To learn and share about dampers / shock absorbers

[RideRate]

speedsense,
The spring effect of a shock is there and is very real and as long as we run single rod dampers charged with nitrogen this will be the case. An ideal damper will exhibit a force output that varies strictly with velocity, which is what I think you desperately want to believe you have. However, the single rod charged dampers we use are actually an ideal damper and small gas spring in parallel. The spring effect is a by-product of the damper and doesn't effect the velocity dependent characteristics of that damper, it's just a necessary component of the system. If your charge is high enough you better factor it into your suspension setup and possible choice of springs. What matters in the end is the total spring rate acting at the wheel and if you add a bunch via your damper you may want to remove some via your spring to maintain your operating point.

There is a real reason base valves and the like were invented. There's a real reason guys prefer to minimize damper charge pressure as much as possible. If the gas spring effect in the damper never made a difference we'd never need base valves and we'd never worry about keeping charge pressures low. We'd just charge dampers with 500 psi with no ill effects and no worry of cavitation. But this isn't the case. 500 psi is going to manifest itself as a huge gas spring that will likely be unwanted in our setup. Therefore we spend time with basevalves, anti-cavitation devices, and achieving low charge pressures. It keeps our damper as close to the ideal damper as possible.

Supporting a car with dampers is far from ideal and not most efficient. However, if rules pigeon hole you into that corner and using the shocks in that manner is the only way to achieve an effect that net makes your car faster would you not use it then?

[Belatti]

When weight and packaging is not a big problem, for some specific racing cars you could use oversized remote reservoirs in the dampers. This would make a "not so progressive" gas spring, please correct me if Im wrong.

Changing subject, lately I have been designing and testing many low speed (needle) valves. I ended up preferring parabolic (grade 4) profiles rather than cones, for a more linear force control through the adjustable range.

Check this graph:



Here you can see tests for a digressive rebound adjusted from closed (hard) to 75 "clicks" in steps of 3 clicks with a parabolic grade 4 needle. Till click 60 the low speed gets choked, the shims open and the force curve "bends". When I reached click numbre 63 the jump is notorious, and the shims dont act anymore. So I went back and tested clicks 61 and 62 (highlighted). There is some dynamic effect I am not taking into account...

[Belatti]

Here, a couple of needle tests but for bump. Guess wich is parabolic shape and wich is a cone

1


2

[Belatti]

Here, one of the things Im very proud of:



A 3 way damper that can REALLY adjust separately low and high bump speed. It can even go from linear to digressive using the same piston although with different shim config.

[speedsense]

speedsense,
The spring effect of a shock is there and is very real and as long as we run single rod dampers charged with nitrogen this will be the case. An ideal damper will exhibit a force output that varies strictly with velocity, which is what I think you desperately want to believe you have. However, the single rod charged dampers we use are actually an ideal damper and small gas spring in parallel. The spring effect is a by-product of the damper and doesn't effect the velocity dependent characteristics of that damper, it's just a necessary component of the system. If your charge is high enough you better factor it into your suspension setup and possible choice of springs. What matters in the end is the total spring rate acting at the wheel and if you add a bunch via your damper you may want to remove some via your spring to maintain your operating point.

I'm not disputing that there's a spring effect in a shock.But a 45 pound measured spring force on a stationary shock is not the same as the "same" 45 pound spring force on a moving shock piston that is producing flow across the piston that is in constant chaotic motion. Depending on the piston design and the valving, the flow will reduce that number to one that may or may not have the same ride change effect that one would see on a stationary car/shock. The author of the article presented states that a spring change may be necessary to compensate for the spring rate of the shock. I believe he's incorrect (actually flat wrong) in prescribing such a change.

There is a real reason base valves and the like were invented. There's a real reason guys prefer to minimize damper charge pressure as much as possible. If the gas spring effect in the damper never made a difference we'd never need base valves and we'd never worry about keeping charge pressures low. We'd just charge dampers with 500 psi with no ill effects and no worry of cavitation. But this isn't the case. 500 psi is going to manifest itself as a huge gas spring that will likely be unwanted in our setup. Therefore we spend time with basevalves, anti-cavitation devices, and achieving low charge pressures. It keeps our damper as close to the ideal damper as possible.

Supporting a car with dampers is far from ideal and not most efficient. However, if rules pigeon hole you into that corner and using the shocks in that manner is the only way to achieve an effect that net makes your car faster would you not use it then?

