However, if you’re serious about performance, you’ll want to upgrade the Ninjas’ stock suspension setup. Factory motorcycle suspension is designed with compromises, so it works with a wide variety of riders and on a wide variety of surfaces. The STX36 shock is designed to be fully adjustable (even length-adjustable) to help fit the suspension to the rider, and to the road or track conditions. It’s also available with multiple spring rates.
The STX36 has a 36mm piston, and all the internals are low-friction for smooth operation. It’s designed to avoid overheating under hard riding, so your suspension won’t fade away in the middle of a wild track session. The STX36 is serviceable and repairable, so while it’s expensive up-front (the shock retails for about $800 in the US), this is something you’ll be able to use for years to come. The shock may not add horsepower to your bike, but if you dial it in correctly, it will help you put more horsepower to the ground, which is even more important.
One other thing: Because the shock fits the Ninja 400, it should be able to fit Kawasaki’s Z400 naked bike as well, as it’s basically the same machine. However, as the shock’s internals have been designed with the Ninja’s weight and dynamics in mind, there may be some extra trickery involved to set the shock up for the Z400. It’s nothing a competent suspension tech couldn’t figure out, of course; talk to Öhlins, and they should be able to tell you if this will work.