Elementary question #2: Pumping tires to correct pressure
 

When I pump up my tires to a certain pressure and release the tire, air escapes. (When I put the pump back on the valve, the pressure indication is less than I pumped it to.) Do people pump a little higher than the intended pressure to correct for this, or not. If so, what's your rule? Have just been wondering about this for a long time.

What type of valve, Presta or Schrader (automobile type)? A Presta valve closes on internal pressure so when you disconnect the pump hose the air is trapped in then tube and none escapes. A Schrader valve has a spring to close it but if you unscrew the pump head slowly some air can esacpe

it may be that when attaching the pump the air in the filler hose is equalized with the pressure in the tire. since the air in the tire is under compression, some will escape the tire.

the air escaping on removal of the pump is probably from the filler hose (that is that same that is in the tire when filling) and is equalized back to atomospheric pressure when the nozzle is removed from the valve. the loss to the tire occurs the NEXT time you attempt to fill it.

so you never can know what the real pressure is unless you install a pressure gauge in the tire, sort of like Schrodinger's Cat paradox.

i'm just guessing... :lol:

It depends on the air pump connection how much air you'll actually lose. With the lever type connections, it seems you lose more air than with a screw on/off type of connection. I've always inflated my tires ( especially the rear tire to about 105 more PSI than what the sidewall pressure guide recommends. My 700c X 35mm tires call for about 5 bars or 70/72 PSI. I usually pump them up to about 80 PSI because in about two weeks, they'll lose that extra air and be right at 70/72 PSI. You can always let excess air escape by checking it a few times with your air gauge until you get it where you're comfortable with the reading.

If y]you take a moment to think about it, what really matters is how the bike rides. So forgetting the number, if whatever technique is yielding a correct (judged by ride properties) pressure, then all is good, and you can continue doing whatever you do. OTOH, if you feel the tires are ending a bit softer than you like, then overfill some to allow for whatever loss you get when removing the hose.

Also read what Hillrider said about the difference in how valves work, to which I'll add that trying to read tire pressure from a just connected hose is problematic, since a decent amount of air flows out of the tire to fill the hose causing a drop. The only reliable reading from a floor pump's gauge is the one taken just before you disconnect the hose.