To learn and share about dampers / shock absorbers

[RacingManiac]

Talking from a monotube shock the spring rate from the gas force actually comes from the displaced rod flow(hence why larger rod will have more effect, since the rod takes up more fluid volume). If you have a rod size similar to the floating piston size, the amount of your shaft stroke(giggle) will be very similar to your floating piston stroke, which will cause quite significant pressure change in your reservoir. Even if the disc valve opened, the displaced rod volume still needs to be accounted for, or else you will be hydrolocking the system.

Charge accumulator with a hydraulic damper is effective how hydrostrut type suspension works, as they neglect suspension spring altogether and their "suspension" comes from the gas accumulator...

[747heavy]

Hi Speedsense,
thx for your reply and the kind words - it´s appriciated.
I think we need to straighten some things out, to make sure we are all on the same page. But at the moment, I have not so much time. I will some back and (try to) explain some things a bit better. I think we have a bit of misunderstanding about some terms and formulations.
I think RacingManiac has made some good comments. So maybe you think about what he has said, and I will try to add to that tomorrow a bit.
To show a bit how the gas spring effect "deforms" the damping graph, I have made a quick simulation.
It may is not 100% accurate, but I hope it illustrates the point, I´m and RacingManic are trying to make.
The damping terms is just simple quadratic damping with an factor to come into a force range, which is a bit realistic. (the formula I used is velocity^2*6/100).
It assumes pure damping and is only velocity dependent (flow through an orfice).
THe gas force is only position(stroke) dependent.
The graphs shown are for 50 mm stoke with different shaft diameters and gas pressures.

first cases shows a damper cycled @ 1Hz with +/- 25 mm stroke and 100 psi gaspressure.
Shaft diameter is 16 mm (5/8") and separation piston diameter is 44 mm.
In this case the loadcell is zeroed at the beginning to take the preload/nose pressure out. Which means the loadcell will read 0 N force at the BTC and 0 mm/s speed. THe dyno does not account for the gas spring rate which will cause on offset to the bump/compression side (positive force).
The yellow graph shows the "pure" damping forces, as whould a TRD damper.
Friction and other "real world" effects are not shown, it´s just a simple simulation, to show the sping force effect in an combined/real graph.

16 mm Shaft / 100 psi gas pressure


16 mm Shaft / 200 psi gas pressure


25 mm Shaft / 100 psi gas pressure


25 mm Shaft / 200 psi gas pressure


now, most dynos will make a compensation for the force coming from the gas spring.
a simple way, to do this and have the graph centered around the 0 mm/s point/axis
is to take the (force in TDC - force in BTC)/2 and offset it from the force data measured. This tilts the football graph around the centre. Some dynos, may used better more sophisticated corrections, but some just do that.
The result will then look like this for the same damper, and a higher springrate will show up in the data as an increase in hysteresis, especially around the 0 mm/s point.

25 mm Shaft / 100 psi gas pressure with correction from the dyno


25 mm Shaft / 200 psi gas pressure with correction from the dyno


this shows the same damper as above 25 mm Shaft/200psi with correction, but in the case of the red line, the gas volume in the reservoir was only 1/2 of the volume in the blue line, making for more progression of the gas spring.



this shows the spring force of the gas spring and the spring rate over stroke, for the last example
These are the position dependent forces which are added to the velocity dependent damping forces as the shock compresses more and more.


O.K. that should be it for tonight, I hope it makes a bit of sense.
I will try to explain a bit better tomorrow.

[DaveW]

Brilliant, 747.

Speedsense: Some damper types don't cavitate (e.g. TTX & DSSV) & so use low charge pressures. TTX is a through rod damper anyway, & so has no charge pressure "preload". Changing from one damper type to another (same open length & same springs) on an F3 vehicle, for example, can change static ride height by up to 5 mm., & this is considered to be significant for an aero vehicle. The ride height change is caused by the different preload & not by a change in damper "spring rate", which would be expected to be negligible in a good damper. "Jacking" a vehicle's running ride height with damping style is another subject altogether, I think.

[marcush.]

Brilliant, 747.

Speedsense: Some damper types don't cavitate (e.g. TTX & DSSV) & so use low charge pressures. TTX is a through rod damper anyway, & so has no charge pressure "preload". Changing from one damper type to another (same open length & same springs) on an F3 vehicle, for example, can change static ride height by up to 5 mm., & this is considered to be significant for an aero vehicle. The ride height change is caused by the different preload & not by a change in damper "spring rate", which would be expected to be negligible in a good damper. "Jacking" a vehicle's running ride height with damping style is another subject altogether, I think.

