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FORK WORKS: MX1 SUSPENSION'S WP AER AIR FORK RE-VALVE

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MX1 SUSPENSION WP AER AIR-FORK RE-VALVE

Regardless of what a small handful of naysayers believe, the WP AER air forks on the 2017 KTMs and Huskys are the best production air forks ever made. They are simple to set up, easy to live with and well damped; however, no fork is perfect. That even goes for Yamaha’s heralded Kayaba SSS forks. Most MXA test riders feel that the AER air forks, like the 4CS and bladder forks before them, are too quick in the mid-stroke, especially in consecutive braking bumps. The damping before that is well modulated, and the ramp-up of the air pressure after that is more than adequate to keep the AER forks from bottoming. But, in the nether region, between the middle of the stroke and bottoming control, the AER forks flutter.

MXA had excellent luck with MX1’s Advanced Progressive System (APS) on the older WP 4CS forks, so we were interested in what suspension guru Dick Wilk could do for the latest AER air fork. First, before we decided to test Dick’s MX1 fork mods, we had to make sure that he was working on the same area of concern as we were, because otherwise we’d be going at cross-purposes. Luckily, Dick succinctly described what his new AER re-valve mod fixed—and since it matched what we wanted fixed, we were off to the races.

First and foremost, this is an inexpensive mod. At $150, it goes against the grain of most current fork mods, which can sometimes run as much as $1000. Dick has been working with KTM, Husaberg and Husqvarna forks for decades, and he doesn’t feel the need to fix what isn’t broken. Dick’s AER mods are focused on the WP mid-speed valve. The compression stack on the WP mid-valve has a built-in leak, which is actually a free-flow pathway around the large 30mm shims (achieved by stacking two 26mm shims under it). The fact that fork oil can bypass the shim stack is the cause of the flutter in the braking bumps. The fix is simple. MX1 removes the two 26mm shims and reconfigures the complete compression Christmas tree to meter all the oil that flows through the mid-speed piston. Dick does the same basic shim-stack change to the rebound side of the mid-speed valve. But, there is a catch. Without the free bleed of the stock shims, the overall stack has to be lightened in both directions, because shutting off the free bleed increases both the compression and rebound damping. It took lots of test hours to get the perfect combination of shims to manage the flutter without making the suspension too slow or too stiff in the mid-stroke. The complete re-valve revolves around an understanding of flow dynamics and shim-stack stiffness. That’s why it doesn’t cost an arm and a leg.

Every test rider felt that the MX1-modified forks followed the ground much better than the stockers. They didn’t chatter off little bumps; instead, they mimicked the feel of WP Cone Valve forks, which are superb at tracking across rough ground without any lift-off. This is a very good mod for a discerning AER rider. It is also a must-do for 2016-17  KTM and Husky owners because it turns out that the MX1 mods were very close to what the KTM R&D department did with the 2018 forks.

One additional MX1 option for WP AER forks is MX1’s external air canister. There is nothing inside the canister; it is empty, save for air pressure. It increases the air volume of the WP air chamber by approximately 15 cubic inches, which acts as a buffer for the rapid ramp-up effect of most contained air spaces. The canister threads into the fork’s Schrader valve fitting and has its own Schrader valve at the end of the canister. It smooths out the feel at the very end of the stroke and allows a wide range of air pressures.

How much does all of this cost? The retail price of the 2017 WP MX1 AER fork mod is $150, plus oil. The canister costs $195. Contact MX1 Suspension at (916) 741-8767 or www.mx1suspension.com.

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