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  • Resevoir Pressure

    Anyone have any info on the pros and cons of low pressure vs high pressure in reservoir shocks?

    What I'm finding is higher pressure has more hysteresis/less grip:
    http://www.penskeshocks.com/assets/T...ing%202-04.pdf

    But what if you combine high pressure with slightly softer springs? Seems that would offer more mechanical grip because of the softer springs while also having improved transients because of the higher pressure reducing body roll.

    Any discussion is appreciated, thanks.

  • #2
    I would think higher reservoir pressure would yield LESS hysteresis. More initial pressure equals less pressure change on the "high" pressure side of the piston.

    Less pressure, more hysteresis. You see this in lower pressure twin tubes that have to run smaller pistons due to packaging. Higher piston pressures, lower initial pressure, more hysteresis. At least that's how I see it, I'm not an automotive damper expert, but that's how it'd work in an air or hydraulic actuator.
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    • #3
      That Penske document pretty clearly states "Hysteresis is reduced when pressure differences across the piston are reduced and when overall internal pressure levels are lowered." I wonder if this is the rule or an exception to the rule.

      The way I see it is that the gas pressure is only increasing the pressure on the compression side of the piston, thus creating a greater pressure differential across the piston. Larger pressure diff to equalize = longer time to equalize = more hysteresis. At least that's my basic concept of it.

      What I'm thinking of doing here is getting rid of my anti roll bars. They're a compromise, as with a lot of things in suspension. Don't like losing grip from lifting the unloaded wheels. My thought is a damper with a nice sharply increasing low velocity compression curve would help slow the roll rate in transitions. That plus a small bump in spring rate and increase in reservoir pressure would help keep the peak roll degree in a steady state turn where it's at currently with bars. I'd venture a guess that I'm losing more mechanical grip from the bars than I would from 50-100lb/in stiffer springs.

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      • #4
        Sway bars don't "lift up the inner wheel" anymore than lateral weight transfer on a spring platform unloads the inner wheel - with the caveat that you have to stay within the droop travel of the suspension. Soft springs and big bars run out of droop travel in a real hurry.

        I would go with soft sway bars and slightly up the rate and see how you like it. Our cars have jacked up geometry, so you really need a lot of roll stiffness to not let the suspension move much. That from just springs is probably not good.
        '18 Chevrolet Volt - Electric fun hatch for DD duty!


        DefSport Koni Sleeve and Spring Perch Buy!!!
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        • #5
          Originally posted by LuckyX2 View Post
          That Penske document pretty clearly states "Hysteresis is reduced when pressure differences across the piston are reduced and when overall internal pressure levels are lowered." I wonder if this is the rule or an exception to the rule.

          The way I see it is that the gas pressure is only increasing the pressure on the compression side of the piston, thus creating a greater pressure differential across the piston. Larger pressure diff to equalize = longer time to equalize = more hysteresis. At least that's my basic concept of it.

          What I'm thinking of doing here is getting rid of my anti roll bars. They're a compromise, as with a lot of things in suspension. Don't like losing grip from lifting the unloaded wheels. My thought is a damper with a nice sharply increasing low velocity compression curve would help slow the roll rate in transitions. That plus a small bump in spring rate and increase in reservoir pressure would help keep the peak roll degree in a steady state turn where it's at currently with bars. I'd venture a guess that I'm losing more mechanical grip from the bars than I would from 50-100lb/in stiffer springs.

          The key to the Penske statement, is that it's referring to the pressure difference across the piston, and not the overall gas pressure, ideally you want to run the lowest gas pressure possible, the limiting factor for lowest possible there is how aggressive the valving stack is.

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          • #6
            This is why a base valve shock needs less gas pressure than a non BV to avoid hysteresis, when the shock is in the compression stroke and is trying to create cavitation due to the stack creating a suction between the piston and seal head section of the shock, whereas the base valve shock uses resistance to the shock shaft displacement to create a portion of the compression damping, allowing a lower gas pressure required to avoid cavitation via a less aggressive shim stack on the piston valving.

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