To learn and share about dampers / shock absorbers

[747heavy]

Thank you !!

If you want to reduce the amount of oil volume the shaft displaces for a given stroke, you will need to reduce the shaft diameter.

How much this displaced volume will raise the gas pressure in your canister/reservoir will depend on the volume of your gas chamber in the reservoir.
As larger this is (ratio between gas volume/displaced oil volume) as smaller the change in pressure and therefore force will be (lower internal spring rate of the damper)

For the function of same dampers it is necessary to use a large shaft (better adjustment range for bump/compression forces), so these dampers will need to use
larger canisters/reservoirs to compensate for it.
As long as you have enough space, and weight is not the most critical consideration, you can design a good damper in both cases.

I will (try to) explain some of it a bit later on.

[Arunas]

Great reading 747heavy, keep posting =D>

[speedsense]

With most of the "gas spring" calculations that been done, aren't the gas spring numbers generated from a stationary point for the piston position?
Whether at a fully open position or some distance of insertion into the shock?
At this stationary point the valving is closed on both sides of the piston (no flow through it) and if the measurement of the gas spring was taken while in this "state" the gas spring effect would be at it's highest.
As seen by 747's numbers, the larger shaft size increases the gas spring, as the volumetric displacement of the fluid by the solid rod (assuming it's solid as it is in JRZ,Moton) provides more force towards the gas.
It would stand to reason that the highest gas spring rate would be a solid rod, solid piston with ample seals and no bleed holes open.
In use (running on a car) the shock has varying fluid amounts flowing through it at the different velocities and the highest flow is at the shock's highest attainable velocity, would contribute the least amount of force/resistance (of the assembly of rod and pistons) towards the gas spring having an effect.
This would have a reduction effect on the stationary gas spring number, especially with a very high flow piston in use...

Hi Speedsense,

It seems to me, that you still have a slight misconception in regards to the gas spring in a damper - with appologize and the upmost respect.
Let´s see if this drawings help, to get my point of view across.

Imagine this to be our damper, in a stationary position, as you say all valves in the piston o closed.

As you can see, we have the same amount of pressure above and below the piston, as it´s allways the case, based on the fact, that within a fluid under pressure, the pressure is equal at any point.

As you can see, we have equal pressure acting on both sides of our piston.
The only difference is, that said pressure has a larger area to act on in one chamber (bump chamber) and as we know that F=P*A, we can see that the extension force (preload), does not result from a pressure difference from above and below the piston.

It purely results from the different surface areas on both sides of the piston.
This difference will allways be there, whatever the pressure, it does not matter if valves in the piston are open or closed.
The force would be there even if we have no piston at all.
(please see the following drawing)


We have a pressure of 20 bar inside the damper, this pressure acts all around inside the damper. And it acts on the shaft. The only place where it can´t act, is the cross section area of the shaft (PI/4*d(shaft)^2)which is outside the damper.
This part has 21 (absolute) bar on one side and ~1 bar (atmospheric pressure) on the other.
This difference * the shaft cross section area gives you the force, you feel and measure as preload on your dyno or spring tester.

If you like you can make the test for yourself, if you have access to a damper and a dyno or spring tester.
Take all the shims of the piston and test the damper, you will still measure the same gas force/preload.
You can even take the piston off, but please be careful, it will shoot the shaft straight out, when you pressurize the damper, and you will have a mess, with oil everywhere .
Therefore, if you really want to do this, you should clamp the damper into your dyno or spring tester first, so that it can´t blow apart -->
Zero, your loadcell, and now pressurize your damper.
The force you will read on your loadcell is P*A, give or take ~10-20 N to account for friction.

The other small glitch in your conception, is about the hollow shaft.
It does not matter, as far as the volume change is concerned, if the shaft is steel or filled with oil.
Both mediums are incompressible, so both will displace the same amount of fluid. The damper with the fluid filled shaft, has a slightliy higher oil volume to start with, is maybe a bit lighter and provides maybe a bit more cooling, as the oil can dissipate heat via the shaft now.
But as far as displacing fluid goes, and as far as our area for the gaspressure to act on is concerned, it does not matter.


Hope this makes some sense, and that the gaspressure vs. preload argument is a bit easier to understand now.

