Why Supple Tires Run at Higher Pressures

One of the benefits of wide tires is that they run at lower pressures. Yet the supple casings of our Compass tires require higher pressure to corner well and not bottom out on big bumps. Doesn’t the higher pressure negate the benefits of wider tires?

Think of the tire casing and the air inside as two separate springs. Let’s call them the rubber spring and the air spring. Both work together to support the weight of the rider. How you distribute the load between the two springs changes with a supple tire: Less rubber requires more air. And vice versa.

When you inflate your tire, you are aiming for a tire drop of about 15-20%. (Tire drop is how much the tire deforms under the rider’s weight.) Whether you use rubber or air as the spring doesn’t change how much the tire deflects. What changes is how the tire behaves as it flexes.

On tires with stiff casings, you use rubber as your suspension, and you need little air. The extreme are airless tires like those found on dock-less bikeshares. They use only rubber and foam to support the bike and rider. However, rubber has a lot of hysteresis, so it consumes a lot of energy as it flexes. That is why stiff tires are slow.

A tire with a supple casing (above) relies mostly on air to support the weight of bike and rider. Air has almost no hysteresis: Little energy is lost as the tire flexes. That is part of the secret behind the speed of supple tires.

A supple tire is more comfortable, too. The thinner casing has less inertia and requires less energy to flex, so it can react more quickly to surface irregularities. A supple tire absorbs the vibrations, while a stiff tire transmits them to the bike and rider. And since vibrations slow you down, that makes the supple tire faster, too.

So you gain speed twice: the thinner casing absorbs less energy (smaller hysteretic losses) and it transmits fewer vibrations (smaller suspension losses). Talk about a win-win situation!

It’s important to remember that supple tires need more air to hold up your bike. After all, you’ve reduced the spring rate (stiffness) of the rubber spring, and you need to make up for this by increasing the spring rate of the air spring. If you run your supple tires at the same pressure as stiffer ones, you risk having the sidewalls collapse during hard cornering. And that can be dangerous.

Put in a little more air, and you’ll find that supple tires offer more grip than stiff ones: The supple casing reacts better to surface irregularities and thus keeps the tire in better contact with the road.

There is no need to go back to the days when we inflated our tires to the maximum indicated on the sidewall. Use Bicycle Quarterly’s tire pressure chart (above) as a starting point, and experiment from there.

Remember that the chart lists the weight per wheel, not for the entire bike. I apologize that the chart stops at 37 mm – when Frank Berto measured the tire drop of dozens of tires, 37 mm was the widest road tire he could find!

On gravel and in mud, there is less traction, and thus the forces on the tire are lower. The tires won’t collapse even if you run very low pressures. On soft surfaces, you get more traction, more comfort and more speed at those low pressures.

Generally, I run about 10-20% less air in my tires on gravel than I do on pavement. You don’t want to run a pressure that is too low – it flexes the tire sidewalls excessively, which robs power and ultimately breaks the fine threads that make up the casing. And of course, you don’t want to bottom out except on rare occasions, even when running tubeless tires.

Finding the right tire pressure takes some experimenting. If the tire feels too stiff, let out some air. If you feel the tire folding under the rim as you corner, add some pressure. Also remember that the gauges on bicycle floor pumps are notoriously inaccurate, so if you pump up your tires to the pressures from the chart and your bike doesn’t feel right, trust your feel rather than your gauge.

Conclusion: Even though supple tires run at higher pressures, they are faster, more comfortable and grip better than tires with stiff casings. The ‘correct’ tire pressure should have some margin of safety above the point where the tire collapses during hard cornering.

Click here to find out more about our Compass tires.

 

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 the high-performance components we need for our adventures.
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25 Responses to Why Supple Tires Run at Higher Pressures

  1. Graham says:

    Any plans for a 650b x 2.3 tire? Lots of Enduro All Road bikes can fit this size now.

    • Our 650B Switchback Hill measures 50 mm on most rims, so it’s almost that wide. An even wider 650B tire would be interesting, though.

      • Graham says:

        Do it! 60mm would be amazing.

      • Jacob Musha says:

        I would ask for wide tires for smaller rims first. If a short rider wants to use a 53mm tire like the Rat Trap Pass, even on 26″ rims, they are forced to choose between significant toe-overlap with fenders or serious geometry compromises. Natsuko’s Hirose, for example, looks well proportioned and appears to fit her well. But it only uses a ~30mm tire on a 26″ rim.

        A 53mm tire on a 24″ [507] rim would be perfect, and you’d have a monopoly on that (admittedly, very small) market, since I’m not aware of any good tires in that size. Now that wide tires and fenders are becoming popular it makes sense to use smaller wheels. For a very short rider, the sub-optimal handling due to the reduced wheel/tire inertia is surely worth having a frame that fits correctly and doesn’t have lots of toe-overlap.

  2. Eric Bailey says:

    The Frank Berto tire inflation chart is a good start. Given that the BQ team has had a lot of real world experience riding the Compass tires, have you considered publishing the pressure(s) you find optimal with the 650b x 42 and 26 x 2.3? Sure, it is weight and surface dependent but you could just mention rider weight and use pavement pressures to simplify as a start.

  3. Keith Benefiel says:

    How would this translate to optimal pressure differences between sidewall weights, for instance, Babyshoe and Babyshoe EL. Would the more supple of the two require higher pressure?

  4. David Gabbé says:

    I wrote a web app based on Jan & Frank’s work. It includes the current Compass tire line. A preview version is https://preview.frame38.com. It’s hosted on a free tier w/a limited # of CPU hours per 30 day cycle. If it’s unavailable, it means the compute limit was reached. Less than ideal, but I haven’t had time to deal w/the hosting.

