When you attach a pump to an air cylinder (tube, tubeless tire, fork, shock, or dropper post air spring, etc) a certain volume of air will escape into the hose and gauge of the pump to equalize the system.
This will be different for every air cylinder and pump combination. For example attaching a floor pump to a tubeless tire with 28psi it doesn’t take much air to equalize where attaching a high-pressure/low-volume shock pump to an air shock with 250psi will result in a drop in pressure to equalize the system. Like wise, all my shock pumps take different volumes to equalize.
One real life example: I have a old RP23 that gets pumped up to 165psi. When I remove my pump (zero loss head) there is 165psi in the shock. If I re-attach my pump the gauge shows 145psi so it takes 20psi to equalize the system. The same pump vs. the Float 32 fork on the bike run at 65psi takes a marginal amount of air to equalize because the positive spring on the fork is much larger.
It’s important to know ~ how much air from a given air spring it takes to fill a given pump if you are going to comment on the air pressure in something you didn’t fill (i.e. don’t have initial gauge measurement)or are trying to measure loss over time.
In my real life example if my pump shows 100psi in my fork when I attach it but I know it takes ~20psi for the RP23 to backfill my pump/gauge then I can say my shock has leaked.
In the case of the reservoir on this Thru Shaft shock I took the measurement on my pump gauge and then experimented with different air pressures to figure out what pressure would give me my initial reading on re-attaching my pump.
Feb. 5, 2018, 2:47 p.m. - Andrew Major
When you attach a pump to an air cylinder (tube, tubeless tire, fork, shock, or dropper post air spring, etc) a certain volume of air will escape into the hose and gauge of the pump to equalize the system. This will be different for every air cylinder and pump combination. For example attaching a floor pump to a tubeless tire with 28psi it doesn’t take much air to equalize where attaching a high-pressure/low-volume shock pump to an air shock with 250psi will result in a drop in pressure to equalize the system. Like wise, all my shock pumps take different volumes to equalize. One real life example: I have a old RP23 that gets pumped up to 165psi. When I remove my pump (zero loss head) there is 165psi in the shock. If I re-attach my pump the gauge shows 145psi so it takes 20psi to equalize the system. The same pump vs. the Float 32 fork on the bike run at 65psi takes a marginal amount of air to equalize because the positive spring on the fork is much larger. It’s important to know ~ how much air from a given air spring it takes to fill a given pump if you are going to comment on the air pressure in something you didn’t fill (i.e. don’t have initial gauge measurement)or are trying to measure loss over time. In my real life example if my pump shows 100psi in my fork when I attach it but I know it takes ~20psi for the RP23 to backfill my pump/gauge then I can say my shock has leaked. In the case of the reservoir on this Thru Shaft shock I took the measurement on my pump gauge and then experimented with different air pressures to figure out what pressure would give me my initial reading on re-attaching my pump.