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.