To learn and share about dampers / shock absorbers

[747heavy]

well, you have a point, and as with most things, there is more then one possible reason for it.
A increase in temperature may also affect the viscosity of your damper oil, and with that depending on your valve design your damping forces.

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.

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.

It´s not to prove anything, and is maybe more academical, but it will help you to
see how your dyno measures things, and what he can or can´t compensate for.

I´m not familar with your type of dyno/software used, so it´s difficult to make
recomendations or statements in this respect.
You will need to see in the manual, as if and how your dyno measures and accounts
for gas force (what the software calls preload) and seal drag/friction.
Same dyno´s measure the value and take the preload(offset) out of the data, and in others you need to provide an value for shaft diameter and gas pressure.
Some people zero the loadcell when the damper is mounted and preloaded in the dyno, others don´t, depending of what your dyno does before he starts to measure.
in some dyno´s you can deactivate the check gaspressure/seal drag/friction feature
befoe each run, to save some time when you just do an adjuster sweep or something like this.

Best is to play a little bit around with it, and then I´m sure you will see, how things show up in the graphs.
The offset Dave mentioned, can have many/other reasons, just wanted to hint onto one possibility, not claiming, that this is surely the reason for the offset.

[DaveW]

Great stuff, Belatti & 747. I would just add that it is useful to know damper preloads, because they can (do) affect static ride height.

[Belatti]

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

[747heavy]

maybe you find some useful info here.
It goes along the lines, of what I have tried to explain.
It´s specific for Roehrig dynos, but most dynos handle the "problem" in a
similar way. Some of the basics are universal true, as in which load/force is your
loadcell seeing.
There are some differences from dyno to dyno manufacturer, as of how to account for
the effect in the software.

http://www.roehrigengineering.com/Techn ... ESSURE.pdf

http://www.roehrigengineering.com/Techn ... lained.pdf

http://www.roehrigengineering.com/Techn ... 0CURVE.pdf

some other stuff, which you may find interesting

http://www.roehrigengineering.com/Techn ... %20CVP.pdf

[747heavy]

I just played around with some numbers, maybe not all are correct, because I had
to take some guesses as far as the gas volume in your reservoir/canister goes.
But (unless I have made a miss calculation somewhere ), I think it should be correct at least at an order of magnitude level.
It assumes a 5/8" shaft (~16 mm) and 200 psi reservoir pressure

For your test on Friday, if I can make a recomendation, I would choose a 1Hz test
with 1" of stroke (I believe, this is your standard test stroke),
this will give you a max. velocity of ~ 80 mm/s.
By doing so, you should be able to use a scaling on your force axis of ~ +/- 40kgf or 500N, and keep damper/oil warm up to a minimum.

If you start the test with 200 psi gaspressure and drop to 100 psi gaspressure, you should see, that your force vs. displacement graph (football) will have an offset of ~ 14 kgf (137N) to the rebound side. Which means that if the centreline was at "0" to start with, the graph is now centered about -14 kgf.

If you do the heat up test of the reservoir/canister, an increase of (gas) temperature from 25°C to 65°C should result in an increased force (offset to the bump side) of ~ 5,9 kgf or 58 N.

By choosing the +/- 500N max force scaling, this should be possible to see.
That are just some "off the cuff" calculations, not taking seal drag/friction or change in oil viscosity into account.
It was just meaned to give you an ball park figure of what to expect. The first test being the more easy and quick to do.

Good luck & have some fun

[747heavy]

as we have talked about cavitation in dampers, and what happens when, due to inadequate pressure balancing, the pressure in the rebound chamber becomes lower then vapor pressure, some may like to have a look at the following videos.
It shows what happens inside a damper under these conditions.

http://www.roehrigengineering.com/downl ... shock1.wmv
http://www.roehrigengineering.com/downl ... shock2.wmv

[Jersey Tom]

Good videos.

[speedsense]

maybe you find some useful info here.
It goes along the lines, of what I have tried to explain.
It´s specific for Roehrig dynos, but most dynos handle the "problem" in a
similar way. Some of the basics are universal true, as in which load/force is your
loadcell seeing.
There are some differences from dyno to dyno manufacturer, as of how to account for
the effect in the software.

http://www.roehrigengineering.com/Techn ... ESSURE.pdf

http://www.roehrigengineering.com/Techn ... lained.pdf

http://www.roehrigengineering.com/Techn ... 0CURVE.pdf

some other stuff, which you may find interesting

http://www.roehrigengineering.com/Techn ... %20CVP.pdf

Maybe it just me, but I have some "real" problems with this article
(http://www.roehrigengineering.com/Techn ... ESSURE.pdf)

When the author made this statement

"If you put 50 psi into your shock and put it on the car, it would raise the car by a small amount because the gas force is trying to push the shaft out. If you added more pressure, say 200 psi, it would raise the car even more. Now, I am not suggesting it will raise it 1 inch, but it will raise it. What do you do then? You take rounds out of the car to get the ride heights you want. Basically, when you scale the car you take into account the effects of the gas pressure from the shock. If you add gas pressure, you re-adjust your spring heights and away you go. This is also the reason that some people use the gas pressure of the shock to act like a spring in the car, thus allowing them to run less spring in the front of the car. It is for this reason that Roehrig decided to measure the gas force and then remove it from the data. Like you do on the car, we remove the static effects of the gas chamber on the data."

