Tire Pressure: Data and Details

Illus.BQ.RollTest

A little while ago, I wrote about how new scientific research has allowed us to design wide, supple tires that offer the speed of narrow, high-end racing tires. The key finding is that above a certain threshold, increasing tire pressure no longer results in lower rolling resistance. While these new data have become widely accepted – witness professional racers adopting wider tires and lower pressures – it’s natural that new ideas are met with skepticism. In order to contribute to a better understanding of how tires work, I’d like to share more data from Bicycle Quarterly‘s testing.

tire_pressure_rolldown

The data above came from Bicycle Quarterly rolldown tests of various tires. The results indicate that above a certain threshold, for clinchers, tire performance increases only very slightly, if at all. It’s not surprising that the 38 mm-wide Mitsuboshi tire rolled faster at 35 psi than at 25 psi – it was hardly rideable at the lower pressure. But increasing pressure to 55 psi resulted in no speed increase.

We saw the same for the 27 mm-wide Rolly-Poly. Somewhere between 55 and 85 psi, higher pressure no longer resulted in significantly increased performance. Going from 85 to 105 psi resulted in only a minimal increase in speed. For the two tubular tires, the effect was reversed: Higher pressures actually reduced performance.

tire_pressure_rolldown_2

We’ve confirmed this finding numerous times. Above is another set of test runs (pressures are in bar/psi). For these 32 mm tires, increasing the pressure from 7o psi to 85 psi brought no significant change in performance. (The apparently slower speed at 85 psi is not statistically significant.) Please note that the values you see here are not corrected for temperature, so you can’t compare different tires. (The pressure runs were done consecutively, so temperature didn’t change from one pressure to the next.)

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We discussed the measurements with power meters in the previous post (above). That data also has been confirmed multiple times, at different speeds.

cypres_pressure

Above is data from running ultra-high pressures, up to 200 psi. Performance did not change with increasing pressures. (Don’t do this at home, 14 bar/200 psi is not safe with these tires!) I could bore you with even more data, but I think this is pretty convincing, especially since we’ve confirmed numerous times that our tests are repeatable (testing the same setup multiple times yields the same results) and statistically significant (meaning we aren’t just looking at random noise in the data). That is important, because all too many studies are based on single test runs, which don’t meet basic scientific requirements. Now that we’ve conclusively debunked the old view of “higher pressure = more speed”, let’s look at some of the details in the data above.

Rough vs. Smooth Pavement

The rolldown tests were performed on relatively rough pavement. No holes or bumps, but the tar between the aggregate had washed away over decades of Seattle weather. Interestingly, high pressures generally did result in slightly higher speeds for some clincher tires. It’s not a lot, but it’s statistically significant.

The track tests were run on very smooth, newly laid asphalt pavement. There, we see the opposite. High pressures of 100-110 psi result in slightly lower speeds. How can we explain this? One hypothesis is that the tire deforms more on the rougher pavement to conform to the surface irregularities. At lower pressure, you create an imprint of the road surface in the tire as it rolls. At higher pressure, the tire bridges the gaps between the high points, and thus deforms less. On the smooth road, there aren’t any gaps to bridge, and so the high pressure loses its advantage.

Moderately high pressure = worst performance

On the smooth track, moderately-high pressure (100-110 psi) is worse than either lower or higher pressures. Why is that? Here is a possible explanation: As you increase the pressure, the suspension losses (vibrations) increase faster than the flexing of the tire (hysteresis) is reduced. So you lose performance as you increase pressure. At a certain point, the bike is vibrating as much as it can, but higher pressures still reduce the flexing of the tire. So from that point onward, higher pressures improve performance – until you end up back where you started at lower pressures.

Tubulars vs. Clinchers

What about the worse performance of tubulars at higher pressures? Tubulars derive much of their performance from their suppleness and low suspension losses. Increasing the pressure increases vibrations faster than it reduces the flexing of the tires. These explanations are just hypotheses – our best guesses to explain what we see. The data itself – higher pressures don’t lead to improved performance – is beyond doubt.

hawaii_01

Take-Home Message

For most riders, the take-home message is simply this: As long as you inflate your tires enough that they are safe to ride, tire pressure doesn’t matter much. Find a pressure that feels good when cornering, and ride your tires at that pressure. When in doubt, let out some air – you’ll be more comfortable and have better cornering grip. (If you ride narrow tires, beware of pinch flats, though!)

If you worry about the last bit of performance, then you can try to use the data above to tease out that last 2%. On smooth roads, low tire pressures yield the best performance. On rough pavement that doesn’t have holes or bumps, increasing your pressure may make your tires a little bit faster. That may seem counter-intuitive, and of course, it also will be less comfortable. But if you are doing a short time trial on a country road, it may be worth considering. Ideally, you’d run a few experiments with a power meter to dial in your tire pressure for that particular road surface.