Being "pigeon holed" is a matter of experience and knowledge, there's plenty of ways to avoid it instead of making a shock act like something it isn't. A balanced car with an capable driver will win more times than a "tricked car" with the same capable driver who's unsure of what the car will do. IMHO

[PhillipM]

It can even go from linear to digressive using the same piston although with different shim config.

You can do that with pretty much any piston with the right shims

How far you can go towards a full digressive setup is the trick I suppose...

[RideRate]

I'm not disputing that there's a spring effect in a shock.But a 45 pound measured spring force on a stationary shock is not the same as the "same" 45 pound spring force on a moving shock piston that is producing flow across the piston that is in constant chaotic motion. Depending on the piston design and the valving, the flow will reduce that number to one that may or may not have the same ride change effect that one would see on a stationary car/shock. The author of the article presented states that a spring change may be necessary to compensate for the spring rate of the shock. I believe he's incorrect (actually flat wrong) in prescribing such a change.

You've got to explain to me how a 45 lbf spring force is not the same as the "same" 45 lbf spring force. You really seem to believe the spring force (aka spring effect or spring characteristic) goes away when the damper moves. Is this correct? Is this what you believe? We've already explained how this is not the case. That spring force in the shock is going to always affect the height of the car just like a normal small spring. The hope is the force stays so low it's mostly negligible, but this isn't always the case.

All I can say is take your favorite pressure tapped shock. Run it on a dyno with a sine wave, track file, or whatever you want. If you don't believe a dyno collect some data on the shaker rig or at the track. Look at your compression chamber pressure data. Multiply that pressure by the area of your shock shaft. If you run a basevalve remove the pressure drop across the basevalve from the compression chamber pressure. Subtract from this the area of the shaft times atmospheric pressure. Plot as a math channel vs time, track position, shock displacement, or whatever just as long as you know it's data from a constant chaotic motion event. What you see live and in front of you is damper force that is independent of velocity of the damper and only occurs due to the position of the damper's displacement (and the acceleration, but that's small and you can remove it too if you like). Look at that data. It's the spring in real life acting on your car straight from your damper and during constant chaotic motion. It's not going anywhere. Sure, there is some lag in it, but every spring exhibits some level of frequency sensitivity which is a totally different topic. It's still a fundamental spring and you can see it for yourself.

Being "pigeon holed" is a matter of experience and knowledge, there's plenty of ways to avoid it instead of making a shock act like something it isn't. A balanced car with an capable driver will win more times than a "tricked car" with the same capable driver who's unsure of what the car will do. IMHO

Sure there are ways to avoid it, some or all of which may not be legal, affordable, or beneficial. But if a method using shocks or anything else not as intended is faster than using the "proper" method, who wins? Not saying it's right, but one method will dominate until the other is proven out to be net faster for whatever reason.

[speedsense]

You've got to explain to me how a 45 lbf spring force is not the same as the "same" 45 lbf spring force. You really seem to believe the spring force (aka spring effect or spring characteristic) goes away when the damper moves. Is this correct? Is this what you believe?

Not go away, but reduces from the stationary measured/calucated amount. The stationary position has no measureable stiction, a moving one does have a measurable amount. As pressure effects the piston seals and increases the stiction amount (along with other influences), one can ascertain that spring rate is the cause of ride change due to pressure but cannot ignore the other acting properties, such as stiction.
Can the author of the article, be sure of a spring rate change to the car as a result of can pressure of the shock as an addition to the overall spring of the car? Higher pressure raises the stiction amount of the shock piston and is not a result of the internal spring rate of the said shock.
Or even the coil spring itself also contains stiction rates. Hyperco makes a hydraulic platform for the coil spring to reduce this stiction amount. As someone who tested these platforms (during their introduction) with live suspension data from a car, the addition of the platforms, reduced the stiction rate of the coil spring and resulted in a drop in spring rate to achieve the same performance of the higher rate springs. This was done with the exact same shock settings. The addition of the platform changed the shock velocities as well, and it was presumed it reduced the shock shaft's (from the coil spring acting on it) stiction as well.
The shock changed it's character without any changes to it's internal workings or pressure.
How can the author be sure, it's not stiction affecting the internal spring rate or whether it's the pressure?