IMHO, simply incorrect to state that shock pressure can increase the car's spring rate OR increasing shock pressure will raise the car. Especially lowering spring rate by running higher pressure. I've never, ever seen this happen on any race car I've worked with.

Another one incorrectly stated, IMHO

"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 one more--
"Another thought would be, as the temperature of the shock increases, the temperature of the gas chamber increases and as a result the pressure goes up and the force exerted on the shock shaft increases. This means that during your 20 or 30 lap run on the track, the pressure in the shock is increasing because the temperature of the gas chamber is getting hotter and the increased spring rate may be affecting your car’s handling. Think what would happen if 20 laps into your run, you added 25 lb/in of spring rate to the front shocks, what would it do to your handling?"

IMHO, never happens under normal running/temp conditions (as far as ride height change goes). Though, it did happen one time, when a shock got heated by a close by exhaust pipe and overheated the shock oil. Caused the shock to hydraulic when it moved and reacted like a hydraulic spring when it moved, IE.- ..an extreme case...

While this guy may have a great understanding of shock dynos, his thoughts of how shocks work in a "real life" application, aren't in touch with reality in my opinion.
Yes, you can effect the SHOCKS internal spring rate with pressure, but you CANNOT effect the car's spring rate NOR will the ride height change as much as he suggests that you actually have to move a spring perch to compensate.

I will add, if you pressure a "static" (on a setup table)on a "normal" set of shocks (coil over car), you can raise the car, that is if don't roll the car and move the shock (apply some velocity to it), the ride height will return to the ride height setting after doing so... You have only effected how the shock works with pressure, NOT how the springs work or their rate or the ride height.

A static shock dyno is good for making sure the shocks are all equal and working as desired. A great baseline for understanding where you are in terms of force vs velocity. Though not much more is useful.

Now, a hydraulic dyno, on the other hand is a real life description of the workings of a shock, as actual chaotic shock movement is possible. The information available is much more useful, but as with all things this expensive, not available to a lot of lower level teams...

[RacingManiac]

The whole gas pressure adding to the rate thing though, wouldn't it depend greatly on the actual ride spring rate, your shock bore and stroke combo, and how that relates the ride spring rate though? The "gas spring" of the shock reservoir is essentially running parallel to ride spring that whatever rate you get is added to the ride, and if they are close rate wise to each other the contribution will not be insignificant I'd think.

[747heavy]

Hi speedsense,

can you please expalin a bit better what you mean here:

Caused the shock to hydraulic when it moved and reacted like a hydraulic spring when it moved, IE.- ..an extreme case...

do you mean hydraulic lock out?
what is a hydraulic spring in your case/opinion?
Thx.

Now, a hydraulic dyno, on the other hand is a real life description of the workings of a shock, as actual chaotic shock movement is possible. The information available is much more useful, but as with all things this expensive, not available to a lot of lower level teams...

I will agree partly with what you say here. Sure, there is no doubt that a hydraulic or electromagnetic dyno is very useful and a great asset for every team.
But it poses a small risk/inperfection when not used carefully. The devil is in the detail of the control method. Most of these dynos (at least as a default setting) are position controlled. Which means that the dyno will force the damper
into a position (creating xxx N force). I a real world application the suspensionis force controlled, which means he can "pause" in it´s movement under certain condition.
Nevertheless, I would use one, every day of the week, if I have the chance, but would probably consider a conversion into a single post rig, which would be the next step up.
It´s as with most tools horses for courses, and every method has it´s benefits and it´s limitations.

As for your other statements.
I don´t think, that what the guy says is "technically incorrect", but would
agree, that in some (most?) cases the effect is rather academical, as far as
performance on the track is concerned, taking other real world effects such as friction and sticktion in the suspension system into account.

The effect will be greater with larger diameter shafts and will depend greatly from/vary with the volume of your of your gas-chamber.
Ratio of shaft dia./separation piston dia. (progression of gas spring)

I think Moton is one of the dampers, where this effect is used as a tuning aid.
That´s one of the reasons that they run such large shaft diameters.

