TheOtherEric

I've heard a number of different recommendations for nitrogen pressure in Moton and JRZ shocks, so I'm trying to understand this. Can anyone explain how changing the N2 pressure will affect the damping rates? Do higher pressures simply move the damping curves in proportion to the pressure? Or perhaps it changes the shape of the curves.

I've heard people say "it acts like a spring" but that makes little sense because shocks aren't springs! They don't store energy as far as I know.

p.s. I could use some good recommendations on N2 psi. Moton suggests 185 F/R and I've heard go lower and also go higher.

LVDell

Actually, you ARE changing the effective spring force by changing the pressure. For every 10psi of pressure you are changing the spring rate ~6# of force on the 22mm shafts and ~2.5# on the 14mm shafts. Essentially it allows you to run a softer spring and then dial in what you would have obtained with a stiffer spring and no pressure. It's not meant to substitute spring rate as there is a defined range of pressure you can run on the cans but you can more easily hit your target from track to track depending on spring needs. Easier to dial in more spring rate with pressure than it is to remove spring rate as you will hit the floor more quickly.

Bill Verburg

Gas shocks do store energy, any compressed gas is just like a steel spring in this regard.

Geoffrey

Not all remote reservior dampers that have nitrogen gas are designed so the nitrogen acts as a spring force. It happens that the JRZ and Moton dampers work that way with 9.8lbs per bar of pressure. For a 911 it is advantageous to have more pressure in the rear than the front. I tend to run 125/175, but the reality is that I'm not so sure it makes all that much difference given all the other components that can be adjusted. If for instance you thought that you might want to try more spring rate in the front of the car, you could increase the pressure in the front dampers and it would give you an idea if that was the right direction.

LVDell

On the Moton's (22mm shafts) it's 8.6# per 1 bar.

Geoffrey

Yep, you are right, the documentation I have says 8.3lbs per bar. Sorry for the missinformation.

Bryan Watts

Moton/JRZ style dampers as opposed to Ohlins/Penske style dampers.

onefastviking

Start with Moton recommended pressure and adjust as needed from there. How much will vary depending on car set up, coil over springs used, driver preference. Don't go below min. or max recommended from Moton.
It does act as spring pressure as already explained here.

TheOtherEric

Thanks guys for setting me straight. Seems to me that the "spring" effect of varying the N2 pressure is pretty small. If I understand correctly, dropping the fronts say 50 psi means around 30 lb/in difference. That's not much.
Does anyone know what happens to the damping curves when you change pressure?
Funny, that's exactly what my shock rebuilder suggested.

onefastviking

I really should have said start with whatever your shock manufacturer recommends, I am not sure if you have Motons or not.

And while that difference seems small it does help a lot to fine tune the car. Try it yourself on a test n tune day. take out a little or add a little depending on different needs and see how it helps or hurts the car. You will understand more after trying it for yourself.

TheOtherEric

But is the difference in handling due to the increased stiffness effect or due to shifting the damping curves? I'm assuming that N2 pressure would shift the damping curves, but I have no idea if that's true.

Geoffrey

Well, that is only part of what happens. When you change the nitrogen pressure, you are also changing the corner weight on the car. For instance, on my car, releasing the nitrogen from one shock makes a dramatic change in the corner weight of the vehicle. Changing it front to rear or changing the ratio front to rear will change the corner weights as well. This will have an effect on the cross weights and uneven cross weights will cause the car to understeer in one direction and oversteer in the other relative to a 50/50 cross weight.

The pressure in the canister is mainly to keep the shock oil consistent and keep it from foaming. It is the internal valving that changes the dampening curve.

LVDell

Well put Geoffrey

AudiOn19s

Old Thread bump, After a quick conversation with my shock guy the other night I'd like to dive into this a little deeper.

I understand changing pressures can effect spring rate. I'm not really so interested in changing one corner or another on the car. I'm looking at effect of changing pressures on all 4 corners at the same time, how it effects shock performance and how the car acts.

Here's the scenerio in more detail. I've always run my JRZ's at 200psi front and 225psi rear canister pressures. This is how the previous owner ran them, the car is setup very well, it's stiff but not jarring. (3000lbs car, 600lb front springs 750lb rear springs). My 200 and 225 pressures are within what JRZ recommends for canister pressures.

My shock guy suggested I'm running too much pressure in the shocks and that they are soo stiff that I'm not letting the shocks effectively do their job. He states in turn that I"m transfering that work down stream and putting way too much load on the tires from running the car overly stiff. He suggests dropping canister pressures 50 lbs both front and rear, letting the shocks do more of the work they're supposed to be doing and increasing compression a click or two if I find the car to be a little soft with 50lbs less pressure in the cans.

I do have data now so that I can try to better quantify the results of doing this, but I don't have a N2 rig to change them on the fly same day at the track to compare back to back so who knows how good the data will really be.

Thoughts on his comments? Does that make sense that I'm possibly not letting the shocks do their job effectively by running them with too much canister pressure?

Andy

Matt Romanowski

The change in spring rate is because of the volume that the shock shaft takes up when it is inserted further into the shock body in compression. While it has an affect, the overall change in dampening is in the high speed and low speed valving (track bumps and chassis changes). If we go by what everyone says that the difference is 8.6 lbs of spring rate per bar, we are only talking about a spring rate change of ~25 lbs.

Is the car stiff under track bumps or under braking/corner (chassis movements)? I would guess you have more of a valving issue than a cannister pressure issue.