To learn and share about dampers / shock absorbers

[DaveW]

Incidentally, I agree that hysteresis should be minimized for "circuit" dampers. However, it appears to be a "good thing" for gravel stage rally dampers. I suspect that a) rally cars are not controlled in the same way as circuit cars & b) the lack of "attack" of a high hysteresis damper helps to reduce the "impact" of very large obstacles.

[DaveW]

I think more like this.

I suspect your plot shows valve dynamics (transient coupling between pressure & valve position)....

[747heavy]

in an extreme case, maybe it looks a bit like this:



Please note:

The damper shown has "build in" position dependent damping caracteristic.
I just did not find a better plot in a hurry.
But the delay in force build up in quadrant 3 is "typical" for post caviation, because it takes time to "collapse" the "gas bubbles" and make the oil an incompressable fluid again. - IMHO
I´m sure Dave will be able to explain a bit more accurate

[747heavy]

Hi Belatti,

maybe you find this a useful site/software for your quest.

http://shimrestackor.com/index.htm

[DaveW]

But the delay in force build up in quadrant 3 is "typical" for post caviation, because it takes time to "collapse" the "gas bubbles" and make the oil an incompressable fluid again. - IMHO

Mine too, 747.

Incidentally, & on the subject of valve dynamics, a persistent problem for road car dampers is what development drivers call damper "chuckle". This, from rig test observations, appears to be caused by a delay in opening of the damper shims, which appears, in turn, to be caused by capillary forces "binding" the sealing shim to its piston. The "chuckle" is caused by a shock wave generated when the shim is finally released. The "fix" is a sintered piston, but that is something of a two-edged sword. Road car development drivers spend much time trimming shim pre-loads to "improve harshness". It appears that the fluid boundary layer holds the sealing shim away from the piston for a short time, long enough with suitable preload for the shim(s) not seal completely at high frequencies (usually between 8 & 12 Hz on a rig). This is observed as a quite sudden loss in low speed damping - frequency sensitive damping for free.... The characteristic is much less pronounced if sintered pistons are used.

Not important for race car dampers, but I thought you might be interested.

[747heavy]

Thanks for your insights, very interesting and much appriciated.

Some manufacturers of racing dampers/piston machined from aluminium,
reduce the contact area of the seal shim with the piston face, for the reasons you mentioned - I think.

appears to be caused by a delay in opening of the damper shims, which appears, in turn, to be caused by capillary forces "binding" the sealing shim to its piston.

a bit like shown with the red piston at the top left

[747heavy]

an SAE paper to complement DaveW´s comments, in regards to the damper "chuckle".

http://www.soundanswers.net/Tech_Papers ... 1-2091.pdf

[DaveW]

an SAE paper to complement DaveW´s comments, in regards to the damper "chuckle".

Interesting paper. Incidentally, I'm told that the "chuckle" phenomenon isn't (so) observable when the damper is tested by a dyno. That raises the possibility that some damper characteristics are impedance-sensitive. Put crudely, a damper is forced to prescribe a specified displacement time history when it is mounted in a dyno, regardless of the force time history. When installed in a car, however, the damper displacement time history is not prescribed; if it wants to pause for any reason, it can do so....

[WilO]

@Belatti: thank you for starting this topic, and encouraging folks to share their knowledge. Your willingness to share and learn makes this forum great.

@Dave and 747: Thank you both for sharing your considerable knowledge, your posts are a masterclass on the relevant topic. Dave, your comments regarding road car dampers and displacement time history are particularly relevant to me right now.

747, I was wondering if I might ask you where you found that illustration of the damper cycle/potato plot.

Thanks again gentlemen.

Wil

[747heavy]

747, I was wondering if I might ask you where you found that illustration of the damper cycle/potato plot.

Hi Wilo,

not 100% sure which one you mean, but if it is the last one, then here:

http://forums.vwvortex.com/showthread.php?2011200


if you are interested in PSD (position sensitive damping)

you may like this as well.

http://www.freepatentsonline.com/6415895.pdf
http://www.freepatentsonline.com/6966412.pdf

regards
747h

[Belatti]

Yesterday my mind was thinking about Dave´s post about hysteresis and hose compliance. Here I plot a graph of a 2 way adj. damper cycled at 3Hz and I added the reference colors of the "clicks" regulations from 3 (tight) to 21 (loose).

There you can see how hysteresis very low when the damper is not under heavy load. Then it increases when you turn the faucet.

[RacingManiac]

Thanks for your insights, very interesting and much appriciated.

Some manufacturers of racing dampers/piston machined from aluminium,
reduce the contact area of the seal shim with the piston face, for the reasons you mentioned - I think.

At work here we do keep that in mind when we design our pistons, which most of them are machined at the moment during prototyping. The effect is definitely something we monitor, and tries to minimize if possible

[DaveW]

There you can see how hysteresis very low when the damper is not under heavy load. Then it increases when you turn the faucet.

I am tempted to suggest that you might like to repeat the experiment after replacing the hose by a machined adapter, but that would assume that you are comfortable rebuilding & recharging the damper. A fairly easy (though not conclusive) alternative would be to carry out runs at temperatures of, say 20, 40 & 60 deg C with settings 3/3.

Incidentally, your plots demonstrate my contention that some damper architectures "like" (i.e. work better with) rebound-biased damping.

p.s. you might also like to refer to 747's Penske reference. Assuming the design allows, moving shims from the head/reservoir valve to the main piston will usually reduce hysteresis, but will also reduce compression adjustability. I should add that my damper expert friends prefer not to break open a reservoir when changing a damper spec., for some (no doubt very valid) reason.

[Belatti]

Here it is... a slightly different internal package but basically the same model of damper cycled at 3Hz at the mentioned temperatures.



BTW, yesterday was talking to a person that told me he saw a local team testing the dampers with the coilover spring mounted on. Whats the point of that?

And talking about coilover springs, I tested some of them, too. Eibachs are 40% more expensive than this local product and have the fame of being built with an expensive swedish steel. Still the results of the test showed not much differece in the "K" curve between both. I wonder if both will keep its properties the same for a long period of time.

[DaveW]

Thanks, Belatti. I've sent you a p.m.

I've no real opinions on springs, except to say that I would test spring & damper separately, & would always use spring bearings between the spring & platform (perch). I would expect rates to remain relatively stable, & any settling should be adjusted out during set-up. If settling is an issue, then pre-working the springs might improve stability.