gas pressures for shocks

[brian]

I decided to start a new thread on tthe issue I raised regarding pressures. Brian mentioned that testing his shock on a scale did not disclose any changes when pressures were changed and that's probably right since he was testing the slow speed circuit of the shock. Long strokes in the shock are the domain of the low speed curcuit. In my previous post i mentioned gas pressures effected "high speed bleed" behavior. If you put a shock on a dyno and change pressures you will see a change in the high speed behavior. Specifically, the high speed blowoff, the point at which the bleed is overloaded and the shims begin to work, will change. My butt translates the high speed circuit as the jiggily feeling you get from the car. It kinda feels that its bouncing across the bumps rather than flowing over them. The high speed circuit really determines how a car will react to curbs as well.

The triple adjustable Penskes have an adjustable high speed circuit that changes the preload of the shims which will change when the shims will blowoff. It is felt that on a shock like the small black fronts or with the relative low spring rates vees use, pressures will have a measurable effect on the high speed behavior.

When we first began using Penskes rears, they came with a recommened 250 psi. Over time, many of us found that 150 worked a lot better. Granted, many of us ran springs that were a lot softer than many today; that will change how pressure changes will alter the behavior of the car. The evolution of drilling bleed holes, pioneered by Joe Stimola in our class, also dealt with the same issue only more drastically than pressures.

Like I said in the previous post, changing pressures on a test day is a very simple quick thing to do and your results may vary but when you don't have time or options, it could make a difference on a rainy day.

[hardingfv32-1]

1) Testing a shock on a scale was done to verify if there is any substantial spring rate change as the shock travels through its operation range. I could not detect any rate change on a bathroom scale. So it is unlikely that a pressure change is going to effect the spring rate.

2) NOTE: I am going to IGNORE any effects of aeration when discussing the effect of pressure changes on any of the valve systems. That said, may I assume the internal pressure is equal on either side of the piston with the shock is at rest? I believe that the shocks valving must have a pressure differential across the piston to function. This is usually caused by the piston's motion. How would higher or lower equalized starting pressure change the flow through the valves?

No seat of the pants comments. We need a hardware explanation. Shocks are very well understood and documented.

Brian

[brian]

When I get home I try to find some dyno graphs that show the blowoff behavior. Someone else will have to do the ngirearing stuff.

[hardingfv32-1]

How or what are you characterizing as blow off behavior? I associate blow off activity with large impacts, like hitting a curb.

Brian

[brian]

My understanding of blowoff is that it is the event when the bleeds or bypasses are overloaded by the rate at which the oil is moving, and the shims begin to deflect. If you look at a shock graph, the slope of the curve starts out at an higher rate, or steeper slope, until the bleed is liquid locked and and the shims begin to flex at which point the slope is less. Adjustable shocks change the timing of this event by either changing the size of the bleed hole, like the Penske 8100 on compression; moving a jet in and out of an oriface which is often used in the rebound adjustments and by preloading the shim pack as in the double adjustable Penske 8760.

I'm leaving for the mainland tomorrow and will get my notes and talk my shock guy. Aloha.