Vertically Compliant

In your head you just followed the title of this post with, “and torsionally stiff,” I’m betting. You may have rolled your eyes as you did it. If there is one more cliched phrase used in bike reviews, I can’t think of it. It also probably stopped meaning anything around the time the economy tanked in 2008.

Back when most of what I reviewed was either steel or aluminum, frame flex was an issue. With the steel bikes, they were noticeably more comfortable than the aluminum ones, but they were also much more likely to twist under out-of-the-saddle efforts. As a result, whenever I reviewed a bike, I was looking for something that was comfortable as it rolled on 23mm tires pumped up to 120 psi, but also stiff enough that I couldn’t make the front derailleur rub the chain under heavy torque. It was the rare bike that both didn’t rub and didn’t rattle the silver from my teeth.

By the late 2000s, carbon fiber bikes were advanced enough in their layup that they did flex more vertically than most steel bikes (i.e. were adequately compliant) but were still stiff enough they didn’t twist much under big efforts. A lot of engineering got carbon frame builders to this point.

But the tire manufacturers were about to make all that engineering beside the point with a veritable revolution in casings and rubber compounds. In the preceding decades, a 32mm tire was slow as hell compared to a 23mm tire, but better testing, in particular the testing performed by the Finnish company Wheel Energy, led to sea changes in casings, compounds and to a lesser degree, treads. Suddenly, bigger tires were rolling faster than their skinnier forebears, and new data showed that many of them were most efficient at pressures far lower than the average rider might suspect.

That’s because we were told that rolling resistance always went down with higher pressure. Wheel Energy was one of the first labs to show that there comes a point when traction is compromised enough that rolling resistance starts rising again. In short, we learned that running a 40mm tire at 80 psi is a bad idea.

On a parallel line, composite frame builders were working that vertically compliant and torsionally stiff mantra into the ground, but what made a detectable difference in my comfort on their bikes was inflating my 23mm tires to 105psi, rather than 120psi. There was an intermediate step at exactly 116 psi (8 bar) before going to 105, and I only chose that pressure because it was easier to line up the needle with the 105 mark than it was the mark for 101 (7 bar) on my floor pump.

I could tell I was a little more comfortable at 105, but that change was less about comfort than it was traction. In charts that tire companies sent in press materials it became apparent to me that the increase in rolling resistance by dropping from 120 psi to 105 was well worth the sacrifice thanks to the increase in traction.

Further complicating this picture is the fact that as tires increase in width, their circumference increases as well and as that increases, rolling resistance decreases, so some of the losses that come with the wider tire patch and lower pressure are offset. Just what pressure is most appropriate to a tire does change with a rider’s weight; the heavier the rider, the higher the pressure required, and vice versa.

Based on what I’m encountering with riders I know and what I’m seeing in many (though not all) bike shops I visit, most riders are over-inflating their gravel tires. In my conversations with the most knowledgeable people I know in the industry what I’ve been told is this: For every millimeter a tire goes up in width, lower the tire pressure by 5 psi.

In my case, if I start with a pressure of 100 psi, it works very well until about 45 psi at which point I think the guideline needs to cut in half—consider that a 5 psi change at 50 psi has twice the effect that 5 psi does at 100 psi. I’m running 35mm tires around 37-40 psi, 38mm tires at 30-35 psi and 40mm tires at 27-32 psi, depending on the tire. I’ve gone as low as 25 psi with a 40mm tire.

Ultimately, there are two ways to judge your lower limit: tire squirm and bottoming out the rim. Tire squirm is more a comfort factor than anything. Most of us experience a coronary event if we feel a sidewall wrinkle as we corner. Bottoming out the rim is unpleasant, but low pressures cope better with the sharp bits that cause flats typically, because the tread and casing have more leeway to flex, without catastrophic failure.

So where does talk of frame stiffness and tire pressure intersect?

In the marketing of current gravel bikes, that’s where. Manufacturers are still talking about frame compliance and noting the things they do to increase a rider’s comfort in the frame, which approaches pointless. Why? Because in many instances far more comfort can be dialed into a gravel bike by dropping tire pressure by 5 psi. And I don’t think many people would think that a viable option without making the case that there’s a fair chance many riders are already over-inflating their tires.

Nothing against ad copy, but they are marketing based on the wrong need. You don’t need a frame with more flex; you need one with more clearance, if anything. And if you’re not planning to drop some cash on a new bike, then let some air out … so you can keep the silver in your teeth.

Join the conversation
  1. khal spencer says

    My suspicion is most of us are not engineers, and frigging a bit with tire pressure makes far more sense than searching for That New and Wonderful Frame.

  2. bart says

    I’m guilty of being a chronic over-inflator of my gravel tires. I came of cycling age during the hey-day of 23s pumped to 120 PSI. I’ve been riding Donnelly X’Plor MSO 36s tubeless for the last 5 years as that is the largest tire I can fit in my “gravel” bike (aluminum Marin Lombard frame) and the sidewall says 40-60 PSI. Out of habit, and some deep-seated, semi-conscious belief, I always just inflate to the top end of the guidance even though I’ve seen Patrick talk about this subject for multiple years. After reading this on Thursday afternoon I decided “f*#% it”, for my ride on Friday morning I would go for it and try riding at 40 PSI. 40 PSI is what I came to with Patrick’s calculation above and it’s the low end of the sidewall guidance so that seemed like a reasonable place to start. I have to say I was worried about tire squirm. I don’t like that feeling. But, I told myself “this is going to feel strange, but just go for it and get through it.”

    The result…it felt really nice! I didn’t get any tire squirm. I made a point of hitting bumps, rocks, etc, just to test things out. I rode on some of worst and best surfaces I could find. The ride on Friday was just 50 minutes, and on Sunday I decided to roll at 40 PSI again. This time for more than 2.5 hours. Again, things felt nice. Less rattling around, and the bike rolled just as fast. I’m going to keep rolling 40 PSI for the next week and then try 60 again just to see what I’ve been “missing” to prove to myself that 40 is the way to go.

    Patrick, thanks for your persistence in continuing to talk about this subject. It’s taken multiple articles by you and multiple years for me to try this. Not sure why I was so stubborn. If any other riders out there can relate to my hesitation, my suggestion would be to just give it a try, see what happens. You may discover this works for you, or you may decide you like your high pressure, but either way, you’ll be more confident with your decision.

    Patrick, I do have a question, if the sidewall says 40-60 PSI, how low is it really safe to go?

    1. Padraig says

      Bart, I’m really happy for you. Truly, the only reason TCI exists is to increase people’s enjoyment of cycling. Whether that’s reading something inspiring or rolling on equipment that makes the ride more fun, that’s all we hope to accomplish.

      I once had someone at one of the tire companies explain to me the engineering rigamarole that goes into setting tire pressure ranges molded into a sidewall. My takeaway was that it was just a matter of math, a calculation, and not actual research on what the tire can take. The number of times I’ve run a tire below the lowest recommended pressure and have not had a problem has been, well … all of them. I’ve never had a problem with going too low. That said, if I manage to feel the rim bottom out on a rock, either I start taking a different line, or I increase the pressure by a few psi.

      A data point to consider: I’ve ridden 28s which had a minimum pressure rating of 80 psi at 60 psi. No problems.

      Not knowing how much you weigh, ultimately, you may be able to go even lower. My experience is that you’ll bottom the rim out long before you get enough tire squirm to risk unseating the tire.

    2. bart says

      Thanks Patrick. I just went for a 30 min ride at 35 PSI. That was starting to feel “soft” to me, but not too much so. Experiments will continue!

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