Minimum Tire Pressure

Hahn_Paso

Over the last few years, the idea that higher pressures don’t make your bike faster finally has become accepted. Many cyclists now run lower pressures to improve comfort and traction, without giving up anything in speed.

On gravel, lower pressures actually make you faster, since the bike bounces less. On soft gravel, like we encountered during our ride across the Paso de Cortés in Mexico (above), lower pressures (and wider tires) allow you to float on top of the surface, rather than sink in. Again, that makes you faster and more secure.

So lower pressure is better in many cases, but how low can you go?

contact_patch

Here is a detail from the photo of Hahn on the Paso de Cortés. You can see how long that contact patch is – there is a lot of tire on the ground, which spreads the rider’s weight over a larger surface area.

Yet the pressure is not too low. The tire still holds its shape: Seen from the side, the tire sidewalls form a nice circle. That is the reason why it still rolls as fast as it did at higher pressures: The flex in the tire is limited to a relatively small area.

Only when viewed from above, can you see the contact patch bulge outward – but even that should not be excessive.

paso_cortes_descent

What happens if your tire pressure is too low?

  1. The tire can collapse when cornering. During our Mexican adventure, we pumped up our tires when we reached pavement, so we could tackle the fast and twisty descent with confidence (above). Even on gravel, a tire can collapse under the forces of cornering, if it’s not inflated high enough.
  2. You can pinch-flat, if the tire bottoms out, and the tube gets crushed between rim and road surface.

BJPASS_result-750x481

3. The tire can get damaged. When the tire gets kneaded too much with each revolution, it’s not only slower. (Yes, lower pressures do get slower at some point.) It also puts very high stresses on individual threads of the casing, which then can break. The tire needs a certain pressure to hold its shape and distribute the stresses uniformly over all the threads in the casing.

In the photo above, you can see a cross-hatched pattern where the casing threads have broken. This tire was tested by a magazine, and they rode these 35 mm tires at extremly low presssures of just 35 psi (2.4 bar).

The tire probably is still fine to ride, but if you try to run it tubeless, air (and sealant) will seep out of the tiny holes caused by the broken threads. (The sealant colored the sidewall where it leaked.) If you see a single zigzagging line in the tire sidewall where one thread has broken, increase your air pressure slightly to prevent further damage.

What is the minimum pressure that is OK to ride?
This depends on many factors, including:

  • Rider weight. Obviously, heavier riders need to run higher pressures to prevent the tires from collapsing.
  • Surface grip: The more grip you have, the higher are the forces generated during cornering. To withstand those forces, your tire needs to be inflated harder.
  • Tire construction: A stiff tire is held up by its sidewalls as much as by the air pressure inside. A supple tire’s sidewalls do little to support the bike’s weight, so you need higher pressure. Thanks to the supple sidewalls, this tire still is more comfortable and faster, even at the higher pressure.
  • Riding style: A rider who has a round spin can run lower pressures. If your bike starts to bob up and down with each pedal stroke, your tire pressure is too low. Fast riders need to run slightly higher pressures, since they hit obstacles with more force. And riders who corner on the limit need higher pressures to prevent the tire sidewalls from collapsing.

I polled the riders on the Bicycle Quarterly team about the tire pressures they ride. I was surprised how consistent they are. Some riders are a bit heavier and use a bit more air, so we equalized the values for weight of 82 kg / 180 lb.

tire_pressure_chart_psi

Or if you prefer metric values:

tire_pressure_chart_bar

Of course, we’ll adjust these values if needed, for example, on rough gravel, we increase the pressure to prevent pinch flats… And remember that different pressure gauges can vary by up to 15%, so your 45 psi may be quite different from our 45 psi! Still, this provides a starting point for thinking about the right tire pressure.

For the majority of riders today, the advice “When in doubt, let out some air!” still holds true, but as we lower our tire pressures, we need to be aware that too little air also can cause problems.