If you run tubular tires, you should definitely run them at relatively low pressures. This provides the best performance and the best comfort on all surfaces we’ve tested. As for me, I run my tires at relatively low pressures on all roads. My rides encompass a multitude of surfaces, and on truly bumpy roads, we’ve shown that lower pressures always are faster, because the bike bounces less. But that is a topic for another day…

About Jan Heine, Editor, Bicycle Quarterly

Spirited rides that zig-zag across mountain ranges. Bicycle Quarterly magazine and its sister company, Compass Cycles, that turns our research into high-performance components for real-world riders.
This entry was posted in Testing and Tech, Tires. Bookmark the permalink.

19 Responses to Tire Pressure: Data and Details

  1. Bill Gobie says:

    Another benefit of lower pressure is fewer flats. I started using pressures 20-30 psi below the maximum sidewall pressure about ten years ago, reasoning there would be less force to drive sharp objects through the tires. My incidence of punctures and ruinous glass cuts dropped dramatically. My speed wasn’t any slower for it. Lightweight tires became almost as immune to road damage as armored tires. Since I made that change I can typically wear the tread on a rear tire down to paper thickness before the casing begins to fail.

  2. EB says:

    Fascinating stuff except who rides Rubinos or Vittoria CX (other than the likes of a Scott Dickson). Do you have a sense of what tire pressure you might suggest for my 32mm wide Compass Stampede Pass extra legere tires? Me and my bike go 215 lbs all in with full rando regalia, which is probably a lot more than your test subjects (you?). I remember seeing somewhere that the suppleness of EL casing actually needs more pressure and I am now concerned that I am running my tires too low but they seem right. The 32 mm sidewalls are stamped 90 PSI and have been running the rears at 80 psi and the 28mm front Shinook Pass EL (105 max psi on he sidewall) at 75 psi. Am I in the ballpark? It would be really intersting to see data on your tires…..like the ones that I buy. Anyways, riding these tires at these pressures (80psi rear and 75psi front) and riding a magnesium frame, roads that torture other rides seem just slightly bumpy to me. As a sidenote, I had zero flats last year running latex tubes at these pressures but I replace them pretty early…..around 2,000 miles on the rear tire. The things that keep us awake a night….comfort, rolling resistence, and how to avoid flats. Any chance that we will see Compass EL CRR data or equivalent? Trying to decide whether Vittoria EVO CX iii clinchers might be faster and as comfy. Hard to tell by seat of the butt which are faster plus I have a theory that central fatigue is partially a result of vibration and probably will stay with the EL tires.

    • Your pressures are in the right ballpark. Generally, for supple tires, we found that as long as the tire handles well, you have enough pressure. At the super-low pressures where rolling resistance increases, the tires also “wash out” in corners and become almost unrideable, so you’ll notice when you get into that territory.

      We plan to test the new Compass tires soon. The standard casings are the same as the Grand Bois, so they’ll roll as fast (or a little faster, since there is less tread on the shoulder that needs to flex with each wheel revolution). The Extralights certainly do feel faster on the road, but we need to do a controlled test to determine just how much faster…

  3. SD says:

    No error bars? No confidence intervals or variance for repeat measurements? How do you know sub – 2s differences are even real?

    If this wasn’t self published it wouldn’t pass peer review….

    • We performed a rigorous statistical analysis. The results can be found here:

      Vande Kamp, Mark Ph.D., 2007: The Accuracy of Tire Tests – A Statistical Analysis. Bicycle Quarterly Vol. 5, No. 3, p. 20.

      The tire test articles were peer-reviewed by Frank Berto, Jim Papadopoulos, Alex Wetmore, as well as the makers of the tires we tested. I doubt you’ll find any other testing in the cycling world that meets such high standards. Of course, I expect you to hold Bicycle Quarterly to higher standards…😉

      This blog is intended to provide an overview of the results, but not all the data. For that, please refer to the relevant articles in Bicycle Quarterly.

    • How do you know sub – 2s differences are even real?

      The statistical analysis shows which differences are significant (i.e., most likely real) and which aren’t. We only report differences that are statistically significant. For example, for the Grand Bois Cyprès, the speed difference between 55 and 70 psi is statistically significant, the difference between 70 and 85 psi is not.

  4. zundel says:

    I do a lot of city riding: start and stop. Accelerating from the stops uses the most energy on a commute ride. Any idea about hard acceleration and tire pressure? I can see and feel the rear tire squat.

