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YZ450FX Free Piston Mod or Aftermarket?

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I got my forks apart and plan to swap out my inner chamber springs.  I had already swapped out my main springs before I even rode the bike, but didn't feel like changing the inner chamber springs at that time.  My main complaint is going over washboard / shallow choppy type terrain.  I'm going to see how the heavier inner chamber springs affects that, if any.  I had purchased some after market (I think it was pro-circuit) free pistons for one of my other Yamaha's, and never installed them (took a while to arrive and I ended up getting replacement plastic ones locally due to I needed the bike operational).  Think I should install them on the FX, Keep the stock, or keep the stock and do the free piston mod?  Pros / Cons, to each? 

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That spring holds the inner chamber in place. It will not change the issue you describe.

You need to revalve the mid valve stack 

Changing the ride springs without changing the valving just means you are farther off than you were stock....especially on the rebound.

Changing the piston means more oil flow.....which might not be what you shim stack would like to see.... they should be both done at the same time or not at all.

Are you opposed to sending your suspension to a pro tuner?

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1 hour ago, Krannie McKranface said:

That spring holds the inner chamber in place. It will not change the issue you describe.

You need to revalve the mid valve stack 

Changing the ride springs without changing the valving just means you are farther off than you were stock....especially on the rebound.

Changing the piston means more oil flow.....which might not be what you shim stack would like to see.... they should be both done at the same time or not at all.

Are you opposed to sending your suspension to a pro tuner?

I had pretty good luck with my 2012 WR by just changing the springs.  I'm not opposed to sending these off to someone and have been planning on it, but I'd like to see how both springs replaced felt.  I have been in touch with a guy who does suspension on the side, and has done a couple of these bikes and mentioned the mid valve was a problem.

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I replace the main spring and ride the bike first too.   When I ride with the right spring, I set the clickers as close as I can to where the damping needs to be and then adjust the stack after riding it.

I found that my damping preferences change when I change the spring.  With a too soft spring I'm dialing in a lot of compression damping to compensate for a soft spring.

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The free piston mod, specifically, drilling drain holes and removing the seal at the top of the free piston, is mostly intended to prevent the free piston from being broken due to an accumulation of oil on top of the piston should the compression assembly rod seal leak.  In its stock form, the free piston adds a third, very small air chamber sealed off from both the inner and outer volumes, by venting the upper area of the piston into the outer chamber, you avoid over-pressurizing the third volume because oil has taken up space there.  Note that your aftermarket pistons have large open slots in the sides and no seal at the top. 

The purpose of the free piston and spring is to apply pressure over the sealed oil volume in the inner cartridge.  It holds nothing in place, per se, other than it keeps the oil from bouncing around and mixing with air. When the fork compresses, the damper rod moves up into the cartridge, pushing the mid/rebound piston ahead of it.  A volume of oil is displaced by the rod entering the the lower section of the cartridge, below the rebound piston and is forced up past the compression, or base valve, and since there is no air in that volume, something has to allow for it to be there.  Just as the high pressure nitrogen fill in a shock bears on the overall spring rate of the shock, a higher rate ICS spring adds to the overall spring rate of a twin chamber fork.

Changing the rate of these has to be considered mostly along with the preload of them.  If you have a high rate ICS spring with very little preload, the overall rate will ramp up more quickly as the fork compresses, whereas a lighter spring with more preload will produce a more constant rate throughout the stroke. 

Another important function is the prevention of cavitation, especially during rebound.  If you have fairly soft rebound, and the fork is "busy" all the time, you'll need more ICS spring rate to force oil back against the retreating rebound valve.  If you stiffen the ICS, you may find you want lighter main springs to compensate. 

As to the small bump compliance issue, most of that will be in the mid valve.  You may need to add a little float, soften it overall, or build in a crossover for this kind of high speed, short amplitude stroke.  Another thing you can try is backing off the rebound adjuster.  This may work for two reasons.  First, the same bypass is used by the rebound and mid-valves, so opening it up will reduce both.  Secondly, while YZ's are well known for having way too little shock rebound from the factory, they are just as prone to have considerably more fork rebound than necessary, and over a series of small bumps, if the fork doesn't recover from one hit quickly enough, the next hit will be harsher, and the next harsher than that, in sort of a miniature "packing" event.

 

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