Further reading:

Photo credit: Cyclocross magazine (damaged casing)

About Jan Heine, Editor, Bicycle Quarterly

I love cycling and bicycles, especially those that take us off the beaten path. I edit Bicycle Quarterly magazine, and occasionally write for other publications. Bicycle Quarterly's sister company, Compass Bicycles Ltd., turns the results of our research into high-quality bicycle components for real-world riders.
This entry was posted in Testing and Tech, Tires. Bookmark the permalink.

39 Responses to Minimum Tire Pressure

  1. Andrew Letton says:

    For heavier rider+bike+luggage, should I scale up the values in your table linearly (as a starting point), or is it not linear?

    • It appears to be linear – for the slope of the curve, see Berto’s tire pressure chart. We are working on a new version of that for a Bicycle Quarterly feature on tire pressure.

      (The problem with Berto’s chart is that he measured actual tires, but back then, wide tires weren’t supple, so his narrow tires are more supple than his wide ones, and his pressures for wide tires are too low for supple tires.)

      • 47hasbegun says:

        I’m excited to see curves for wider tires and heavier loads. I was trying to use that chart for the Rat Trap Pass tires a while back, but had handling troubles because I got it too low for the load I was carrying in addition to my own weight.

      • The wide tires Berto tested really have nothing in common with the supple tires we ride today. Just for comparison, when we rode the Jones 29er with Schwalbe Super Moto tires, we ran them at 17 psi even on pavement. The Compass Rat Trap Pass is almost the same width, yet it needs at least 27 psi to prevent sidewall collapse during fast corners. That is almost twice the pressure!

      • Robert Perks says:

        FWIW, as a big guy (220lbs) who rides with loads (kids), the equation can also be adjusted at the tire. What Compass sells as standard casings, or my current fave the Soma Shikoro, allow the pressures light folks ride to translate. I have also found that the relatively heavier casings have a much wider latitude in pressure, e.g. the 40mm shikoro is fine for me from 25psi up to 42psi. The light casings have a very narrow window in the mid 30s where they want to roll off at the low end, or bounce like a basket ball at the high end. When I am at the sweet spot the fancy tires are indeed nicer to ride, but I much prefer the lack of attention that the Shikoro affords for most riding.

      • You are absolutely right – a stiff sidewall does much more to support the weight of bike and rider. I once rode a 1950 René Herse with cheap Chinese 650B tires in Paris. I checked the tires with my thumb, and they felt fine – I didn’t realize that I was pushing on the stiff casing more than the air inside. Then I rode in a fast paceline for two hour. Only when coming back to my friends did I check the air pressure with a gauge: 17 psi. The 42 mm tire worked fine, but my workout was much greater, not because of the low pressure, but because of the stiff casing!

        Basically, tires with stiff casings begin to approach airless tires, where the air pressure is less important for supporting the bike. Of course, that means also less comfort and less speed.

  2. noglider says:

    There is another factor you didn’t include, which is periodic seepage. The lower pressure you create when pumping, the sooner you will have to top up your tires. This is important not just for convenience but also because of forgetting. I might normally re-inflate every week, but I might forget from time to time, leaving me with a decreasing margin of error. When people ask me what pressure to use, I recommend something higher than they need because I assume they won’t re-inflate regularly enough.

    • You make a good point, although at the lower pressures we use now, the leakage is less than with high pressures. Just make sure your Presta valve closes all the way – there is less air pressure to push it closed than with higher pressures. (Schrader valves have a spring that pushes them closed, so they don’t rely on tire pressure, but they are large and heavy, hence not popular on performance bikes.)

      • Edwin says:

        I know shrader vs presta is an endless debate, but when you say they are “large and heavy” it got me wondering: how much do they weigh? What is the difference? Might be a good project for BQ. A confounder would probably be that there are more lightweight tubes with presta valves. And don’t forget to subtract the grams from a slightly wider hole that will make the same rim slightly lighter! Shraders also don’t seem to have as many available lengths, in case you have deep rims. Now might those 80mm long presta valves might be classified as “large and heavy” as well?
        Get your precision scales out!

