pressures. The top air chamber, known as the main chamber, pushes the fork downward to resist the forces imparted when hitting bumps or landing from jumps. It is simple to understand because it works identically to a steel coil spring. It is possible for the main chamber’s air pressure to create a condition known as “topping out.” Topping out is caused when the forks are fully compressed and the air pressure inside the forks exerts its pressure in the opposite direction of the main chamber’s air. In short, the compressed air in the forks makes the forks extend rapidly back to their full length. It is called “topping out,” because when the forks extend to their full length, they make a clanking noise when they hit the end of their stroke.

The solution to “topping out” on air forks, especially the Kayaba PSF and Showa SFF-TAC air forks of 2015–2018, was to add a second air chamber on the opposite side of the over-pressurized main chamber. The air pressure below the main chamber was used to stop the forks from topping out on the rebound stroke. This second chamber was called the negative chamber. On Showa and Kayaba’s air forks, it actually was a second chamber with its own piston and Schraeder valve to tune it. But, it was a nightmare to deal with and eventually Showa and Kayaba gave up on their production air forks.

KTM, on the other, didn’t jump blindly into the air-fork craze. Instead, KTM watched the Japanese fork brands crash and burn, and then came out with an ingeniously simple idea‚ especially when compared to the complicated balance chambers of Showa and Kayaba air forks. KTM’s concept was to harness the air pres sure in the main chamber to do double duty. Basically, they use the main chamber as the negative chamber

Now to answer your question about a “simple fix.” WP forks can go flat when they are left sitting. It is not common, but it does happen. Why? The air in the main chamber leaks into the negative chamber through the cross-over bleed slot, and the unbalanced air pressure pulls the fork tubes up. The simple fix is to cycle the fork legs up and down so that the pressure in the negative chamber can transfer back to the main chamber, and when it does, the forks will extend to their full length; however, it often requires two or more people to cycle the forks up and down over their full length. One person sits on the bike and pushes the forks downward, while a second person straddles the front wheel, helping apply even more downward force.

Once you get the forks going in a rocking-horse motion, the air trapped in the negative chamber will cycle through the cross-over bleed slot over to the main chamber, bringing the forks back to their original length. All you have to do is reset the air pressure and take the bike for a short test ride.