TTX is two tube design this is also key element in getting rid of cavitation as the fluid is not succed throu the orifices but pushed through in compression and rebound.When these were introduced we tried to run them completely unpressurized and they worked quite well even in that condition.

[speedsense]

Brilliant, 747.

Speedsense: Some damper types don't cavitate (e.g. TTX & DSSV) & so use low charge pressures. TTX is a through rod damper anyway, & so has no charge pressure "preload". Changing from one damper type to another (same open length & same springs) on an F3 vehicle, for example, can change static ride height by up to 5 mm., & this is considered to be significant for an aero vehicle. The ride height change is caused by the different preload & not by a change in damper "spring rate", which would be expected to be negligible in a good damper. "Jacking" a vehicle's running ride height with damping style is another subject altogether, I think.

Yes, Nascar Cup Cars are another subject all together and in a world of their own ....

All of my experience has been with pressurized shocks.

The overheated shock was on a RT-41 Atlantic car (full ground effect car) and the shock in question was 24mm from an exhaust pipe, in one of the first tests with the car, I missed the fact that it would heat the shock.....

"The ride height change is caused by the different preload & not by a change in damper "spring rate", which would be expected to be negligible in a good damper."

This I agree completely, though not what the author of the said article presents. Either the article is written poorly or the author is truly mistaken.
Especially were he states that some people actually lower their spring rate because of the spring rate of the shock. It's just beyond me that someone with experience with race cars would think this way (IMHO, wrongly so)

As for static ride height changes, when switching out a shock, the ride change is apparent, as stated before the shock statically is solid and could cause the change, though would the ride change (only because of swapping the shock) be present after running the car for a few laps and putting the car back to static, would it still be 5mm? Even with the shocks at heated (running temp).

I believe that some of the "theories" (for valving, settings,chassis effect, etc.) that some believe, revolve around static position shocks. Yet the shock, once in use are never (rarely?), in that position where both sides (Reb, bump) are closed. And some are contrived on a shock dyno, measuring a constant length stroke with known values, yet the shock rarely is performing in such a manner and chaotic in velocity in reality.
IMHO

[DaveW]

... the author is truly mistaken. Especially were he states that some people actually lower their spring rate because of the spring rate of the shock. It's just beyond me that someone with experience with race cars would think this way (IMHO, wrongly so)

Mmm.. Rockers on race cars frequently have a "rising rate" characteristic. Hence reducing damper preload will, before adjustment, cause an increase in average spring rate. I have watched a race team recover the change in static ride height by adjusting push rod lengths, rather than adjusting spring platforms. I wonder....

[marcush.]

I´m pretty sure there are guys out there who will not appreciate that a ride height change caused by a spring change is to be treated differently than a rideheight change you do for aero or CoG reasons...in F1? naa....but there are a lot of bellcranked cars out there..

[speedsense]

... the author is truly mistaken. Especially were he states that some people actually lower their spring rate because of the spring rate of the shock. It's just beyond me that someone with experience with race cars would think this way (IMHO, wrongly so)

Mmm.. Rockers on race cars frequently have a "rising rate" characteristic. Hence reducing damper preload will, before adjustment, cause an increase in average spring rate. I have watched a race team recover the change in static ride height by adjusting push rod lengths, rather than adjusting spring platforms. I wonder....

Yes, many have progressive rate rockers, though not sure what you mean by "reducing damper preload,... cause an increase in average spring rate"? Adding preload or reducing it does not and cannot change the spring rate. A progressive rate rocker only changes the motion ratio.
Adding preload/subtracting it, only "places" the work rate in a different area of the spring's range, the spring rate is a constant. Nor does the progressiveness of a rocker change the spring rate, only the motion ratio...
Preload on a spring, connected to a suspension that is not droop limited, will raise the ride height of the car from the perches. However, a droop limited suspension, that is just below (say 4mm as example) the set ride height of the vehicle, will only change the ride height the difference of the actual ride height to the droop limitation. Adding further preload will compress the spring/shock, and the only way to correct the ride height is through the push rods.
The same is true if you running zero preload and full droop suspension, the only way to prevent an unwanted preload, is to adjust the pushrod for ride height adjustment.
What I question is exactly how much springing is added by the shock's spring rate, specifically once the shock is in motion. We know it's there when the shock is in a static position, the following are true:
1) A force produced by pressurization times the rod area
2) A stiffness from pressure rise due to rod insertion.
3) A static (Coulomb type) friction arising rod seal and piston friction.
- quoted from John C. Dixon, The Shock Absorber Handbook, page 256 on gas spring.
As 747 pointed out and as Dixon does, the gas spring has a stiffness that is increasing according to piston, and shaft size increases. And as both point out, the Coulomb friction can be quite small with good design, Dixon " 5n or less (1 LBF)
So again according to Dixon, several conclusions regarding damper force production (edited without the math analysis):
1) The reservoir pressure acts on the rod area, to give a static force (independent of velocity
2) The foot valves give a pressure drop that acts directly on the rod area. 3) The piston valves give a pressure drop that acts on the annulus area, for both compression and extension.
4) ...solutions to math analysis, left out due to length)
5) Forces proportioned to areas squared...Because the area produces the volume flow rate, which produces the pressure, which then acts on the area (for linear valves.
So according to number 3, which when the piston is in motion would drop the pressure on the annulus in both motions, the question becomes how much the "gas spring" has an effect. It would stand to reason that the faster the velocity is, the lower the acting pressure on the moving annulus becomes.
Unless the shock is of an inferior design (most shocks today in use, are of very good quality) then the gas spring effect acting on preloading the annulus once in motion is a subtraction from what it would be in the static position. The greater the flow rate of the piston/valving,the less the gas spring has effect.
AAIMHO

