fatbillybob

Gents,

What happens when you have nitrogen filled remote reserviors for adjustable shocks and you start at 100psi then add 50psi then another 50psi. Does that just increase the "felt" spring rate? Is there a relationship to how much the rate increases per psi? In a double adjustable you got rebound and compression control. The canister pressure controls what? And does it effect the results of the compression and rebound settings?

Thanks!

Larry Herman

Every shock brand is going to work a little differently due to the characteristics of the internal valving, but overall as you increase the cannister pressure you will add to the spring rate a little, increase the amount of compression damping force and reduce the amount of rebound damping force.

ProCoach

FBB, the Penske and other folks will tell you that the canister pressure has a profound effect on the ability of the fluid to keep from becoming aerated.

In the years that I've been working with these things, no one has suggested lower canister pressures than about 100-125 psi and most suggest between 150-225 psi.

The canister pressure is not the controlling factor in valving. The shim pack is primary, with the external damping djustments being secondary. I have not been able to quantify in shock shaft motion histograms a 50 psi difference in canister pressures.

The least bs, simplest and easiest to understand advice I've heard since Joe Stimola passed away is from Anthony at Anze Suspension in NY. Call him.

Larry Herman

Could be due to the type shock. For what ever reason, I felt a profound difference in 50 lbs on my Motons, and what I stated was taken directly from a conversation about that with Lex Carlson.

Geoffrey

To expand on what Larry mentions, in a JRZ or Moton damper, 1 bar of canister pressure is equal to 8.33psi of lifting force (spring rate). So says the manufacturer.

TheOtherEric

I assume that's a typo since neither force nor spring rate is measured in psi.

Veloce Raptor

Like Larry, I have experimented with differing pressures on my Motons. I noticed them.

Land Jet

You may want to read "Vehicle Dynamics and Damping" by Jan Zuijdijk, the founder of JRZ's. He covers all aspects of dampers including cannister pressures.

va122

what do you think is better and advantages to both? Low pressure/high spring rate or vice versa?

Larry Herman

Victor, as Geoffrey illuminated, the increase in spring rate is small, and only for tuning. 100 psi increase in cannister pressure will yield about 55 lbs/in increase in spring rate. IMHO, run the cannisters at a middle pressure (140-180 lbs) and pick the right springs.

fatbillybob

Thank you Gentlemen. As always time to experiment.

va122

Ok makes sense. Thanks.

PedroNole

I would agree with this and would offer the following relative to spec cars (GTC PCA classes or GT3 Challenge spec cars):

The later 996 and 997 based Cups run very stiff spring rates...1300/1500lb springs. The can pressures have really very little effect on the overall tuning that you could otherwise get through say a 200lb difference in spring rates. In fact at Sebring,
where you would really like to be able to run significantly softer spring rates than say Watkins Glen even, I don't think you can reduce the can pressures significantly enough to have a noticeable effect without probably compromising the overall efficiency of the dampers.

chris walrod

There is a minimum amount of can-pressure needed to avoid cavitation. That said, there has been instances where LS compression damping was reduced by increasing can-pressures. This is a case where can-pressures were too low and allowing LS fluid surge and not forcing fluid flow through the piston/shim stack. A complete opposite reaction.

Cory M

As another data point, Joe Stimola told me that a 50# increase in can pressure added about 15 lb/in to the spring rate (Motons).