Gotta love Currie!

[PStov98TJ]

The first thing that happens when a shock cycles is it starts building heat from the friction of the piston moving inside the shock. The second thing is you have to have a way to deal with the variable displacement of the fluid due to one side of the piston having a shaft and the other side not. If you take a shock and fill it completely full of fluid with no room for expansion, the heat from movement and the piston shaft displacement will soon blow fluid out the seals and ruin them. There is also a variable reaction to the valving because you have displacement from the piston shaft and the movement of the piston acting upon the valving. So, you need room for the fluid to expand and the only way to do that is with a high pressure gas charge above the fluid. After a short period of hard work, the charge will get emulsified into the oil which then changes it's density which again acts upon the valving and changes how the shock responds. If you ever watch an emulsion shock on a shock dyno, the response is all over the place and nowhere nearly as predictable as a reservoir shock which has a piston with the oil on one side and the higher pressure gas charge on the other.

That makes sense. So with reservoir shocks, the fluid from the shock itself displaces into the gas res? And if I'm correct in that thinking, it would allow the fluid to expand but still remain the same density?

 

[Chris]

Thanks Blaine, now I've added reservoir shocks to my never ending list of modifications!

 

[mrblaine]

That makes sense. So with reservoir shocks, the fluid from the shock itself displaces into the gas res? And if I'm correct in that thinking, it would allow the fluid to expand but still remain the same density?

The reservoir is both gas and fluid separated by the piston in there. As the shaft moves the shock piston up it is displacing fluid so it needs a place to go so it doesn't blow up the shock. The reservoir is where it goes and there is a piston in the reservoir with gas on the other side of it that is charged after the shock is assembled. It acts as a spring to keep pressure on top of the fluid to stop it from mixing with gas and emulsifying.

The pressure in the reservoir also serves to keep a bit of pressure on the shock oil to stop the seals from weeping.

The fluid expansion from heat is minor, the fluid expansion from the shaft entering the body as the piston moves upwards is much higher.

 

[Chris]

I'm going to stand on the side of the road with a sign that says, "WILL WORK FOR TJ PARTS". Hahaha

 

[PStov98TJ]

Got it. Thanks Blaine!