dynamic gas test and afects - Roehrig Engineering

ROEHRIG ENGINEERING TECHNICAL UNDERSTANDING SERIES 

THE ROEHRIG GAS TEST 

THE DYNAMIC GAS TEST AND EFFECTS OF GAS PRESSURE 

Test: Used the same shock and ran it on 2” stroke, 100 F warm up at 10 in/sec using a Roehrig 

Engineering 5VS crank type dynamometer. We collected data at three different pressures 

without changing anything else. For the test, we zeroed the load cell with the shock hanging 

from the upper crossbar, pre-loaded it 2” for the test. We ran a standard Roehrig Engineering 

gas test, stopping at mid-stroke for 2 sec. 

One feature of Roehrig Engineering’s (REI) Shock software is the ability to measure the gas 

force of the damper and remove its value from the data. REI does this by moving the crank to 

mid stroke and measuring the load cell force. We do this in both the compression and the 

rebound directions. We also allow the user to set how long the shock stops before measuring the 

load cell. This becomes very important when the shock has very little bleed. 

Why does Roehrig feel that this important? The main reason is that with a gas shock, there is an 

effect of the gas chamber on the damper. Statically, if you compress the shock by hand you are 

compressing the gas chamber. The pressure in the chamber pushes back against the shaft with 

some amount of force. The higher the gas pressure, the more force it produces on the shaft. The 

gas chamber acts like a spring, the more you compress it the more force it exerts on the shaft. 

If you put 50 psi into your shock and put it on the car, it would raise the car by a small amount 

because the gas force is trying to push the shaft out. If you added more pressure, say 200 psi, it 

would raise the car even more. Now, I am not suggesting it will raise it 1 inch, but it will raise it. 

What do you do then? You take rounds out of the car to get the ride heights you want. 

Basically, when you scale the car you take into account the effects of the gas pressure from the 

shock. If you add gas pressure, you re-adjust your spring heights and away you go. This is also 

the reason that some people use the gas pressure of the shock to act like a spring in the car, thus 

allowing them to run less spring in the front of the car. It is for this reason that Roehrig decided 

to measure the gas force and then remove it from the data. Like you do on the car, we remove the 

static effects of the gas chamber on the data. We only want you to look at the dynamic effects of 

the shock. 

Removing the static gas force at mid stroke allows the user of a Roehrig dynamometer to be able 

to compare shocks with different gas pressures and not have to worry about the static force from 

the gas chamber. There are some dynamic effects from the gas chamber that will show up in the 

data. 

ROEHRIG ENGINEERING INC. 

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Figure #1 shows the same shock with three different gas pressures (100, 175 and 250 psi). Each 

time we added gas force the graph moved up the force axis by an amount equal to the extra force 

on the shaft. If you did not know what changed, you might say that we added compression and 

took away rebound. It could lead you down the wrong path. As you add more gas pressure, i.e. 

spring rate, you add a static force that continues to raise the overall force of the graph. You have 

added a force offset based on how much gas pressure is in the chamber. But, has the graph shape 

actually changed or does it only seem like it? 

Figure #1: Gas Force Kept in the Data 

RED TRACE: 250 PSI, BLUE TRACE: 175 PSI, GREEN TRACE: 100 PSI 

What happens when we remove the static gas force from each test, so that we only compare the 

internals of the shock and the dynamic effects of the gas chamber? 



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The graph below shows how when you remove the static gas force, as measured by Roehrig’s 

gas test, the graphs line up just like you would expect. Since we did not change the internals of 

the shock, it makes no sense that the curves would change. Just like if you added 200 psi to the 

front shocks on your car, the front end would raise a small amount due to the added spring rate 

from the shocks. You would then, I hope, re-adjust your spring perches to get your ride height 

back to where you wanted it. 

Figure #2: Gas Force Removed 


We have removed the static gas force from the shock. Are there any dynamic affects of the 

increased pressure? 



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Notice how the test with 250 psi in the chamber (red trace) looks just a little stronger on 

compression closed side of the cycle. Figure #3 shows a close up version of the three runs. The 

run with the most gas pressure (red) shows extra compression force on the closing side. This is 

due to the effect of the dynamic addition the gas force makes. In the compression closing stage, 

the shock is still compressing and it is slowing down. Since it is still compressing, the gas 

chamber is adding its spring rate to the force curve. Basically, the higher the pressure, the larger 

the spring rate and the larger the force offset to the curve. 

Figure #3: Gas Force Removed Close up View 




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For those of you wondering why we do it at mid stroke and not some where else, you ask a good 

question. The further you compress the shock, the more you compress the gas chamber and the 

bigger the pressure build up in the shock. Doing it at mid stroke is just a good average. Future 

versions of software may allow the user to do a gas test at several different stages of the stroke 

and compensate at each stage by removing the force. This is for the future. 

If you realize that depending on how your shocks are mounted on the car, the shock may be 

compressed more on one corner than the next corner. You also see how the gas chamber can 

affect the car’s behavior over time. If your right front shock was more compressed than the left 

front, they would each have a different static gas force, provided you put the same pressure in 

them to start. 

Another thought would be, as the temperature of the shock increases, the temperature of the gas 

chamber increases and as a result the pressure goes up and the force exerted on the shock shaft 

increases. This means that during your 20 or 30 lap run on the track, the pressure in the shock is 

increasing because the temperature of the gas chamber is getting hotter and the increased spring 

rate may be affecting your car’s handling. Think what would happen if 20 laps into your run, 

you added 25 lb/in of spring rate to the front shocks, what would it do to your handling? 

We will be producing several more papers discussing temperature, adjustment and cavitation in 

shock absorbers. Stay tuned and keep watching the Technical info section of the website: 

www.roehrigengineering.com for future papers to help and maybe confuse you more. Any 

suggestions for technical info can be submitted to Michael@roehrigengineering.com. 



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dynamic gas test and afects - Roehrig Engineering

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