To learn and share about dampers / shock absorbers

[747heavy]

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

It will depend on your damper design and the max. bump/compression damping forces you want to achieve.
As an guide line, you can use this formula:



for your damper (assuming a 45mm piston) this would make:



This would be more or less the "worse case" but it will be save, and limiting the risk of cavitation.
But to answer this question more acurate, we would need to know a little bit more about your damper design.
Is it a monotube or twintube damper, does the damper has a valve or restriction inbetween the oil in the compression chamber and the separation piston, or does the separation piston sees the full pressure in the compression chamber?
The formula above is for the case that the separation piston sees the full oil pressure in compression/bump.

Such designs include for example: typical Bilstein dampers,some TEIN, KONI, some SACHS, ÖHLINS and Penske designs, most "cheap" monotube dampers such as AFCO and others.

The internals would look like this:



or this:


In this case the separation piston will need to support all the pressure in the compression chamber.
With high damping forces in compression/bump direction this dampers will need very high internal gaspressue.
Normal values for these types of dampers are ~20-25 bar.
This damper design is not very suitable for large diameter shafts, as the gasforce, preload or "nosepressures" will be very high.

This is one of the main reasons, that you normally see BILSTEIN dampers with small 10-12 mm shafts.

If the gaspressure is too low, the separation piston will move on impact (shockwave) more then the shaft, this leads to an increase in internal volume and therefore a pressure drop in the oilcamber (rebound side), this can lead to possible cavitation inside the damper.

See the following illustrations:

correct pressure:


gaspressure too low:


If your damper has a valve or restriction (orfice) between the volume of the compression chamber and the separation piston, then you can run lower pressures.
The principle of such a damper will look like this.




Most monotube racing dampers with external canisters/reservoirs will work like this. (for example Penske,Proflex or SACHS RDS dampers etc.)
As the valve will create a pressure drop between the oil in the compression chamber and the oil in front of the separation piston, the pressure which acts on the separation piston will be lower.
Therefore less gas pressure is needed to suppot the piston.
How much less will depent on the pressure drop at the valve, and will change with the settings if the valve is adjustable.
One would need to do a calculation when the valve design in question is know, it´s difficult to make an "rule of thumb" statement, but such dampers normally use 10-20 bar of pressure, with 12-16 bar beeing a common value for most of them.
If you use a value calculated by the formula above, you will be on the "safe side" as far as cavitation goes, but will need to live with increased friction, hysteresis (caused by the gas spring effect) and preload/gasforce especially if large shafts are used.
How much of an problem this is, will depend on your application and the spring rates used.

[747heavy]

@ Belatti

For your twintube experiments, maybe this gives some ideas.

[Belatti]

Thanks!

I wish I could make that racing twin tube damper work. Now, I wont be able to use a dyno for a while (I left you a PM)

DEFOE, it would be nice if you could tells us where you work and where are those dampers used.

[747heavy]

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

Sorry DEFOE,
I´m ,like DaveW, don´t really understand the question.
There is not really a "maximum rebound velocity" design limit for the damper.

The maximum rebound velocity will be defined by your spring rates, the amount of compression of this spring, your masses and your rebound damping.
Your spring will store energy and then release this energy to accelerate a mass.
If your tire is still in contact with the ground, it will accelerate the sprung mass/body upwards.
As lower your mass is, as higher will be the resulting velocity for a given energy.
Seeing that there is normally a large difference between the sprung mass of the car and the unsprung mass of the car, the resulting velocities will be quite different.
If your car/tire looses contact with the road, the spring will accelerate the wheel/unsprung mass downward, because the unsprung mass is lower and the wheel moves in the same direction as gravity the resulting velocity for a given amount of energy will be higher.

This in return means, that the damper will produce more force (as this is normally a function of velocity).
This is one of the reasons, for digressive rebound damping curves, because at a certain velocity the wheel will be not in contact with the ground, and we would not like to keep it up in the air for too long.

In off road racing there are acceleration sensitive valves used and systems called "fast rebound".
They will decrease rebound damping at high velocities to allow the wheel to return to the ground more quickly.

The sprung mass of your car and the amount of energy stored in your spring (springrate x travel) will define what the max possible acceleration is, as long as the wheel is still in contact with the ground.
Higher values mean, the wheel has lost contact with the road.

Depending on your damping rate you can influence the max. rebound velocity your suspension will see, what a good value for this is, will depend from the application.

It´s different for a F1 car then it is for a Paris-Dakar style car or off road race buggy.

[DEFOE]

sorry，Belatti

I am in china ，and I can't provide help to you。

[DEFOE]

Sorry，I should make a statement that our company manufacture shock absorber for Aautombile，currently not in sportvehicle.

