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Yamaha FZ-09 Fork Piston Kit Breakdown

Following up to Tuesday’s fork post…with pictures and some explanations.

As I mentioned, the GP Suspension kit includes the following components:

  • 20 mm rebound piston w/ custom shim stack
  • 20 mm compression valve w/ custom shim stack
  • Rebound needle

Stock needle on the left, GP’s shiny new brass 7.5 degree needle on the right. The larger taper angle allows for more adjustment and better flow characteristics through the rebound bleed circuit.

Here is where the real magic happens in the rebound circuit. The stock piston is on the left, the GP 20 mm piston is on the right. The larger oblong holes you see are the ports used for reverse flow – oil flows through during the compression stroke so that the piston assembly minimally adds compression damping. The 3 large(r) holes in the recessed/dished areas on the stock piston are the high speed rebound feed ports (these are what load the shim stack). As you can see, the GP piston uses double the hole count on a larger hole circle to both flow more oil and obtain more leverage on the shim stack. The ports are nicely machined to improve flow through the port. Now, you’re probably wondering why we’d want to flow more oil through the FZ-09’s rebound circuit. Short answer is, we don’t. The stock setup doesn’t have enough rebound, so we’d actually want to reduce oil flow; the heavier oil used previously with the heavier spring rates was done in the same light. GP, like Penske, RaceTech, Traxxion, K-Tech, etc. make various pistons that are used across multiple bikes. This port design was done to optimize the geometry across a wide variety of applications. So how do we add damping back? Good question…

The stock rebound piston and shim stack is on the left. GP’s setup is on the right. Though the pictures aren’t great, you can see the heavier shim stack. Varying the shim diameter, thickness, and order in the stack controls the mid-high speed damping response. The combined effect of the needle, piston, and new shim stack is a wider adjustment (ranges from less than stock to MUCH more). No more pogo-ing down the road.

Here’s another view of the shim stack, this time assembled. From the left to right, top out spring (takes up unsprung mass when the wheel is in the air), rebound piston holder (needle assembly inside), new check valve (provides ‘unrestricted’ flow during the compression stroke), GP piston, and custom shim stack…all tucked under the factory nut.

These are the compression valves. Similar deal as the rebound piston, but a little different. The larger oblong holes provide the same increased functionality over the stockers. The high speed ports are actually a bit smaller to work in conjunction with the high speed shim stack. Not shown here, but a bleed port has been added to the piston to provide good low speed compression response in light of not having a dedicated/adjustable bleed circuit.

This is the final assembled compression base valve. You can see the shim stack here.

While inside the forks, I did a light polish on the damping rod to reduce sliding friction. I also chamfered the cartridge tube ports to flow a little smoother.

Before:

After:

Some FZ-09 trivia, unlike most modern forks, these cartridge tubes are steel, not aluminum.

It’s a good thing I had the bike out when I did. Got a few inches of snow shortly after the ride and the roads are back to shit. Yay…

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