Coilover Shock Nitrogen Pressure Question

[SLOWPOKE693]

Guys, im trying to clear something up and need the help of the shock experts here.

Is changing the nitrogen pressure in the shock reservoir a way of tuning the shock? As an example: Fox factory recommended setting on this particular shock is 200psi. Shocks get sent out to a tuner for custom valving and DSC adjusters. Customer gets the shocks back, charges them to 200psi and installed them. Car handles great except for the ass end bucking over large jumps. Customer calls the shock tuner, explains the situation and is told to drop the rear nitrogen pressure down to 100psi by the tuner. Does this sound right? I was always under the impression that the nitrogen pressure was to keep the shock oil from aeration and cavitation, am I way wrong?

Beat95yj mobil1syn @bkw1.

 

[ThePanzerFuhrer]

Changing pressure will change the dynamics of the shock. However with what it’s doing the shock builder took out too much on the rebound side.

 

[SLOWPOKE693]

Changing pressure will change the dynamics of the shock. However with what it’s doing the shock builder took out too much on the rebound side.

I understand that.

My question; Is it a bandaid for improper tuning or perfectly acceptable to lower the nitrogen pressure to below the factory spec in the offroad world to change the dynamics of the shock?

Wouldn't a lower nitrogen pressure on a dedicated off-road race rig cause aeration and shock fade during a race vs the recommended pressure and proper tuning?

 

[jeepkevin]

I race, I agree 200 psi is some what of a guideline. I like stay stay right at that number and change internals. Dropping to 100 does not seam right to me. If you overcome nitrogen charge then the shock is bypassing everything internal that is is supposed to do. At that point the shock is inefficient. Time for revalve.

 

[YotaAtieToo]

I really don't know much about this, but do you think maybe the tuner was just trying to get an idea of what needed to be done? Ie: customer said 100 psi works perfectly, so we need to use xxx shims to fix it. Or 100 psi and it didn't fix the problem so we'll need xxxx shim.

 

[snivilous]

Is it a part of tuning? Yes. Is it the way to fix that issue? Highly unlikely. Tweaking nitrogen pressure is the fine tuning level, if the ass is bucking around that's more than just tweaking the charge pressure. My guess is dude thinks (or hoping the easy way out) is to lower the pressure and that decreasing that slight push of the shock extending will stop it kicking up so much. Maybe he's hoping that will change it enough that he won't have to revalve it, but as anyone who has compressed a shock by hand knows (especially on a single shock rig) that's a minimal force. I've had some instances where I had to adjust nitrogen pressure because the combined pressure from the 8 shocks on my 4500 kept fucking with where ride height would settle to, but it wasn't trying to solve a valving issue which that is (imo).

 

[mobil1syn]

NO NO NO NO ... if the tuner tells you to drop the pressure on a reservoired shock, they dont know what they are doing and need to be fired.

shock pressure should remain at 200psi for 99.99% of people. i can think of 3 reasons to adjust the pressure, but its never less than 200 psi on 2 of them.

 

[bdkw1]

If lowering pressure changes your tune, you are getting cavitation and need more pressure. Pressure on anything other than an airshock should not be used for tuning. Lack of pressure is the biggest issue. For a resi out the top shock like a coil over, 200 is the minimum. If you have a heavy vehicle with a lot of valving it only goes up from there.

 

[HYDRODYNAMIC]

A long time ago, King told me to do the same thing, lower pressure to reduce harshness. What it does is, when you hit a bump the piston moves and instead of the oil going through the piston and valves and doing work, the piston pushes the oil into the reservoir and compresses the air pocket for a short period of time and the oil eventually works its way through the piston. It is a buffer in a sense. The issue is that as the air on one side is compressed, a vacuum is pulled on the backside of the piston and an air pocket is formed only to be slammed closed as the piston reverses direction. As the vacuum pocket is slammed closed, the rushing in oil is also slamming into the metal and causing damage from impact.

 

[westracing01]

I've been wondering this myself for a while. On an offroad rig, I always thought it was to prevent cavitation and fade due to heat, however on the oval track dirt race car, shock pressures are adjusted to change the timing of what the shock is doing.

It's not the way I would go about solving the issue, but it does work. In the OP's application, I'd be revalving the shock.

Did you try messing the with the DSC adjusters to try to dial it in a little?