  5. Jaro Franta says:

    It’s not a big deal to extrapolate Berto’s chart: Just notice the spacing between the lines on his chart, and add new lines accordingly.
    For example, the Barlow Pass 38’s are just over 40mm wide on my Mavic rims, so I added a line for 40mm — see linked graphic.
    Then, using weight estimates for front & back wheels, my personal tire pressure estimate comes to about 35psi rear, 32psi front.
    Makes sense ?
    It actually “feels right” too.

  6. mtbvfr says:

    Hi Jan et al,

    If you will be carrying the great majority of weight over the rear tyre, would loading up the bike and then sitting on it be the best way to determine a 15% drop for both tyres as a starting point.

    Thanks, MTB.

    • Determining the weight distribution of your bike isn’t hard, but you need a bathroom scale and a block of wood or something to lift up the other wheel to make the bike level. Then have a helper steady you on the bike. They also can read the scale. Place the scale under the front and under the rear tire, and you get the weights.

      However, the old recommendation of adjusting your tire pressure accordingly is outdated with supple tires: The front tire can collapse under hard braking. So we recommend inflating the front tire to almost the same pressure as the rear. More about this can be found in this blog post.

  7. Stuart Fogg says:

    I’ve used quite a variety of road tires and noticed that when flat the treads can be pressed to the rim with very little force from my thumb. I’d be surprised if any of those carcasses contribute much to supporting the ~100 lbs per wheel on my bike. However I have no experience with gnarly mountain tires, 100,000 km touring tires, or high-load e-bike tires.

    • Try riding some really inexpensive tires some day. I once rode an old René Herse in Paris that had more recent Chinese tires. I pumped up the tires until they felt firm when pushed with my thumb. I rode the bike in a fast paceline ride, and it seemed a bit hard to pedal. When I got back to my friend’s house, I checked the pressure. It was 17 psi – for a 42 mm tire. With supple tires, the bike would have been unrideable at that pressure.

  8. Rick Thompson says:

    Thanks for the, as usual, clear explanation of what is happening. I pump my Snoqualmie Pass EL to 45 psi, then ride until they go down enough feel squishy. That turns out to be about 25 psi. In between, I don’t really notice much difference. The ride is smooth and cornering fine. Then again, I don’t notice much difference between a $25 and a $100 bottle of wine….

    • I don’t notice much difference between a $25 and a $100 bottle of wine

      I like that analogy. I am sure you’d notice the difference between a $ 10 and a $ 25 bottle, though! In bicycle tires, I can’t think of any that are as rarified as a $ 100 bottle of wine. The Snoqualmie Extralights are probably the same as a $ 25 wine – very nice, but still with the realm of daily enjoyment. (And since they’ll last thousands of miles, they are much cheaper yet!)

  9. Bruce d says:

    Can conservative pavement riders potentially use lower pressures than the charts indicate?

    • We suggest that you experiment with tire pressures until you find what works for you.

      However, keep in mind that when you brake really hard, all your weight is on the front wheel. You’ll be going straight, so the tire can (and will) deflect a lot more than is safe in cornering, but there is still a limit – you don’t want the tire to deform so much that your braking power isn’t transmitted to the road surface any longer. It’s easy to check – ride in a straight line on an empty road and brake really hard. You’ll feel if the tire starts to collapse. As long as you go straight, nothing will happen – just release the brake and inflate your tire a bit more…

  10. Björn says:

    “I don’t notice much difference between a $25 and a $100 bottle of wine”

    You’ll notice the difference betwen a 100$ and a 25$ bottle.
    Like you”ll notice much more going back to 23mm wide tyres then going from 23mm to something wider.

  11. ViveLemond says:

    I just put my 42mm BSPs at 40psi, and pump up every few weeks, for unloaded riding. Anything over that starts to feel “firm” and less comfortable.

  12. paul says:

    Does a “roll-down (hill) test” work well enough for establishing pressure? I try 10 psi pressure differences coasting down a short hill, then check the max speed. On the 32c tires I just got, 50 psi was the fastest. (Sidewalls are on the thick and stiff side.) 40 psi was much slower, whereas each 10 psi increase was about 0.2 – 0.3 MPH slower.

    • That is a great experiment – similar to how we first established that wider tires and lower pressures don’t slow you down. If you run each pressure several times to make sure it’s not just noise in the data, that approach should give you an indication of which is fastest. Depending on the steepness and profile of the hill, the differences between setups can be exaggerated. During the first meters at low speed, rolling resistance is pretty much your only resistance, whereas at higher speeds, air resistance (which should be constant) comes into play. The faster you go, the less rolling resistance makes a difference.

  13. Larry says:

    Just curious… I rides on Track/Velodrome with clincher tires. I see you do test on velodrome too.

    I wonder if compass tires would be give any advantage on Track specific use? (AKA I want killer tires!).

    Usually, on Track I pumped to the maximum pressures as clincher tires will hold on the rims so Crr will be lowest as much as possible because Track’s surface usually smooth as glass without bumping anyway.

    Mind to comments on this? I knew this probably wrong place to ask lol I come this far……

    • We tested on a paved surface on the infield of an outdoor track. It’s a good example of a newly paved, super-smooth road. A wooden track is a different matter. Without testing on that surface, it’s hard to say how many vibrations are transmitted by such a smooth surface. I suspect it’s still more than most riders think. In any case, a supple casing makes your tires faster on all surfaces, which is why professional racers all use supple tires. So among clinchers, I’d go with the most supple ones.

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