Whenever I have an early morning ride planned I typically inflate my tyres the night before and call it good. However, that may be putting a little too much faith in the tyre system’s ability to retain pressure. Have you ever thought about the rate of air loss and whether it may be enough to impact your ride?
This question got me to run a few quick tests on a few popular types of tubes to see just how much pressures drop when there is no mechanical leak or flat. And while I’m sure the findings won’t surprise many, there’s no denying that certain tube types need more attention than others.
All pneumatic bicycle tyre systems suffer from permeation, a phenomenon that happens when the molecules of a gas diffuse through the materials containing them. However not all containing materials are equal, and some certainly allow more permeation than others. Compared to a car’s tyre system, bicycle tyre systems often need to be more supple and lower-weight, and in turn, permeation is a far greater factor in cycling.
“Some believe that loss rate increases over rougher surfaces but we’ve never conclusively shown that,” said owner of Silca and pro team technical consultant Josh Poertner. “It is key to remember that the mechanism here isn’t exactly mechanical, it gets portrayed as gas molecules escaping through pores in the tube, but it is actually a solubility problem when the gasses dissolve into the rubber and then move through the rubber and out the other side.”
The goal of my little test isn’t to put exact numbers to a tube type – my sample size is simply too small and your results will vary based on the exact products you have. However, my test did repeatably reveal trends that I’ve experienced and heard from others over the years.
To test this I set up four wheels with the same model and size of tyre, all with the same internal rim design, internal width rim, and rim tape.
While there are specialist tools to measure permeation, my method was simply to focus on pressure loss. For this I used an EVT Bleed’n gauge as the valve design reliably lets me put it onto the valve without significant pressure loss (i.e. no audible air loss) and it’s not hungry at all for the air it takes onboard. In repeated tests it consistently showed less than a 1 psi difference in connecting, removing, and reconnecting the gauge – good enough for this test.
Three main tube materials
This simple test sought to measure a small number of common tube materials that are popular in road cycling. For this, I tested a fresh Maxxis Welterweight as the regular butyl rubber tube, a Vittoria Latex tube, a regular Tubolito (TPU construction), and then the Tubolito S (super light TPU construction)*. In a separate test, I measured a Specialized Turbo lightweight butyl tube against the same Vittoria Latex and Tubolito S as above, over a 24-hour period.
All tyres/tubes were pumped to 80 psi and left for 24 hours at room temperature. Below are the initial measured results:
- Tubolito S: 79 psi
- Tubolito: 79 psi
- Vittoria Latex: 73 psi
- Maxxis Welterweight (regular weight butyl): 77 psi
The tyres were then left for a further six days to measure the loss over a week-long period:
- Tubolito S: 72 psi
- Tubolito: 76 psi
- Vittoria Latex: 53 psi
- Maxxis Welterweight (regular weight butyl): 73 psi
As noted, the separate test involving the Specialized Turbo tube was only run for a 24-hour period (using the same pressure gauge, wheels and tyres as above). The pressures after the 24 hours were:
- Vittoria Latex: 73 psi
- Tubolito S: 79 psi
- Specialized Turbo (lightweight butyl): 75 psi
From this, we can see that the regular-weight Tubolito, a TPU-based tube, is the most effective at holding onto air. Tubolito’s lightweight version and regular butyl tubes aren’t far behind, while lightweight butyl and to a greater extent Latex, suffer from significant permeation.
Unsurprisingly thicker tubes of the same material hold air better than thinner (lighter ones). Do keep in mind that this is not a statistically valid test and that even identical models of inner tube could present differences in air retention.
“Latex tubes historically [lose] around 1% per hour, but modern additives like graphene have them down to generally less than 0.5% per hour,” said Poertner. “Of course, this is variable, so there will be differences between ‘identical’ tubes, but in general, if you have a six-hour event and pressures started at 90 psi, and the mechanics inflated the tires ~1 hour before the start, you are likely finishing on 84 psi.”
What about tubeless?
Trying to find permeation trends in tubeless systems is somewhat like trying to explain chess to a cat: it’s not going to happen.
Even on the assumption that the tubeless system is completely void of physical leaks, there are simply too many variables across tyre construction, rim tape, rim design, and sealant used. And even more so than tubes, tubeless tyres in an identical manufacturing batch can have significant differences in porosity.
Broadly speaking, a well set up tubeless system can offer comparable air retention to a regular butyl tube – it should still be rideable after a week. Some brands push their tyre construction to the limits and rely wholly on liquid sealants to create a system that holds air. Meanwhile, some other brands, such as Goodyear and Giant (whose tyres are made by Maxxis), have been equipping their tubeless tyres to offer superior air retention – and I can attest that it makes an appreciable difference.
For example, it was a few years ago that Giant moved all of its performance bikes, regardless of discipline, over to tubeless. Part of that move was creating a tubeless tyre system that would sit inflated on a shop floor for a decent period of time without the use of tyre sealant (which may dry out before the bike is sold). While it typically works as intended, such a feature isn’t solely without trade-off and the tyres that typically offer the least permeation also tend to be a little heavier, and in some known cases, slower-rolling.
Planning for permeation
Say you have an epic ride or endurance race ahead, what can the rate of permeation tell us and how do we plan for it? Well, it’s a topic that Poertner knows all too well, and while permeation is a factor, it must be considered in balance with the weather forecast, road conditions, rider power, and the structure of the event itself.
“The process we run with our teams is to look at what we deem to be the critical point in the race,” he said. “We then account for loss/time x hours between that moment and the bike prep and then we look at the weather to determine if we adjust further for temp variation between those same times.”
Poertner’s advice is often to err on the low side of pressure for most surfaces, especially if there is rain in the forecast. However for an event such as Paris-Roubaix, that advice is commonly flipped, and it’s all about finding the lowest pressure without risk of breaking wheels, and then tweaking it up slightly from there.
“For an event like Roubaix, we actually test the tires and write the loss rate on the sidewall as it is so important to get it right on race day,” he said. “You also have two completely different surfaces which might have an optimal pressure difference between them of 25 psi, so we have to choose which direction to optimise for. [It’s] not an easy question.
“Spring Classics are also interesting in that you can have some pretty wide temperature swings, so an hour before the start temperatures can be quite cold. In 2019 we saw temperatures rise almost 30 °F (approx 18 °C) throughout the race – that’s roughly a 3 psi increase over the day which directly offsets your loss due to permeability.”
There are also alternate gasses that permeate more slowly or are less reactive to temperature change. “Nitrogen is a cheap and common one though you will still have some loss,” Poertner said. “None of the [alternative gasses] really fix the situation.”
So where does all that leave us? There are certainly trends in how well various tube types hold onto pressure. Put simply, those running latex and superlight butyl tubes will need to keep a closer check on pressure loss compared to those with regular butyl or fancy TPU tubes. However, it’s important to remember that even identical models of tubeless tyres and tubes can vary in air retention effectiveness.
Before a big event, it’s worth figuring out the approximate permeability of your selected tube or tubeless system and assessing whether it’s something you should factor into your starting pressures.
*Note that the feature image shows different tubes to those used in the test. The feature image shows a Continental Race butyl rubber tube (far right) and Michelin Latex tube (second from right).
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