***Please excuse the erasing of your great images to shorten the posts***

Perfect sense and excellent explanation!!
So with me being a Wabbit with a bad Habbit and with a much greater understanding of the gas spring, thanx to 747, the following thoughts:
The piston assembly surrounded by a constant gas spring rate, accelerates through the fluid, opening the valving and allowing the flow through the piston. The passages in the piston have a venturi effect with a flexible flap on the end of it (the flow ports exit of the movement of the piston) controlling how much fluid can flow. The port would accelerate the oil through the passage, and according to Bernoulli's principle have a lower pressure inside the port. Depending on the acceleration of the flow, we could achieve zero pressure vs the outside gas spring pressure of the internal shock.
Now the question, if I achieve zero pressure internally within the piston, do I effectively cancel the gas spring effect, for the area of the port? Obviously the "solid" areas of the piston, still have gas spring rates acting on it.
And further more, if I achieve negative pressure within the piston port(s), does the negative minus from the gas spring effect for the areas of the piston that have gas spring pressure acting on it?
And one more, would zero or negative pressure in the port, cause cavitation upon exiting back into the higher pressure?
Last one, as I understand it, hydraulic fluid (assuming that shock fluid is hydraulic fluid) is slightly compressible, so do I use Bernoulli's compressible fluid equation or non compressible equation?

Happy Thanksgiving to all who celebrate the holiday...especially thankful for having 747 here in this forum...

[marcush.]

=D> =D>

I feel very much honoured to have met someone like him and it shows ,forums can be more than what they usually are....and for sure he is one of those who add considerable value to this place.

[autogyro]

+1 A forum diamond.

[Belatti]

If you like to see the "drift" in the football graph, due to an increase in gaspressure/shaft extention force, while trying to keep the other parameters in check (as constant as possible) you may consider the following.

choose a relativ slow speed (~ 50-100mm/s) and a dampersetting which is not extreme stiff. By doing so, you will be able to scale the graph in a way that it shows even slight variations more clearly, and the damper/oil does not warm up to much, therefore the viscosity effect of the oil will ne small, and you can rune the damper longer without worrying too much that it gets too hot.
now while cycling the damper on the dyno and looking at the live data, warm up the canister/reservoir of the damper with an electrical heat gun.

Great ideas to test heavy! It will definitively help me to get the offset thing in the software.

I will try to test them on friday

Well, Friday it is (a week later, though). I had a difficult weekend but finally at last the team won the race and then I took some national holiday days off this week. Still I was reading the interesting discussion between speedsense and 747 from home.

Here I post the graphs from the "academical" test 747 suggested to do.






Sorry I wont post force values, but at least they are in the same scale so I think the "line counting" will serve our purpose. They are screenshots of the live data at different temps while I was pointing the heat gun at the cycling damper with one hand and the IR themometer with the other.
(25mm stroke, 1Hz, 80mm/s speed)

[Belatti]

By doing so, you should be able to see the effect of rising pressure.
Alternatively, and maybe even more simple.
Pressuerize the damper the damper to ~200 psi and run it, as above.
Now, while looking at the live data start to bleed off some pressure. You should see, that your graph is drifting towards the rebound side, and that at the end it will not be centered around the "0" force line.
It will/should be centered around a -xx N/kgf value.

Now I plot the bleeding pressure offset drift...



I dont know the pressures in that graph because, as the live data will only plot "the last 5 spins" only, I had to do it quickly before the offset correction erase the "old" cycles and get again arround 0. However, just to have a reference I started at 150psi and ended at 100psi.

[speedsense]

If you like to see the "drift" in the football graph, due to an increase in gaspressure/shaft extention force, while trying to keep the other parameters in check (as constant as possible) you may consider the following.

choose a relativ slow speed (~ 50-100mm/s) and a dampersetting which is not extreme stiff. By doing so, you will be able to scale the graph in a way that it shows even slight variations more clearly, and the damper/oil does not warm up to much, therefore the viscosity effect of the oil will ne small, and you can rune the damper longer without worrying too much that it gets too hot.
now while cycling the damper on the dyno and looking at the live data, warm up the canister/reservoir of the damper with an electrical heat gun.

Great ideas to test heavy! It will definitively help me to get the offset thing in the software.

I will try to test them on friday

Well, Friday it is (a week later, though). I had a difficult weekend but finally at last the team won the race and then I took some national holiday days off this week. Still I was reading the interesting discussion between speedsense and 747 from home.

Congrads on your win, stellar mate!!! Makes the holidays even better no doubt.. =D> =D> =D>

[747heavy]

Well guys,
I´m not quite sure what to say. I´m a bit blown away and feel flattered.
Did not think, that it is all that special, but if one or the other get´s
something useful out of my humble efforts, then I think it has served it´s
purpose.
Thanks for the kind words, it´s truly appriciated.

