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-   -   How much air pressure loss is normal? (http://www.bikeforums.net/clydesdales-athenas-200-lb-91-kg/668679-how-much-air-pressure-loss-normal.html)

markdavid570 08-04-10 11:37 AM

How much air pressure loss is normal?
 
I recently bought a decent stand pump with an attached guage so that I can make sure that my tires are properly inflated. The max pressure listed in my tire (700x28) is 85 psi. The other day, I made sure the air pressure on my rear tire was right around 85 psi and I went out for a 22-mile ride. The next day when I checked the pressure, the gauge showed that it was down almost 10-15 psi. I expected some loss, but is that much normal?

It's a new tube and tire and I've only gone for a couple 20-mile rides on it so far. This is also the first time that I've had a decent pump with a built-in gauge. Before this, I was using a little hand pump and just pinching the tire to see if it felt right...not good...I know.

jonathanb715 08-04-10 12:53 PM

Some pressure loss is normal. If it's more than 5 psi or so overnight, I'll check the tire very carefully to be sure there's not a thorn or piece of debris piking through the tire and causing a slow leak. If I can't find it, though, I will still go for a ride on it.

If it's down 20 or more PSI overnight I know something is definitely wrong and will pull the tire off and check both tire and tube very carefully before riding on it.

BTW, if the tire was filled from a CO2 cannister it will lose a whole bunch of pressure overnight - that's normal, just pump it up each day - once the CO@ has leaked out and is replaced by air it will act normally again.

JB

Mr. Beanz 08-04-10 01:12 PM

Not sure if others experience the samething but seems that everytime I install a new tube, it seems to lose more air the first couple of days/rides.

My tires are 120 but overnight on average may lose 10-15 psi. If I don't go directly on with the head and hit the core, I may measure 20psi or more. But every little tap of the valve lowers some, more with higher pressure I would think.

Over the years, I'e had several pumps with gauges that react differently. Sometimes they measure straight on, sometimes 20 less till the first stroke clicks the gauge into a proper reading. Some measure 20 psi till the first stroke clicks the gauge into a proper reading.

Maybe experiemnt with other inflated wheels/bikes as well.

markdavid570 08-04-10 02:11 PM

Thanks guys! I'll keep an eye on it.

DoubleTap 08-04-10 06:47 PM

Each time you connect your pump, you lose some pressure to filling the hose on the pump. If I check the air pressure with a gauge independent of the pump, I may have lost 5 lbs or so. Connect the pump, and it shows I've lost about 20 lbs. It takes a few lbs. of air to fill the pump hose.

TomD77 08-04-10 07:24 PM

When you compress air to 125 psi, the temperature of the air is going to rise considerably. The amount of temp rise will depend on your pump but 70-80 F degrees certainly isn't out of the question. The air will then equalize temperature to ambient and pressure will be lost. If you're just topping the tire off, just a little hot air is going in so the drop won't be much but if you're filling the tire for the first time, the corresponding drop will be much greater. Of course some of the heat energy will go to heat up the tube, tire and rim.

Theoretically a contained ideal gas will lose pressure in direct ratio to its temperature drop when measured in degrees Kelvin (which starts at absolute zero). So a tire originally pressurized to 125 psi with air at an effective 130 F and ambient at 80 F will work like this: 80 F is 300 K and 125 F is 328; so 300K/328K X 125PSI = 114PSI So the tire will lose 11 PSI due to heat loss.

StephenH 08-04-10 10:50 PM

I think with my tires, most of the loss is actually from connecting and disconnecting the pump to check the loss. One little SPSSSST in the process can lose 10-20 psi. And that's at 80 psi with 35x700's.

Wogster 08-05-10 08:45 AM

Quote:

Originally Posted by jonathanb715 (Post 11231641)
Some pressure loss is normal. If it's more than 5 psi or so overnight, I'll check the tire very carefully to be sure there's not a thorn or piece of debris piking through the tire and causing a slow leak. If I can't find it, though, I will still go for a ride on it.

If it's down 20 or more PSI overnight I know something is definitely wrong and will pull the tire off and check both tire and tube very carefully before riding on it.

BTW, if the tire was filled from a CO2 cannister it will lose a whole bunch of pressure overnight - that's normal, just pump it up each day - once the CO@ has leaked out and is replaced by air it will act normally again.

JB

There is an easier way to get this, when you get home, let the CO2 out and refill with air. :D

Mr. Beanz 08-05-10 08:57 AM

Quote:

Originally Posted by Wogsterca (Post 11236762)
There is an easier way to get this, when you get home, let the CO2 out and refill with air. :D

That would be too easy! This is bikeforumsdotnet! :p


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