Bicycle Mechanics - Why do tubes leak?

Is it the permeability of the rubber, the valve, or both? I'm only riding my bike 100-150 km per week and I have to top up the tires every Saturday. I'm running 700C x 23's @ 120-130 psi, and in a week it'll lose 10-15 psi.

It's interesting, there has been a wide adoption of exotic materials in the bike world - carbon fiber, titanium, lots of Alu and steel alloys, now carbon nanotubes, etc.

You'd think that with all these advances we'd have a way to do something as mundane as keep air in a tube. But maybe it's just me.

Do slimed tires hold their pressure better? On my hybrid with bigger, lower-pressure tires the tubes are slimed, and it does hold pressure better, but that could be down to the lower pressure, too. In any case, I'd rather pump up the tires every week than add the weight to my skinny-tire bike. I'm just curious.

http://www.kenkifer.com/bikepages/humor/air.htm

I believe it's mostly due to the higher pressure.

While we're on that subject, why do you run 120-130? Unless you weigh over 200 lb, you're way too high. Read the risks associated with improper pressure here (http://www.sheldonbrown.com/tires.html#pressure) and check this post (http://www.bikeforums.net/showpost.php?p=5320226&postcount=3) for a good rough estimate on the pressure you should be running. Also, are your tires rated for that high of pressure? Going too far past that risks unseating the tire, which has a very ugly result.

Rubber does get thinned when it's stretched - Der! :D - and become more permeable. As a series of long-chain and complex molecules, it holds air in a remarkable way. For tires and tubes to do what they do, they have to accomplish the following: Be able to maintain a shape. Withstand variations in pressure. Be malleable to allow it to curve to fit forms. Maintain a pressurized atmosphere for a fairly lengthy period of time. Be able to do these things in a predictable way many times. While other materials can do this better - only rubber, and more modern synthetics built of the chemical physics of rubber (latex), meet all of these criteria. Steel would withstand high-pressure and be able to maintain this for longer times - it's not form-fitting without extreme heat. And it would make a lousy inner-tube.

But yes it does leak. Check your pressure before you ride, and top it off to your favored pressure. Failure to do this can cause damage to the rubber and, by proxy, it's surroundings - your wheels. And you should to prevent the tire from flying off while your going 30mph down a hill. Take a proactive stance with your tires & tubes. :thumb:

It's interesting, there has been a wide adoption of exotic materials in the bike world - carbon fiber, titanium, lots of Alu and steel alloys, now carbon nanotubes, etc.

If you ever tried to mount a tire with a CF or Ti tube, you'd know why most of us use rubber ones.:(

Idea:
Would using a slightly wider tube than needed by your tire, result in less inner tube stretching when inflated and therefore less gradual leaking from pores in the rubber?
It'd be heavier. You'd have to be careful about installing it, don't get the excess tube width trapped under beads...

Will it hold air longer due to less stretch? Get a tire-gauge. We'll be looking forward to your report. But I rather doubt this. I believe it's the nature of the rubber-compound itself. A few p.s.i. isn't likely to have much measurable effect. This is, after all, a large chain of molecules - dense ones - that are only allowing x-amount of air-escape.

But it would be a good experiment to conduct. Until then - top off your tubes before each ride.

I don't know why, but I think it is a good practice to top off the air before a ride, and it takes about 5 seconds? I don't even have a fancy air pump like Panthers007!

I would stick to 110-120 on a road tire, not sure why you would be going to 120-130, are you racing or running track? Your tires have pressure ratings for a reason.

This pump might take 3 second to top-off the rubber. It moves an amazing amount of air - real FAST! No wonder the SKS Rennkompressor is the pit-crew choice in European racing circles.

http://i424.photobucket.com/albums/pp323/nagognog/IMG_0370.jpg

http://www.kenkifer.com/bikepages/humor/air.htm

Cold scientific analysis is always helpful at explaining complex problems.

Keep in mind, that you can easily let out 5# when checking the pressure. It doesn't take much, especially at any pressure over 75#.

That being said, I find that Schwalbe tubes hold pressure better than any other tube i have ever used. Not very expensive at the Schwalbe web site.

answer lies within one of these threads

http://www.bikeforums.net/showthread.php?t=546439
http://www.bikeforums.net/showthread.php?t=499922
http://www.bikeforums.net/showthread.php?t=496199

something to do with the solubility of air in butyl and latex rubber.

Keep in mind, that you can easily let out 5# when checking the pressure.

Ahhh...the old Schrödinger's Tire Paradox.:(

Easily avoided by using a compressor with an adjustable regulator, so there is no need to check it afterward.

Keep in mind, that you can easily let out 5# when checking the pressure. It doesn't take much, especially at any pressure over 75#.

That being said, I find that Schwalbe tubes hold pressure better than any other tube i have ever used. Not very expensive at the Schwalbe web site.

I don't even bother to check the pressure. I just hook up the pump every other day and top them both off.

I'm surprised the OP can go the whole week on only 1 fillup. After 3 days I can feel the difference but I'm also 220#s and run both tires at 120psi.

