Damper Gas Pressure
Damper Gas Pressure
(OP)
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.
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.





RE: Damper Gas Pressure
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.
RE: Damper Gas Pressure
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.
RE: Damper Gas Pressure
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".
RE: Damper Gas Pressure
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.
RE: Damper Gas Pressure
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Damper Gas Pressure
Assuming the same 1:1 MR scenario, every pound of rod force from gas pressure, is a pound not sitting on the road spring.
RE: Damper Gas Pressure
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?
RE: Damper Gas Pressure
RE: Damper Gas Pressure
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.
RE: Damper Gas Pressure
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.
RE: Damper Gas Pressure
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.
RE: Damper Gas Pressure
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
RE: Damper Gas Pressure
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.