Coilover Charge

[redneckrapunzel]

I know that the charge in a coilover is not designed to hold the weight of the car at all but it does no doubt therefore I'm wondering how much will the charge effect the amount the springs will compress vs uncharged? Im assuming that it will be a negligible amount but just wanted to make sure that im not crazy for thinking that.

[BadAssEddie]

It adds to the effective spring rate by the amount of the pressure times the surface area of the piston. In an ideal damper, force is only developed through the resistance of the fluid when the piston is moved through it. But, IRL you need a charge on the fluid to prevent things like shock fade and emulsion. I did not include any effects from charging in my design and it works well. I think it will be okay either way.

[willhf1011]

You've got the right idea. It will typically make a measurable difference, 1/2"-1" depending on the coilover, rig weight, charge pressure, and intended ride height.

[redneckrapunzel]

ok that all makes, my worries is that the area of the piston in the reservoir i believe is around 2.36 square inches (1.7 bore) which at 175-200 psi is a very large amount of added spring rate it seems. The pressures i wrote are what i read were standard pressures for a coilover are they way off? I felt like the pressures would be specific to each brand coilovers depending on there design but could not find anything on this.

[willhf1011]

The charge pressure doesn't act on the piston area of the shock though. It acts on the floating piston in the reservoir which pressurizes the fluid which in turn acts against the cross sectional area of the shaft (probably 5/8" shaft?). Your charge pressures are set at full extension, so to figure your pressure at ride height you need to calculate the added pressure (i.e. reduced volume) from the shock collapsing 7 inches, assuming you are looking for 5" ish of uptravel, which it sounds like from your spring rates.

[redneckrapunzel]

ok that makes much more sense and now that I think about it it makes sense that that is how the pressure should be applied.

[BadAssEddie]

Yeah sorry I mispoke a bit. If you want a real nber and not just an idea you have to do the trickle down calculations like Will is saying. It explains it in the bible with pictures. Many standard HVAC nitrogen regulators have 150 psi safety blow off valves. I'd just charge it to that and call it good.