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How To: Shock absorber basics and oil change

You know it’s time to change the oils in your shocks when you notice there isn’t enough oil or when you’re making changes to the damping setup or when the shocks are damaged and require taking apart. In any case, the shock oils should be changed 2–3 times during a season. The properties of the oils might fade or change over time. One problem is dirt and dust that gets into the shocks especially when driving in dusty conditions. When there’s dirt in the oils, the seals and other parts of the shocks will wear down faster compared to driving with clean oils.

When you’re changing the oils, start by removing the shock and cleaning it carefully. By cleaning the shock, you can make sure no sand grains or other particles will remain the shock that can damage it from the inside.

Do the following:

  • Remove the shocks.
  • Remove the spring from the shock to make handling easier.
  • When cleaning the shock, also clean under the rubber boot on the shock shaft, if there is one.

Now, empty the shocks. You can use a single-use cup and place a paper towel in the bottom that will absorb the oil in itself. Remove the cap from the shock and place it on a paper towel. Now, pour the oil from the shock in the cup. Move the shock shaft up and down a couple of times to remove oil from under the piston. When most of the oil is removed, place the shock upside down on a paper towel so the remaining oil will come out. Do the same with the other shocks.

  • If the oil that’s come out of the shock is dirty (dark and thick like jelly), remove and clean the seals at the bottom of the shock and rebuild the shock. By cleaning the seals, you can make sure the dirty oil containing sand and metal particles won’t wear down the shocks.
  • If the oil that comes out of the shock is still “normal”, i.e. runny and transparent, it’ll be enough to clean the cylinder with a rolled-up piece of paper towel. Clean the cap as well and if you’re using caps with bladders, check the condition of the bladder and that it fits properly.

Now, you can add the oil. I’d recommend using a work stand to keep the shocks upright during the assembly.

  • Pull the whole shock shaft out of the shock body before adding the oil.
  • Fill the shock about halfway.
  • Now, move the piston in the oil so that all the air gets out. Wait for a moment and move the piston again.
  • Next, add more oil until it’s 1-2 mm from the top of the cylinder.
  • Move the piston up and down and let them sit on the desk until all the air bubbles in the oil has disappeared.

The right way to proceed depends on the type of the shock. With all shock types, it’s essential that the shock is in an upright position so that the air bubbles can move right upwards.

There are several different cap types:

Emulsion shocks:
Air is allowed to mix with the oil. With this type of shocks, it’s particularly important to make sure there is no air in the shock when the shaft is pushed all the way up.

There are several types of caps for emulsion shocks
With the Bleeder type, the air is removed using the bleeding screw on the cap.
With the other cap types, the excess oil and air will come out through the threading on the cap.

Bleeder Cap on the left and without on the right

With both cap types, fill the shock cylinder up to the brim to make sure the space inside the cap will also fill up and any extra oil can come out.

  • Fill the cylinder
  • Prepare the caps: Remove the screw from the bleeder cap. Place seals

With bleeder screw. Screw in the cap. Screw in tightly using moderate force. Now, push in the shock shaft and the air and excess oil should come out of the bleeding screw hole. When you see that all excessive oil came out, you’ll know there’s only oil left in the shock. Screw the bleeding screw back in. Compress the shaft a few times. If there’s clear signs of air still in the shock, add more oil and try removing the air again.

If there’s no bleeding screw, fill the shock all the way up with oil. Before screwing on the cap, you can add a few drops of oil in the cap. Now screw in the cap loosely and carefully move the shaft all the way in. All excess oil and air should come out through the threading on the cap. Hold the shaft in and tighten the cap to get rid of the remaining excess oil. Now, compress the shock a couple of times and see if there is still air in the shaft.

There should be no rebound on an emulsion shock, i.e., when the piston is pushed all the way in it shouldn’t move outwards by itself once it’s released. It’s essential to have similar reaction in both shocks at one end (front/rear) of the car, so their functioning would be identical on both the left and right side.

Bladder shocks
If you’re using bladder caps, building the shock is similar to that in emulsion shocks without bleeder caps. Fill the cap with oil up to 1–2 mm from the brim, not all the way up because the bladder takes some space in the cap. With bladder caps you can adjust the rebound to make damping more progressive – the longer the shock travels, the stiffer it gets. A bladder will also keep the oil from foaming which makes the car move more smoothly during a longer drive.

When it comes to building, the only difference to emulsion shocks is that the rebound can be adjusted by how much of the piston shaft you push in before screwing the cap on.

  • The farther you push the shaft in, the less you’ll have rebound.
  • So, if you push the shaft all the way in, there’s no rebound and the shock works like an emulsion shock.

Adjust the rebound the way you want and hold it against the desk while you screw the cap on so the shaft won’t move outwards. When both shocks are ready, pump the shocks a couple times. Do the same on both shocks and check that the shaft comes back out the same on both shocks. If the one of them comes out more, there’s too much oil in it. Or if it doesn’t come out enough, there’s too little oil in it.

Tip! You get a suitable amount of rebound for a basic setup when you leave about 1/3 of the shaft out. You can build the shock with the shaft pushed all the way up. Both will result in a smooth and neutral outcome.
Note! See, however, that the shock shaft doesn’t damage the bladder and the shaft doesn’t squeeze up the bladder.

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