If we make a simple calculation and take a 5/8" shaft (~16 mm), like a Penske damper and set the gas pressure to 100 psi (~6.9 bar) the extension force on the shaft is ~14 kgf (137 N).
If we double the pressure to 200 psi (~13.8 bar), the extension force will be ~28 kgf (273 N).
The difference is ~14 kgf (137N).
If we do the same on a damper with an 1" shaft (25,4 mm)the difference is ~36kgf (350N)

For the sake of an simple example, let´s assume a 1:1 motion ratio and a spring rate of 100 N/mm the ride height difference in theory (ignoring friction and other effects) should be 1.37 mm in the first, and 3.50 mm in the second case.
If we have 200 N/mm spings it will be half of it, and probably within the measurement tolerances in the first example.
If this is a "problem" or worth consideration, will depend on you application IMHO.
It may be different, if you race a StockCar or an Formula Ford.
Also keeping in mind that changing gas pressure by 100% is quite a large step, which you normaly don´t do (at least in my expirience).

This would be the effect on static ride height or "preload".
The contribution to the running spring rate would be ~ 1,9 N/mm for 100 psi and ~3,8N/mm for 200 psi with the 5/8" shaft. So that means the effective running spring rate in our example changes from ~102 N/mm to ~104 N/mm.
I would agree, that this in real world conditions is rather academical.
For the 1" shaft it would be 5.5 N/mm and 11 N/mm. If we take our 100N/mm main spring example, then an pressue change from 100 psi to 200 psi would change the effective spring rate from 105,5 N/mm to 111 N/mm or a ~5% increase. If our main spring is 200N/mm the increase will be only 2.5%
While I agree, that the first example is rather academic, the second one, especially if we run main springs in the 80-120 N/mm range, should be perceptible
for a good driver.

The spring rate characteristic will change with reservoir/canister gas volume and is stroke dependent, especially for dampers with large dia. shafts.
For the calculations above I used and estimate of 44mm dia and 80mm length for the gas chamber, and asumed a stroke of 2" (~51 mm).

If the effect is perceptible will depend on the application and spring rates used IMHO.
For shaft diameters < 16 mm (5/8") gas pressures up to 250 psi and springrates
> 100 N/mm the effect´s are probably negligible in most cases, I would agree with that.

I does poses a problem/challenge from a damper dyno manufacturer point of view, as the gas spring effect will distort the force vs. displacement graph, when large dia. shafts, high gaspressure and large strokes are used.

[RacingManiac]

At least in the case of our in house dyno, the typical test will measure friction and gas force at the extremities and mid point of the stroke, and the output data will subtract that from the data to get true damping force.

[747heavy]

yes, this is a quite typical approach, which is used by some (most?) dynos I have
worked with.
The way I have seen it, is that you measure the force difference between BTC (0°) and TDC (180°) to define the spring rate of the gas spring (assuming a linear relationship), this will help to keep the graph centered.
As 90° and 270° represent the same position (mid stroke) the (spring)force should be the same.
Any difference can be attributed to friction, this value is often halfed and taken off the data, assumimg that pure mechanical friction is symetric.
There are pro&con arguments as of to stop/pause the dyno in this positions to take the measurements or to circle it very slowly. (Slip Stick problem)
Here different dyno manufacturers take a different approach, some let the customer decide, if he want´s to pause or not, and which velocity to use for the friction cycle, if he preferes not to pause at 90/270°.

[RacingManiac]

We uses hydraulic actuator based dyno here at work, the program basically cycles the shock by millimeters at top, middle and bottom of the stroke to get the gas force and friction value at each point.

[speedsense]

Hi speedsense,

can you please expalin a bit better what you mean here:

Caused the shock to hydraulic when it moved and reacted like a hydraulic spring when it moved, IE.- ..an extreme case...

do you mean hydraulic lock out?
what is a hydraulic spring in your case/opinion?
Thx.

Not sure what you mean by hydralulic lock out? If you mean flow stoppage, yes, the shock started to work as though the valving was no longer operating, at least partially, as the velocity grossly slowed. The shock was literally taking almost a sec to do a rebound stroke. Over bumps it would actually jack that corner up.
A hydraulic spring in this case, would be akin to shoving a solid piston into fluid with no relief flow.

I will agree partly with what you say here. Sure, there is no doubt that a hydraulic or electromagnetic dyno is very useful and a great asset for every team.
But it poses a small risk/inperfection when not used carefully. The devil is in the detail of the control method. Most of these dynos (at least as a default setting) are position controlled. Which means that the dyno will force the damper
into a position (creating xxx N force). I a real world application the suspensionis force controlled, which means he can "pause" in it´s movement under certain condition.

Agreed, except this statement. What do you mean exactly? I can only think of one instance and that would be a suspension corner(s) that is no longer in contact with the ground.

