Air-space is similar to spring and gives static resistance to load.
- Add 35-lbs, fork compresses 1-inch.
- Add another 35-lbs, it compresses 1-inch.
- Near end of travel, air-space gets compressed and adds to spring-rate. Add another 35-lbs, it only compreses 0.75-inch.
- Add another 35-lbs, it compresses only 0.5-inch.
Air-gap functions as rising-rate spring near end of travel. Decrease that air-space and you get more increase in resistance to compression near end.
This is completely separate function from damping. Which is resistance to flow of oil through holes. This is 2nd-order function on top of static compression and it affects the
velocity of the movement (integration function). Without damping, after you hit bump, fork uncompresses fully quickly and tosses you up, then comes crashing down again and up... and down.. like pogo stick! Damping absorbs energy from hitting bump so fork only bounces once. If it bounces too quickly and multiple times per bump, increase damping by making holes smaller or increase weight of oil. Your diving under braking can be fixed by increasing damping with thicker oil along with stiffer spring.
Common upgrade is to chop off some of spring and replace with something not compressible, like PVC spacer. Depending upon your weight, 4" is typically common amount to remove. This will increase spring-rate by +33%. Works by ratio of original length:
L1 * R1 = L2 * R2
16 * 0.65kg/mm = 12 * R2
R2 = (16*0.65)/12 = 0.87kg/mm
