Got hysteresis? This is why you need the FULL shock dyno graphs for your dampers!
... and understand what they mean!
THIS is what hysteresis looks like, and fortunately (in this case) can be corrected by adding nitrogen pressure during a subsequent rebuild of the H&R Street Performance (Bilstein monotube-based) coil-over I'm working on for our Porsche Cayman S customer, Edge Motorworks in Mountain View.
But in some cases whether through a shock body that's too short, and/or a shock shaft that's too long, or choosing a twin-tube design that isn't able to flow sufficient fluid your damper may have poor hysteresis. To avoid hysteresis, many damper manufacturers err on the side of TOO MUCH nitrogen pressure.
The reason you need nitrogen pressure is so that the 'air column' under the oil chamber won't collapse as the damper moves in the compression direction. You want the nitrogen to have enough resistance so the oil is forced to move trough the pistons, instead of having the shock piston displace the nitrogen chamber downward.
For the majority of situations you want to avoid hysteresis, since it causes a phase delay in the build-up and release of compression force. The damper is partially operating as an 'air spring' when you're experiencing hysteresis, so the damping force is building more slowly as you move into compression. Whether for street comfort or track performance, hysteresis will tend to prevent your vehicle from staying in sync with what the road is throwing at it.
So how do you avoid it? Take care when choosing either a new damper or making changes to your existing damper. A competent shock builder and suspension engineer will understand the reasons to minimize hysteresis for street and race setups and will be willing to communicate / share information. I say this having kept information to myself for years (and some companies still do) but I always had stringent internal standards and made sure to minimize hysteresis so if/when I showed the dynos, I could be proud of the end result!
For our own FCM Elite-tuned suspension, being the process by using our Elite contact form:
www.fatcatmotorsports.com/cont...
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Where you say "this isn't cavitation" at @3:42 or so, that actually IS cavitation. Hysteresis is an umbrella term that refers to any kind of history-dependence of a characteristic curve of any description - when applied to dampers it's usually to describe acceleration/displacement sensitivity (also called frequency sensitivity) which causes a lag in the buildup of damping pressure, generally in proportion to the pressure itself (ie the phase shift is usually roughly twice as big in terms of time/displacement if the pressure reached is twice as high). Cavitation however doesn't just cause a pressure-proportional delay in the buildup of pressure, it causes the sudden vaporisation of oil into gas behind the damper piston, followed by a sudden collapse of that gas cavity after the shock changes direction - but until such time as the gas has collapsed back into liquid, there is zero (or even negative) damping force being generated, usually causing a significant knock. While this technically does fit the definition of "hysteresis" in the sense that what the shock was doing beforehand does affect it, that isn't the typical semantic use of the term damper hysteresis, nor is it the reason hysteresis needs to be considered, because in this case it's just cavitation.
@SuspensionTruth
3 жыл бұрын
Thanks for your comment. I do appreciate semantics and also illustrations/examples. Some graphs I've seen showing 'cavitation' have different characteristics than what I pointed out here. If the governing principle behind cavitation is vaporization of oil into gas and zero / negative damping force being momentarily generated, the behavior at 3:42 shows a wider envelope of damping / delayed force build / release, but not a drop off or flattening. I attribute that to nitrogen chamber partially collapsing but not to the degree that prevents the damper from building force. As a comparison, c.f. illustrations of cavitation in this paper here: www.diagnostyka.net.pl/pdf-64482-17802?filename=Experimental%20diagnosis%20of.pdf and lower down on this page under 'degassing shocks' farnorthracing.com/autocross_secrets6.html When running dampers on my dyno, I've never seen a graph such as Dennis's site or the paper show.
@VorsprungSuspension
3 жыл бұрын
@@SuspensionTruth cavitation does still build force if the nitrogen pressure is building (ie oil is being displaced and significantly changing the nitrogen chamber volume). Depending on the dynamic scenario involved, you can see force that is lower than damping alone would give at a given speed (indicating the cavity increasing in size) OR higher, which occurs when the cavity is decreasing in size, which is what happens as the damper piston slows down (and potentially also after it's reversed direction), where you see unexpectedly high damping forces at low (or negative) speeds due to the nitrogen pressure effectively acting over the full piston area instead of just the shaft area due to the vacuum behind it. The reason I pick on the semantics is that cavitation is a serious functional problem that can cause extremely unpredictable behaviour (and potentially damper damage), which is distinct from "normal" damper hysteresis which is (for roadgoing vehicles) not optimal but fortunately is generally quite predictable from the driver's point of view.
@SuspensionTruth
3 жыл бұрын
Thank you for the added information. Would you say then a monotube damper may cavitate more easily than a (conventional) twin tube, while the twin may have more hysteresis, especially on the compression stroke? Do you have graphs / references showing 'normal' hysteresis vs. cavitation? Would help educate myself and clients further on the nuances of non-ideal vs. damaging behavior.
@VorsprungSuspension
3 жыл бұрын
@@SuspensionTruth we don't work with automotive emulsion-type twin tubes in our industry, only sealed TTX type twin tubes which are close to cavitation-proof. Emulsion dampers often have a ton of hysteresis just from foamy oil alone. Read up on Penske and Ohlins' literature on pressure balancing and hysteresis, they have a ton of good resources you can find with a bit of googling.
@strongme80
2 жыл бұрын
@@VorsprungSuspension Is this you Steve Mathews from Canada? If it is, damn right, Steve knows his shit for sure!
PvP for marketing, cvp for winning.
@SuspensionTruth
5 жыл бұрын
That's T-shirt-worthy, Mr. Dover!
How much nitrogen in these? Also how long of a cycle at how many inches per second have you ran them?