The effect will be greater with larger diameter shafts and will depend greatly from/vary with the volume of your of your gas-chamber.
Ratio of shaft dia./separation piston dia. (progression of gas spring)

I think Moton is one of the dampers, where this effect is used as a tuning aid.
That´s one of the reasons that they run such large shaft diameters.

JRZ and Moton are very alike, with JRZ being the first to use such shafts. The theory, as I understand it, is to displace more fluid through the piston valving.

If we make a simple calculation and take a 5/8" shaft (~16 mm), like a Penske damper and set the gas pressure to 100 psi (~6.9 bar) the extension force on the shaft is ~14 kgf (137 N).
If we double the pressure to 200 psi (~13.8 bar), the extension force will be ~28 kgf (273 N).
The difference is ~14 kgf (137N).
If we do the same on a damper with an 1" shaft (25,4 mm)the difference is ~36kgf (350N)

For the sake of an simple example, let´s assume a 1:1 motion ratio and a spring rate of 100 N/mm the ride height difference in theory (ignoring friction and other effects) should be 1.37 mm in the first, and 3.50 mm in the second case.
If we have 200 N/mm spings it will be half of it, and probably within the measurement tolerances in the first example.
If this is a "problem" or worth consideration, will depend on you application IMHO.
It may be different, if you race a StockCar or an Formula Ford.
Also keeping in mind that changing gas pressure by 100% is quite a large step, which you normaly don´t do (at least in my expirience).

I've worked with both types of cars. Neither one, had the "effects" the author relates. Even a Cup car with High Rebound valving numbers (some 10 years ago) and staggered side to side (valving wise) for ovals. Even with the high rebound and intentionally driving the car with the shocks to the ground by shock jacking forces, the spring rate of the shock could not keep it there and it returned to ride height when the car was at rest. The springs on the car pushed the shock back to ride height (though rather slowly) and was dead on at the set ride height every time.

This would be the effect on static ride height or "preload".
The contribution to the running spring rate would be ~ 1,9 N/mm for 100 psi and ~3,8N/mm for 200 psi with the 5/8" shaft. So that means the effective running spring rate in our example changes from ~102 N/mm to ~104 N/mm.
I would agree, that this in real world conditions is rather academical.
For the 1" shaft it would be 5.5 N/mm and 11 N/mm. If we take our 100N/mm main spring example, then an pressue change from 100 psi to 200 psi would change the effective spring rate from 105,5 N/mm to 111 N/mm or a ~5% increase. If our main spring is 200N/mm the increase will be only 2.5%
While I agree, that the first example is rather academic, the second one, especially if we run main springs in the 80-120 N/mm range, should be perceptible
for a good driver.

Most shocks are recommended to be run within approx. 100-250+ lbs range (depending on the shock), under is cavitation, over and you chance blowing the seals. With the larger shaft size, one needs to understand that the flow rate is with greater force, so smaller valving changes can make large changes vs. it's smaller shaft shock companies.
I agree with most of what you say here, except as soon as the shock has a velocity, the piston valves open and you have a body moving in fluid. As far as I know, when calculating flow rates, even in pressurized fluid containers, don't have spring rates in the equation. Shocks are velocity dependent and only function with velocity and any "spring rate" that the shock has is lost as soon as it moves.
A good driver, would only notice that the ride has changed in most cases, not much else and no where near what changing the settings will do for driver feel.
Consider that watching shock velocities in data acquisition and comparing the velocities of a high pressured shock to a lower press. one, there's no perceivable difference in the velocity slope/shape/wave.

The spring rate characteristic will change with reservoir/canister gas volume and is stroke dependent, especially for dampers with large dia. shafts.
For the calculations above I used and estimate of 44mm dia and 80mm length for the gas chamber, and asumed a stroke of 2" (~51 mm).

If the effect is perceptible will depend on the application and spring rates used IMHO.
For shaft diameters < 16 mm (5/8") gas pressures up to 250 psi and springrates
> 100 N/mm the effect´s are probably negligible in most cases, I would agree with that.

I does poses a problem/challenge from a damper dyno manufacturer point of view, as the gas spring effect will distort the force vs. displacement graph, when large dia. shafts, high gaspressure and large strokes are used.

[/quote]

See, here in lies the problem. When you start the dyno, the shock contains a spring rate, as the flow ports are closed, and the shock statically is a hydraulic pump. You start the run, the data records (even though the load cell has been zero'd for the nose pressure, hopefully) and records the static spring loaded shock. As the shock is accelerated and cycled, it again stops at the end of the run, effectly closing off the ports again (though still recording data) and returning the shock to a spring or hydraulic pump that has a spring rate.
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.
AS Always In My Humble Opinion...

[speedsense]

BTW, I forgot to say, how much I appreciate this thread and the information you have provided, 747, great stuff you have posted here...