Doesn't ics spring length change piston height and purge?

Wouldn't a shorter or longer ics spring change the position of the floating piston in the chamber?  This in turn would alter the purge of oil as you are riding correct? The problem is my tuner put 107mm long 1.6kg ics springs in my cartridges that originally had 90mm plus 15mm spacer springs that were 2.0kg.  Im thinking the 2mm spring length increase causes the piston to have to travel 2mm farther up to the taper where they purge which with 8:1 rod ratio is 16mm farther of rod travel before my forks purge excess oil.   Problem I'm having is the forks work great when the carts are purged by hand and the rod just barely returns to full extension.  But after and hour of riding the carts add oil through the seal head and do eventually stabilize to where they start purging the oil as they should at the free piston, but the carts at this point extend with force and are harsh.  Ideally when I pull the carts out after riding the rod should extend the same as when I purged them and assembled the forks.  I do bottom the forks while riding but I don't think the rod goes far enough in the cartridge to purge oil until the piston is pushed up farther by added oil through the seal, at this point they purge and stay at this level but the rod has significant nose pressure and the plushness is gone.

I know with showa or kyb forks, I assume yours are WP closed cartridge?, a longer spring should keep the piston further down and I guess cause it to not purge as soon or as easily, same as a stiffer spring would cause it to not purge as soon. So maybe it's the stiffer ics spring causing your problem?   

With my showa sff fork I have the opposite problem where I have more rod pressure in the cartridge after doing the oil change procedure correctly and the extends fully. But then after I ride it loses pressure, purges out more oil, and the forks feel softer then and when I pull the cartridge out of the fork the damper Rod doesn't fully extend on its own it still has a little pressure but not as much pressure as it has when I'm done doing the oil change and pumping to full stroke a few times to purge the excess oil and air. I like the feel better when I first ride it before more oil ends up getting purged out. I talked to a suspension company that said this is how that tac air forks work and it's normal, to help soften up the initial movement of the stiff air fork at the top of the stroke. I converted to spring fork conversion though. 

24 minutes ago, tom02cr250 said:

I assume yours are WP closed cartridge

From this recent thread, the OP is having this issue with an '08 YZ250.

Just some guessing here but I would think no matter the small difference in spring length,

the oil displaced from the rods entering the cartridge remains the same,

meaning it will fill the same volume behind the base valve and always move the ICS piston the same distance, no matter the spring rate or length.

Edited by mlatour

Yes it they are 2008 yz250 cartridges but fitted to WP cc fork outers.  They are the best forks I have ridden the first hour or so but get harsh.  The longer ics spring causes the piston to start lower in chamber and when the rod compresses the piston needs to travel farther to get to the taper where it would purge.  Spring rate does not matter as the volume of oil displaced determines piston travel.  I'm pretty sure my thinking is correct but maybe I'm wrong.  If I'm right a shorter ics spring will help if not solve my issue

I'm not familiar with those custom assemblies, KYB inside / WP outside.

 

Is there a chance the internals and externals don't bottom simultaneously,

meaning by time the outers / springs bottom out, the cartridge rod still has some rod travel left ?

 

If that's the case, would it mean that no matter if you bottomed the suspension regularly (as Mog mentionned earlier)

since the cartridge does not fully cycle, it would never travel far enough to self purge ?

 

Again, not familiar but throwing possibilities out there.

 

 

2 hours ago, 2000ktm200exc said:

Yes it they are 2008 yz250 cartridges but fitted to WP cc fork outers.  They are the best forks I have ridden the first hour or so but get harsh.  The longer ics spring causes the piston to start lower in chamber and when the rod compresses the piston needs to travel farther to get to the taper where it would purge.  Spring rate does not matter as the volume of oil displaced determines piston travel.  I'm pretty sure my thinking is correct but maybe I'm wrong.  If I'm right a shorter ics spring will help if not solve my issue

the longer spring does not start the piston lower in the chamber. the piston always starts and finishes in the same position. a longer spring gives more preload on the piston which can add to small bump compliance (stiffer). you have a longer more preloaded spring which now gives a very slightly harsher low speed compliance but the spring is softer which adds to cavitation as speeds go up. your fork is going harsh as you ride. it is pumping up and needs to purge on a regular basis. the sss forks pump fluid to the internal cartridge at a very rapid rate when used  offroad. they need to bottom on a regular basis, like every lap of a course.  your set up is probably too stiff. I bet your main spring rate is way too stiff. do you have an oring on your fork to see that you bottom?

I think I understand now, the ics spring length has nothing to do with piston position once the cart is final purged.  I believe the guy that fit the SSS carts into my forks wanted the dampening rod to full extend instead of stopping short like they would with negative spring preload.    I would probably be better off with shorter ics springs and a higher rate like the stock 2.0kg ones.  My current main springs are 4.6N preloaded 5mm and of course the  1.6kg  ics springs 300cc oil in outers.  I am 5'10" 195lbs without gear.  Vet A rider.  Maybe I could try a set of 4.4N  

take some compression clicks out to soften it up. add and oring or a zip tie to see if you actually bottom on your course.  offroad racing?

Yes, actually a mix of single track and motocross but more singletrack than anything.  Already at 14 out on compression and I don't think I should go any lower than 300cc of oil.  It must be the spring rate, it's pretty hard to bottom forks on miles of single track until there is an obstacle I can hit hard enough or jump off of.

The purge happens when the piston travels to the correct distance based on volume displaced by rod , with a softer ICS even 2mm longer it should purge easier , are your pistons drilled ?it won't make any big difference but might allow the pistons to purge easier

Sent from my XT1650 using ThumperTalk mobile app

1)  The piston is shouldered in place and kept from moving down too far in the cartridge, but it does float on the shaft when bled right.

2)  The piston height is set absolutely by the height of the purge taper.  Once that purges, the piston will move down by the volume of the rod, regardless of the spring force or spring length.

Can the spring be a little preloaded
And if so? Can i just do one spacer? And how long?

Im intrested becose i have the same fork and i want the hydralic
To come on soner!
And not have the first 5-10cm so soft?

Sent from my D5803 using ThumperTalk mobile app

You can yes

winning by 4:1 lol

how much freeplay do the ics spring need? or can it have 0?

or can you even preload it?

I would not preload them

winning by 4:1 lol

Ok but can i have a longer spacer then? To take som slack away?

Sent from my D5803 using ThumperTalk mobile app

Sure and others have preloaded them , I just wouldn't

winning by 4:1 lol

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