To learn and share about dampers / shock absorbers

[Belatti]

Thanks for the inputs.

Dave, I know that dampers are multifunctional, too. Thats why it called my attention that this guy (and the data acquisition companies that programs their softwares) pays attention to this type of graph that, too me, only seems to show how much time the dampers are working in each speed, wich is something that tells you nothing, for example, about transients.

It bugs me that they try to change damper "clicks" til they get a normal Gaussian curve for the whole lap instead of looking at particular cases.

Take for the example this track:



Here it only matters 1 y 2. The rest (3&4) are flat out. 3 includes a "left wheels" jump that will make the graphs look biased. Why would I look at "whole lap" data there?

On the other side, I have looked at those curves of lap record-breaking cars and they are almost Gaussian normal and in the same proportion at left and right car corners. Then, looking a badly tunned car I noticed the pattern doesnt follow Gauss.

What I intend to do with all this is to use the tool I can afford for now (damper sensors) for something else than looking at downforce, roll angle, dive, squat, etc. In otherwords I want to see if there is something more (like damper tunning) I can obtain from it.

[DaveW]

Interesting....

A tip. If the probability of x is plotted on a log scale against x.abs(x) plotted on a linear scale, then a normal distribution will be triangular in shape and will be symmetrical about the average value of x. The absolute values of the slopes of the two arms will be proportional to the (inverse of the) rms value of x.

Your comment about obtaining a normal distribution of damper velocity is interesting because, in my experience, the probability distribution of road inputs is not normal (Gaussian). There is usually an increased probability of encountering both low values & high values of road input velocity (the probability distribution is cusp-shaped). Not quite sure what that implies about damper set-up....

[Jersey Tom]

Like I said in the other post... these values, be it % Rebound vs % Comp, or % High Speed vs % Low Speed.. are just identifiers and indicators. They are useful because they are simple and don't require any in depth system knowledge.

The idea is you identify what range of parameters work well, be it for your car, your driver, or a given track. Then the next time you go somewhere, you know "Ok my combination of things likes 50/50 Comp/Reb and 70/30 Low/High speed.. I should aim for these splits in my setup". No real science behind it.

I suppose you could say there is SOME logic... ie matching reb & comp so that the car doesn't "jack down" over bumps, etc. However, there are some cases where having the car ratchet itself down is a good thing...

[Belatti]

I see Tom, the logic of seeing if there is a probability of "jack down" is the only I have found...

So my partial conclusion till now is that the tool is aimed to people that works in a way they dont care about undertanding the system in depth, but just sticking to some magic numbers and stay there as long as the car works "more or less"...

Dave, having a little fraction of your experience I can guess by intuition that the probability distribution of road inputs is not normal, too. Such is the case of the track I ploted.

Anyways, if Im allowed I plan to run a couple of laps to gather data arround a track and then change Bump/reb and low/high speed clicks on dampers, ask the driver to try to follow the same trajectories a couple laps more and, if the laptime and GPS trajectories are within what I can consider equal, see what changes in the data.

I´ll tell you later

[Jersey Tom]

So my partial conclusion till now is that the tool is aimed to people that works in a way they dont care about undertanding the system in depth, but just sticking to some magic numbers and stay there as long as the car works "more or less"...

I wouldn't exactly phrase it like that. It's a low investment, high return tuning method. Just the same as accurately cataloging tire pressures, cambers, bar and spring sizes, etc.

IMO everything in this gig is a trade-off of payout to time invested. You can go totally off the deep end in developing the end-all be-all simulation tool that won't be finished for 3 years... and doesn't do any good for next week's race. Or, you can just focus on the short term stuff that might get you ahead now, but will get stagnant over time.

Or, you find the balance of the simple and the extreme.

[Belatti]

Thats a good point of view and sometimes it may depend on the "size" of the team one is working with...

I try to balance my work between trying to do the things to improve for next race without loosing focus on the whole system understanding.

[DaveW]

I seem to have been posting across JT recently. I didn't mean to do that, apologies. BTW, I hope you avoided the recent tornados, JT (or, rather, I hope the tornados avoided you).

Fundamentally, I agree with JT's thoughts. I would just add that quick "rules of thumb" are what race engineers use during a race weekend. A good race engineer will then review what happened in some detail away from the track to try to improve performance &, at the same time, keep his driver on side. That is a good strategy, I think, & it is what I like about Belatti's approach. I look forward to hearing about the outcome of his "experiment".

[RideRate]

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.

Does the wheel moving in the same direction as gravity matter? Single DOF model. If initial displacement position is +4 or -4 in the vertical direction does it make a difference? Does this translate to real world? Just a thinking question for ya.

The wheel dropping out will give relatively higher velocities by a fair margin until lots of damping is present and then the gap closes. How much damping is dependent on the mass ratio of sprung to unsprung.

