Effects of gas pressure on automotive dampers

[Tim Jones]

thread800-179294

Hello,

In reference to the above thread: I appreciate that this thread is years old but I stumbled on it a while back while trying to find answers on this subject, and having pondered it and many other sources at length with the ‘what’s happening, when and why’ method, I’m certain the OP was correct in his questioning of what appears to be the ‘standard ’ of subtracting the gas force from dyno curves in order to arrive at the actual damping forces. (although this is half-right it appears).

Now, if we were talking about the damping being done by the valving inside the damper, then this would be wholly correct, but we want to know the damping values as applied to the car, and when fitted to the car the gas pressure remains, and some of the damping provided by the valving is ‘used up’ controlling the gas spring. It therefore has a large effect on the rebound damping applied.

My reason for researching this was that I realised that it has to be the case that a damper that extends on it’s own at speed 'x' due to gas pressure cannot exert any damping to the vehicle it is fitted to until that speed is exceeded.

The effect is also there to see on every gas-pressurised dyno curve i.e. there is no rebound damping until you exceed the natural extension speed (in the OP’s hypothetical example, two inches per second). Below that speed, the damper is actually assisting the rebound of the suspension - vastly different from a non-gas damper. And in my experience you really feel this on the road in the form of better ride, traction and grip.

Until now I’ve only mentioned rebound, and that is because apart from extra seal friction/stiction, the gas pressure only affects the rebound response. It took me a long time to see this but if you follow a typical rebound curve, all of the gas force is accounted for by the time the damper has slowed to a stop in the rebound direction, (resulting in an increased static and dynamic ride height, compared to without gas pressure) meaning that from there, in compression, the only force is pure compression damping (after overcoming seal stiction as mentioned). A dyno curve shows the static rod force as the starting point in compression but that is not the case. Well, it is the case as measured (total force) but is misleading as a) it has already been accounted for, and b) even if it wasn’t, the gas force is not a damping force and should not be factored as such.

In ‘no mans land’ between natural extension speed and zero, the energy of the gas spring is divided proportionately, depending on speed, between ride height increase (potential energy) and damper valving dissipation.

So in other words, to obtain the actual damping values, as applied to the vehicle, the gas force should be subtracted for compression, and should remain for rebound.

As the rod force causes a ride height increase it is easy to see why comparisons have been made to main spring pre-load. After all, the gas spring inside the damper is of course a highly pre-loaded, low rate (in my experience) spring. But, it operates in parallel to the main spring and therefore unloads it, not pre-loads it, and therefore treating the rod force as main spring pre-load does not seem correct. Also, as described above, it has very real effects on the rebound damping forces as applied to the vehicle and therefore cannot be ignored?

I realise this is contrarian but I’ve tried my best to argue with it and cannot!

Maybe you can…..?

Cheers,
Tim.

 

[gruntguru]

Am I missing something or is the gas pressure simply a constant force tending to lift the car? If so it has nothing to do with damping. If total force supporting the car is given by:

F = A*dY/dt + B*Y + C

A is damping
B is spring rate
C is lifting force (including gas pressure effects)
Y is vertical displacement

je suis charlie

 

[GregLocock]

+1 on that. The gas pressure will increase the seal friction I expect. Olsen say that above a certain point all it does is change your ride height.

Roehrig used to have a pdf on this very subject

https://www.yumpu.com/en/document/read/6787882/dynamic-gas-test-and-afects-roehrig-engineering

Cheers

Greg Locock


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[Tim Jones]

Cheers guys for your replies and the great link, and apologies for the delay in responding.

I feel I may have been a bit long winded in my initial post but I like to fully explain myself…I will try to be more concise so that my overall argument is not lost.

As far as I can see, the link from Roehrig reinforces my understanding of the situation, i.e. that the apparent industry standard of simply subtracting the gas force from the whole damping equation is wrong. Simply put, it is still there when fitted to the car, and although it is indeed accounted for by a ride height increase as they say, this is only true when static and in compression. In rebound, the gas force is accounted for by the initial part of the rebound damping which results in the natural extension speed of the damper. Crucially, as is my argument, this results in no damping force available to the vehicle up to that speed.

In rebound, as the example curves with gas pressure appear to clearly show, there is no damping until we reach rebound speeds of approx. 1 in/sec for the low pressure and 2.25 in/sec for the higher gas force.

To push my point, if we take these 4 examples here, (no gas + 3 different pressure pre-charges) and we corrected for ride height so according to the document we have now equalised things. Do these dampers now behave the same on the car? The answer to me is a resounding no because the rebound curve is very different on all four. The gas pressure has not only delayed the onset of damping, it actually helps the main spring force until that point.
If we expand this out to compare the same examples at different motion ratios, in a high motion ratio example, the gas force will result in a bigger ride height increase, but the effect on rebound damping will be less pronounced(comparatively). On low motion ratios like double wishbone however, the gas pressure effect on ride height is smaller but the loss of low speed rebound control is amplified to the vehicle/wheel motions.

