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KYB SSS: Drilled free piston OC oil level considerations

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I currently run '06 YZ450 forks on my '11 WR450 and when I first got them (earlier this year), I read up on as much as I could and went ahead and drilled the free pistons (two 4-ish mm holes at the bottom of each piston) while going through the forks.  I then put everything back together using 355 cc of Maxima 85-150 in the outer chambers.  This is with the stock '06 YZ valving - I weigh about 220 lbs with all of my gear on and run .50 kg/mm springs in the front and a .58 in the back.  I ride primarily whooped-out desert sand, rocks, and hard pack off-road - not on a track at all, though we do have some decent sized jumps here and there.

 

So does a drilled free piston drive a requirement for additional fluid in the outer chambers to account for the volume of and access to the free piston?  I never rode those forks without the drilled piston and have nothing to compare them to (other than my gold valved WR forks), but I'm wondering if maybe I'm a little short on fluid in there since I would bottom with a moderate flat landing (or get a harsh jolty feeling once in awhile) and don't know that I've ever felt the true plushness of the fork through the whoops and small stuff with where I had the compression and rebound previously (10 clicks out on comp/10 out on rebound), despite trying many different clicker combinations.  And I definitely find myself working too hard to keep any kind of decent pace for very long. 

 

From reading previous posts about this, if the free piston volume is about 70 cc, does running standard OC fluid levels mean I'm running low right off the bat?  I know it's not likely the equivalent of 70 cc low and it depends on which part of the stroke I'm in, but what about adding another 50 cc?  Any thoughts? 

 

And Dave, if you read this, yes, I'm the one you talked to the other day who ordered the Phase 4 kit for the stock WR forks and shock.  We only got to talk about this briefly and I knew you were busy.

 

Thanks a lot.

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Yes you should try more oil but only a little , try 370

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I don't like the forks when they get to needing over 360cc's of oil.
 

When that's the case - I find more valving and getting the oil height in the 340 range is better - even with drilled pistons.



 

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Thanks guys for your help. I'm gonna pull them again and readjust the fluid level...I was at 355 cc, which I think was too much to begin with (I put that much in because of my 225 lb riding weight), and put an additional 50 cc in, thinking it would compensate for the extra volume of the free piston (and since I already had the forks off for something else). I'm gonna remove 60 cc a side and see what I get.

FWIW, the ride with the current oil level, wasn't much different than with the 355.

This oil level, plus the .50 springs don't make it overly plush.

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Thanks guys for your help. I'm gonna pull them again and readjust the fluid level...I was at 355 cc, which I think was too much to begin with (I put that much in because of my 225 lb riding weight), and put an additional 50 cc in, thinking it would compensate for the extra volume of the free piston (and since I already had the forks off for something else). I'm gonna remove 60 cc a side and see what I get.

FWIW, the ride with the current oil level, wasn't much different than with the 355.

This oil level, plus the .50 springs don't make it overly plush.

 

In my experience riders of your size need a huge addition of damping.  The harshness is very likely a soft issue.

 

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this is a good thread. i recently got back into riding, have a YZ with SSS, but also looking at an sff rmz. I'll have to poke around to see what the latest tricks are for either.

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harrperf, thanks a lot for the reply - I appreciate your inputs on this.

 I removed all fluid out of the outer chambers the other night and readjusted the level to 345cc a side.  Went out and rode it yesterday evening, all on hard pack (dry, concrete-like desert hard pack mostly, not groomed dirt hard pack) with lots of deep rutted two-track, loose rocks, short flat landing jumps, flat turns, couple of sandy sections, but no whoops at all.  I'm at 14 clicks out (of 24) on compression, and 10 out on rebound.  It bottoms with a sharp hit off a big prairie dog mound at higher speeds (4th-5th gear) or if I slap the front end down off even a small jump (as opposed to landing front wheel slightly first or both wheels at the same time), but does very well in tracking through and avoiding climbing out of the ruts and allowing me move through the rough, uneven stuff without upsetting the chassis at all (which my Scotts stabilizer helps immensely with).

I'm going out there again after work this evening to keep working the clickers and taking laps around the whole place (10 mile loop) with each setting.

Also, as I alluded to in the original post, I'm going to build up the stock WR forks and shock with the Smart Performance Phase 4 kit, and then I plan to dig into these SSS forks and try some different stacks until I find something that works well for me for what I'm riding and hang onto them for awhile as either a backup or for different conditions.

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If your bottoming at 14 at least try going in to say 6 clicks to see how it feels

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The original fork configuration is that it has an inner chamber which is bled free of air, an outer chamber with a nominal 350cc of oil, and a third chamber that is only air above the free piston.  By drilling the FP, you convert it to a two chamber fork by linking the ICS volume with the outer fork chamber.  On paper, that would seem to indicate the need for slightly more oil in the outers, as you will appear to have reduced the amount to which the ICS air is compressed, and you will have added that much to the outer air volume.  The thing is that the free piston only moves about 1.2", so the ICS air doesn't a very high "compression ratio" as originally built unless there's a lot of oil on top of it.  With the piston originally being plastic, I doubt anyone thought there would be a big pressure differential from the ICS to the outer chamber, or the walls of it would have been more robustly made.  I'm not so sure that the net effect of drilling them doesn't come closer to zero change in internal fork pressure than you might think.

 

In the end, I agree with Harperf: lower the oil to reduce the effect of the progressivity of the air spring, and rework the damping/spring rates to make the other problems go away. 

 

Another matter with this fork in general is that I find them to work better with more ICS preload than a lot of them come with, and I mostly use the SPI "EPNP" spring kits for them.

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