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 :rolleyes:.

****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 :rolleyes:.

****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! :rolleyes: I guess they enjoy pinch flats?? :confused:
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. :rolleyes: 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.

[ehirsch83]

I check my air about every 2 rides, so every other day usually. I am usually down about 20psi. From 120-100.

Mr.Silver, what is the recommended psi for your tires? 100 seems low for a road bike...

[Grog]

Some tubes do let more air go than others. My front and back tires have different tubes that behave differently. One of them is a Schwalbe tube with extra-long stem (easy to recognize), I should take note to see if it's the one that looses more air... Next time I pump my tires I'll let you know.

[Mr. Bloom]

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?


Karen: Such big words! Are you sure you're not a scientist?

ehirsch: My tires are 100 psi. bikeleague training says not to overfill, so I don't (although I used to...)

The bikeleague training also says to check air pressure before each ride. I do since I'm alone during the week and don't have many to call for help if I get a flat and get stuck out past dark.

[kiwi girl]

I'm only riding my road bike on the weekends at the moment. My tires seem to go down from 120 to 80 or even less over the course of a week - but I agree with the slowing down of the rate of loss theory - after a couple of months they haven't got much below 50.

Note to self - pump up the tires on the commuter.

[sundial]

Mr. Silver, is there a chance you have a bad batch of tubes? Our LBS had cyclists coming in by droves with flats. I believe the bad batch of tubes were Specialized. They switched to Bontrager and it reduced the incidents of flats.

I had something weird happen not long ago. I purchased new Specialized Captain tubeless tires and it would go completely flat over night. The bike mechanic took soapy water to it and air bubbles developed all along the sidewalls. :eek: Needless to say I switched tires.

[redrhodie]

Am I the only one who pumps before every ride? It's part of my pre-ride ritual. If I didn't do it, I think I'd feel like I forgotten to brush my teeth.

Okay, I admit it, I'm a little like Monk.

[sundial]

I do. But then I've been accused of OCD. :rolleyes:

[bmccasland]

Don't forget the laws of physics involving air pressure also includes ambient temperature. So when my bike sits in my car all day on a sunny day, I rarely have to pump the tires before my afternoon ride. But on cool mornings when I head out from the house, I need to add air.

More fun with air pressure - when I used to live and work at 7000 ft elevation, and we'd get an order of lab supplies, the boxes would be almost bursting open. All our pipets, tubes, and other such plasticware would be packed in sealed plastic bags, the difference in elevation change from the factory site (sea level) to the lab would make the sealed plastic bags expand. This also happens with air splints, so if you have an air splint and go up elevation - more pressure on your broken limb. Air splint going down elevation, loss of pressure, loss of splint function.

So Mr. Sliver - are you checking your air pressure the same time / every day? Has it been warmer or cooler than the day before? You could have all kinds of road trip fun checking pressure at various temperatures and elevations.

[OakLeaf]

Even more fun with air pressure:

if you bring a flexible water bottle with a straw onto an airplane, then before takeoff, take a big drink and re-close the bottle WITHOUT letting air enter (so the bottle is slightly concave.

If you don't do this - or if you have a rigid bottle without a bite valve - then before you open the bottle during flight, warn your seatmate of the impending geyser.

DAMHIK.

[parity]

Am I the only one who pumps before every ride? It's part of my pre-ride ritual. If I didn't do it, I think I'd feel like I forgotten to brush my teeth.

I do this as well before every ride. I first let a little air out to avoid over inflating and then pump my tires to 110 PSI. I've also heard latex tubes will leak more air then butyl tubes. I guess its also possible ultralight butyl tubes will leak air faster then a heavier butyl tube. I use Michelin Aircomp Ultralight tubes and overnight they will lose around 5-10PSI.

[VeloVT]

I give them a "pinch-test" before every ride :D. If I'm riding every day, I pump up probably every other ride, maybe every 3rd if some of the rides are very short (under 25 miles). If the bike is sitting around longer between rides, I might have to pump up every ride. I pump to 120 front and rear. I think my tires are rated to 145 but I've never tried riding them at such high pressure (not to mention that I'm not sure I'm strong enough to pump that much air into the tire).

[Mr. Bloom]

Yes, I check before every ride. I've made the bikeleague mantra my routine:

A - Air
B - Brakes
C - Chain/Chrank
Quick Release.

I think what I'll do is check before and after each ride to see what a difference that makes.