I think that the gas filled into the shock absorber is to resist the "Cavity" of oil.

Twin Tube Gas filled Shock Absorber is our type, and I would post a photo later

[DEFOE]

[747heavy]

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?

Hi Speedsense,

Sorry mate,

I know I still owe you some answers, did´nt forgot about it, but still trying to come up with an good explaination.

Being a bit "lazy" I feel that perhaps this will answer most of your questions, and maybe some others as well, better then I could formulate it.

If not feel free to come back, and I will try to explain the little that I know.


to sum some things up.

- creating low/zero or negative pressure/vacuum inside the damper is not a good idea, as it will lead to caviation.

- the gas spring force is allways there, but surely, you can create a higher opposing force during the opperation of the damper.
Your net force (in rebound direction) will be negative then, but it would be "more negative" if your damper would have no gasspring i.e. beeing a TRD damper.
It´s a bit like spring preload vs. dynamic ride height. Sure your spring will still compress and your car dive at the front under braling (as an example).
But it will be higher at the end of the braking, then it would be with less/no spring preload. I hope this comparsion makes some sense.

- you have to see the contribution of the gas spring in relation to the overall forces generated by the damper.
If the velocity is high and let´s say the damper produces 2000N damping at xxx mm/s 100 N more or less may not be such a big deal.
On the other hand, if your damper velocity is very low and your damper produces 300N damping forces at xx mm/s 100N more or less, are quite a sizeable contribution, towards the overall forces.
So you will need to see it in context, and then decide for yourself if it is significant/a problem for you or not.

- is damper fluid compressible?
Depends on the forces and the situation, for some calculations/models you can assume it to be incompressible.
For others not, see the file I provided. It depends of what you try to model and which level of accuracy is required.
I think in most cases you will be o.k. if you assume it to be incompressible.

Have a look at this graphic (it´s from the file above), it shows the same damper at the same velocity but with a different characteristic/damping level.



So if you assume, that a damper is only velocity dependent and will allways produce the same force at the same velocity you may make a mistake in your assumption.
The magnitude of the error will depent on the damper in question and the conditions.
This damper is a fairly "standard" damper, there is no specific frequency or amplitude/stroke sensitive valving advertised/mentioned.
For this type of test a hydraulic or EMA dyno is very useful and valuable.

[Peter-RC]

first of all... this topic gives me several mental errors... all saying "information overload"

Since I don't come from a engineering background... I was thinking to start with the very basics.. The OptimumG pdf files were quite helpfull.
But at the very first file they talked about ride frequencies, based what parameters you would define a ride freqency?
And in pdf 2 he talked about calculating springrates based on the ride frequency (+motion ratio and corner weight)

Now I want to apply those calculations to my 1:8 onroad r/c car... and since it is radio controlled... it hasn't need for comfort (sort of, i don't like it shaking my electronics to pieces)

So, how would i determine the ride freq. for this little car, and thus lateron spring rates?

[sorry to have a post with 'toy' cars]

[747heavy]

Sorry，I should make a statement that our company manufacture shock absorber for Aautombile，currently not in sportvehicle.

I think that the gas filled into the shock absorber is to resist the "Cavity" of oil.

Twin Tube Gas filled Shock Absorber is our type, and I would post a photo later

O.K. DEFOE,

I see where you are comming from.
Your damper basicly works like this, as far as the displacment of the oil in the damper is concerned.



So our first excercise where we calculated the gasforce in relation to your gaspressure and stroke for a given shaft diameter still holds true - IMHO.

How much pressure you need to fill into your damper will depend on the design of your foot valve and the flow/pressure drop relation of the footvalve to your main piston valving.

5-6 bar pressure is not uncommon in this type of damper (it´s allready a bit on the high side).
You don´t normally use much higher pressures in this typre of dampers, therefore forget about the min. pressure calculation we did above, as this is mainly for monotube dampers.
And seeing that it is for a automotive application your bump/compression damping forces are probably not that high anyway (compared to some racing applications).

I assume, that you will generate most of your compression damping in your foot valve anyway, and that there is no large pressure drop between the compression and the rebound chamber in your damper (not in bump/compression direction anyway).

You use the gaspressure, as you say, to prevent cavitation and to help with the re-absorbation of micro bubbles/gas solubility in your damper, especially around your footvalve.

A good twintube design will work without any pressure, but adding some pressure will give you a bit more freedom with valve design and increase the response of the damper/valves a bit.
It will help to make the damper perform more consistently under harsh conditions.

But seeing that this is a commercial opperation, I´m sure you have the R&D facilities in place to evalute the effects on different seal designs and materials on overall friction in your damper.
Therefore, there should be no need to discuss this here in great detail, as this may goes a bit far.