    • Eric says:

      Unless you are using tubeless with a slow leak, your tires lose pressure because the gas is going into solution into the rubber. The rate of diffusion through the rubber will increase with a higher pressure difference (head), and so your high pressure tires will lose air faster than low pressure tires. However, with newbs, it’s better to be too high than too low because they won’t own the consequences of casing a rim on a pothole and ruining their wheel.

  3. SmoothestRollingBike says:

    Jan, you are right, that different pressure gauges can vary by up to 15%, so your 45 psi may be quite different for every rider. Do you have any experience with Kappius pressure gauges? They seems to be professional Dwyer gauges adapted for bicycles.

    And one more time, please test Rat Trap Pass tires with Michelin C4 Latex 26×1.50/2.20 tubes. I know that you have tested latex tubes with narrow road tires, but wide tubes are really different. Huge amount of butyl rubber really slow you down. I find latex wide tubes almost comparably with tubeless with Stan’s latex milk, and wide butyl tubes really slow, even those extra light.

  4. Brage says:

    Really looking forward to an updated chart. Being on the heavy side (Pushing 135kgs including bike and backpack) it would be great to see some conversion tables for heavier cyclist also.
    I’ve ordered Barlow Pass. Really excited!

  5. Zed says:

    In the caption for Frank Berto’s chart, it says use the actual tire width. In addition to tire label discrepancies, will measuring the width adjust for rim width? I recently changed from 15 mm inside width rims to 20 mm rims.

    Thanks!

    • Really, the chart just takes an average of tires in that width. It certainly doesn’t have the accuracy to even worry about rim width. A 38 mm tire from 20 years ago can be run almost without any air pressure, since it was a stiff “touring” tire. You’ll need higher pressure if you run a Compass tire.

  6. Bill W says:

    You seem to recommend slightly higher pressures than I run on my Compass Bon Jon Pass 35s. I run 60 lbs rear, 45 front for on-road rider+bike weight of 220 lbs. I get a bit of bobbing sometimes but a smooth spin prevents that. (it may be that my pump is not accurate and my pressures are higher?)

    • If it works for you, then that is great. Don’t change it! We tend to descend very fast, and thus the danger of pinch flats is greater, so we ride slightly higher pressures than you may need. Or, as you say, it could just be different gauges.

  7. Andy Stow says:

    I have a Google sheet that implements the original plots published in BQ, with interpolation and extrapolation. I’d be happy to share it if that’s okay with Jan.

  8. Jay Guerin says:

    I run 40 psi comfortably in my Grand Bois Hetres 42s on my Riv Bleriot with me weighing 190 lbs. It’s interesting that the sidewall calls for a minimum so 55 psi. By the way the rear tire is now bald with several small cuts. I started to order a replacement (in tan) but see you offer only black or white. So I’m holding off till I can buy two matching.

  9. Willem says:

    Riding the Rat Trap Pass with a camping load at the rear and a total weight of some 115 kg, I used almost 4 bar in the rear tyre and 3 bar on the front, and that really was as low as I dared to go. So this does indeed suggest a linear relationship.
    When touring, you do not always have enough of a chance to maintain pressure. I think that we really need a new and better pump design. My classic Zefal HPX is optimised for inflating narrow tyres to high pressures. We now need a design that prioritises volume over maximum pressure (or rely on gas stations for anything other than puncture emergencies).

  10. Joe Kopera says:

    Can’t agree with this article enough, especially for heavier riders– I’m typing this with some pretty nasty road rash all up my left shoulder, and a not-too-pretty elbow after having my tire collapse at the D2R2 this past weekend. I was running my front Babyshoe Pass tire at 35 psi, as the course was alternating between pavement and pretty hard gravel, and anything higher than 40 was feeling a bit too jittery for me (completely subjective). I didn’t take into account the combined rider + bike weight of about 290 lbs for my rig when I took a very hard corner on pavement. The tire is OK and I didn’t get a flat, but my skin is worse for the wear.

    Also worth noting, that at my weight, even having the front tire at 35 psi on a steep and very rocky descent (it *is* the D2R2) had the front rim hitting rocks a few times, and it was hard to find at out-of-the-way place to stop and inflate– something I should have stopped and considered beforehand!

