o.k. let´s see, if we can wrap up our small excurse into the gas spring effects in a conventional monotube damper, by looking at the influence of temperature.
We have seen, that the ratio between the volume which get´s displaced by the shaft and the gas volume in our damper, are important to define the progression rate of our gas spring.
As larger our gas volume compared to the displaced oil volume, as less the spring rate will be affected over the stroke of the damper.
For most racing dampers this ratio is sufficient to not have an large change in gas spring rate over the stroke of the damper, so the effect will be small.
But there are maybe cases, where larger shaft diameters are used and where, due to packaging, weigth or cost constrains, the gas volume is a bit marginal, then the gas spring progression comes into play and adds spring rate to our main spring rate.
This effect will be less perceptable in a stiffly sprung car, but maybe leads to an perceptable change in a lightly sprung car, which uses larger strokes.
(such as Sprint Cars on Dirt, or-off road racing buggies etc.)
When we talk about temperature, we have to consider three effects.
The first one, is the change in oil viscosity and it´s effect on flow rates and forces generated by the valves.
This can/will have a effect on the overall damping values of the damper, as seen in Belatti´s experiment.
Depending on the damper in question, it´s design, it´s valve technique and the oil used, this can have major effects on overall damper performance and overshadow the effects from the gas spring.
But this is a different topic, and not directly related to the gas spring effect.
For our assessment on the gas spring effect we need to consider the follwing two effects, the temperature has on our damper.
The first one is the increase in pressure inside the gas chamber in the damper - canister.
This is again governed by the "ideal gas law".
But to make matters worse, we have to consider, that our oil will expand, due to the increase in temperature, as well.
By doing so, it will take away volume from our gas chamber, by moving the seperation piston further inwards (towards the gas).
This change alone would increase the gas pressure slightly.
How much does our damper oil expand by temperature?
Well, the precise value will depend on the oil used, it´s density and CTE, but for most practical applications an estimate of 0,01%*oil volume*delta T can be used, and will give sufficient accurate values - IMO
Applying this to our example JRZ damper, this means.
Let´s say our damper warms up during use from 25°C to 65°C, which is a delta T of 40°C.
Based on our dimension, we estimate the oil volume with ~152 cm^3 and we have the same value as our gas volume.
When our damper heats up , the oil volume increases to ~158 cm^3 and our gas volume decreases to ~146 cm^3.
If our damper had a gas pressure of 150 psi when it was @ 25°C it will now have a pressure of:
Phot= (339K*150psi*152cm^3)/(299K*146cm^3) = ~177 psi
when @ 65°C (note that the calculation uses absolute temperature in K)
This means our preload increases from 393N to 465N, a increase of ~72N or 0,1 lbs
If our car would be fitted with 70N/mm springs and would have a motion ratio of 1:1
the car would be ~ 1mm higher now.
The average spring rate increases by 0,27N/mm or 1,52lb/in, so this is a secondary effect, which is neglectable in this case.
some graphical representation of the values:
note: the preload, was taken off the data in both cases, to show only the difference in force @ the shaft vs. stroke.
The pressure/force of the "hot" damper whould have an offset of 72N to the "cold" damper value.
