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!
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