wle

simple question
i'm sure there will be raging flames and controversy
but
here goes
assume presta valve and pump

you know how the pressure goes up, then back down, for each stroke?

which one is the real tire pressure, the high or the low number?

it can vary as much as 20 psi in my experience
so
?


i would guess the low number is correct..
discuss among yourselves..
(surprised to have never seen this come up)

also complicating matters: depending on the seal at the nozzle/valve junction, the pressure may go on down to zero
as air leaks from the hose, though the presta valve has closed by then
.. obviously the tire pressure isn;t zero..



wle

txags92

If you are seeing the pressure bounce up before dropping back with each pump, then you probably don't have your pump seated on your valve far enough. In any case, it would be the lower pressure that represents the tire pressure, as the higher value is just what is needed to push the valve open and let the air go into the tire.

RPK79

I use a formula. Assume H = the high number and L = the low number:

((H-L)2)(-.5)+H

TrojanHorse

You have not even begun to consider the accuracy of the gage!

What about ambient pressure?

dtrain

I 'hear' my wife, but what is she really saying? Does it matter? I mean, as long as she thinks I was listening...

nondes 

I have this idea of setting up a wheel, tube and tire with an extra valve hole and valve so that I can put a pressure gauge on one and see what happens when I remove the pump connection from the other one. Until then I don't think we can tell what really happens, as air could be released when you re-attach a gauge to a single valve. Sort of a Schrodinger's cat problem.

Elvo

Are you talking about gauge pressure or atmospheric?

wheelreason

Could be simplified, much like this thread...

RJM

4...the answer is 4

canam73

In your description it's probably the low number. On the down stroke the hose loads and you get a reading higher than what is in your tire. The pressure will quickly equalize between the two giving the pause at the 'low' number. If the pump isn't sealed well the hose will continue to lose pressure and the gauge will go down, but this won't effect the tire pressure.

Now back to the cat theories.

hueyhoolihan

you know the N S A reads this stuff, don't you? it could get back to her one way or another.

Bandera

No, I don't know.

1) Get a good quality pump.
B) Seat it properly on the valve.
iii) Inflate away to desired PSI.

Treat yourself: Silca Super Pista pump review - BikeRadar

-Bandera

Reynolds 

Pump gauges aren't very precise IMO. They might as well have Low, Medium and High zones, not psi.

gsa103

That'd be fine. As long as its within +-5 psi who cares.

hueyhoolihan

IMO, and i have recently used a number of complimentary bicycle pumps, with gauges, at LBS's, they have all been with 3-5 PSI of one another, which has surprised me. whether or not the PSI they have collectively shown would match your typical automobile tire gauge or and industrial gauge, IDK. i suppose it could be a conspiracy...

RollCNY

The pressure has to increase to move air. So the pump is a cylinder, you rapidly compress the cylinder, create a high pressure area, which flows through the system, with the system including the pump cyclinder, the hose, and the tire. How fast you pump is how fast you spike the system pressure before it equalizes.

Read the low number, or pump incredibly slowly.

FLvector

Sounds like you have a cheap pump. Get a good one. This shouldn't happen.

Dudelsack 

42i.

thump55

I'm not.

eja_ bottecchia

For every fact there is an infinity number of hypotheses.

Pirsig smiles upon your endeavors.

wle

formula, is that a joke?

Bob Dopolina 

Buy a floor pump.

wle

i am using a floor pump.
not sure what makes anyone assume otherwise.
wle

canam73

Don't sweat the high/low. I have 2 floor pumps with gauges, a Lezyne and a Planet Bike and they both do it. If others aren't seeing it either they pump slow, don't have sensitive gauges or aren't paying attention. The only gauged pump I have that doesn't show a momentary spike is a Topeak Morph and that one reads very slow.

If pressure continues to go down after the 'low', you may need a new grommet in your pump head or your pump may not be sealed well. If you can still fill your tires in a reasonable amount of time I guess it isn't an issue but I would keep an eye on it.

carpediemracing 

With a presta valve there is no spring in the valve. The air pressure pushes the little nib thing out, closing the valve.

When you pump up the tire you have to overcome whatever is holding the little nib in place. At low pressures it's not a big deal because the nib is loose. When the pressures get higher then every time you stop pumping there's a lot of pressure pushing the nib out. It'll actually push the nib up hard enough that you have to use some effort to get air into the valve.

So when you are pumping up your tire and it's already at higher pressures (so not at 10 psi, we're talking more like 80-100 psi and up) you're doing the following steps (and keep in mind that the gauge reads the pressure at the end of the hose, if you will, not the pressure in the tube):
1. Increase pressure in chamber in pump up to and beyond pressure in tire. The gauge shows rising pressure on each pump here.
2. Once the nib moves then the pressure drops in the pump chamber and goes up in the tire. The gauge will drop at this point.
3. Keep pushing on the pump and you increase the pressure that little bit (5 psi or so once you're up there in psi). The gauge starts to go up again on each pump.
3a. If your pump head is not seated well air may leak out from the pump chamber/hose, reducing the gauge reading to zero. However since the valve nib got pushed shut by the pressure inside the tube the tire still has air.

With a shraeder valve the pump holds the valve open (using the nib inside the pump head) so the pressure fluctuation is less significant.

With most valve extenders (for aero wheels) the valve is so far in the rim that it cannot be closed. Air pressure is enough to hold the valve open. There are instances where people have theorized that the rider hit a bump when the valve was lined up perfectly, allowing the valve nib to depress momentarily. If your tire goes soft on a bumpy race and you have valve extenders (which simply screw onto your valve, forcing you to leave the valve nib unscrewed), it may be that the nib depressed momentarily.