Damper Gas Pressure

[Krazan]

I have a question regarding gas pressure in a monotube shock/damper.

Conventional shock dynos measure the rod force due to gas pressure, and subtract that force from the measured force values as the shock is being cycled.

The reasoning (As explained to me) is that the rod force is essentially a spring force, and not damping, and so should not be considered.

Here's the catch (Or question)

Let's say for instance that I wish to valve a damper with a linear curve that provides 50% of critical damping of the sprung mass for rebound.

Just to put some numbers on this, let's say that I need a damping constant of 25 lbs per inch per second.

Let's also say that the rod force due to gas pressure is 50 lbs.

If a damper was built with these specs, I could compress the shock, and when released, the rod/shaft would extend at a rate of 2 inches per second.

Once attached to the suspension, would I effectively have zero rebound damping at a velocity of 2" per second, and "negative" damping below that velocity?

Would my effective total rebound damping force be reduced by 50 lbs at all velocities?

On to the compression side, would my effective damping force have 50lbs added to it at all velocities?

Let's look at this another way. If I were to valve a non pressurized shock so that the forces were 25 lbs per inch per second, the spring mass damper system should respond more or less as the mathematics would predict.

If I were to valve a pressurized shock to have the same damping characteristics as measured on a conventional shock dyno (With rod force subtracted from the measured results) the response of the SMD would be different.

So on to the big question. Should the gas pressure/rod force really be ignored when making damping calculations?

Does anyone take this into account when valving shocks?

Sorry this was so long. Any input would be appreciated.

 

[GregLocock]

I treat the end force as a precompression in the road spring, as such it is no longer part of the damping side of things.

That's an interesting perspective, either way. I think your way of thinking may work OK at constant velocities.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Krazan]

I have always looked at it in the same way, but have been questioning this lately. I typically calculate a damping curve for a particular vehicle, valve the shocks, and then make adjustments at the track until the driver and tires are happy.

For the most part, my calculations get me quite close, but every once in a while, I'm further off than I would like to be.

I'm starting to see a trend that may relate to rod force.

For instance, when using a Strut at the front of the car, the shaft diameter is large, and so the force is high for a given gas pressure. These cars seem to want more rebound damping. Especially in the low speed range.

My calculations are based on basic SMD principles, and no matter how I look at it, the rod force certainly throws this all out of whack.

I'll supply another "for instance" and see what you think of this.

This requires some "what if's" If I decide I need the previously stated damping constant of 25lbs-in-sec, could I simply add 50 lbs to that "curve", valve the shock accordingly, and have its behaviour match that of a non pressurized shock?

In other words, at 1 ips, I would need 75lbs of damping force (25 * 1 + 50) and at 10ips I would need 300lbs of damping force (25 * 10 +50)

If I were to do that, would I get the same SMD response (In rebound only) as with the non pressurized shock which has 25lbs damping force at 1 ips, and 250lbs damping force at 10ips?

On to the compression side, I really can't make these corrections, because at 1ips, the rod force is greater than the required damping force, however, if the rod force were say only 10lbs, I could account for that. At least on paper.

I guess what I am getting at is in an SMD system, does the spring/mass really care where the force comes from? Gas pressure, or damping, it is still a force.

Any ideas? I could really use some help. I'm having a hard time getting my brain wrapped around this.

 

[Fabrico]

Rod force brought about by gas pressure should be ignored when making damping calculations. As measured on a shock dyno the results should differ as you are essentially testing one shock with a spring and one without.

I'm curious about your mention of obtaining "critical damping of sprung mass for rebound".

 

[Krazan]

OK, Let's try this another way. Another "for instance."

An imaginary car has 25 lbs of rod force, and a 1:1 motion ratio.

Push down on one end of this imaginary car, compressing the springs one inch, and then abruptly take your weight off, letting the car rise back to ride height.

Now increase the gas pressure so that the rod force is 100 lbs, and do the same thing again. The car rises faster.

The response time of the vehicle has just been altered, but it has the same springs, and damping (As measured on a conventional shock dyno)

As I see it, there are two ways to look at this. Either the car has less effective rebound damping force, or higher spring rate.

I have already related this to the damper, now let's consider the spring. If we "rate" the spring with the damper attached, we get a digressive force displacement curve. A digressive spring.

Now our SMD is completely non linear, and even with the help of Matlab, I'm not sure I can properly account for this.

