Tire Pressure Issue

[Mr. Bloom]

OK, this perplexes me...

• Before I ride, I fill my tires to 100 psi
• Next day, before the next ride, my air pressure is consistently about 80psi
• New tube, confident there are no links

Is there some law of physics that causes road bike tires to dissipate so quickly on psi? Is my observation unique?

I know I really should check the psi immediately after a ride...would bumps cause this big a loss of pressure??

Thoughts?

[KnottedYet]

Even my low-pressure tires do something like that. (I run about 75#)

I haven't checked to see *when* it happens, I just assume it will always happen and check my tires at least once a week.

[VeloVT]

I think some of it is that unavoidable "pffft!" of air when you release the pump from the presta valve. Frustrating .

****If you inflate to 120psi you get an extra day maybe.

[motochick]

Fill them with nitrogen and they won't leak as fast. We use nitrogen on our motos.

Brenda

[Pedal Wench]

I think some of it is that unavoidable "pffft!" of air when you release the pump from the presta valve. Frustrating .

****If you inflate to 120psi you get an extra day maybe.

I always thought that last 'pfffft' is the air that's in the pump's hose, not air from your tire. Presta's self-seal with each pump -- well, at least that's what I thought.

[bike4ever]

Inner tubes are porous. They will slowly loose air. The higher the pressure in the tube - the quicker it goes down. Your tubes are a good example with 100 psi down to 80 psi. I pump my mountain tire up to 35 psi. I can check it a week later, and it will be at 32/33 psi.

This is why I always sell a floor pump with a road bike sale.

[Mr. Bloom]

It's interesting that we're having the same experience, but that ya'll are checking your air weekly. I'm doing it for daily rides and adding 20 lbs each time.

I'm certain that the bumps I hit knock some air out, the porousity of the tube contributes, but at the rate of reduction, it would imply that I'd be flat after a 10 hour ride...but I'm not...that's the contraction that perplexes me.

So, we will refer to this as "THE AIR LOSS PARADOX"

[OakLeaf]

Well, for one thing the rate of reduction you're talking about is over 24 hours, not 2 - the tube doesn't stop being porous when you get off the bike.

But more importantly, it isn't linear. The higher the pressure, the harder the air molecules are being forced through the interstices of the tube. I'm not sure what the curve looks like exactly, but you must have a rough idea of it already - don't you ever take a day off riding? On the second day, the pressure will be 70-75, on the third day it'll be 70, and if you store the bike long term it'll be months before it's completely flat.

[7rider]

Inner tubes are porous. They will slowly loose air. The higher the pressure in the tube - the quicker it goes down. Your tubes are a good example with 100 psi down to 80 psi. I pump my mountain tire up to 35 psi. I can check it a week later, and it will be at 32/33 psi.

This is why I always sell a floor pump with a road bike sale.

Interesting on the pump sale. I hope you instruct them on the importance of using it and how to use it. I know folks who have a pump, and never seem to use it! I guess they enjoy pinch flats??
I check my tires before every ride...I can't say I lose 20 pounds in a day, but I definitely need 2 or 3 pumps from the floor pump (more, of course, if I haven't been on the bike all week). I run my road tires b/t 105-110 psi.

[Mr. SR500]

It's losing too much air. Try a different brand of tube or another new. Currently my rear tube will hold pressure, only losing a couple psi a day, but the front will lose about twice as fast and it gets inflated to lower psi.

[Tri Girl]

I've wondered about this myself. I rode my tri bike on Tuesday and pumped the tires up to 135 (the rec. inflation is 130-160). Yesterday I get it out to ride and it's down to 120. Who knows? I just consider the pumping my pre-ride warm up.

[Tuckervill]

I'm no scientist or anything.

Shouldn't there come a point when the tube is under pressure that the interstitial spaces are too small for air molecules to go through?

Thus they would never go completely flat. The curve would end at some point.

I can imagine a manufacturer would try to perfect their product to balance between the natural properties of the rubber and the amount of stretch at pressure that increases the size of the interstitial spaces.

Anyway, that's my thought experiment for the day.

Karen

[SouthernBelle]

I ride a 22 on the front, 23 on the back. I think they have the same tube. The front seems to lose faster though. And by a fair amount.