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 majority of the fast rebound systems are mechanically load sensing (from what I can tell) and not an inertia valve. It knows that the tire is unloaded, not that the wheel is or wants to accelerate downwards. I'm trying to think if an inertia valve could be made to work well for this instance and don't really think it would.

747,
I'm just picking on you. I know this is an old thread, but I was cringing reading speedsense talk about the gas spring effect of a damper and how it disappeared in motion. If he hasn't seen a damper alter ride height then he just hasn't seen a damper pressurized enough. If the gas spring effect wasn't there, how would an airshock work with light valving? Glad you set him straight and put the correct information out there for people to see. You do a great job of passing out accurate information. Kudos.

[GSpeedR]

Heh, one of the articles 747heavy references way back in page 1 is a paper I wrote a long time ago while in school, I'm surprised to find it here. It was cleaned up and published as an SAE paper (2006-01-3641) soon after it was finished, but the larger thesis is apparently available. There are a couple details included in those papers I admit I no longer agree with.

This was a pretty incredible thread. It's a shame that a lot of the contributors don't seem to post here anymore.

[Jersey Tom]

I hear one of those authors works on the 88.

[hardingfv32]

Penske High Speed Rebound Blow-Off:

What does it mean when the nose is not at 0 the force line, Force vs Velocity graph?

What is the purpose of this type of nose profile?

http://www.penskeshocks.com/PRESS_2010-01-21.php

Brian

[GSpeedR]

Penske High Speed Rebound Blow-Off:

What does it mean when the nose is not at 0 the force line, Force vs Velocity graph?

What is the purpose of this type of nose profile?

http://www.penskeshocks.com/PRESS_2010-01-21.php

Brian

The non-zero force at zero velocity is a product of damper hysteresis caused by fluid/system compressibility. It is essentially damper "lag" caused by energy storage (remember the series spring from earlier). You'll find this to be very frequency sensitive and thus will appear different for varying speed sine tests.

The highspeed shaft is varying shim "preload" and increasing low speed slope while keeping high slope the same (since the shim stack is the same).

I hear one of those authors works on the 88.

Not sure, I'll ask him.

[hardingfv32]

How is it used to tune the suspension?

Roughly for what situations?

Brian

[RideRate]

They call it high speed "blow off" because that's what it looks like on a sine input dyno run. It builds force quickly then "blows off" and builds force much slower. This isn't exactly the effect on a car however mainly because this is rebound. The increased blow off point the plot shows is just more and more preload. Typically on the car increasing rebound preload lets the shock hold a higher pressure differential across the piston as it returns from compression and thus bleeds back to ride height slower and slower. More or less it's a method of holding the car's height lower longer. In basic suspension terms it's packing.

As for the non-zero point. It's a function of hysteresis and damper lag as stated. Look at the plot as Force vs Velocity and not Force vs Absolute Velocity, follow the trace from compression opening to compression closing to rebound opening to rebound closing and this lag in response is more obvious and intuitive.

[speedsense]

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.

Does the wheel moving in the same direction as gravity matter? Single DOF model. If initial displacement position is +4 or -4 in the vertical direction does it make a difference? Does this translate to real world? Just a thinking question for ya.

The wheel dropping out will give relatively higher velocities by a fair margin until lots of damping is present and then the gap closes. How much damping is dependent on the mass ratio of sprung to unsprung.

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 majority of the fast rebound systems are mechanically load sensing (from what I can tell) and not an inertia valve. It knows that the tire is unloaded, not that the wheel is or wants to accelerate downwards. I'm trying to think if an inertia valve could be made to work well for this instance and don't really think it would.

747,
I'm just picking on you. I know this is an old thread, but I was cringing reading speedsense talk about the gas spring effect of a damper and how it disappeared in motion. If he hasn't seen a damper alter ride height then he just hasn't seen a damper pressurized enough. If the gas spring effect wasn't there, how would an airshock work with light valving? Glad you set him straight and put the correct information out there for people to see. You do a great job of passing out accurate information. Kudos.

Seen ride changes many times due to pressure, and in almost all cases extension of rebound. As far as pressures I've been all over the board with them, too high- too low...
My questioning, and sorry to make you cringe over it, is reflective of the article written in the beginning of the thread about changing of springs due to the shocks increase in pressure.
Has 747 set me straight? though I know a lot more than before thanks to 747, not sure about whether I'm straight,according to hoyle, on the subject. Though I still continue in a winning manner with help from my shocks, I still question the usefulness of a static dyno other than making sure the shocks are equal, and having use of a hydraulic only a couple of times, impressive.
The things that make me cringe are when the shocks are used to support the car and having empathy for the driver who has to drive such a car, especially the author of the article who suggests changing springs because of what the shocks are doing. Not an individual I would seek out to help me with my shock questions...IMHO.