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Why do bicycle tires lose air over time?

We get the question quite often from our fans: Why do my tires go flat when I'm not using them?

 

We thought we would take a minute to explain the physics of the phenomenon that can get frustrating. While we are at it too, we nerded out a bit on the subject. We looked at what you can do to slow the process. We also looked at what things you might be doing that is making it worse.

 

THE SCIENCE BEHIND TIRES LOSING AIR

 

Rubber is comprised of long elastic strands of polymers. An inherent characteristic of those elastic polymer strands is micro pathways through which gasses are able to pass. The air in your tube eventually escapes through these micro pathways. Voila - lost air pressure in your tires.

 

This process is called diffusion. The higher the pressure and lower the volume, the more quickly the tire will lose pressure. Even with the same amount of air passing through the rubber, the effect is more significant because of the lower volume. On average, tires will lose 1-2 psi per day. For low volume, high pressure tires, you will most likely need to top off your tires every 4-7 days to keep it within the recommended range.

 

WHAT CAN SLOW THE LOSS OF AIR

 

In theory, a heavier gas would have a harder time passing through the rubber, which you'd think would result in your bicycle tires maintaining pressure longer. So, why not use CO2 --which is heavier than either O2 or ambient air -- all of the time?

 

Studies have been done on the subject, and the key here is the solubility of the gas. CO2, it turns out, is very soluble in rubber: CO2 passes through rubber almost 5 times faster than O2. So unfortunately, CO2 is not the answer. In fact, if you use a CO2 cartridge to inflate your tires, they will go flat faster than if you inflate it with a normal pump and ambient air. So, if you need to rapid inflate your tires on the road with a CO2 cartridge, it is a good idea to replace the air in the tube once you are home.

 

There hasn't been much done to decrease the rate of diffusion in bicycle tubes to date. In theory, a thicker tube, like a thorn-resistant tube, should have better air retention, holding its pressure longer due to the additional time it would take for the air to diffuse through the tube. We haven't found any scientific studies on the actual effects, though.

 

In any event, the down side is that thicker tubes add weight to your bike as well as increasing rolling resistance. So, do you want to pump up your tires a little more often, or work harder with every pedal stroke?

 

CONCLUSION

 

Air loss in your bike tires are a natural fact of life. Currently, there is nothing that can replace the feel of riding on air.

 

You can get solid rubber tires, like these new ones from Tannus, but you are trading a supple and fast ride for less maintenance.

 

There are also a few groups working on a 3D printed tire that isn't solid and feels more like riding on air. However, this solution is likely far in the future, if it ever comes to market. According to the makers, these tires take 24 hours for the machine to produce. Because a single 3D printer that makes these tires costs, on average, $40,000, with a maximum output of 365 tires per year (assuming no down time), simply recouping the machining cost alone within 2 years would be about $55/tire. Factoring in the cost of materials, labor, or electricity, along with a standard mark-up suggests these tires would almost certainly wind up being priced at more than $200/tire. Perhaps some day this technology will become more efficient and may become a reality on most bicycles, but not likely anytime soon. Pretty cool though! We just wonder how well they actually ride.