Nitrogen Charge effect on pre-load

[MXTex]

I just finshed servicing the shock on my 98KX250. Previously, I had been running a 150psi Nitrogen charge. But this time, per a recommendation from Jer a while back; I brought the Nitrogen charge up to 175psi. (This is a MX-Tech valved shock)

With the 175psi charge in lieu of 150psi, the spring needed much less pre-load on it to net 4" of race sag. (maybe 4 or 5 turns of the ring adjuster) Is this normal for a 25psi difference to make this kind of difference? With the shock charged at 175psi with no spring on it, you could push it through it's stroke with significant effort. More than at 150psi.

Shocknut - Marcus,
I'd love to hear your comments among others.


Thanks in advance.

 

[Jeremy Wilkey]

MX-TEX,

Force can be calculated out its the ratio of areas.. 16mm vs 46mm dived it out and it is a direct substituion for spring, or preload..

Regards,
Jer

 

[marcusgunby]

jer makes a logical point but i dont think if we did the calculations that it changes the preload that much-4 to 5 turns of the spring is a huge amount.I also now charge shocks to 175psi and havnt needed to alter the sag hardly at all.

 

[Papakeith]

Maybe this is just me being dense, but why would changing the pressure in the shock affect the spring setting?
Wait, never mind :o. It's the volume of the oil that is allowed to compress the air in the bladder isn't it. With the added pressure it takes more force for the piston to push the oil through the compession valve.
Am I even close?
Either way I don't see how it would affect preload. When sitting still, ie. setting the sag, the piston isn't forceing oil into the resevior. So I don't see how the added pressure could make a difference in that respect.
I can see where it would make a difference under load now though.

 

[MXTex]

Maybe my pre-load wasn't set correctly at 150psi. It's been a long time since I set it and I have lost some weight. Does it make sense though that it is harder to push the piston through the body at 175psi?

 

[Papakeith]

1st, don't confuse me for someone who actually has knowledge about suspensions.
2nd - with the disclaimer officially submitted:

Does it make sense though that it is harder to push the piston through the body at 175psi?

To me?. . . not really. I wouldn't think that the added pressure would affect the fluid flow past the valve stack in the piston when compessing the shock by hand.

 

[Jeremy Wilkey]

Originally posted by marcusgunby
jer makes a logical point but i dont think if we did the calculations that it changes the preload that much-4 to 5 turns of the spring is a huge amount.

Guys nothing logical about it.. ITs a fact.. Now if you want to get serious you would allso calculate in the pressure rise.. As nitrogen volume (not gas volume but space) is reduced..


PapaKeith, the piston cares less wether its in 200 or 1000 PSI... Oil Techincally oil gets hard over say 450 PSI and that impacts damping, but to limit the amount of cavitaion 145-200 should suffice and give a decent ride character in our current aplications..

The rod is the only thing that notcies a diference in pressure.. ITs a simple hydralic relationship of areas.. Cut and dried. Now Marcus, your right in terms in terms of that was lame of me for a answer... I was snikering to myself late last night that maybe Tex had over filled his shock and had a very very low Nitrogen volume, that was exponetianlly rising when he got on the bike and compressed the small nitrogen voulme by 30%.... I know Tex and was contimplating e-mailing him today to check oout some stuff. (as a way to avoid embrassement) Hopefulkly iyts just a spring sack / chain postion deal..

Regards,
Jer

 

[Jeremy Wilkey]

Guys,

Just read my string.. I don't want to change anything, but after being away for last 6 weeks, I'm struggling to find my pace.. Sorry no rudness intended... I'll get back in the zone.. This DRN stuff takes focus.. And this is why I love this place so much.. IT really makes you stay sharp.. And I'm on it like butter knife to a crispy peice of french bread.. :think:


JEr

 

[MXTex]

Update. Jer and I spoke this morning and figured out that my servicing technique resulted an oil over-filled shock. What I did wrong was not equalizing the bladder to atmospheric during the final bleed. I did release all the pressure before opening up the bleeder; but the key mistake was not keeping the schroeder open while the bleeder was out. So, per Jer's instruction, I performed the final bleed again. This time I 1st moved the shock through 5 complete strokes then let sit upright for 3 minutes. Then I released the 175psi charge and removed the schroeder valve. Next I removed the bleeder (keeping the shock fully extended) to find that quite a bit of fluid came out with bubbles. Finally, I recharged up to Jer's updated Nitrogen charge spec; 160psi. Should be set to go. Thanks a million for all the help.

Jer, I know your intentions and would never be offended by your responses. Thanks for the thought though. :)

Jim