To learn and share about dampers / shock absorbers

[747heavy]

deal - a good steak and some Malbec will do the job for me.

[Belatti]

NP mate, my family owns a Restaurant!

[autogyro]

If anyone deserves a free lunch its Jumbo.
I would be pleased to buy him one.

[DaveW]

Thanks are also due to Belatti for starting this thread, for stimulating discussion, & for providing an excellent demonstration of Penske's damper set-up methodology.

[ubrben]

Seconded - great thread.

Ben

[Arunas]

Seconded - great thread.

Ben

Malbec was definitely not the worst part of this thread

[747heavy]

maybe this fit´s nicely into this thread, even if it is mainly about road car damper technology.

I hope, it helps the interested reader, without indeep knowledge, to get an overview about the different concepts (monotube vs. twintube etc.) and working principles of dampers.

This perhaps helps, to follow the other parts of the discussion in this thread more easily.

[DEFOE]

I have a question：

our damper'S diameter is 22mm，and we charged 6 bar gas，and according the caculation，the gas rebound force should be 228N。But in the production process，we see the sensor （detecting the robound force）is defined by (230N,270N),Why it isn't (210N,250N)thanks.

[747heavy]

Sorry DEFOE,

I maybe don´t understand your question correctly.
The calculated value for a damper with a 22 mm shaft and 6 bar gas pressure would be 228N, as you say.
If you measure 230N in your production process, this sounds very sensible and propable to me.
The difference of 2N is probably friction, which is not included in the calculation.
I would not be too surprised to see up to 15-20N friction forces, but this will depend on the seals you use, and other design factors of your damper.
Other possible reasons for a different force reading include loadcell calibration and resolution.
So 230-250N readings for 6bar gaspressure would not surprise me, and I don´t think that they are out of the ordinary.

>250N would indicate that friction is a bit high, or that maybe the damper is not charged exactly with 6bar.
It´s a bit difficult to say from the distance, where the difference comes from, without knowing more about the damper or the test method.

But why would you expect to see lower values then 228N?
Where comes the 210-250N idea from?

Sorry if I may understand your question wrong, maybe you can discript the measurement process a bit better.
The 228N would be the preload/break free force at end stroke.

If you measure the force at midstroke or fully compressed, you would measure a higher force.
How much more, will depend from your gas spring progression (volume of your gas volume) and at which stroke you measure.
Hope this helps a bit.

[DEFOE]

Thank for your reply.

At first, I don't understand the concep of the filled gas with 6 bar in the file.

whether it should becaculated with 5 bar or 6 bar to plot with the area of shaft(

I think not the area of the piston).If it is 5 bar, the rebound force should be

190N,otherwise should be 230N.

But the process file defined the force of the front shock absorber(the shaft diameter is 22mm) which is measured at the exend status after filled the gas( the largest distances of the shaft-10mm),the qualifiled force is 270±20N,and the rear shock absorber (the shaft diameter is 12.5mm) is 75±15N.

I can't understand the logic of the force.

you have mentioned the calibration, I try to find some clues to validate it.

[747heavy]

o.k. I can see where you are comming from, and I appologize for the confusion.
It´s the difference between absolute and relative pressure.
I showed the 1 bar atmospheric pressure in the graphic just to show the pressure difference which acts only on the shaft area.

The way most filling gauges are setup, you will need to calculate with 6bar (not with 5bar).
So if you fill your damper with 6 bar, this is the value you should use for your calculation.

As you measure the force at 10mm stroke, the value will be higher then the pure preload value.
How much higher will depend on your ratio between gas volume/shaft volume.
As higher the gas volume in relation to your shaft volume, as smaller the change will be.
For the example shown (JRZ damper) you would measure ~6N more then preload (234N instead of 228N).
If your gasvolume is smaller this value will become higher.
For half the gasvolume in the example it would be +11N (239N force at 10 mm stroke).

Any friction force, would be on top.

The friction will (among other things) be dependent from your seal and shaft guide design, from the diameter of your piston, the material of your piston band/seal, the diameter of your seperation piston, the size and material of your seperation piston seal/O-ring, it will be different for a damper which uses a sliding seperation piston compared to an damper which uses an membrane type of seperation (rubber bag/sock) etc.
If the damper sees any form of side load during the test the friction value can change quite dramaticly.

If you use a dedicated machine to only measure the gas force/seal drag/friction, then loadcell resolution and calibration is maybe not a problem, but if the test is carried out on a "normal" damper dyno which perhaps uses a 10kN loadcell, trying to measure 10-20N is maybe not so easy/accurate.

The rear value makes perfect sense to me, as the calculated gas preload would be 74N, and taking the 10mm stroke into account, I come to 75N for a standard gasvolume/shaftvolume ratio.
The ±15N are to account for friction, loadcell resolution and production tolerances.

I´m not sure why they would choose 270N as the reference value for the front, 240-250N would be more logical for me.
Unless you have a very high progression value from your gas spring.
What are the total strokes for these dampers?

[DEFOE]

Thank for your reply.

The stroke（avilabe top to bottom）of front is 143mm，gas volume is [118.5ml,172.8ml];the rear is 144mmand ,the gas volume is [20.58ml,38.25ml].

we checked the force caused we have found some sealing is broken，and we doubt have filling more gas than needed.But the original designer is leave from current position，no others knows how exactly the amount of gas should be filled and why the front damper's force is larger than the caultion.(if 5bar, it is 190, and if 6 bar,it is 230).and the rear(if 5 bar, it should be 60N,and 6 bar, 75N)

I should give more informatio: the supply pressure is 8-9.5bar(cosider the leakage), and we don't know how exactly the amount of gas have been actually filled in the damper. and the calition force is 80N without any force in the front rear(currunt change to zero,and found the measured force is decrease relatively).another infomation is: some design modification has been happened on the force in the file.
fornt: [30N,50N]>>>>>[60N,90N]
Rear: [below 200N,tomorrow checked]>>>>>[250,290],no one know why?(calibration?)

Could you give me more information about the rebound froce. thank you.

[747heavy]

O.K. based on the informations you have provided, I calculate the following forces:

for the front:



for the rear:



(please note, that I take the base gas pressure out, and only show the pressure increase vs. stroke in the tables above.
So when it says 0bar at 0mm stroke it is equal your base pressure, in this case 6bar.
You would need to add the pressure and force values shown to your base value)

So for the front you should see 228N + 5,1N = ~233N at 10mm stoke with 6bar gaspressure and a 22mm shaft.
I don´t know, why they would quote 270N as a reference/base value (235N ±20N would make more sense for me)

For the rear it would be 74N +2,4N = ~76N, so to quote 75N ±15N as a reference does makes sense and sounds quite logical to me.

That the allowed friction tolerance at the front is slightly higher with ±20N compare to ±15N for the rear, does make some sense for me, as (all other things beeing equal) you will have more friction at the front due to the larger shaft 22mm vs. 12,5mm and a greater seal(friction) area.

To have 270N @10mm stroke in the front damper, would mean 6,95bar gas pressure for a 22mm shaft and the gas volume you provided - IMHO

[DEFOE]

thank you for your have provided so much information to me,and help me a lot。

I'd like to ask two questions：

1.how many gas should be filled in shock absorber？and the relations between sealing。

2.The design maxium rebound veloticy for shock absorber（3m/S or 4M/S).

[DaveW]

The design maxium rebound veloticy for shock absorber（3m/S or 4M/S).

I think the maximum rebound velocity will depend mainly on the parallel spring rate, the preloads of the spring & damper, & the rebound load-velocity trajectory of the damper. I'm afraid I don't really understand why you ask....