The nitrogen gas pressure in today's shocks serve two purposes.
1) Nitrogen is used as an inert gas.
2) Nitrogen is pressurized in the shock the same reason your cooling system is pressurized. It raises the boiling point of the fluid.
As the shock is worked, if the piston speed travels fast enough, or enough force applied, the fluid through the valving can reach a point where cavitation can occur.
When that does occur, the damping goes away.
By raising the pressure and therefore the boiling point, it prevents cavitation and loss of damping.
The same happens with forks, but due to economics, gas pressurization is not usually done.
If you look closely at the Ohlins forks used on WSBK and MotoGP, you'll notice a small cylinder piggy-backed onto the lower fork leg.
That's the nitrogen gas cylinder for the pressurized fork cartridges.
Traxxion-Dynamics also makes gas pressurized fork cartridges for bikes, including the RC51.
Nitrogen is used since it's an economically extractable atmospheric gas and is inert at lower temperatures.
At high temperatures it can chemically combine with other elements, but those temperatures are far higher than any reached by the suspension.
It is possible to substitute gases other than nitrogen, such as argon, but isn't really necessary.
Concerning pressure amount, the gas pressure in the shock should just be high enough to prevent cavitation, but not too high, since it could overly exert an air spring effect on the shock.
The typical value is around 150 PSI.
Most shocks are unbalanced in that internal pressure can exert differential forces between the sides of the piston due to the piston rod area.
This can cause the shock to extend fully if not constrained (air spring effect).
Balanced shocks, such as the through-shaft design used in F1 have internal pressure applied equally on both sides of the piston.
So the shock shouldn't try to extend regardless of internal pressure (no air spring effect).
Last edited by SubSailor; 02-18-2013 at 12:21 PM.