[DaveW]

speedsense: I think that you are referring to the rate of the spring itself. I was referring to the spring rate at the wheels (arguably, the only one that matters).

[speedsense]

speedsense: I think that you are referring to the rate of the spring itself. I was referring to the spring rate at the wheels (arguably, the only one that matters).

Yes I was, and the shock. Though it would be relative, as the wheel sees (from the rockers) leveraged force rate and not a change in spring rate per say IMHO..

[747heavy]

Hi speedsense,

Sorry mate, I did not forgot about you, but needed to get some othe things out of the way first.
I will answer or comment to your questions/comments in seperate replies. I feel that way it is easier to follow, and we don´t need to requote the whole lot, when replying.

What do I mean with hydraulic lock out?

For me it´s the same as for RacingManiac.
In a conventional damper (e.g. no constant volume dampers such as TRD or Rotational dampers) you need a reservoir (volume) to take up the volume displaced by the shaft
(PI/4*d(shaft)^2*stoke) plus the volume increase due to the expansion of the oil volume with temperature (~0,00064*Volume_Oil*delta_T).
If the volume in the reservoir can´t accomodate this increased volume, the damper will "lock out" and the force is approaching infinite (assuming oil to be incompressible (~0,07%/Mpa-for pure oil), and nothing in the damper to yield).
If this happens on a dyno, it either triggers the overload protection, or something is going to give way.
If happen in the car, it may have a similar effect to the damper bottoming out - instant loss of grip is a very likely outcome. But I have seen dampers in rally and off-road applications explode under these conditions, after jumps.

About Moton and JRZ dampers.
I think the are the same, as far as te working principle and philosophy behind is concerned. I don´t know the finer details, but it probably goes back to the same man beeing involved at different times - Jan Zuijdijk
This is quite common within the dutch damper industry, that they are all somehow "connected" with each other, in one way or the other.

some info for the interested reader can be found here:

http://www.janzsuspension.com/
http://www.vehicledynamics-expousa.com/ ... ijdijk.pdf
http://www.jrzsuspension.com/uploads/20 ... og2010.pdf
http://www.motonsuspension.com/index.ph ... ilosophyms

As far as the function goes, they are the same damper. If you read there product discription, it´s like a carbon copy of each other.
I post that only as reference, it does not mean, that I agree with everything which is said in there publications, but we will come to that later.
They both use a 22mm shaft (not 25mm, as I initialy thought), but the underlying concept remains the same nonetheless.

[747heavy]

some tables I did,
I will explain a bit later, but you can start to look at it, if you like

amount of preload as a function of gaspressure for different shaft diameters



percentage change of spring stiffness due to gas spring rate for different main spring rates.



you may need to right click and select show gaphic and zoom it out, so that you can read it.

As you see, and said before, the contribution to the overall spring rate is small in most cases, and the change in preload is the dominating factor.
with shaft diameters >20 mm it starts to be worth a consideration. - IMHO
(the springrate measurements are based on 100 mm stroke and an gas reservoir volume of 152 cm^3. piston diameter 44 mm)

Enjoy

[DaveW]

Yes I was, and the shock. Though it would be relative, as the wheel sees (from the rockers) leveraged force rate and not a change in spring rate per say IMHO..

I'm sure you don't really need me to explain why that is incorrect, speedsense....

[747heavy]

as for my comments in relation to the use of hydraulic dynos.

You have discriped one condition when this can happen, the other would be, if the damper produces so much force, that it would start to lift the whole corner of the car. In this condition, that damper will not compress futher in the car, and as it does not compress the velocity will not increase. The limiting factor is force not position.