    • We haven’t tested hard acceleration from a stop. Generally speaking, rolling resistance is most important at lower speeds, since wind resistance is almost zero (unless you have a big headwind). However, even at high speeds, rolling resistance is an important factor. Reducing your wind resistance by 5% is very difficult and has drawbacks (usually, this can be achieved only via a lower, narrower riding position). On the other hand, reducing your rolling resistance by 50% often is easily achieved by choosing the right tires.

      • zundel says:

        Can you begin to profile energy loss and rebound from bicycle tire flex? Your research highlights how subject feel misleads, and urban riding has too many variables to isolate.

      • As you’ll read in the Spring 2015 Bicycle Quarterly, tire flex can play an important role in how the bike planes. So it’s probably another case of a little flex being good, but too much being detrimental again.

  5. a biologist says:

    Hi, thanks for performing these tests. I’ve been using your tires for some months now and I am very happy with them.

    In this post, you use the words “statistically significant” several times, but there is no way for the reader to assess this assertion. I’m sure you left the error bars off the graphs to make them easier for the lay reader to read. However, it would be really nice if you maybe made a post for the scientifically / statistically proficient reader, with error bars on the graphs and some description of the test statistics you used. Even better would be to make the actual raw data available, but I don’t know many people who would do that🙂

  6. cpkestate says:

    so there’s no more reason to build a new ‘road’ bike with 700c wheels anymore no? other than body height maybe, larger frames might be better served
    being short, I enjoyed my 26″ equiped fast ‘road’ bike. looking forward to see compass new thread and EL casing in 26×1.75 itteration.

    I hope I’m not the only want to dream of non-dished, 130mm rear hub with short shimano-style screwed freehub body that fits 5 ten speed cog…. or even better, a whole compass grouppo!!

    • so there’s no more reason to build a new ‘road’ bike with 700c wheels anymore no?

      It depends. For racers, where even a small advantage can make a significant difference, 700C x 25 or 28 mm tires probably offer the best performance, at least on smooth roads. They roll as well as wider tires, but they are a little lighter. On 15-minute climb, losing or gaining 6 seconds can make the difference between staying with the group or getting dropped, or even better, between saving enough energy to make a successful attack, rather than just hanging in the bunch. Of course, this assumes that you already have tires with the lowest rolling resistance (which most professional racers do). (Racers could climb even faster if they used smaller wheels, but the handling disadvantages on the downhill don’t seem to make this advisable.)

      If you ride by yourself or with a group of friends, 6 seconds over a 15-minute climb is very little (0.7%), and the added comfort and safety of wider tires make them a good choice even on smooth roads. However, putting a value on these characteristics is a personal choice. For some, knowing that they don’t give up even a tiny percentage on a long mountain pass is important. There also is the aesthetic appeal of a “road” bike with narrow tires.

      In general terms, you are right – for most riders, there is no clear advantage to narrow tires any longer. And 700C wheels work best with narrower tires. With wide tires, smaller wheels keep the bike nimble, make it easier to design a bike without toe overlap, and also reduce the weight.

      Regarding a super-high performance 26″ tire, that would open up a lot of interesting possibilities…

    • nic says:

      What? Everyone knows that “the more speeds, the better the bike!” I’m puzzled by the fact that these days, cyclist are stuck between single speed and too much speeds. Remember shimano airlines? A downhill group with seven cogs (nine speed spacing) that did not went to market. Flange spacing was huge . Ah! Five speeds…! The right ones …

  7. Andy Sutterfield says:

    Hi Jan, I’m curious about the tradeoff between rolling resistance and aerodynamics. I read an article geared towards time trial racers that claimed that although wider tires do have less rolling resistance, the aerodynamic penalty makes them slower at race speed. (“Wide” in this case meaning 25mm instead of 21-23mm.) I imagine there must be a relationship between ideal tire width (to minimize the combined effects of rolling resistance and aerodynamic resistance) and bike speed. I would think for most of us who don’t ride at race speed all the time, the aerodynamic penalty of wider tires is dwarfed by their rolling efficiency advantage. Have you or anyone else done research on this?

    • We tested 25 and 32 mm tires in the wind tunnel, and the difference was too small to be statistically significant. So if the wider tires are less aero, it’s by only very little. Lowering your stem by 20 mm had a much greater effect.

      On the other hand, if you are only after speed on smooth roads, 25 mm is probably your best bet – wider tires don’t roll faster on smooth roads, and you get less a little weight and perhaps a little less wind resistance.

      For the rest of us, even a 42 mm-wide tire doesn’t appear to be much less aerodynamic once you have a rider on board of the bike, even when coasting down steep hills at very high speeds.

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