Bicycle Mechanics - Tire Filling

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Stumprofig
07-31-04, 11:33 AM
Here's a real dumb question, but...

When you fill up tires with a floor pump, is it normal for there to be a rush of escaping air when unhooking the pump from the valve? I pump to the rec. pressure (125 psi) and then when I remove the pump's hose from the valve, there's the hiss of air leaving and I feel as though it's losing several pounds of air. Is this my fault? What can I do?

Thanks

beatle bailey
07-31-04, 11:37 AM
That's normal....go a little over the amount you need. My tires need 100 lbs., so I put in 102 and then I feel it's about right after I remove the valve.
Tom Pedale
07-31-04, 11:41 AM
Here's a real dumb question, but...

When you fill up tires with a floor pump, is it normal for there to be a rush of escaping air when unhooking the pump from the valve? I pump to the rec. pressure (125 psi) and then when I remove the pump's hose from the valve, there's the hiss of air leaving and I feel as though it's losing several pounds of air. Is this my fault? What can I do?

Thanks
It's O.K. The tire valve is one-way, it only lets air in unless you depress the valve. The rush of air you hear when you detach the pumphead is the leftover air inside the pump.

d_D
07-31-04, 04:15 PM
It depends on the pump. Some do release air from the tube when removing the pump.

Mine does because it has a t shaped head with a schrader hole on one side and presta on the other. When the lever is in the release position air can pass straight through from on side to the other so air escapes when you remove it from the valve.
It's not really a problem, just pump it up a bit more and remove it quickly.

My pump was cheap, ones that have a better designed head that don't lose air on release are no doubt more expensive.
supcom
07-31-04, 05:34 PM
When you disconnect the hose from the valve, the pressurized air in the hose escapes to make the hiss. If you are using schrader valve tubes, the pin in the valve is released when you flip the lever and should seal off the tire. If you are using presta valves, then the sudden drop of pressure in the hose will seat the sealing plug inside the valve.

Don't worry about the small amount of air that escapes. Even if you lose a ound or two of pressure, you're not going to notice it.
froze
07-31-04, 09:49 PM
Tom is right, that air your hearing is not coming out of the tire except for maybe 1/4psi at the most, most of the air your hearing releasing is the pressure that is still in the pump. Think about it, if you have a tire pressure guage and put the guage on correctly then release it you will hear very little if any air being lost, that's because there's no air stored under pressure in the guage.