Why are inner tubes not airtight?

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ericonabike
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Why are inner tubes not airtight?

Postby ericonabike » 28 Oct 2015, 7:50pm

Some 50 years ago, when I got my first proper bike, I was sternly advised on the need to 'put a little air in' every other week or so. Fifty years on, we have put men on the moon, computerised the world, and cloned sheep. But I still have to pump my tyres every other week. Given the advances made in every other aspect of bicycle technology, why has the inner tube remained so stubbornly inefficient?
Motorists' mantra: Cyclists must obey the law and the Highway Code AT ALL TIMES. Unless their doing so would HOLD ME UP.

Mark1978
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Re: Why are inner tubes not airtight?

Postby Mark1978 » 28 Oct 2015, 8:24pm

Because physics hasn't changed in those years.

Valbrona
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Re: Why are inner tubes not airtight?

Postby Valbrona » 28 Oct 2015, 9:30pm

Same reason why the HIV virus can get through condoms. :lol:
I should coco.

Samuel D
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Re: Why are inner tubes not airtight?

Postby Samuel D » 28 Oct 2015, 10:00pm

Sounds like you need a better pump! It is not such a hassle with a good track pump.

I suppose the reason is that inner tubes are necessarily made of thin and inexpensive material (usually butyl rubber, a synthetic rubber). With a NASA-like budget there may be alternative materials or manufacturing techniques that would hold air somewhat better if not otherwise work better.

jb
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Re: Why are inner tubes not airtight?

Postby jb » 28 Oct 2015, 11:37pm

To make a flexible material so dense as to stop even the most determined air molecule would result in a wheel that felt like it was permanently submerged in cold porridge.
Also,the great advantage spending a few seconds topping up a tyre is you can give it a quick inspection.

So, all things considered a good trade off being slightly porous.
Cheers
J Bro

greyingbeard
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Re: Why are inner tubes not airtight?

Postby greyingbeard » 29 Oct 2015, 10:58am

Im sure the tyres I had in my yoof didnt need pumping up as often as the new ones.
could it be that thick cheap rubber holds air better ?

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NUKe
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Re: Why are inner tubes not airtight?

Postby NUKe » 29 Oct 2015, 11:06am

greyingbeard wrote:Im sure the tyres I had in my yoof didnt need pumping up as often as the new ones.
could it be that thick cheap rubber holds air better ?

You are right the reason being two fold. Older inner tubes were thicker but more importantly we ran at lower pressures,
it was only with the advent of hooked rims that tyres started being pumped harder If you go back to days when racers used Tubs which were the high pressures tyres of old they need to be pumped up every other ride.

There is a new technology starting to emerge, tubeless tyres, perhaps these will not need as much attention, I doubt it though
NUKe
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ericonabike
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Re: Why are inner tubes not airtight?

Postby ericonabike » 29 Oct 2015, 4:13pm

Tongue was slightly in cheek when I posed the question, but it still feels as though this is a forgotten area of bike tech. Especially if we are serious about making bikes an everyday form of transport, rather than a tool for the 'keen cyclist'. Imagine selling a car with a reminder to pump its tyres up every other week...
Motorists' mantra: Cyclists must obey the law and the Highway Code AT ALL TIMES. Unless their doing so would HOLD ME UP.

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Mick F
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Re: Why are inner tubes not airtight?

Postby Mick F » 29 Oct 2015, 4:21pm

ericonabike wrote:Imagine selling a car with a reminder to pump its tyres up every other week...
I thought all cars came with that warning?

Look at your average Haynes Manual.
Weekly Checks are near the front, and one of the checks is for the tyres - tread, condition and pressure.
Mick F. Cornwall

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CJ
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Re: Why are inner tubes not airtight?

Postby CJ » 29 Oct 2015, 4:54pm

ericonabike wrote:Tongue was slightly in cheek when I posed the question, but it still feels as though this is a forgotten area of bike tech. Especially if we are serious about making bikes an everyday form of transport, rather than a tool for the 'keen cyclist'. Imagine selling a car with a reminder to pump its tyres up every other week...

A car tyre is not only massively thicker than a bicycle tyre, but also contains a hugely greater volume of air at lower pressure, so the expected rate of pressure loss for identical technology will be the product the first two ratios divided by the third. Let's say twice as thick, six times the volume and half the pressure = 24 times the loss rate. So expect to pump up your bike tyres once a week and your car tyres twice a year. Sounds about right to me.

A bike is a very special thing, miraculously light compared to the weight of what it carries and how hugely it enhances the range and speed at which a human may go, with no more effort at all! Many parts of a bicycle are consequently stressed more closely to their breaking point than the parts of other vehicles and bicycles are designed in what may see old-fashioned ways but which are actually necessary in order to accommodate the consequent flexing of the bicycle's very light component parts. Attempts to 'modernise' the design of the bicycle frequently fail miserably, resulting in something far too heavy, too inefficient, too expensive or that simply falls apart.

