KYB SSS Fork - Can't get inner chamber rod to extend fully

[RMK800]

I'm changing the oil on some 2006 SSS forks.  I did the free mod on the piston with the holes and then filled it up with oil to the recommended height.  I cannot get the inner chamber rod (whatever you call it) to extend fully.  When I pump it down it won't extend all the way, it goes to about an inch left.  I have tried this three times thinking that I have air in there and made sure to pump the rod to get the air out.  I don't know what is going on.  I didn't have any problems on the other one.   I'm frustrated.

 

Thanks for any help.

Edited September 23, 2014 by RMK800

[cj_wai]

I'm changing the oil on some 2006 SSS forks.  I did the free mod on the piston with the holes and then filled it up with oil to the recommended height.  I cannot get the inner chamber rod (whatever you call it) to extend fully.  When I pump it down it won't extend all the way, it goes to about an inch left.  I have tried this three times thinking that I have air in there and made sure to pump the rod to get the air out.  I don't know what is going on.  I didn't have any problems on the other one.   I'm frustrated.

 

Thanks for any help.

I,m pretty sure 1" left is ok.

 

post this over in the suspension forum..the bright guys will help out right away

[RMK800]

I,m pretty sure 1" left is ok.

 

post this over in the suspension forum..the bright guys will help out right away

I'll try that, but when I set the inner chamber down with the rod on cement. The rod compresses about an 1 1/2 inches by itself.  My other fork doesn't do it at all. 

[RMK800]

After thinking about this.   The issue is probably the valve is leaking air.   I'm thinking I need to replace all the o-rings?  

[grayracer513]

Is this only on one of the damper cartridges?

 

Ironically, what it can actually mean is that the one that does extend fully isn't entirely free of air.  Some combinations of the inner chamber pressure springs (ICS springs; those that sit over the free piston) will not force the rod all the way back once the oil has lifted the piston to the bleed port.

 

Review the process in the link below, and if you are still convinced there's a problem, replace the free piston shaft seal and O-rings.

 

https://www.thumpertalk.com/topic/737910-smart-performance-inner-cartridge-bleed-method/page-2#entry7613090

 

Also read: https://www.thumpertalk.com/topic/680824-base-bolt-jam-nut-clearance-06-yz-450-fork/#entry6860151

[RMK800]

Thanks I'll look at that. It's conflicting information, the service manual says fully compress when bleeding air and oil and the guys in the suspension forum says KYB tells them that you should only compress to about 3 inches from the bottom If I do that on both forks, the action is smooth on both of them and extend fully. You do have me thinking about the fork that can compress all the way without issue.

Edited September 24, 2014 by RMK800

[grayracer513]

The cartridge should be compressed all the way until it stops on the lock nut.   If you don't do this, the result will be an overfilled cartridge.  That will not harm the cartridge or its operation in any way, but what will happen is that the first time the fork runs the full length of its compression travel (bottoms out), the excess oil will be exhausted into the outer chamber, raising the oil level there to some point beyond where you set it.   This could be significant in terms of oil level, or a bigger problem if using two different weights of oil in the inners and outers.

 

The fork is designed to do this because bottoming also puts a high level of pressure against the rod seal, which is biased toward keeping oil in, not keeping it out, and that will force outer chamber oil into the inner chamber.  The design allows the fork to blow off the excess so that it can't hydro lock.

[BGoyins]

When a fork bottoms what is it bottoming against? How much of the cartridge rod is still extended when the fork bottoms? I'm guessing that's where the KYB recommended 3 inches comes from.

[grayracer513]

The 2006 and later SSS fork that's still being used bottoms against oil.  If there is no oil, it bottoms metal to metal.  No bumpers.  Cast into the bottom of each axle lug (the part the inner tube screws into) is a cylindrical extension about 1.5 -2" tall, centered right down the tube axis, and with a machined bore that is only a small amount larger than the oil lock collars on the ends of the cartridges.  The oil locks are those aluminum collars at the bottom of the cartridge, held on by thin retaining rings.  They can be replaced and resized for either tuning or repairs, BTW.

 

As the fork compresses, the cartridge runs deeper into the outer chamber oil, displacing it around the nose of the damper.  When the cartridge nose is driven into the last 1.5" (more or less) of travel, it enters the oil lock cylinder and suddenly has to move oil out of the way through a much tighter clearance gap.  Most of the locks have tapered noses to soften the impact, and the shape can vary (or be varied).  When it's set up correctly as to clearance and oil weight, the metal parts never come into contact, because the fork rebounds before the oil has time to be completely emptied from the oil lock.

 

All of that effectively turns the end of the cartridge, including the seal, into a piston crown.  The pressure that results can be extremely high for a moment.

[BGoyins]

Ahhhh! That explains a lot. I wondered what those collars were for and now that makes perfect sense. So I would have to agree that 3 inches is probably too much. And yes, the pressure must be super high for a bit. Thanks for explanation!

[grayracer513]

If you want to know exactly how far the rod runs up in yours, you can slip an O-ring or a small rubber band around the rod, and assemble it without the main spring.  Compress it all the way once, pull it apart, and you'll have your answer. 

[RMK800]

This is all good stuff. I did ask this question in the suspension forum. Ya I know that bad to post two places. But here is a response from someone over there.

This is perfectly normal for a stock KYB ICS setup. There is freeplay in the ICS spring (negative preload). When the cartridge is assembled, and short cycled (not bled) the rod will extend because the trapped air is an additional volume that is compressing and also taking this freeplay out of the ICS system. Once the cartridge is bled properly, the rod wil not extend because it can only move as far as the ICS spring will move the piston X the area ratio of the piston/rod (approx 9). In other words the small freeplay in the ICS system causes a larger freeplay in the damper rod. If you shim the ICS spring to remove this freeplay, the rod will then extend fully. You can calculate this and verify it by looking how far the piston moves when you manually extend the rod fully after the spring is relaxed. Should be around 2.5 - 3 mm.

So is he taking about a different fork? Should the shaft extend fully all the way out on its own? Does your Grey extend out all the way on its own? I know you probably answer that above but just want to confirm.

Also it was mentioned you can shim by the piston to extend fully.

Thanks!

[grayracer513]

Mine does, but I don't have stock ICS springs.  I'm running the EPNP (early pressure non-progressive) spring set from SMART Performance, which instead of free play has pre-load.  That means the piston will travel the entire length of the guide rod as the cartridge extends.  With OEM ICS springs, there are a number of variations year-to-year, some of which have quite a bit of free play, and some of which have a slight preload.  The ones with slack in the assembly will act as described above.  Those with less will typically extend at least almost all the way out. 

[RMK800]

Ok, can you comment from a performance perspective with riding the difference with the shaft fully extended to where one is a few inches short. What difference would we feel?

[grayracer513]

Having the springs preloaded applies pressure to the oil in the damper from the first millimeter of travel, which makes the valving more responsive in the first 2".  The advantage of the EPNP spring set is that while the preload and early pressure is greater, the rate is lower (Yamaha ICS springs run at around 20N/mm while the EPNP sets are between 10N and 14N.  This reduces the pressure on the oil when the fork is deep in the stroke, which in turn makes the fork less harsh when deeply compressed.  Without any pressure over the oil column, the valving tends to "lag" a bit high in the stroke.

 

I should mention, before anyone gets any ideas about shimming up their existing ICS springs to preload them, that some of the springs used will coil bind before full compression is reached, and that is quite unhealthy (the fork would hydro lock at that point and something, usually the free piston, would necessarily break (even if it's drilled).  Besides, you'd only make the deep stroke harshness worse.