A racecar body exhibits some of the same restraints as shock pressure, as it also moves in liquid that is under constant pressure (atmospheric), yet there are differences in how that constant pressure affects a better aerodynamic design, reducing the pressure's impact. Why wouldn't the same be true with the shock piston with higher flow rates? It only stands to reason that if my shock piston, that is placed in motion, was a ring with nothing in the center of it (except a piece to hold it on the shaft) will have less impact from shock pressure than a solid piston with a few holes in it. Would it not?

We've already explained how this is not the case. That spring force in the shock is going to always affect the height of the car just like a normal small spring. The hope is the force stays so low it's mostly negligible, but this isn't always the case.

All I can say is take your favorite pressure tapped shock. Run it on a dyno with a sine wave, track file, or whatever you want. If you don't believe a dyno collect some data on the shaker rig or at the track. Look at your compression chamber pressure data. Multiply that pressure by the area of your shock shaft.

With no consideration for the shock piston?

If you run a basevalve remove the pressure drop across the basevalve from the compression chamber pressure. Subtract from this the area of the shaft times atmospheric pressure. Plot as a math channel vs time, track position, shock displacement, or whatever just as long as you know it's data from a constant chaotic motion event. What you see live and in front of you is damper force that is independent of velocity of the damper and only occurs due to the position of the damper's displacement (and the acceleration, but that's small and you can remove it too if you like). Look at that data. It's the spring in real life acting on your car straight from your damper and during constant chaotic motion. It's not going anywhere. Sure, there is some lag in it, but every spring exhibits some level of frequency sensitivity which is a totally different topic. It's still a fundamental spring and you can see it for yourself.

The question isn't whether there is a spring rate in the shock with say 45lbs measured rate. The question is: How much is the reduced effect by piston design/valving and a reduction of stiction of the piston seals (by design), reducing the impact of the 45lbs of spring rate to a number low enough to not be a consideration as an acting spring or influence on the car's sprung mass?

Sure there are ways to avoid it, some or all of which may not be legal, affordable, or beneficial. But if a method using shocks or anything else not as intended is faster than using the "proper" method, who wins? Not saying it's right, but one method will dominate until the other is proven out to be net faster for whatever reason.

I guess at that point we should rename the dampener to something else.....

[GSpeedR]

speedsense: damper stiction is certainly a real effect, and as you have experienced, is related to damper pressure. However, stiction is a distinct effect from the gas spring and can be treated separately. In the simplest form, stiction can be represented as a Coulomb Friction element (related to the sign of velocity only), and this is in parallel to the 'damper' element, which is in series with a spring representing damper compressibility.

Overall we have a nonlinear damper in series with a lag spring, both of which in parallel with a stiction element and the gas spring. If necessary the gas spring will be in parallel with a gas damper which adds frequency sensitivity and energy dissipation within the gas chamber.

[GSpeedR]

When weight and packaging is not a big problem, for some specific racing cars you could use oversized remote reservoirs in the dampers. This would make a "not so progressive" gas spring, please correct me if Im wrong.

Increasing the gas volume is increasing its mechanical capacitance and thus reduces the spring rate effect. However, many remote reservoir dampers use narrow hydraulic lines to connect to the main damper, filled with hydraulic fluid (rarely gas, due to more difficult sealing). Because the lines have such a small flow cross-sectional area the effective compressibility is very significant (most volume change occurs in the line-axial direction). Thus they can add lag. If there is a base valve then the pressure (and thus the flow) is reduced, which reduces the lag effect, but it can be significant. They do allow larger gas volumes and often larger diameter piston, both of which are good things.

[RideRate]

The stationary position has no measureable stiction, a moving one does have a measurable amount. As pressure effects the piston seals and increases the stiction amount (along with other influences), one can ascertain that spring rate is the cause of ride change due to pressure but cannot ignore the other acting properties, such as stiction.

No, let's not ignore important properties, but let's not lump them together either. Stiction and the gas spring are two different effects with minimal coupling. The stiction doesn't eliminate the gas spring, it's just another force that needs to be included in the summation. It effects the gas spring the same as a-arm friction effects the coil spring. Does the suspension friction change the rate of your coil spring? We know it doesn't, it just causes at net effect at the wheel but all the spring rate and the friction must be accounted for to properly sum. Friction and stiction are separate effects than the gas spring.

