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Followup to Pumps and Tire Pressure Thread(11 posts)

Followup to Pumps and Tire Pressure ThreadFez
May 16, 2003 6:19 AM
Assume a good functioning floor pump with accurate gauge.

Once you pump up a tire and REMOVE the pump head, that sound of air upon removal is the change in pressure within the pump head from tire pressure to atmospheric pressure, correct? In theory, no air is escaping from the tire.

When you install the pump head the next day (or even 5 minutes later), is there any loss in air pressure caused by the attaching the pump head?

Over the years, I find that when I check my tire pressure every day, I "lose" about 1 bar per day. However, when I extend that interval to 5-7 days, I only "lose" 2 bars.

This tells me more air is probably lost upon reinstalling the pump head than to natural causes.

P.S. I still think it is a good idea to check tire pressure frequently, but I think some of the air loss claimed is due to the installation of the pump head.
NopeMel Erickson
May 16, 2003 6:42 AM
With a conventional presta valve you do not lose pressure by attaching the pump head to the valve. The way a presta valve works prevents this. The air pressure in the tire keeps the presta valve closed. A pump must exert a pressure greater than the pressure in the tire to open the presta valve and put air into the tire. When you attach the pump head to the valve there is no pressure and the valve doesn't open. The valve only opens when you pump. Therefore, you don't lose tire pressure just by attaching the pump head. This is different in a schraeder valve. A spring holds that valve closed and the spring must be depressed through mechanical means to put air into the tire. This is done when a schraeder pump head is attached to the valve and air from the tire is allowed to flow into the pump. In this case you do lose some air from the tire before any pumping begins.
Just a question, then94Nole
May 16, 2003 7:34 AM
If the presta valve only opens when air is going in, how does the pump gauge register the constant pressure in the tire when you stop pumping?

Just a little confused
Just an answer, thenMel Erickson
May 16, 2003 7:55 AM
Once pumping starts the pressure is equalized throughout the tire and pump, it's all under the same pressure and the guage simply reads it.
re: Followup to Pumps and Tire Pressure Threadgeeker
May 16, 2003 7:06 AM
I have an old Silca floor pump w. pressure gage. In order to pump up tires, after loosening the Presta nut I have to tap the top of the valve 1 or 2 times to loosen things up. If I don't tap the valve/nut until some air escapes, pumping will just exert force without any air going into the tire. The tapping process seems to let out 5-10 psi.
Yeah, that can happenMel Erickson
May 16, 2003 7:59 AM
Happens to me, too. Sometimes I depress the valve and let some air out before attaching the head but most of the time I simply wiggle the head after I attach it and give it a couple of pumps. The valve is sticking and needs to be freed.
re: Followup to Pumps and Tire Pressure Thread, My Takejamesau
May 16, 2003 8:03 AM
I've the Topeak Joe Blow and presta stems.

Before attaching head to valve stem, the gauge registers 0 psig. After attaching the head (before I start pumping) the gauge reads about 85 pounds; that 85 comes from the tire pressure. Basically, the tire pressurizes the head and hose leading to the pump chamber; the tire loses pressure until it and hose equalize at about 85. After pumping up to around 105, I disconnect and hear the whoosh; the whoosh is pressure loss from the pump hose and head. The hose goes back to 0 and the tire remains at 105.

Funny, I've got the same pumpMel Erickson
May 16, 2003 8:11 AM
and, if I'm not mistaken, when I attach it, but before pumping, it reads 0. I'll test it at lunch time and report back.
Checked at lunchMel Erickson
May 16, 2003 12:03 PM
When I hook up the head the guage doesn't budge until I start pumping. Maybe you're pushing the head on so far it's depressing the valve. Otherwise, the guage shouldn't register until the valve is actuated by pump pressure.
Checked at lunchjamesau
May 16, 2003 12:17 PM
it's the way I've always done it: put the head on, flip up the tab, see the pressure, then stroke away. The behavior occurs on shraeders too.
Haven't got an answer.Mel Erickson
May 16, 2003 2:00 PM
Curious that it works differently with the same model pump. Mine is older and maybe they changed the head so it depresses the valve mechanically now. Anyone else?