Let’s use the 250psi gas pressure example, with a 2:1 wheel to damper motion ratio. Dyno’d then gas force subtracted to calculate vehicle damping ratio’s. The numbers used for calculation showing rebound damping from zero, yet in reality will not actually be presenting any slowing force to the vehicle ride or wheel movements until >4.5in/sec!

Surely to keep the vehicle damping ratio the same, the gas pressure must be compensated for with tighter valving in rebound. We’ve added a spring to the system, so it will need extra damping to control it and maintain the status quo?

Alternatively, the gas pressure offset can be used to advantage as it allows you delay the rebound damping until a designed in speed thus giving an extra tuning variable?

So much for being more concise…!

Cheers,

Tim.

 

[gruntguru]

In rebound, the gas force is accounted for by the initial part of the rebound damping which results in the natural extension speed of the damper. Crucially, as is my argument, this results in no damping force available to the vehicle up to that speed.

You are equating damping force to the force in the damper rod. If you built a "damper" that incorporates the spring that supports the car, the rod force would be positive almost all the time. This does not mean there is no rebound damping.

je suis charlie

 

[Tim Jones]

Right i think I get it now, the fog has cleared. I put a 25kg weight on top of a damper which was equal to approximately the mid-stroke rod force and it all became clear. When everything is in equilibrium, you still need to exert a force in the rebound direction to get it to move...it behaves exactly as it would without gas pressure. Instead of the damper initially producing a tension force, it just reduces its extension force at exactly the same rate, which amounts to the same thing?

Cheers,

Tim.

 

[BrianPetersen]

TL : DR

The gas pressure is there to avoid the effects of cavitation on the side of the damper piston that is extending. In bump travel, that's the bottom. In rebound motion, that's the top. By raising the total pressure on both sides of the piston, it avoids that pressure attempting to go negative when the piston is moving quickly enough, which would result in cavitation, which is perceived as a loss of damping.

Of course the piston rod area combined with the gas pressure means there is a net force attempting to extend the damper, which is a function of whatever the gas pressure is, which is a function of where in the travel the piston is and how much reservoir volume is available. (It is not a significant function of how fast the damper is moving in either direction, extension or compression, unless cavitation is happening.) Ordinarily this is a pretty small component relative to the force going through the springs, but it is non-zero, and acts to raise the ride height a little bit.

When the damper loses its gas pressure, it loses most of its high-speed damping, because it can no longer stop cavitation.

 

[sierra4000]

So why is ride much harder when more pressure is use?
Is ONLY due to extra seal friction?

 

[BrianPetersen]

1. Certainly, increased internal pressure will increase the seal friction.
2. If internal pressure is insufficient to prevent cavitation of the fluid, it may be perceived as a "soft ride", i.e. reduced high-speed damping, at least initially.
3. The slight increase in ride height associated with having higher internal pressure (due to the differential area, i.e. the piston-rod area), may affect whether you are hitting travel limits on bumps or dips. Note the use of the word "affect", that word was chosen carefully, I did not say it would increase or decrease the probability of hitting travel limits ... it could go either direction depending on circumstances.
4. There is undoubtedly some effective spring-rate increase due to the internal gas pressure acting as an additional air spring.

How much all of these things matter ... varies.

 

[sierra4000]

OK,
thanks
seal friction depends on:
a)seal construction
b)piston rod diameter (friction area)
?

 

[GregLocock]

That's your lot. Going to a bigger piston rod increases weight, increases the gas springing effect, increases friction. Seal friction is the #1 reason I prefer smaller motion ratios. Just thinking about it you get lower velocities as well, so should be able to reduce the pressure.

Cheers

Greg Locock


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[sierra4000]

Thanks,
I am limited with McPherson

 

[GregLocock]

Indeed, and for racing I doubt it matters much.

Cheers

Greg Locock


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[sierra4000]

Yes,
but during cornering friction increase again,
exist some study McPherson friction during cornering?

 

[GregLocock]

Sure, strut bending is well understood by those who design such things, which I don't. Basically you need to make sure that the spring force is well centred.

Cheers

Greg Locock


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[Tmoose]

I'd say 4 or 5 times over the years installing high pressure gas struts has increased the ride height noticeably and a few times even objectionably, like almost an inch.
For those cars with struts in the front ( large shaft Ø ) and more normal shocks in the back ( small shaft Ø) that can be like .5° less caster.
And maybe even less desireable aerodynamics and the perception of some lost high speed stability.
(Darn that Roger Huntington Nascar article)