To the ones of you who celebrate Thanksgiving, a happy Thanksgiving and to
the rest, I hope you have a great weekend

Many thanks
747h

[747heavy]

@ speedsense

About the Moton story, this can be true, but I thought, that it was a dutch company

Headquartered in Uden, The Netherlands, with more then 15 years of experience in damper design and manufacturing Moton was founded in 1999

so maybe this ex JRZ fellow, just headed "across the road" to set up his own shop.
Seeing that JRZ was founded in 1995, this is maybe a possible explaination.
Happens quite often within the industry.

as for your other questions, I will try to come back to it, a little bit later, as I would like to finish some thoughts and comments regarding the gas spring inside the damper first.

I´m not sure, I can be of much assistance, if you want to discuss the flow dynamics through a piston in great detail.
As this may exceeds my level of knowledge and trying to explain it definitly exceeds my command of the English language.
But I can try to explain some concepts, using an a little bit simpler model/approach.

not sure what an "Wabbit" is, only ever heard about a "Wombat", but this is maybe something completely different in meaning.

In any case, I did not mean my comments, and responses to you in any disrespectful way, nor is it my intention to prove a point, or to prove you wrong.
From a practical perspective, I can see where you are comming from, and I partly agree with you.

But I was feeling compelled to make a comment on it, because I see a slight danger if we generalize it to "gas springs in dampers have no effects on real cars".
I know, that this was not what you may wanted to say, but the aim of my explainations here, was to lay down the principles involved.
So that everybody can make his own assessment, and decide if it is a "problem" for him or not, and if it warrants further consideration for his/her application.

Why it may has little effects for a racing damper with 14-16 mm shaft and strokes of ~ 2"/50 mm, when we consider small pressure variations of up to 100psi in a heavily sprung car, the story could look completly different if we talk about cars with much lighter spring rates and more suspension travel, especially if shaft diameters >20 mm are concerned.
Rroad cars being the extreme example for a lightly sprung car, but some off road racing cars/bikes may fall into this category as well, keeping in mind that in the later case, large differences in operating temperatures adding into the equation.

I think, that at the end, both you and the Roehrig guy could be right, it depends from which end of the scale you look at it.

Just to put some numbers out, and as I have zero knowledge about Formula Atlantics or Indy cars, which sort of spring rates and motion ratios do we talk about?
I don´t want you to post your "secret championship winning" setup, but maybe you could provide an "order of magnitude" value, similar to the information in the Dallara F302 manual.

It would be very much appriciated. Thank you.

[747heavy]

@ Belatti

Well done !! and congratulations to your win, I´m sure you worked hard for it.
Thanks for doing the tests and posting the results here.
The pressure test is very much logical, and as I would have expected it.
In the temperature test, we may see that the effect on viscosity of the oil, is overriding the gas spring effect.
What it looks like, is that you loose quite a bit of damping overall, with temperature. I will look into it a bit more closely over the weekend.
Did you warm up only the gas canister or the whole damper body?
Sorry, I did not mind about it before, it would have been good to measure the
gas pressure in the damper before and after the warm up.
This way, we could calculate how much increase in preload, due to the higher pressure we would see. The underlying assumption is that the gas pressure increases with increasing temperature, seeing that the volume remains the same.

Based on the ideal gas law.



-->



assuming your damper has a 5/8" (~16mm shaft) and that you started your test with 150 psi, we should see a "hot pressure" of ~166 psi after heating up the gas from 23°C to 55°C.

In combination with a 5/8" shaft this should lead to an increase in preload from 206 N in cold condition to 227 N in hot condition or a force offset of 21 N.

Now 21 N, keeping changes in friction levels and resolution of the load cell in mind, is maybe difficult to see on the graph, and probably overshadowed by the change in damping forces, due to the change in oil viscosity.

There is another interesting "event" happen during the test.
When you tested the damper @ 23° you have a small "blip" at -12mm on the bump side, this "blip" becomes less with increasing temperature and allmost disappears at 55°.
To me it looks like that your valve opening is maybe a bit "sticky" at first, and becomes smoother with higher temperature, leading to a smoother transition in the force graph around this point.

[747heavy]

so now that we have established, that we need to compensate for the shaft volume, and that our shaft volume depends on our shaft diameter and it´s stroke --> meaning, that if we have two dampers with equal stroke (let´s say 100 mm) the one with the larger shaft diameter will displace more fluid, and that our preload depends of our shaft diameter and the pressure inside the gas reservoir, let´s see what this means for our damper.

The increase in pressure inside the dampers gas reservoir can be calculated, using the "law of the ideal gas".



substituded for P2 -->



which means, that if we start with an given volume, and assuming that the temperature does not change, our pressure will increase when our volume decreases.

As an optical representation, it looks like this.



