How often do you pump?

[yamcha]

How often do you pump air into your tires to keep the psi at the level you like? I find myself having to add air about once a week. Do you think that is too often?

[14R]

Every time I ride (about 3 to 4 times a week)

[linux_author]

- depends on your tires and width... e.g., 700x23s will need to be topped off prior to every ride... i don't know about 20" tires... (45psi?)

[makingmark]

Weekly. I use a Black & Decker Airstation which makes it a snap. About the same price as a quality pump, and usable for your car also (which came in handy when the Zipcar I rented had a nearly flat tire...)

I find that checking a Brompton tire can let out a good amount of pressure on its own (5-10 pounds?) because the tires are so small, so it's easier to just fill up on a pretty regular basis.

[2manybikes]

There is no set schedule for a size or anything like it. Different tubes will leak differently. Check every time before you go. I don't mean fill them, I mean just check them every time. Each tube may be different, or they may all be the same, no way to know.

[Simple Simon]

Plan to do at least once a week, but often end up once a fortnight

[yamcha]

Can they make tubes that don't leak? How does the air permeate through the rubber anyway?

[EvilV]

I think that apparently solid materials are not solid at the molecular level. Under massive magnification we can see that stuff we think is smooth and impermeable has microscopic spaces between the stuff that makes it up. The rubber tubes are made of molecules of rubber bound together. The funny property of elastomers like rubber is that you can easily pull the molecules further apart causing it to stretch, unlike steel which does not expand in this way. I think the air molecules under pressure, slowly find their way through the thin curtain of rubber molecules one by one over time reducing the pressure gradually in the tyre. Some probably get away at the valve too. Some gasses like hydrogen have such small molecules that they are very hard to keep inside enclosures like balloons. Come to think of it, you get the same effect in party balloons inflated with air too don't you. They start to look really sad as they deflate over time.

The picture below is a microscope image of a rubber spring - it doesn't show what I am talking about above, but it does show that rubber isn't as smooth and flawless as lit looks under the naked eye.

http://www.le.ac.uk/engineering/svh2/images/rubber.jpg

[stevegor]

I vote EvilV as Sheldon's heir-apparent....that is if Sheldon ever abdicates :eek:

[eubi]

Good Job EvilV.

This property of rubber is called permeability. In general, molecular leaks are governed by the cube of the atomic weight, but for some reason, CO2 flies through rubber! It leaks through faster than H2 or He!

The most common rubber to use for tubes is butyl. It has the lowest coefficient of permeation. Nitrile is next best.

Polyurethane has very good permeability numbers, almost half that of butyl, but its relative rigidity has proven it to be inadequate for tire tubes.

Oh, and I pump up about every week. The rolling resistance on these 20" tires really goes up when they get low!

[2manybikes]

Can they make tubes that don't leak?


Nope.

[EvilV]

I vote EvilV as Sheldon's heir-apparent....that is if Sheldon ever abdicates :eek:

You have got to be joking though Stevegor!! :eek: I'd be well out of my depth trying to take on that role.

You can see a much better explanation below from eubi. He obviously knows what he's talking about exactly. I'm just a bull****ter. I did read about butyl this morning. Apparently before ww2, they used to use natural latex from rubber trees, which leaked much more than butyl which is some kind of artificial rubber.




Good Job EvilV.

This property of rubber is called permeability. In general, molecular leaks are governed by the cube of the atomic weight, but for some reason, CO2 flies through rubber! It leaks through faster than H2 or He!

The most common rubber to use for tubes is butyl. It has the lowest coefficient of permeation. Nitrile is next best.

Polyurethane has very good permeability numbers, almost half that of butyl, but its relative rigidity has proven it to be inadequate for tire tubes.

Oh, and I pump up about every week. The rolling resistance on these 20" tires really goes up when they get low!

[makeinu]

I pump every other week or so. I don't find much difference in leakage between large (20") and small (8") tires. Maybe the leakage in smaller tires is balanced out by the lower surface area?

I can't check the pressure on my 8" tires though. Checking loses way too much air.

[yamcha]

Thanks Evil.


I am seriously starting to think about airless tires. I'm sure they coudn't be that bad for 20 and 16" tires.

[EvilV]

Thanks Evil.


I am seriously starting to think about airless tires. I'm sure they coudn't be that bad for 20 and 16" tires.

I read somewhere that they were poor. I think tehy feel rough as far as ride goes and have higher rolling resistance - not sure though. Some expert will be along in a minute and give you the proper info.

[folder fanatic]

I make it a strict habit of mine to pump up at least once a week for each of my bikes both folding and nonfolding. I check before each ride and make it a point to maintain a very hard tires indeed (better to protect against sharp debris and premature wear on the tires themselves). I very rarely get flats (the last one was in 2000 or 2001) when I do take the time to check the tires at home.

[awetmore]

How often do you pump air into your tires to keep the psi at the level you like? I find myself having to add air about once a week. Do you think that is too often?

I do it about once a month, but there is a wide range of pressures that I'm comfortable with. With low rolling resistance 35mm tires I pump them up to around 65psi and pump them up again when they drop to around 30psi (where handling becomes an issue). Dropping half the pressure like that takes about a month.

When I rode 23mm tires everywhere I'd pump them up weekly or more frequently because a 10psi drop was problematic.

alex

[eubi]

Can they make tubes that don't leak? How does the air permeate through the rubber anyway?

Well, you can make them out of metal....

We have many designs at my work where we have to seal helium with zero leakage. Forget anything rubber or other polymers (plastics) in this case. You have to get creative and find ways to get metal to flex without breaking, such as long, thin tubes, corrigated diaphragms, and bellows..

...and this is why those mylar balloons filled with helium and float much longer than a rubber balloon filled with He. Mylar is coated with a thin film of metal! However, they don't stretch very well, and would be inadequate for used in a tire.


You can see a much better explanation below from eubi. He obviously knows what he's talking about exactly. I'm just a bull****ter..

Thanks for the props, and I don't consider you a BSer at all. You gave a perfectly good explanation. Nice microphoto you posted.

Thanks Evil. I am seriously starting to think about airless tires. I'm sure they coudn't be that bad for 20 and 16" tires.

You can try, but when I tried them on my 27" wheels years ago, it felt like I was pedalling through mud by comparison.

[eubi]

I pump every other week or so. I don't find much difference in leakage between large (20") and small (8") tires. Maybe the leakage in smaller tires is balanced out by the lower surface area?

Yes, that and lower pressure and lower thickness than say, a 27" tube. The units of permeability are expressed as the rate of gas flow in atmospheric cc's per second, through an elastomer measuring one cm square (there's your area term) and one cm thick.

For instance, at room temperature, the permeability of Butyl (our inner tube of choice :D) is about:

0.62 *10^-7 atm cc/cm^2/cm/sec

asleep yet?

Not very darn much, but it asdds up over time.

This is why most of the bikes in the USA are sitting in the garage with flat tires!