A Shore Thing

Sometime back in the 1990s, probably about the time that elastomer suspension was a thing, I learned about the Shore durometer scale. At a time when journalists would get frothed over the possible differences between open cell and closed cell foam, Shore durometer helped define one of the behavioral parameters of rubber in a way that allowed those frothing journalists to pretend to understand what was going on. Sometime after that, probably about when 5.10 Stealth rubber showed up on Intense tires, Shore durometer entered the discussion with regard to tire performance.

The Shore durometer scale, named after the measuring device developed in the 1920s by Albert Ferdinand Shore, is actually 12 different scales; A, B, C, D, DO, E, M, O, OO, OOO, OOO-S, and R. Each scale has a value range between 0 and 100, with 0 being the softest expression of that scale and 100 being the hardest. If we are talking about, say, bicycle tires, we’d be in Shore scale A. If we were measuring the hardness of chewing gum, we’d be using Shore scale 00.

Sticking with Shore scale A, and staying in our lane regarding bicycle tires, the method of measurement involves steadily pressing a hardened steel rod between 1.1 and 1.4mm in diameter – with a truncated 35 degree cone and a tip diameter of .79mm – into the material being tested with a force of 8.064 N for a duration of 15 seconds. The amount of measured deformation gives us our final Shore A rating. If the indenting tool presses 2.54mm or more into the material being tested, the durometer for that material would be 0. If it does not deform the material at all, the durometer would be 100. For reference, car tires often rate out around 70A, hard skateboard wheels would be 98A, rubber bands might measure around 25A, and the Specialized Purgatory T7 I reviewed last week has a 50A durometer.

When I was riding this tire, I was thinking about the differences in ride characteristic between T7 rubber and T9 rubber, and assumed that the T7 compound was a harder durometer than the T9 compound. Because, according to Specialized, T7 is a more durable and faster rolling compound. Thus my assumption. This is the point in the narrative where the booming voice of Frank Campbell, my high school woodworking teacher, interrupts; “Now, Ferrentino, you know what happens when we assume things, don’t you? You make an ASS out of U and Me!”

My assumption has always been that stickier tires will have a lower Shore A number than high wear tires, that softer rubber is how tires derive traction and ride characteristic. So it was a bit of a shock to find out in an email exchange with Matthew Bialick at Specialized that both T7 and T9 compounds share the same Shore 50A durometer. Well, shit. Frank Campbell can be heard laughing heartily in the background.

As it turns out, hardness is only one component of the grip equation. Compound matters too. So, with regard to the difference between Specialized’s T7 and T9 compounds, the 50A hardness, according to Matthew, “is the ideal balance between ground adaptation and rider support.” But, “for us, it’s hard to talk about compound hardness and not discuss the damping characteristics as well. Comparing tire compounds between multiple manufacturers isn’t straight apples to apples, especially with different compound layouts, mono versus stacked multi-compounds, et cetera.”

Matthew continued; “What makes mountain compounds special is tuning the damping of the compound. For grip, it’s about damping the vibrations from the trail as the tire rolls over it, essentially quieting the ground in turn giving you a constant connection for grip and control. Durometer still very important but only one piece of the formula.”

Two compounds. One feels more grippy, but what I am feeling is really the sensation of a tire being less reactive, more damped. The other rolls faster, wears better and is more “lively.” Same durometer. Huh.

Naturally, this led me to bring up a whole barrage of other questions. How do you change the damping behavior of a tire with compound, if the durometer of the tire remains the same? Does the harder wearing tire have more carbon black, and does that make it a less damped tire? Or is there more silicone or urethane on the more bouncy, less damped tire? Given that each compound is essentially the same durometer, is the knob shearing potential the same, or does the difference in damping also imply a difference in wear rates and lifespan? And if so, how is that achieved?

Meanwhile, thinking about how durometer does not matter in as central a way as I thought it did, I ended up also wondering about hysteresis. Hysteresis matters in tires, a lot. But, as with hardness, it turns out what I thought hysteresis was is completely erroneous. Shit, I’ve even been mispronouncing the damn word my whole life. It’s pronounced hiss-tur-eee-sis. I always thought it was hiss-tair-uh-sis. No wonder tire compounders always shake their heads and chuckle amongst themselves when I’m asking them questions. Aaaanyway, I always thought that hysteresis was a measure of how bouncy a rubber compound is. And, in a way, that’s true, but not the way I thought. According to Merriam-Webster, hysteresis is “the slowing of an effect when the forces acting upon a body are changed.”

In the case of rubber, that change is best described as deformation getting converted to heat. The forces being fed into a tire as it deforms over a surface are not immediately responded to equally. There is a lag, and some of that energy gets absorbed and converted into heat, and then the tire returns to shape somewhere near what it was before. This is damping – energy being absorbed and converted. And this is why elastomers were considered good candidates for suspension way back when. Unlike metal springs, rubber has a pronounced hysteresis that does not return all its energy when released from compression. This is also a huge factor in how a tire generates traction, or friction. Hysteresis is a major component of rolling resistance, because it is dissipating energy as heat instead of returning it.

So, with Specialized tire compounds, the durometer difference between super tacky, highly damped T9 rubber and ultra fast rolling T2 rubber is negligible. T2 is about 51A and T9 was somewhere around 45A, when I was watching Wolf VormWalde, Specialized's tire research wizard conducting a Shore test during a Zoom call. The T2 rubber rolls fast because it has a much lower hysteresis - the amount of energy that gets transformed into heat between loading/deformation and unloading/rebounding than T9, in spite of being almost the same durometer. And it has a much lower hysteresis because different polymers are used in each tire.

There are up to 15 different polymers, oils and fillers in a given Specialized rubber compound. The rubbers can be defined as natural and synthetic, and there is a whole pile of varying viscous polymers in the latter range that are called Styrene-Butadienes. The more styrene in one of these, the more plastic and less viscous it will be. The more butadiene, the more squishy it will be. Then, the fillers get added in. Carbon black does not adhere to the polymer chains in a given compound, and therefore allows that compound to flex more freely in all directions. Silica does adhere to the polymer chains, and that in turn helps bind them together and limits how much they can squirm around. More Silica in a compound then means that a tire will be more bouncy, faster rolling, more resistant to shear and tear forces, and probably more durable. More carbon black and the tire will be more damped, more tractable, but slower rolling and less resistant to tear and shear forces.

T9 tires use a whole lot of carbon black as filler and no silica. T7 and lower only use carbon black for color, and then rely on silica and other fillers. This, along with the particular ratio of Styrene-Butadiene being used in the compound, is why T9 will result in a grippier tire than the others.

All of this – durometer, compounding, hysteresis – is in play when you are ripping down the trail. Those slow pedaling Assegais? It’s not necessarily just weight that makes them feel that way. Hysteresis, how much of your energy that tire is absorbing while at the same time increasing the traction you reflexively depend upon, is making itself felt. This is a head explody design paradox. Traction is king, but increasing traction is almost always going to also increase rolling resistance. The people who design our tires are using compounding, durometer, tread patterns and casings to try and thread the needle between traction and efficiency, in the hopes of gaming physics and delivering tires that offer the traction you need without sacrificing all your pedaling efficiency.

As I was climbing up a section of grippy sandstone slabs yesterday, I could feel my rear tire squirming for bite. I imagined each of the knobs deforming and reforming, all these tiny actions and reactions, energy being converted to heat and resulting in adhesion, tiny slivers of watts being lost but at the same time forward motion being maintained. Instead of reducing assessment of my tire’s performance to the binary of “awesome” or “sucky”, I quietly thanked my tires for doing their incredibly demanding job.