Idea:
Would using a slightly wider tube than needed by your tire, result in less inner tube stretching when inflated and therefore less gradual leaking from pores in the rubber?
It'd be heavier. You'd have to be careful about installing it, don't get the excess tube width trapped under beads...
I've heard of people considering this for better puncture resistance, but not for air leakage. Either way, it increases the chance of the tube folding inside the tire and getting pinched until you get a pinch flat. Better to just use thicker tubes, and since those seem to leak air as well, I doubt running a wider tube would be any different.

As recommended above, just fill them up before the ride. We cyclists need the upper body workout anyway. :D

I believe it's mostly due to the higher pressure.

While we're on that subject, why do you run 120-130? Unless you weigh over 200 lb, you're way too high. Read the risks associated with improper pressure here (http://www.sheldonbrown.com/tires.html#pressure) and check this post (http://www.bikeforums.net/showpost.php?p=5320226&postcount=3) for a good rough estimate on the pressure you should be running. Also, are your tires rated for that high of pressure? Going too far past that risks unseating the tire, which has a very ugly result.
I'm about 210 without my backpack and the tires are rated for a min of 100 and max 145. By some chart on Sheldon's page I came up with a number of about 125.

Just because I rock skinny tires doesn't mean I'm not a ******. :p

I can appreciate that an inner tube has a very hard job to do (especially at higher inflation pressures and potentially higher local momentary pressures when hitting bumps etc.), and that being compliant to mechanical stresses but hold the air trapped are kind of opposing requirements. I'm just a bit surprised that with all of the other funky materials we've got on bikes we're still dealing with rubber.

Of course, the same is true in money-no-object racing like F1 and MotoGP, so I don't know why I'm so surprised. :)

I'm about 210 without my backpack and the tires are rated for a min of 100 and max 145. By some chart on Sheldon's page I came up with a number of about 125.
Just because I rock skinny tires doesn't mean I'm not a ******. :p
23mm tires at 125psi? 210lbs? :eek: Get yourself at least some 25mm tires (28 are even better) and pump 'em to 95 lbs and enjoy life.

23mm tires at 125psi? 210lbs? :eek: Get yourself at least some 25mm tires (28 are even better) and pump 'em to 95 lbs and enjoy life.

no way. Im 220 and run on 23mm @ 120. Anything less than 100 and it feels like I'm dragging something.

23mm tires at 125psi? 210lbs? :eek: Get yourself at least some 25mm tires (28 are even better) and pump 'em to 95 lbs and enjoy life.
What makes you assume that I'm not enjoying life at 125 psi? I love my bike and I'm enjoying my life a lot more with it than I would without it. My old commuting/touring bike is a Giant Cypress R hybrid with 700c 32's at 85 psi (or something like that), and I like my new Jamis Sputnik in a way I never enjoyed the fat heavy Cypress.

This pump might take 3 second to top-off the rubber. It moves an amazing amount of air - real FAST! No wonder the SKS Rennkompressor is the pit-crew choice in European racing circles.

http://i424.photobucket.com/albums/pp323/nagognog/IMG_0370.jpg

Looks alot like my recently restored Meidai pump!
I call modern tubes and tires "job security". I spend more time topping off tires on my bikes AND cars. It's like they're MADE to slowly leak, so you can wear them out sooner and have to buy more.
I had an old Western Flyer with the original tubes that had to be 30 years old - and they still held air. They just made stuff better int he old days. (Gulp - can't believe I'M talking like this!)
And I agree - almost all tires should be checked and topped off before each ride. Way too many low running tires out there = pinch flats and slow walks home!

And I agree - almost all tires should be checked and topped off before each ride. Way too many low running tires out there = pinch flats and slow walks home!

I betcha' most of us cyclists/wrenches are more conscientious about checking our CAR tire pressure, too.

Just a good habit to form.

Well, if you're comfortable at 125 and it hasn't harmed the rim at all over bumps, keep enjoying it and don't worry what the others say. Just be content to fill your tires at least twice a week (I do every other day) no matter what tire and pressure you use.

Road bike tires need to be pumped up before each ride. This is SOP.
pityr said it right.

Al

The thickness of the tube makes a big difference as well. Lightweight tubes will leak faster. Heavier-duty tubes that weigh 2-3x as much leak a lot less slower.

The thickness of the tube makes a big difference as well. Lightweight tubes will leak faster. Heavier-duty tubes that weigh 2-3x as much leak a lot less slower.
Less slower...meaning faster?

Sheesh Dan, you're slipping these days. :p

Less slower...meaning faster?

Sheesh Dan, you're slipping these days. :pSorry, must the the liquor talking! ;) Yah, thicker tubes leak a lot less and slower.

I have training/commuter wheels with 25c kevlar-belted tyres with Tuffy liners and thick thorn-resistant tubes. There's a good 5mm of material to penetrate before a thorn can get through. The 100psi I pump into these tyres last at least 2-months before needing to be topped-off.