WP's new air-spring fork is already available on European KTM motocross models, and it will likely be introduced in the US on 2017 KTMs. The new design is a major departure from their controversial 4CS fork. WP ditched the forward-thinking design for one that is, in many ways, much more conventional. The fork is filled with design features we are accustomed to seeing on Japanese products. For example, it uses a spring and floating piston reservoir pressurization system, large pistons with heavy shim stacks, and of course the appearance of an air spring in place of steel coil springs.
The new design divides the spring and damping duties into separate legs. The left leg contains an air spring cartridge, while the right leg houses all the damping components. Separating the suspension functions like this is nothing new in motocross, or mountain bikes for that matter. When the fork, triple clamp, and wheel are all bolted together they function as a single unit.
On the negative side, the new fork will require a little extra track-side maintenance. You will need a special tool to accurately set air pressure (I got the Works Connection Digital Shock Pump). Forget about pumping it up with a compressor and using a pressure gauge to bleed off excess air - the air pressure setting is way too sensitive. It needs about 150 psi and even a couple psi change makes a big difference.
On the positive side, the new fork is really lightweight. It's easy to feel the difference when riding. More importantly, unlike a stock 4CS, it actually absorbs bumps! With a reasonably compliant feel and good bottoming resistance, this is easily the best performing production fork to date from WP. Still, it's not perfect. It has a bouncy feel at the top of the stroke that give it a wiggly turn-in feel, and lots of brake dive for a motocross fork.
We have a ton of testing yet to do with the AER 48, but so far it looks like a step in the right direction for WP.
The internals of the air spring side are extremely simple. Air pressure is fully contained within this internal cylinder, so a leaking main seal won't cause the fork to go flat.