Of all the questions I get regarding bike maintenance, questions regarding CO2 canisters for inflating tires pop up surprisingly often.
A quick scan of forums and Reddit posts reveals differing opinions, and advocates of both methods generally back up their preference with “well I’ve never had any problems with it…” etc.
This post aims to clear up some objective differences between pumps and CO2 canisters that are important to be aware of when deciding between the two.
As a bonus, we’ll even dip our toes into the science behind CO2 inside bicycle tires, so stick around if you’re interested in learning why tires deflate faster when filled with CO2.
- 1 Dealing With Punctures on The Move: Pump or CO2?
- 2 What are the Advantages and Disadvantages of CO2 canisters?
- 3 Why Tires Filled with CO2 Deflate Faster Than Air
- 4 The Effects of CO2 on Sealant in Tubeless Tires
- 5 To Summarize
Dealing With Punctures on The Move: Pump or CO2?
Attention spans are short these days, so before we dig into the individual differences, let’s answer the question at hand:
When you’re on the move and a puncture strikes, is it better to have a hand pump or CO2 canisters at the ready?
If time is of the essence at the roadside, choose CO2; otherwise, choose a hand pump or a frame pump.
CO2 is lighter to carry, makes puncture stops shorter, and will probably get you over the line (or through the door) faster. Hand pumps (for mountain bikes) and frame pumps (for road bikes) are otherwise more convenient, cheaper, and reliable, all of which we’ll touch on more in the next section.
Of course, you could also carry both, and be spoiled for choice next time a puncture strikes!
What are the Advantages and Disadvantages of CO2 canisters?
Let’s begin by looking at a few reasons why people might prefer CO2 canisters to a regular old pump.
CO2 Canister Advantages compared to Pumps
- Fast inflation speed. CO2 canisters inflate tires far quicker than any pump. If you’re racing, or can’t afford to be late for work, then CO2 can get you back on the move significantly quicker.
- Smaller size and weight. Canisters are tiny and weigh less than a pump unless you carry loads of them. Even the most miniature hand pumps are still a noticeable size and weight, so CO2 is the clear winner here.
- Can reseat tubeless tires. If something drastic happens and you need to reseat a tubeless tire, a sharp burst of CO2 is your best hope. In the upcoming section on how CO2 affects sealant, I’ll touch on why this usually isn’t a good idea.
- Require less manual effort. If you have issues with your hands or wrists, CO2 can solve the problem with little effort once you get to grips with the process.
CO2 Canister Disadvantages compared to Pumps
- Require forward planning. One-use CO2 canisters can run out quickly, especially if you need to use a few to get your tires back up to pressure. You’ll need to order more when you get low on them and be sure you have enough with you to cover your journey at all times.
- More expensive. CO2 canisters aren’t cheap, and unfamiliarity can lead you to burn through them without reaping the full benefit of a canister. Pumps are a one-off cost, and hardly a sizeable one at that.
- All or nothing. Get a tubeless puncture that self-sealed and want to top up a little? That’ll cost you a full canister.
- It requires replacing with air later. CO2 will get you home, but once you get there, you’ll want to replace the CO2 with air, because of our next point…
- Tires filled with CO2 deflate faster. The explanation for this deserves its own section, so keep reading to find out more.
Why Tires Filled with CO2 Deflate Faster Than Air
When CO2 canisters first came to market, they came without explanation of how CO2 differs from regular air when inside a tire made of butyl rubber.
Most of us are aware that CO2 is in the air we breathe, and topping up a puncture would result in a mix of air and CO2 inside the tire, so it’s reasonable to expect the difference in pressure retention to be minimal.
I’m sure I’m not the only cyclist to have inflated a tire with CO2 and wondered if I’m going mad when they’ve dropped pressure at an alarming rate shortly afterward.
As it turns out, filling tires with CO2 is vastly different from filling them with regular air. Carbon dioxide (CO2) accounts for a measly 0.04% of the air around us, while nitrogen and oxygen account for roughly 99% of it.
How Much Quicker does CO2 Deflate when Compared to Air?
Because of its significantly higher solubility in rubber when compared to oxygen and nitrogen, CO2 permeates ordinary rubber roughly 5 times faster than oxygen, and about 15 times faster than nitrogen.
To answer just how much this translates to in bike tire pressure terms, well, it’s hard to put an exact figure on something that depends on so many variables.
I can, however, speak from experience and say that I’ve had a tire filled with CO2 deflate from 90psi to just 50psi in just three days. That’s a pretty rapid drop.
But Why does CO2 Deflate Faster than Air?!
All gasses leak through (or permeate) materials. The rate at which they do so is governed partly by the size of the gas molecules. It won’t surprise you to learn that larger molecules tend to permate slower than smaller molecules, but it may surprise you to learn that CO2 molecules are actually larger than those of nitrogen and oxygen.
I know what you’re thinking – that’s the opposite of what you’d expect! Why would CO2, with its larger molecules, leak out faster than nitrogen and oxygen who both have smaller and more permeable molecules?
The answer to that is solubility. As well as the size of molecules, the solubility of the gas also affects the rate of permeation. It turns out that CO2 is far more soluble in rubber than oxygen and nitrogen are, and as a result, leaks out much, much faster.
Next time someone tries to tell you that tires don’t deflate faster when filled with CO2, or cites CO2 molecules being larger than oxygen and nitrogen as the reason for it, you’ll know better!
The Effects of CO2 on Sealant in Tubeless Tires
Using CO2 on tubeless tires can wreck sealants by hardening the latex present in them. If you’ve ever heard of sealant turning to rubber balls when CO2 is used, you’re about to find out why that happens.
When CO2 is introduced into a tire from a canister, it rapidly expands. This rapid expansion is why tires inflate so quickly using CO2.
Expanding gas is an endothermic reaction, meaning that heat is lost in the process. The higher the pressure of the gas in the canister, the greater the thermal loss, and the colder the air inside the tire will be.
This sudden change in temperate can cause the polymerization of latex sealants, resulting in the sealant “balling up” and turning to rubber.
Sealant mixtures vary in ingredients, so this may not be the case for all sealants. But it’s worth keeping in mind that before using CO2 on tubeless tires, you should be very careful to check that it won’t cause sealant issues.
When using CO2 for installing tubeless tires, it’s generally recommended that you seat the tire using a quick burst of CO2, deflate before adding sealant, then reinflate with a pump.
Personally, I’d always go with a pump over CO2 canisters if you aren’t going to carry both. There are situations where CO2 just isn’t as reliable (long trips, multiple punctures, messing up the application), and I value reliability over a few saved moments at the roadside.
With CO2, you’ll still need to repump the tire with air when you get home to avoid that rapid pressure loss. I view it as stealing some time at the roadside in expense for more time back home.
Unless you frequently encounter punctures, the cost probably isn’t going to be what makes up your mind, but rather the convenience; still, for me personally, a pump wins every time.