Have a nice day

[DEFOE]

OK，Thank you very much for your suggestion

Would you pls give me some forum or address，caused I am lack of such knowledges。Thank you

[747heavy]

And in pdf 2 he talked about calculating springrates based on the ride frequency (+motion ratio and corner weight)

Hi Peter,

Not sure where you would get your target ride frequencies from (for your application), but I did a example calculation for you showing the resulting spring rates.

Now I did some assumptions for car weight, weight distribution and unsprung weight.
They are probably not 100% correct, but it should be good enough for an "order of magnitude" estimate.

If you can povide some data, for spring rates, sprung and unsprung mass and motion ratios, we can calculate your current "ride frequencies".

Not sure if this is all that useful for your application, but we could try together.

So what do you think? Does that makes some sense? Or is it totally "on the moon"



P.S.: Let me know, if you need some help/tricks to measure corner weight, motion ratio and sprung/unspung weight for your model.
I have some ideas, how you can do this relative easy.
What makes the OCP/DAX rear axle? Did you test it yet?

[747heavy]

OK，Thank you very much for your suggestion

Would you pls give me some forum or address，caused I am lack of such knowledges。Thank you

Sorry DEFOE,

I did not mean to be rude, nor did I want to offend you.
But trying to define design parameters for damper seals, or making accurate calculations for seal friction in a damper, is not something that can be explained in a couple of sentences.

First and foremost, it would exceed my level of knowledge, as I don´t design dampers for a living, just trying to use them in a sensible way to win some races. Secondly, it´s an area where every damper manufacturer has it´s own way to achieve what he wants in this respect.

I don´t know how or for whom you produce the dampers, but there are normally some requirements beeing outlined by your customers.
They will specify the damping levels/forces they want, the operation conditions (temperature range, mounting positions, stroke, length etc.etc) and most likely they will tell you, what levels of friction they will tolerate, or which levels of friction they need.

To explain just two things very simple:

Why has a damper with a 22mm shaft more friction then a damper with a 12 mm shaft (all other things beeing equal)?
Because the seal area is larger, PI*diameter shaft will give the circumference value for the shaft.
As larger it is, as more friction you will have, if all other things remain the same.
The same goes for your piston diameter.
A larger piston has a larger circumference, therefore more area where the piston seal/band will touch the inner cylinder wall (bore).

Now, how much overall friction you have will depend (amount other things) from the materials you use (is your seal rubber or PTFE, is your piston band teflon or does metal rub on metal?).

As I don´t know, what you use, I can´t say anything about it.
What lubrication you use?
What coatings you have on your shaft?
What is the surface roughness of your shaft?
All these things (and many more) will affect your friction.

As for why does the friction increases in most cases with higher internal pressures?

Because normally, if you use a lip seal, one part of the seal is exposed to the pressure in the oil --> higher gas pressure means higher pressure inside the oil.
Therefore the seal get´s pressed with a larger force against the shaft.

This is quite logical as a higher internal pressure will require a "better sealing".
So doing it this way, the seal will "adapt" the amount of "sealing" in relation to the internal pressure.
I made a quick sketch of an lip seal type shaft seal.
I hope this will help to explain it a bit.



Now, how much does the friction increases, depends on you seal design.
How much area is exposed to the oil?
Which materials do you use (Rubber, PTFE etc.), not all seals are equal, not everybody uses a lip seal, not everybody let´s the pressure inside the damper affect the seal preload.

There are means to an end, but I don´t know which "end" you have in mind.
Is low seal friction important to you?
Is good sealing important to you?
What´s the required service life of your seal/damper?
What´s the pressure inside the damper?
How much does the seal need to deal with shaft imperfections (pitting) etc.
It´s impossible for me to know, what you would like to achieve and why.
A F1 damper has other design criteria, then a rally or off road damper etc.

I´m not sure, you will find a website or forum where these type of questions get answered or discussed in great detail.
I don´t know any - sorry.

People who design and built dampers, normally do it, to sell them and to make a business out of it.
So I doubt that many will share their "secrets and tricks" of the trade.

It´s a bit like asking Coca Cola to post the recipe for Coke, so that you can make your own.

If you ask a large manufacturer of seals for dampers, I´m sure they will be able to answer some of your questions, but also would want to sell you there seals.
Normally, there is no free cheese in life.

Good luck on your way.

[DEFOE]

hi 747heavy,I am in shock absorbers only four months,I can't find no words but thank in my hearts that your reply so much to me caused your warmheart.
Thanks a lot.

[747heavy]

Thanks - you are welcome
Good luck