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A typical car might have 2.5 bar tyre pressure, and use the Schrader valve system.

A typical mountain bike might also have approximately the same tyre pressure, perhaps a bit more, and also use the Schrader valve.

Why is it that a car tyre seems capable of maintaining most of its pressure after a year, while a few weeks might be enough to notice a significant difference in bike tyre pressure?

There are other related questions here, such as:

Do tire / tube / valve combinations that don't leak air exist?

How long should a road tyre hold its pressure?

These questions have answers that discuss how air can leak mostly through the rubber itself, but also in the valve system.

This only brings me back to my own question: Is the rubber that different on a car? Are there any tires I can get for my bike that will not leak? I'm sure I'm not the only one that finds it annoying to maintain the bicycle tire if there is really no need for it with the right kind of material properties and valve systems to prevent that. When people can justify spending large sums on relatively minuscule gains, why not more of a focus on maintenance free tires?

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    Consider the volume to surface area ratio of the tube. – Daniel R Hicks Dec 13 '19 at 14:03
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    (And there are also some hoity-toity ultra-thin bike tube that will barely hold air for 24 hours.) – Daniel R Hicks Dec 13 '19 at 14:04
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    Bicycle tires, especially for road bikes, are inflated to a significantly higher pressure than car tires which means their state is further from an equilibrium than in car tires, resulting in relatively more loss of air. Car tires are much harder to remove/install than bicycle tires so the tire bead will seat more firmly against the rim creating a good seal. (even though I realise this might not be a good comparison since most car tires are tubeless and most bicycles tires are not). – Maarten -Monica for president Dec 13 '19 at 14:31
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    This isn't really relevant for the question, but: 2.5 bar is actually rather a lot for an MTB tire. In many situations, riders may opt for as low as 1.5 bar for better grip. Only in edgy and dry rocky terrain should one go to >2.5 bar (to avoid punctures), or when driving on {urgh} tarmac (to reduce rolling resistance). – leftaroundabout Dec 13 '19 at 23:42
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    @Mazura I suspect there's a large variation in valve quality too. A $5 US Schrader valve makes a lot more sense for a $100 US car tire or $20000 US worth of HVAC equipment than it does for a $5 US bicycle tube. – Andrew Henle Dec 14 '19 at 13:14
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A car tyre has much thicker rubber than a tubeless bike tyre, thicker still than a tube. The steel wires in the belts of a car tyre are impermeable to gas and occupy a significant fraction of the area of that layer. So there is a material difference. In both cases the rim can be regarded as impermeable, and the sidewall more permeable than the tread.

Car tyres, despite their much squarer cross-section, have a larger ratio of volume to surface area. The volume determines the remaining pressure, while the surface area determines the leak rate.

Even taking that into account, car tyres don't really hold enough air for a year. The fact that they may give acceptable handling for months on end has a lot to do with how forgiving driving a car can be compared to running on two wheels. Try asking a motorcyclist (dimensions and construction close to car tyres) how often they top up the air. But still, a few months should be OK on a car, though topping up once a month should improve your fuel consumption.

I don't run tubeless, so the following assumes tubes. My tourer and hybrid, pumped to the max pressure, gives an acceptable ride for about 3 weeks on ~32mm tyres with cheap tubes. That's starting from nearly 6 bar (85 psi) so over twice what most cars take. My MTB doesn't get ridden much at the moment, but again with cheap tubes (though obviously fatter) can hold trail pressures for a little longer than that. In practice I pump it up almost every use, because I tend to ride on road to the trails then let out some air.

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    Also, you don't propel a car using muscle power, so the increased rolling resistance isn't immediately noticeable with a car in the same way that it is with a bicycle. – Sean Dec 13 '19 at 22:11
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    I disagree with your statement that car tires don't hold air for a year. The only time I have ever had a car tire lose air is if it has an actual leak in it. My truck has maintained a constant pressure in all 4 tires for a year now and my SUV for over 2 years (since they were installed). Certainly if your car tire needs air monthly you have a hole in it or a faulty valve. – jesse_b Dec 14 '19 at 21:13
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    @whatsisname: I do. I also have been religiously checking the air in my truck tires at least every other month for the past several months as my TPSM is broken and I expect the pressure to change slightly due to it being winter but they have not moved a single PSI. – jesse_b Dec 14 '19 at 22:05
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    I find it dubious that the steel wires would have much to do with this. Also, 85 psi is ≈5.9 bar, not almost 9. – leftaroundabout Dec 15 '19 at 0:15
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    A local tire selling chain offers a free "air check" service. I use that once every couple of months, and they usually have to add a little air. All four tires behave the same, so I don't think it is leaks. – Patricia Shanahan Dec 15 '19 at 17:54
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The biggest drivers are:

  1. The pressure itself: bike tyres usually run much higher pressure than car tyres, and this pressure is the driving force for leakages, hence bike tyres leak more.

  2. The pressure remaining inside the tyre is essentially the amount of air still remaining, and car tyres having a significantly higher volume. That means for the same absolute amount of leakage, they lose less air relative to the the amount of air they contain (hence retain higher pressure).

This effect is also visible for different bike types - a road race bike running at 8..10 bar needs re-inflating a lot more than a mountain bike running at 2..3 bar

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I think you have a wrong starting point.

At this time there is an ad on the Dutch radio, reminding car owners to top up their car tires once every second month. As the loss of air makes the car spend more energy to drive, even though in the car you do not notice the difference.

On the other hand, when I get a good top up in my bicycle tires, (tube in tire system) I do not need to repeat that for quite a while. At the moment I am at about 3 months and the tires do not feel as if they need a top up. There have been times I can get longer out of it, depending on temperatures and use of a bike.

Of course, build of tubes and tires will make a difference. There will be a big difference between the lightest versions for racing bikes and the sturdy tubes and tires used on bikes for rough surface use, that might be a bigger difference than between the average car tire and bike tire.

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Assuming we are talking about tubeless vehicle tyres against bicycles using inner tubes then I would think the biggest difference is mostly that it's a thick tubeless tyre designed to hold air vs a thin layer of rubber.

A car tyre has a thick carcass of varying materials that are all part of keeping the air in but a bicycle has just the inner tube and that is one relatively thin layer of butyl rubber.

As mentioned in the linked questions and answers, inner tubes themselves are not 100% airtight. The butyl rubber is permeable and will allow gasses to escape. What I haven't found is data to show how quickly you should expect to lose air from a bicycle innertube. A car tyre, however, is expected to lose about 1-3 psi per month.

It also depends on what you filled the inner tube with. CO2 is soluble in butyl rubber, so when you use a CO2 cartridge to inflate your tyre you should expect it to need topping up very soon.

You'd think tubeless bicycle tyres would be better but in my experience, they lose air faster than my tubed tyres.

So the lowest maintenance combo would probably be a good quality thick innertube in a mountain bike tyre, filled with argon gas.

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