  11. DG says:

    An “interesting” effect I’ve had a couple of times now is quite noticeable wobbling at high speeds when tire pressure gets too low. Both occasions were steep descents at 65-80kph with slow leaks.

    Certainly seems as though the lateral forces can come into play at the extremes of pressure and speed.

  12. Roberta says:

    Are your recommendations based on tire usage with a tube, and if so how might your finds be different in the cases of tubeless setups?

  13. Andrew Squirrel says:

    It would be great if Compass sold compact, portable, high accuracy 0-60 psi gauges fit to presta valves. Do you have any recommendations? Fred Blasdel often carries a Meiser pressure gauge on some of our longer bike camping trips but I wonder how accurate the analog gauge can remain after bouncing around in a bag for years. It also takes up quite a bit of bag real estate for a tool of limited use.

  14. Ed B says:

    I had casing issues on three of my four Bon Jon EL tires and never went below 40 psi although I should note that Compass does not list a minimum pressure specification on the tire or on the website.

    • I am sorry to hear that you had problems with your casings. You probably need to increase your tire pressure. We don’t list a minimum pressure because it depends on too many factors. If we listed a minimum pressure, Panaracer’s recommendation would be 60 psi – sort of a worst-case scenario of heavy rider, rough terrain, choppy pedaling style, etc. Obviously, that value would be meaningless, so we don’t put it on the tire.

  15. John Duval says:

    Cornering at speed must be a big driver. The vast majority of my riding is in the city (since it is several hours of riding to get out), where the roads are rough and curves are few. At 100kg I inflate to 50psi and usually don’t air up again till it hits 30. The cyclometer clearly shows a change in average speed over a weeks worth of commuting each time.

    I will be sure to raise the pressure next time I hit the mountains, and see if doesn’t return some of the spirit to my descending that I thought was just me being a little older, and more out of practice than I once was.

  16. Gert says:

    Last year at P-B-P I used Stampede Pass EL at 3.8 bar front and 4.1 bar rear. At a weight of about 105 kg bike+rider+bags. With no promblems cornering on downhills. Actually I have never felt so comfortable and in control on downhills before.
    Before that I tried 3.8 bar rear which felt unstable.
    I now ride Bon Jon Pass EL at 3.6 bar front and rear at about 100 kg
    So I think there is plenty of room going lower than Your chart suggests and still be safe.

    • I am glad you have such good experiences on Compass tires. The optimum pressure really depends on the rider. We always say: “Try different pressures and see what works best for you.” It’s just when we see sidewall failures due to excessively low pressures, we realize that some people are pushing the low pressures too far. And thus we tried to provide some guidance.

  17. robertkerner says:

    Nice post. There is a subjective component to all this: ride feel and actually looking at the tire to see how much the contact patch has spread and then test riding. I’m a bigger guy with bikes ranging from “light weight” to awesomely heavy and the charts or equations can be misleading and fail data points at the extremes of charts. There’a popular equation out there that says for my 210 body and 30lb bike I should inflate my 32mm tires to 135psi!! I don’t think there’s one equation or chart that will work for everyone. You have to test ride.

    • I agree that test riding is the best approach. The charts and equations are a good starting point for experimentation, but no more. And if it seems like you need to inflate the tires you plan to use to very high pressures, then it may be better to use wider tires.

  18. Michael says:

    What would you say is the ballpark low for 175lb. rider and 25lb. bike with Berthoud GB25 front bag loading only on good roads for Stampede 700c x 32 and Chinook Pass 700c x28 tires? Both standard casings with tubes.

    Been running the Stampede at 55 and been great so far. Very cushy.

  19. x1 says:

    I have a wheel where the clincher will partly unseat under ‘low’ pressure (like below 45psi, with a few different cx tires). Or at least that’s what the shop told me the cause was. So that’s another reason for a minimum, with another variable (the rim/tire combo of course varies).

  20. Ray says:

    Are the pressures listed in your table for people using tubes?

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