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Shock seal leak and rebound problem

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We knew something was wrong with the shock on my kids YZ125 because it started handling bad and the rear end was "packing". So I turned the rebound clicker out to speed it up. Even after turning the adjuster all the way out, it still had way too much rebound damping.

So I removed the shock and found the seal was leaking. (Yes, I use a seal 'bullet' when I rebuild them) After removing the spring I ran it thru it's stroke a few times and could hear the tell-tale sound of air and oil bubbling through it.

Question 1- Why would air in the shock slow down the rebound? I would of thought it would speed up.

Question 2 on a tangent subject- How is it possible that "over filling" a shock with fluid can cause handling problems? Seems to me that the more oil you have in the resevoir, the better. It would stay cooler and fresher.

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q'n'd answers


air can be compressed, oil not.

you know this principle from your fork, where this feature is desired (and required), as the airchamber acts as additional spring.

a shock is a closed system, no air, just oil an N2 which gives quite some pressure (usually 145psi) to the oil.

The rebound neddle, sits inside the (piston) rod. This rod has a hole on the top where the oil transfers the pressure onto the top of the rebound needle and presses it down as far as possible, depending on the position of the rebound adjuster.

If there's air in the shock that results in first getting the air compressed and then getting pressure on the oil. Depending on the amount of air and, more important, the pressure of the N2, there won't be enough force to press the rebound needle completely down resulting in a not perfectly working of the rebound.


if the shock gets compressed, ie the rod moves further into the shock body, oil flows through the ports. But there won't be the equal amount of oil on both sides of the piston. Why not?

The rod enters the body as well and where's a rod, there's no oil :thumbsup:

ie you have to have room for the volume of oil which gets displaced by the rod on the other side of the piston. And that's where the N2-chamber comes into play. It must be big enough to compensate for the displaced amount of oil of a totally compressed shock.

And that's why the N2-chamber has to have a certain size.

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OK... This is starting to make sense.

#1 When the shock compresses, the piston is forced up, making high pressure on the resevoir side and low (neg.?) pressure on the other side, where the rebound needle is. Since there is some air in the shock, the bladder cannot exert enough pressure on the fluid on the rebound side of the piston to keep the rebound needle in place (and stop cavitation) and it gets sucked up, closing it off. And on the rebound stroke, the air compresses enough that the fluid can't build enough pressure to push the rebound needle back where it belongs.

Ahhh... I think.:thumbsup:

#2 I understand that as the shock rod enters the body of the shock it displaces an equal amount of fluid into the resevoir. So if the volume of the inflated bladder is greater than the volume of fluid the shock rod displaces, then the shock is not over-filled? Right?

I gave myself a headache! Need beer...

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