We know, there are no stupid questions. But there are some questions that you might not want to ask your local store or your cyclist friends. AASQ is our weekly series where we get to the bottom of your questions, serious or not. This week, it’s all about tire pressure, higher volume tires, and wider rims with Zipp. This is a long one, so let’s dive right into the topic:
General tire pressure
Hello Team BR, I love this Q&A format, some have been very informative. After seeing Zipp approaching the questions and answers, this morning I tried their recommended pressure settings, and they suggested less than 5 bars for my 26mm setup… and I got a flat tire on the first pothole (which wasn’t even big, just a rough pavement joint). Now that I think about it, those numbers don’t make much sense for normal road conditions, or is there something I missed? Also, the bike was going quite slow. – Cesar
Zipp: Hi Cesar. We have published tables specifically referring to the new 303 Firecrest disc brake wheels and 303 S disc brake wheels, which have a wide rim and are tubeless specific. If you use these tables, it’s important to know that the indicated pressure should not be used for any other setup. However, you might be referring to the Zipp Tire Pressure Guide on the SRAM AXS website, which takes into account additional parameters to calculate tire pressure. It’s important that all the information is entered properly in the guide to get the most accurate tire pressure recommendation: rim width, tire size, tire type, tire interface on the rim, riding conditions, surface, etc. Make sure all the information is entered correctly to get the most precise tire pressure recommendation. However, please note that the pressures we recommend are lower than what you might be used to because our Total System Efficiency (ETS) study clearly shows that lower pressures are more efficient on all surfaces. You can find more information about our Total System Efficiency study here.
Can all tires be used with lower pressures?
Zipp: All tires should roll at the appropriate pressure – some will be capable of lower pressures than others. It depends on your complete setup: a larger tubeless tire on a wide rim will allow for lower pressure without squirming compared to a narrow tube tire on a narrow rim. In general, cyclists have overinflated our tires for too long, and it’s time for everyone to return to more appropriate measures. We suggest you use a starting point and slowly reduce the air pressure a few psi at a time to experiment with what air pressure works best for you. You can start by using the Zipp Tire Pressure Guide as a starting point. With an overall efficiency gain, properly lowering tire pressure is simply faster!
My tubeless carbon road/gravel rims have a maximum recommended pressure of 90 psi for 28mm tires. However, my tires have a minimum recommended pressure of 95 psi. Which one should I follow? On the other hand, I weigh 75kg and have ridden with 80-85 psi without any issues on typical urban surfaces. – Larry
Zipp: Hi Larry. Our stance on this issue is clear. If the minimum pressure for a tire is higher than the maximum pressure for a rim, then they should not be used together; in other words, they are incompatible. In your case, the minimum pressure for your tire seems to be overly conservative. 95 psi is a lot of air in a 28mm tire by current standards.
Do the pressure tables apply when using inner tubes? Or is there a general rule to follow when using inner tubes in a hookless setup? – David
Zipp: The pressure tables you may have seen published only apply to our 303 Firecrest disc brake wheels and 303 S disc brake wheels with tubeless tires. However, we also offer a digital pressure guide that provides recommendations for any type of setup – you just have to select the options in the dropdown menu of the tire pressure guide. As for using inner tubes in a hookless setup: if your tire has tubeless-ready beads, you can safely use a tube 100% of the time in a hookless rim setup. This allows those who don’t want to deal with the sealing requirements of a tubeless setup to use a tube in a hookless, tubeless-ready setup.
So why not lower the tire pressure until you bottom out or feel that they twist too much, and then increase the pressure until the problem goes away? – Chris
Zipp: Hi Chris. Your method is a bit extreme, and we wouldn’t go as far as bottoming out the tires, but this is the general approach to figuring out what works best for your setup. Our digital pressure guide can eliminate some guesswork by providing a starting point. After that, users should feel free to adjust the pressure as needed for their personal riding style and environment. You can hear us delve into air pressure here, on our Zipp podcast.
Considerations on tire pressure based on surface
Many of the data you have published about “wide-low pressure is more efficient” seem to take conditions that are not “normal road riding”, but rather what in Europe we would call “extremely degraded” roads (or gravel). So, is there any gain for those riding on good roads? – Jordi
Zipp: It’s true that the greatest efficiency gains with larger tires and lower pressure are achieved when the road surface is less optimal. However, we have also tested those setups on perfect roads and found that larger tires are still more efficient. Tests show that the efficiency increase is up to 40 watts on poorly maintained roads and about 3 watts on smooth surfaces.
Blue: 23mm tire – Red: 25mm tire – Green: 28mm tire
The reason is that power losses due to vibrations occur even on roads classified as good or smooth. The asphalt is rough and creates resistance: the only surface where vibrations are not a factor would be the wooden track of an indoor velodrome, for example. There is much talk about lower pressures being faster, especially on rough terrains. I have used Silca’s tire pressure calculator and found that the recommended pressures are very good (apparently, they are quite similar to Zipp’s recommendations). However, I can’t help but feel that when out of the saddle, and especially when sprinting, the increased tire squirm when the rider’s weight moves up/down and side to side would add quite a bit of resistance. Has this been tested? – Danny
Zipp: Thank you, Danny. Yes, lower pressures are truly beneficial both on rough roads and smooth surfaces. The thing is, most cyclists have inflated their tires to maximum pressure for so long that the feeling of overinflated tires is ingrained in us. As a result, going back to more appropriate pressures will take some getting used to. The potential power loss when sprinting out of the saddle is insignificant compared to the gains in rolling efficiency and vibration damping. You…