 

[SLOWPOKE693]

I've been wondering this myself for a while. On an offroad rig, I always thought it was to prevent cavitation and fade due to heat, however on the oval track dirt race car, shock pressures are adjusted to change the timing of what the shock is doing.

It's not the way I would go about solving the issue, but it does work. In the OP's application, I'd be revalving the shock.

Did you try messing the with the DSC adjusters to try to dial it in a little?

I'm familiar with adjusting pressures on circle track shocks to change the dynamics of them, but like you, had never heard of this practice in the off road world.

Not my shocks, asking for a friend that was told to do this by his tuner. I thought it was jankey and we discussed it, but wanted to make sure we were in the ballpark before he said anything to his tuner.

 

[bdkw1]

A long time ago, King told me to do the same thing, lower pressure to reduce harshness. What it does is, when you hit a bump the piston moves and instead of the oil going through the piston and valves and doing work, the piston pushes the oil into the reservoir and compresses the air pocket for a short period of time and the oil eventually works its way through the piston. It is a buffer in a sense. The issue is that as the air on one side is compressed, a vacuum is pulled on the backside of the piston and an air pocket is formed only to be slammed closed as the piston reverses direction. As the vacuum pocket is slammed closed, the rushing in oil is also slamming into the metal and causing damage from impact.

You just described cavitation. This should be avoided at all costs. When you loose pressure on the back side of the piston, the boiling point of the oil goes way, way down. The "air pocket" as you call it is the oil boiling. This will kill your oil in short order. I have serious doubts anyone at King would recommend this. If one of their employee's did, they should be fired.

 

[HYDRODYNAMIC]

You just described cavitation. This should be avoided at all costs. When you loose pressure on the back side of the piston, the boiling point of the oil goes way, way down. The "air pocket" as you call it is the oil boiling. This will kill your oil in short order. I have serious doubts anyone at King would recommend this. If one of their employee's did, they should be fired.

Hopefully it was explanatory enough for those who are not used to how cavitation works. You call it boiling, I call it cavitation since I come from the hydraulic world. It’s a phase change either way due to sudden localized pressure change or due to temp change. Both are combined factors. The higher the oil temp the easier it is for the oil to cavitate near the piston.
It was around 2004 when King told me to drop the pressure on the reservoirs, I believe they said to drop to 100psi. They were 2.5”x18” rear shocks on a F150 setup for desert but I could feel the paint on the road they were so firm.

 

[Beat95yj]

Typically I like a minimum of 200psi. The exception is when running minimal compression. Tune with the discs/shims. Running low nitrogen pressure prevents compression valving from working effectively. The discs will push the column of oil and not dampen. It will feel completely soft. Also the piston cavitate and it will probably pull nitrogen into the oil. Turning your shock into a milkshake even though it has a divider/floating piston.

the dsc adjuster complicates things a bit. If it is really stiff in compression it can force oil through the discs to a point. In a worst-case scenario you would still have some damn thing because of the piston rod displacement working against the adjuster. But you would go from a 2 inch piston to a 7/8’s.

everything I have just said applies when you have a top mount reservoir hose. If you have a low mount reservoir hose, the problems would be in rebound and not as likely because you don’t see the shaft philosophies and rebound that you see in compression.

Basically when people do this they turn really expensive shocks in the crappy 1970s twin tubes. I’d rather run heckathorns then not have enough nitrogen.

 

[Beat95yj]

I understand that.

My question; Is it a bandaid for improper tuning or perfectly acceptable to lower the nitrogen pressure to below the factory spec in the offroad world to change the dynamics of the shock?

Wouldn't a lower nitrogen pressure on a dedicated off-road race rig cause aeration and shock fade during a race vs the recommended pressure and proper tuning?

Shitass bandaid

 

[Dougal]

Gas pressure does two things. Firstly it stops the shock cavitating on faster hits (not always necessary depending on the valving setup. Secondly it keeps positive pressure on the seals to prevent the shock sucking in air via the main shaft seal.

The more gas pressure you run the more friction you add as it loads the main shaft seals more. As long as you're in a window of pressure where you don't cavitate or suck air and don't have excess shaft friciton. You're good. It doesn't affect damping at all.
There is a spring preload effect also. It doesn't affect spring-rate but it does affect ride height.

On MTB shocks 200psi is fine for most coilovers. But lockout shocks we have to use up to 500psi.