Having to pump up the tyres once a week is a small inconvenience for boosting one's pedestrian range and speed by a factor of four, thanks to a device that costs no more than one week's wages to buy and almost nothing to maintain.
Chris Juden
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Mick F
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Re: Why are inner tubes not airtight?

Postby Mick F » 29 Oct 2015, 5:01pm

CJ wrote:A bike is a very special thing, miraculously light compared to the weight of what it carries and how hugely it enhances the range and speed at which a human may go, with no more effort at all! Many parts of a bicycle are consequently stressed more closely to their breaking point than the parts of other vehicles and bicycles are designed in what may see old-fashioned ways but which are actually necessary in order to accommodate the consequent flexing of the bicycle's very light component parts. Attempts to 'modernise' the design of the bicycle frequently fail miserably, resulting in something far too heavy, too inefficient, too expensive or that simply falls apart.
VERY nicely put.

Thank you CJ.
Mick F. Cornwall

Brucey
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Re: Why are inner tubes not airtight?

Postby Brucey » 29 Oct 2015, 5:18pm

for some weird reason the leak rate through holes often goes as the square root of the pressure difference, so twice the pressure does not necessarily mean twice the leak rate. It'll still be faster at higher pressures though.

If you compare (say)

-a typical old-style 26 x 1-3/8" tyre

- a modern puncture resistant 700 x 35C tyre

- a 700 x 23C tyre

then they have approximate volumes of

3.14 x 59.0 x 9.62 ~ =1780cc
3.14 x 62.2 x 9.62 ~= 1878cc
3.14 x 62.2 x 4.15 ~= 810cc

respectively.

Suppose that the first tyre is pumped to 70psi and then leaks at 1.8litres a week (to start with), and that the other tyres have similar sized holes in them. The 700 x 35C tyre is also pumped to 70spi and leaks a similar rate but loses pressure more slowly because of its greater volume. The 700x 23C tyre is pumped harder (lets say 100psi) so leaks faster; to start with ~x1.2 times faster. However the volume is smaller so the pressure drop is even faster yet.

After a week or so the tyres will be at ~55psi, ~57psi, and ~70psi respectively. After a little over two weeks the pressures are estimated to be ~40psi, ~40-45psi, and ~40psi respectively.

Now the 700x23C tyre became too soft after a little over a week. The 26 x 1-3/8" tyre (of memory) might just be rideable still at 40psi, especially if you are youthful and svelte. What about the modern 700x35C tyre? Well here's the thing; I think that because it is puncture resistant, it will be hard work and slow going as soon as it gets below about 50psi and you will want to pump it up again probably before that.

So you can argue that even if inner tubes are the same now as they were then, you may need to pump many modern tyres more often; not because they leak faster necessarily, but because the time taken to leak outside of their useful working range is shorter, and that this can occur for a variety of reasons.

Cycle tyres work in the reverse way to car tyres; large volume/low pressure vs low volume high pressure means that leaks are far more problematic for cycle tyres.

As it happens I think that there might be some selective memory going on; as a youth, two weeks seemed like an age to me, and a year would have been a good fraction of a lifetime. Some (older) inner tubes in those days were made of natural rubber (rather than synthetic butyl rubber) and would leak rather quickly in fact.

cheers
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Mick F
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Re: Why are inner tubes not airtight?

Postby Mick F » 29 Oct 2015, 5:30pm

Brucey wrote: ....................as a youth, two weeks seemed like an age to me, and a year would have been a good fraction of a lifetime.
I remember someone on the radio suggesting this.

He was a mathematician, and suggested that you are "halfway" through your lifetime at 18 years old.
Maybe wrong about the "age", but I'm sure we all get the idea. :shock:

If you are five years old, a single year is 20% of your life.
If you are ten, it's 10% of your life.
If you are 20, it's 5% of your life.
If you are 100, it's 1% of your life.

Maybe someone needs to draw a graph?
Mick F. Cornwall

karlt
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Re: Why are inner tubes not airtight?

Postby karlt » 30 Oct 2015, 8:59am

It's also because the brain is taking on board far more new experiences when you're younger, so there's more to remember so the time seems longer.

Any road, I remember as a lad (70s/80s) having to regularly pump tyres up; often lived for a while with a slow puncture that could flatten a tyre in a couple of weeks but you just couldn't find. I suspect it was often the valve.

mercalia
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Re: Why are inner tubes not airtight?

Postby mercalia » 30 Oct 2015, 11:28am

on a related matter do slimed inner tubes need less pumping up than non slimed ones? or is the slime molecules too big to fill in the gaps where the air gets out? May need for better slime that has variable size molecules?