I made this comment, because I see/meet some/many people who get all excited when they get their new Roehrig EMA dyno (or hydraulic).
The first thing they want to do is replay track data. Loading there Motec o PI damperposition vs. time data diectly as a drive file.
( I know, you did not say, that you would/want to do that. I just saw the "danger" and wanted to comment on it)

Now, they use this displacement vs. time profile to test there dampers (some test springs and dampers together), and make changes to the damping (&/or springrate).
The dyno (assumimng it is powerful enough) will just force the damper in the position. Independent of the force.
It will show that the damper now generates more force for a given velocity (or displacement when testing with springs), because the dyno just forces the velocity and providing enough energy to get there.
In the case of the car (tire). The energy is limited/defined by F=m*a. So your wheel (unsprung mass) gets accelerated vertical by the roadinput and your driving speed (simplified). Now if you drive the same car (mass) with the same speed over the same road, teh vertical accel is the same.
If you increase your damping, the wheel will travel "slower" upwards and not at the same velocity generating more force. Because at one point, the corner will start to lift, therefore providing a limit to the max. force.

I think an EMA or hydraulic dyno is a extemely usefull tool, but some people IMHO don´t use it to the best effect.
A sine sweep at constant velocity varying the amplitude and frequency will tell you a lot about the damper you use.- IMHO
As the ideal damper would only be velocity dependent, the force should stay constant. Print force vs. frequency or force vs. amplitude and see what happens.
Compare different damper designs/architecturers and draw your own conclusions.

This is just my opinion. It´s not set in stone.
I´m happy for others to take a different approach and to have a different reasoning.

[747heavy]

now let´s see where we go with our spring rate and preload discussion:

"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."

IMHO, never had to, not even once on some 60+ cars I've worked with...this statement is also incorrect. It does change slightly, but not enough to have to move a perch......coil over or rocker suspension....

and

So in summary, the shock has a spring rate, but only when stationary, and is the only time it "could" effectively change the ride height on a car or add to the spring rate. Just add velocity, and there's isn't a spring rate to be concerned with.

O.K. first I thing we need to agree on some terminology (wording)

perfect/ideal damper = a device which generates force in relation/dependent from
it´s velocity.
This force is position independent, wich means at zero velocity the ideal damper will produce zero force. And it will be zeo force at any position of his stroke.

spring = a device which produces force in relation to it´s position/displacement.
More displacement equals more force (F=k*s) k=springstiffness s=displacement

Now, if we are clear with this (and I hope we are), then we can conclude, that a conventional pressuriezed racing damper is a combination of the two.
The gas spring in the damper has a spring rate (they is small in most cases).
Don´t believe me? It´s only preload not springrate. O.K. - let´s make a test

Take your damper (without a spring) of choice, with whatever damping setting&gaspressure you see fit and put it into a "SPRINGTESTER".
Start to compress it. What will happen?
It will not compress, but the rading for the force will increase. - Good
Now comes a point where it starts to move. - Good
Pause here and zero your loadcell, buy doing so, you have taken the preload component out.
Now, all things beeing perfect, we have a good loadcell with no drift, and jumping between digits. We should read 0 N/lbs force, because our damper doesn´t move and has zeo velocity. Makes all perfect sense.
Now, let´s compress our damper 1 or 2" (or whatever stroke you like, more make the calculation more accurate) and pause again.
What happen?
Our damper has zero speed now --> zero damping force, we have taken the preload force value (nose pressure) off.
Why do we have a reading on the loadcell? Does not makes sense, if we have a "perfect/ideal" damper.
Take the reading from your loadcell and divide it by the amount you have compressed your damper.
Voila -> it´s a spring stiffness value (for a 5/8" shaft and 200 psi it should be ~ 0,4 N/mm or 2,3 lbs/in - the actual value will depend on the volume ratio of your shaft vs. canister/reservoir)

So in my world, it´s a spring and it will add force to whatever damping force the damper produces in relation to the stroke(position).
Do I care for an additional 2,3 lb/in in my car spring rate?
That´s not up to me to decide.
It´s less of an variation if I use a 1000 lbs/in spring in my car, but if I use a 80 lbs/in spring it may is worth a consideration.

If you look in the second table I have posted before, you can choose your shaft diameter, and your pressure. It will give you a average springrate for 100 mm (4") stroke. And you can see how much this is a percentual change for your main spring depending from the value of your main spring.
The influence get´s less with higher main spring rates.

Hope it makes some sense.

I will come to the temperature issue at another time.
Need to put some values together, as I prefer to talk "numbers".