Things we'll need to bear in mind:
- Boyle's law and specifically: p1 x v1 = p2 x v2
- Constant Force Spring: Force remains the same through spring compression.
- Rising Force Spring: Force needs to increase through spring compression.
But why use air?
- It's simple: if you can make a piston gun, you can make an air spring.
- It's adjustable: changing the pressure changes the force needed to compress the spring.
- It's calculable: if we know the piston size and the air pressure we can work out the force of the spring.
So what's the stuff about Rising and Constant Forces?
Boyle's Law says that pressure and volume are linked by a constant. So in an air spring, piston travel is also linked, we'll call this swept volume and what is important is the ratio of swept volume to overall volume.
- If the swept volume is a small proportion of overall volume, we have a near constant force spring
- If the swept volume is a large proportion of overall volume, we have a rising force spring.
Why's any of this important? Because we use lower pressures, and so we only have a lower forces to play with.
For instance in a blow back we want a large travel but we don't want the spring force to increase over that travel. So we'd want a near constant force spring.
But in a piston hybrid we might want the opposite, and have a rapidly rising force spring so that our piston only travels a short distance. So that's a rising force spring.
Because we know Boyle's law we can make just the right spring for each and every scenario.
EDIT: As btrettel points out below I got my rates and forces the wrong way round, now corrected.