Spring vs Nitrogen To Pressurize Shock

[spdingtkts]

Good evening.

So you have a shock with a floating piston in a chamber charged with nitrogen used to pressurize the fluid.

What are the advantages/disadvantages to using a spring behind the piston in place of the nitrogen?

 

[patprimmer]

There are no advantages. All functions of spring are best handled by the actual springs.

The disadvantage is that without the pressure from the nitrogen you no longer get the raised boiling point in the oil so your shocks may see loss of power due to cavitation as the liquid passes through the valves at high speed.

Regards
Pat
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[murpia]

Pat,

Does a nitrogen charged damper have a static fluid pressure at full extension? If so how does that work?

I had assumed that the floating piston would bottom out against a stop at full damper extension, thus the fluid pressure would take a 'step' from close to atmosphere to the charge pressure as soon as a small amount of shaft enters the damper. Now I think about it, that doesn't sound right, so what's the truth? Is the floating piston kept off any internal stop by some excess fluid content during build?

If so, a spring would work too, but would need a hefty pre-load and therefore in practice gas would win out (as it indeed does).

Thanks, Ian

 

[GregLocock]

It certainly does, you get a thrust equal to the nitrogen pressure * shock rod area. It's about 200N. There's also a gas spring rate but it is pretty small.

Cheers

Greg Locock


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[patprimmer]

I never looked inside a gas shock. I am just relaying what I understand as their principle of operation. The static pressure pushing on the shaft is I believe a consequence, not a design objective.

This touches on it without detail

http://en.wikipedia.org/wiki/Shock_absorber

This explains anti cavitation by the static pressure principle but does not explain the load that applies to the shaft to push it to full extension.

This explains it as the pressure is equal both above and over the piston, but the projected area of the piston is reduced by the cross sectional area of the shaft on the shaft side.

http://www.monroe.com/support/Technical-Training/Shock-Absorbers

Regards
Pat
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[murpia]

Here's an attempt at an ASCII art sketch of what I mean:
[tt]
--------------------
| [tab][tab][tab][tab] |
| [tab][tab][tab][tab] | <- Gas Pressure in here
| [tab][tab][tab][tab] |
| [tab][tab][tab][tab] |
|######Piston######| <- there is a step here that the piston bottoms out on
.|[tab][tab][tab][tab]|
.|[tab][tab][tab][tab]| <- Fluid Pressure in here < Gas Pressure
.|[tab][tab][tab][tab]|
.-------- ---------
[tab][tab]| |
[tab][tab]| | <- Hose to damper body)
[tab][tab]| |
[/tt]



If the piston is bottomed out on the step, the fluid pressure must be < the gas pressure.

So, the damper must be built with the right amount of fluid to keep the piston off the stop, before the gas pressure is charged, if you want the fluid pressure to stay = gas pressure at full extension.

Regards, Ian

 

[gt6racer2]

Murpia, There is no step in the bore and the piston can move freely dependent simply upon the pressure differential experienced. A operational travel limit isn't needed as the fluid volume is calculated such that the piston will not escape the desired the operational position range even at full extension or full compression.
To the OP question - the base need is simply for a compliance device to accomodate volume change caused by the piston rod entering/leaving the cylinder. Assuming a simple monotube (ie w/o restricted path to a reservoir) said compliance must have enough preload to resist the pressure generated by the hydraulic valve during a compression stroke. If it does not then oil will not flow across the valve and a gas pocket will be created above the piston, causing a loss spot in the ensuing rebound stroke. Ideally the compliance would have zero rate, and not be affected by temperature - but that's not easily achieved.
Typically we've found the best solution to be a gas pocket with a preload pressure 240-280 PSI, kept separate from the oil by a floating piston, but there's no basic reason why the gas could not be replaced by a steel spring. However, I'd suspect such a spring would be rather hard to package. ( 280PSI in a 46mm monotube is around 700 lbs - ie about the corner weight of a small car ).

 

[Tmoose]

Some gas shocks operate on the reservoir/expansion volume in conventional twin tube shox. Pressure is "low". the extension force and seal friction/stiction can be low.

Some mono tube Decarbon shox (like Bilstein) have a large gas volume on the compression damping side of the circuit. The gas pressure always has to be higher than the hydraulic compression damping's pressure for the motion to be hydrualically controlled. Bilstein literature used to mention the charged pressure, but don;t seem to anymore. I believe it was a few hundred psi. I always wondered but don't know if Bilsteins over a sharp bump ever hydraulically "lock" leaving the gas pressure as the compression resistance, since that would require cavitation on the "other" side of the piston, although the high pressure gas would quickly try to refill the missing oil.

the Bilsteins I had (1980 VW Rabbit) jacked the car in the air very noticeably (what looked like an inch) , and I felt the seal stiction was very apparent as harshness. I wishfully tried a few magic greases on the shock shaft hoping to improve things, but with no success.

 

[TC3000]

GT6racer2 has touched on most points IMO

You will find replacement kits for gas charged dampers on some racing shows.
The main claim is, that the coil spring is insensitive to temperature changes,
and thereby keeping the ride height more constant.

How much of an "advantage" this is, depends from your application and PoV on the
subject.

In a "normal" damper, the pressure in the reservoir/canister increases with temperature, which will lead to on increase in extension force on the shaft.
The additional pressure will try to raise/lift the corner. This is more of an issue with dampers who use large diameter shafts and light springs (off road racing etc.).
If your car uses extensive ground effect aerodynamics, this change in ride height, can
upset the balance of the car, over the course of a race. (dampers heating up during the race etc.).
But it depends on the application, and how much a change in ride height will affect your handling.
Normally these type of cars are very stiffly sprung, so taking 50-100N force off the spring, will not result in a huge change in ride height.
As I said, it will depend on your application, most of these cars today will use constant volume dampers a.k.a. through rod or through shaft dampers.
In these dampers, the piston area is equal on both sides, and therefor there is no resulting force in on direction, which makes them insensitive to pressure/temperature changes.

That's about the only "advantage" I could see, but it comes with a couple of disadvantages.
Higher weight/mass, difficult to package, if you want to keep a low spring rate, you would need a very soft, but highly preloaded (long) spring.
It does work, the question is, is it worth the hassle, that you will need to decide for yourself, based on your requirements, preferences and application - IMHO.

 

[nobog]

Here you go: