Optimal damping
Optimal damping
(OP)
I read few books about vehicle dynamics and according to all I have seen (specially RCVD by Milliken) the damping have to be 2 times higher in rebound than in bump. I read that road have 2 times more bump than holes and also that the stiffer rebound allow a higher energy dissipation but some research paper usually describes road as a gaussian signal with 0 mean value. The other thing is that high speed bump and rebound must be softer than low speed. I guess the reason is related to resonance frequency of sprung vs nonsprung mass but I am not 100% sure. Can someone gives me a clean and scientific explanation about these 2 dogmes:
1- damper must have a rebound damping 2 times higher than the bump damping
2- High speed damping must be softer than low speed damping (both in bump and rebound)
Thanks in advance
Sam
1- damper must have a rebound damping 2 times higher than the bump damping
2- High speed damping must be softer than low speed damping (both in bump and rebound)
Thanks in advance
Sam





RE: Optimal damping
2) The second point is strange, as all dampers make more force at higher speeds. ( OK so there are exceptions- but they're rare). Typically high speed damping is generated by an orifice limit - which increases damping rapidly with velocity. The high speed damping is used mainly to control unsprung mass shake on live axles ( 10- 12 Hz ), or to prevent crash through.
RE: Optimal damping
2) At very low piston speeds, the damping is usually almost equal to slightly more rebound. At theese low speeds(just barely turning the steering wheel on the freeway, or very slight dips in the pavement) the damping is controled by slit valves(sometimes called notch leafs or bleed valves) where the oil is simply "leaking" throung the piston or bottom piece.
At High piston speeds (sharp impacts, or extreme transition limit handling) the damping force is controled by the size of the hole in the piston and how much oil can pass through it. I think what the most books are trying to say is that it is prefered if the slope of the force/piston speed graph is less than the mid speed slope. This is because the oil will reach a point where it can't flow any faster and the damping force will start increasing VERY rapidly(2-3 m/s piston speed). So basically they want to see as big of a hole as posible in the piston, so this sharp rise in damping force doesn't occur. But the trade off is that sometimes if the hole is too big, the damping for limit handling is not enough.
RE: Optimal damping
Not only 'sort of'. If you measure the ride height over rough surfaces you'll actually see it suck down onto the springs as the rebound damping dominates.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Optimal damping
As far as rebound damping energy, shouldn’t body mass be subtracted from spring force instead of being added to it? How can rebound address anything more than spring force? The tires aren't glued to the ground.
RE: Optimal damping
1) the maximum velocity is nothing like vehicle speed
2) upward and downward velocities are similar.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Optimal damping
You must have some pretty smooth roads :) We have a section or 2 that we can get the piston speeds up to 2.5 m/s. It is a section of very rough/broken asphalt with undulations that we drive at about 35-40 mph. It is used for flatteness evaluation and body control on rough surface. Mainly to see the limits of the high speed oriface of the dampers.
RE: Optimal damping
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Optimal damping
A force/displacment curve(shaped like an American football), gives a little bit better picture of what is going on, but it still leaves out the accelerations. If there was a way to acurately measure and record a graph that shows exactly what you are feeling, that would be a great tool, but the current dyno's are just not very capable of doing that. It takes a large amount of energy to reproduce what is actually happening on the car.
RE: Optimal damping
So my question is, what's the best way to learn the intricacies of this tuning? Build lots of dampers? It does sound like it's possible improve performance by trying to eliminate friction effects (maybe less shims rubbing against each other) and deflection distances (stiffer shims). Sort of the right idea?
RE: Optimal damping
The BEST way is to spend some time with your damper builder! Ask him to show you every part and how the damper goes together. Or, if you have a boken shock laying around, open it up and check it out...as an engineer, I'm sure you have taken things apart to see how the work. :) It is pretty difficult to explain in writing what all the effects of the internals are, that is why it is best to do a tuning session and build a bunch of dampers and see how the feeling changes. During a typical 1 week tuning session with a supplier, I will spend about 2 days on volume and 3 days on the "Black art"/feeling of the damper. I think you have the right idea about friction, deflection, but one of the most important parts is the smoothness (balance) of the damper, to eliminate the sharp increases in focre build up. This is the part of tuning that the dyno has really no use, and it becomes all subjective. A really good damper builder should "know" what is needed and be able to adjust the valveing based on your subjective comments about the stroke and balance. Like I said before, it is easiest to have him build a couple sets(of the same damping force) and tell you what is different between them, so you can feel the difference in the valve code. The hardest part is finding that "perfect" combination of the leaf valve stack, to get the stroke to feel linear, to have the right amount of body control, to have the right amount of "damping feel", and to make sure it transitions well with the bleed area and the hole in the piston. The other part is to balance the car front and rear, to make sure the dampers work together and not create any strange pitching feeling.
RE: Optimal damping
What I'm getting from that is that a force/velocity plot essentially presents the response to some arbitrary constant rod acceleration rather than to whatever a real-world rod a(t) might be.
Norm
RE: Optimal damping
When a damping force machine takes data, it usually runs 3 or 4 strokes at each specific rod/piston velocity(0.02, 0.05, 0.1, 0.3, 0.6, and 1.0 m/s). It takes it's data point at the specific velocity at the peak of the force. The problem with that is that it doesn't show how it gets to that force. Is it a linear progression, is it a really sharp spike, or is it always just really high force??? That is the part that is all feeling, because the dyno's don't show this very clearly according to feeling. This is why you spend 2 days getting the volume (area under the force/velocity curve) correct and 3 days fine tuning the way the valving transitions between one another(what is not shown by the dyno)... to get that feeling correct.
RE: Optimal damping
Here is a thread that gets into the "Black Art" portion of damper tuning.
http://www.eng-tips.com/viewthread.cfm?qid=107747
RE: Optimal damping
With regards to dyno testing, it seems like you could use a shock dyno that is essentially a linear servo to generate more useful data (i.e. feed it a waveform then measure the resulting force), then run that data through some sort of algorithm to process it into a more useful form, maybe a surface plot of force vs. both velocity and acceleration. I realize this itself would not be easy, then you would have to actually do something useful with the output.
I guess if you're not in F1, stick to simple calcs and testing.
RE: Optimal damping
See, to me this adds another layer to the black art, and I completely get it. But should some cycling info be built into the damper before the black art portion of develop0ment begins? By cycling I mean how the front and rear work together based upon knowns such as front to rear weight bias.
Thanks, Michael