- Shock absorber and how an earth it works
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    1. 09-24-2019 05:15 AM #1
      Of course I know basics of shocks yeah. But one thing I cannot understand is for example monotube gas shocks. So the chamber is full of oil where piston and rod is. Then there's floating piston and under it there's pressurised gas. When you press shock to the bottom floating piston moves slightly also down. But tell me how an earth piston comes itself all the way up? Floating piston moves 10mm and piston/rod moves 150mm? Weird? Check this how small movement floating piston makes and how large movement piston/rod makes: It just doesn't make sense to me.

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    3. Member
      Join Date
      Jan 19th, 2013
      North County San Diego, CA
      2006 Jetta TDI Special Edition, 2018 A3 Quattro, 1995 Porsche 911
      10-03-2019 12:27 PM #2
      You can think of a shock absorber as a hydraulic cylinder with a hole in the main piston, since that is essentially what it is.

      During the compression stroke the cylinder shaft enters the oil cavity and increases the volume of steel inside the cylinder. This volume needs to be accounted for, so the nitrogen "accumulator" is there to give the displaced oil somewhere to go.

      The stroke of the nitrogen piston is proportional to the volume of the shaft. Let's do some math and assume you have a cylinder (shock) with a piston diameter of 30mm and a shaft diameter of 14mm and a stroke of 100mm.
      We will start in the fully extended position (rod all the way out).

      When the piston compresses the rod enters the cylinder displacing oil. The volume of that oil is the same as the volume of the rod, or pi * (Diameter^2) * Stroke / 4. Using 3.14 for pi we get 3.14*14*14*100/4 = 15,386 cubic millimeters.

      To account for this volume change the nitrogen piston will move down and further compress the nitrogen, but since the diameter of the nitrogen piston is much larger than the rod the distance it needs to move is less.

      Since we know the volume change is 15,386mm we can calculate how far the piston must move: Stroke = Volume / (pi*D^2/4) = 15,386 / (3.14*30*30/4) = 21.8mm.

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