Whatever the case, the cars response time is clearly different.

 

[GregLocock]

"The car rises faster." No it doesn't. The car either sits higher in the first place, or you adjust the ride height via the road spring. Either way the rise time is controlled by the damping(which is unchanged) , the vertical rate (which is unchanged) and the mass (which is unchanged).



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Fabrico]

Assuming the same 1:1 MR scenario, every pound of rod force from gas pressure, is a pound not sitting on the road spring.

 

[Krazan]

Thanks guys. I get it now. For some reason I was having a hard time with it. I plotted a force/displacement curve in Excel and it finally became clear. If I caclulate the rate of the spring and gas pressure combined starting from zero load I get a digressive rate. But...If I calculate the rate from the point where the force overcomes the gas pressure, to a further point on the curve, I get a linear rate matching that of the spring.

My conclusion is that the gas pressure/rod force acts the same as preload on the spring.

Does that sound correct?

Am I correct in assuming that an SMD including gas pressure/rod force will respond the same as wihtout gas pressure, as long as the spring never extends to the point where the load is less than the gas pressure?

 

[edstein]

I think the rod presure that is measured depends largely on the shaft size (volume displacement) and the gas chamber volume. On some of the economical gas charged racing shocks, the gas chamber is so small the gas presure increases drastically under compression. On the higher end shocks the larger gas volume makes gas presure increase much less. In the form of racing I am involved in we occasionally use small gas chambers and more gas pressure to get the sprung mass to rebound faster.

 

[gt6racer2]

Hi,
Gas pressure does increase with compression travel, but I wouldn't put in a big rod to tune for that since temperature changes will start to affect your ride height. This is why you don't see monotube struts. ( unless they're the upside down variety where the "tube" is the "rod".)
The gas does contibute some spring preload, and in some cases is compensated by changing spring length, for example when moving from twin tube to monotube.
How are you modeling friction ? If you're getting this complex you need to consider it as 100N is not unusual for a monotube. Twin tubes are much lower, say 40N, although some misguided OE's are currently putting friction devices in to increase it.

 

[PTwizz]

When comparing theoretical forces to track experience (Krazan) it is easy to underestimate the effects of gas compression (causing rising rate) and temperature (causing increases in rate and preload). If the gas reservoir volume is (typically) 3 times the volume displaced by the shaft, then the gas spring rate will increase 50% fromm full droop position to full bump position.
Temperature has two effects. One is to increase gas pressure directly (Boyles law) and the other is to reduce the effective gas volume (increasing pressure and spring rate) due to the expansion of the damper oil.

 

[Krazan]

GT6Racer - We sometimes deal with large shaft monotubes, both as a strut, and the dreaded JRZ/Moton dampers. In the case of the JRZ/Motons I will get the majority of my compression forces from the cannister so that I can run lower gas pressure. Loads of hysteresis, but less rod force and the cars seem to like it. In some cases we deal with large rod monotube struts with no canister, and just have to live with the high rod force. This particular POS strut also has very little nitrogen volume, and so we normally add a "dry" canister just to keep the nitrogen pressure from increasing with travel.

I do not include damper friction separately as the dyno will include that in the results. Unless...the friction is substantially higher when there is a spring attached. Also I have no idea how much the friction increases on a strut when it is loaded. That may be something to look into.

PTwizz - This is certainly worth considering in my model, and may explain some of the recent head scratching. I will start taking hot pressures, and add that to my model. (That should have me talking to myself for a while...)


Thanks guys.

 

[ubrben]

In MotoGP - Ohlins (like Sachs in F1) are using a through rod rear damper. In this case, the only expansion you need to compensate for is the thermal expansion of the oil with temperature.

The Ohlins guy for our MotoGP team pointed out that they can run a very wide range of compression valving with a very small range of internal pressures, making for a more versatile damper in addition to consistancy advantages due to removing the vast majority of the gas spring effect.

Ben

 

[RigTest]

The gas spring effect from reservoir pressure is in parallel to the spring rate of the traditional suspension spring, not in series. So the overall vertical rate of your SMD system is changed and thus your "rise time" would change. Imagine a huge reservoir pressure and think about the additional energy that is stored assuming you compress the pressurized and unpressurized system 1".

That said, I agree with everyone else, that you should measure the gas force/displacement effect of the damper and subtract it from your dyno curves. If it is significant relative to your traditional spring rate then add it back in.