Can the author of the article, be sure of a spring rate change to the car as a result of can pressure of the shock as an addition to the overall spring of the car? Higher pressure raises the stiction amount of the shock piston and is not a result of the internal spring rate of the said shock.
Or even the coil spring itself also contains stiction rates. Hyperco makes a hydraulic platform for the coil spring to reduce this stiction amount. As someone who tested these platforms (during their introduction) with live suspension data from a car, the addition of the platforms, reduced the stiction rate of the coil spring and resulted in a drop in spring rate to achieve the same performance of the higher rate springs. This was done with the exact same shock settings. The addition of the platform changed the shock velocities as well, and it was presumed it reduced the shock shaft's (from the coil spring acting on it) stiction as well.
The shock changed it's character without any changes to it's internal workings or pressure.
How can the author be sure, it's not stiction affecting the internal spring rate or whether it's the pressure?

Because the gas spring is real and undeniable independent of any friction. Friction is not the subject of this discussion and has no bearing on the gas spring effect just on the shock's friction characteristics.

A racecar body exhibits some of the same restraints as shock pressure, as it also moves in liquid that is under constant pressure (atmospheric), yet there are differences in how that constant pressure affects......

Nope not the same. The race car body is immersed in a fluid with equal pressure acting on all surfaces. A shock is not. Everything inside the shock is acted on by the fluid pressure in the shock, but the rod which is connected to all of this is acted upon by 14.7 psia outside the shock. The fact that the rod is always acted upon by the 14.7 psia fluid outside AND the compression chamber pressure inside is the fundamental reason behind the gas spring. If you want to expand this to a racecar then the car needs some surface area acted on by a constant force independent of flow velocities and flow fields. Don't know how you'd do this, but the two are not the same and it's not right to think they are.

With no consideration for the shock piston?

Exactly!!! With no consideration for the shock piston. The shock piston has no effect on the gas spring effect. Run the shock with heavy valving, no valving, little bleed, no bleed, hell run it WITHOUT a piston and the gas spring characteristics are the SAME. It's a function of mainly the rod's position (and accel) and therefore is a displacement dependent force of the shock, hence the definition of a spring.

The question isn't whether there is a spring rate in the shock with say 45lbs measured rate. The question is: How much is the reduced effect by piston design/valving and a reduction of stiction of the piston seals (by design), reducing the impact of the 45lbs of spring rate to a number low enough to not be a consideration as an acting spring or influence on the car's sprung mass?

The 45 lbf gas force is at ONE point yes, it varies with shock position just like a gas spring, in fact just like the gas chamber volume and initial pressure changing by the volume of rod insertion into the body since that's exactly what's happening. Valving doesn't change it and neither does friction, it's a real effect summed into your suspension rate, period.

I guess at that point we should rename the dampener to something else.....

Like damper?

[Belatti]

It can even go from linear to digressive using the same piston although with different shim config.

You can do that with pretty much any piston with the right shims

How far you can go towards a full digressive setup is the trick I suppose...

At least I couldnt do that with Penskes, they have two different pistons. In mine I can go to all the "full" you may want. The digressiveness in the green curve in the first graph can be increased with other shim pack, to 200kg / 20mm/sec. and then 250Kg / almost 280mm/sec. Of course, "big brands" may have other benefits...

[WilO]

I received a question the other day regarding high-speed damping, specifically how best to optimize it given logged track data. I referred the questioner to the considerable talents of Dave Williams and Multimatic Europe, given the reason to optimize high-speed damping must be to minimize CPL variation.

The questioner indicated that he often did not have sufficient time to place a car on the rig, and wondered if any progress could be made with damper data logged at the track. Does anyone have any experience with this? It's an interesting question, given the increase in tire vertical rate with speed and so forth.

Thanks in advance, as always.

Addendum: Turns out this is a discussion taking place on a linkedin board. The OP has looked at shaft displacement in the frequency domain, and eventually would like to model CPL variation.
My question would be how does one derive CPL variation (reliably) from damping coefficient and stiffness. (For interest, it was suggested that the OP place two accelerometers on the shock to obtain pos., vel., accel. traces, and a load cell on the shaft, and accelermometers at both the control arm and at the chassis to determine shock loads and g-forces).

[Belatti]

Wilo, Im curious to know how all this turned out.

[WilO]

Belatti,

I'm curious as well.....I will pursue the topic further with the people involved, and let you know what the results were. I do know that a section from Jorge Segers book on data analysis techniques was referenced (dampers and frequency domain analysis) as a possible method; again, I'll report back with what I find.

Wil