This shows the gas volume in our example (JRZ) damper, and a staring pressure of 10 bar. I have taken the 10 bar (the preload) off the graph, so that it will show only the increase in pressure.
(But I increased the shaft diameter to 45 mm, to show the effect. This is not a true representation of the characteristics of an JRZ damper. I will show some real values later)

According to our "ideal gas law" every time we half the volume, we double the pressure inside our reservoir, and thereby doubling the force on our shaft

Let´s see:
We reduce the volume from 160 cm^3 to 80 cm^3 and our pressure goes from 10 bar to 20 bar. (20 bar - 10 bar offset = 10 bar) and we see our pressure increases by 10 bar.

If we multiply this pressure by our shaft area, we have a force increase over the stroke ( force incease/stroke = spring rate)
Now if we reduce our new volume of 80 cm^3 by 50% again, we double our pressure again.
At 75 mm stroke we have 40 cm^3 volume and (2x 20 bar - 10 bar offset) = 30 bar.
As you can see, that spring rate of an gas spring is extremly progressive.

Everybody who has calculated the ancient "rice corn game" on a chess board, will know that if you keep doubling your values, things get interesting towards the end.

So what does it mean for our damper.
That the effect of the gas spring depends a lot on the ralationship of gas volume/shaft volume.
As larger the number, as smaller the effect.
If our gas volume only reduces by 10-20% with the stroke of the damper, the change in force is small, and depending on the shaft size (pressure x shaft area = force)
the change in overall springrate (main spring+damper spring rate) is perhaps insignificant for stiffly sprung cars, which only using small amounts of damper stoke.

It´s simple:
small stroke = small change in volume (for a given shaft size) --> small change in gas volume = small increase in pressure/force /damper spring rate
small spring rate change in comparsion to a stiff main spring = very small change in percent of overall vehicle spring rate.
This is perhaps one of the examples Speedsense is refering to - and rightly so

On the other hand, if our gas volume is a bit marginal in comparsion to our stroke, we could
(Disclaimer: I DONT ENCOURAGE/ADVOCATE THIS PRACTICE)
use the very progressive increase in springrate as a tuning aid for our car.
A change in gaspressure can tune our damper gasspring into a sort of bumpstop.
I know of at least one team, which used this practice with an Porsche 996 during a 24h race at the Nurburgring.
They increased the gaspressure in the front dampers (25mm shaft) to stop the car bottoming out at some parts on the track.

You can do things like this, but you walk on a very fine line, as things can get out of hand very quickly, when we add temperature to our equation.

some examples:
this would be a example close to what happened with the Porsche Damper.
A increase in pressure = a increase in force on the shaft
pressure/force / stroke would give the spring rate which gets added to whatever your main spring rate is.



comparsion of two dampers with the same stroke, gasvolume and gas pressue, but different shaft diameters.



now let´s see what would happen, if we take our example damper, and reduce the canister volume by 50%:



You can see, that the size of the initial gas volume in relation to the shaft volume has an influence on how our gas spring force will change over stroke.
This is an important point to keep in mind, and a possible differentiation factor inbetween diffeent damper designs, when it comes to determine gas spring effects due to stroke or temperature.
While the JRZ damper shown has a gas chamber length equal to it´s stroke length, this is not allways the case.

(but it´s not uncommon, and a good "rule of thumb" for a "safe" relationship, between shaft and gas chamber volume, for most applications.
If you increase the stroke, and you increase the reservoir/canister length by the same amount your relationship/progression value will stay the same)

with packaging and weight constrains, damper designers are sometimes forced to decrease the relationship.
In such dampers the progression in relation to stroke, and the effects due to temperature incease will be more pronouced.
As allways you have to find the best compromise for your needs.

An example for a damper with a smaller gas volume:

[Belatti]

Did you warm up only the gas canister or the whole damper body?
Sorry, I did not mind about it before, it would have been good to measure the
gas pressure in the damper before and after the warm up..

I heated up the whole (damper and canister) in the most homogeneous way I could. I tried to maintain a stable carcass temp for a while to make sure the same was true inside the thing...

The drifting graph was from 150 to 100psi, however the temp test was performed at 205psi... here, the gas pressures:
23°c ~ 205psi
39°c ~ 220psi
55°c ~ 235psi

There is another interesting "event" happen during the test.
When you tested the damper @ 23° you have a small "blip" at -12mm on the bump side, this "blip" becomes less with increasing temperature and allmost disappears at 55°.
To me it looks like that your valve opening is maybe a bit "sticky" at first, and becomes smoother with higher temperature, leading to a smoother transition in the force graph around this point.

Yep, I had never seen that event in one of our dampers before. I performed the test with an old damper that was stocked "just in case" (spare). It was that or nothing
I doubt about the state of the oil the too.

[DaveW]

Case of damper "chuckle", I think - see my earlier (earliest, I think) post in this thread.

[Belatti]

Oh yeah I find it. Page 3
Thanks.