Is the density of a shoe’s midsole the least appreciated factor in understanding its personality? It seems like it might be.
When running shoes are discussed, what usually gets a lot of attention are the various plates, posts, etcetera that shoe makers design into midsoles to protect your foot, manage pronation, or help you with your taxes. Or the description leans heavily on relatively broad terms like “neutral,” “minimalist,” or “burly.” What often goes missing from the discussion are the relatively simple measures of midsole thickness and density. That’s too bad, because these two elements are incredibly revealing.
Perhaps a concrete example is in order. One of the many reasons I love La Sportiva is that they are very transparent about the thickness and density of the soles of their shoes. They also happen to make two well-loved models — the Wildcat and the Fireblade — that could hardly be more different given the amount of shared DNA. They will work perfectly for the purposes of this comparison.
Your eyes tell you the Wildcat (top) differs significantly from the Fireblade. A closer examination of their specs confirms this first impression and indicates quite clearly just what to expect from either shoe. Our focus is the shoes’ soles, but let’s quickly outline the differences in the uppers to complete the design story.
The Wildcat is categorized by La Sportiva as a Performance shoe. As such, LS has given it a light, breezy upper. Its Tempo last provides a medium-wide fit from the heel forward, with plenty of vertical room in the toe box. By contrast, La Sportiva labels the Fireblade a Racing shoe, which means it is formed on a last which is much narrower through the rear foot, with a wide but fairly low toe box. Now, on to the soles:
Wildcat
Fireblade
In contrast to the pictures of the complete shoes, a glance at sole construction doesn’t really convey how much of a difference there really is. The main thing left unsaid in the graphics is that the MEMIex midsole compound of the Wildcat is single density; the Fireblade’s is triple density. That is huge; my unscientific estimate is that it is roughly the same as the difference between a sponge and a stone.
The final telling specs relate to midsole thickness. The Wildcat is 28mm thick at the heel, 16mm at the toe (for a delta of 12mm). The Fireblade is 26mm thick at the heel, 16mm at the toe (for a delta of 10mm).
Now what exactly should all this tell you, the savvy trail runner? Quite a lot, as it turns out:
- The Wildcat will feel somewhat similar underfoot to a “typical” road shoe. The lower-density midsole compound and slightly thicker heel make for a cushy heel strike and easy transition to toe-off.
- You should expect a shoe like the Wildcat to wear out more quickly, since the midsole’s single density compound will collapse into itself with the pounding delivered by every mile. By definition, a denser midsole has less “inner space” and so will keep its original cushion (or lack thereof) much longer.
- The Fireblade will feel, above all, firm. With its lower heel and very dense midsole, the Fireblade discourages a heavy heel strike. Instead, your foot strike will be coaxed slightly forward and you will get a bit more trail feedback compared with a thicker, cushier shoe. It is this combination of a relatively low heel and very dense midsole that gives the Fireblade its racy manners.
- Neither shoe should be expected to provide an extremely tactile feel of the trail. Both are “go fast” shoes, but they are not designed to deliver a “naked” feel. The sole specs communicate this loud and clear.
- It also turns out that this comparison is an excellent example of the proposition that shoe weight, by itself, indicates only a little. The respective weights of the Wildcat and Fireblade are practically identical — within 10 grams, depending on size and gender — but each shoe gets to this same destination from radically different directions through a series of design decisions driven by the final intended purpose of each shoe.
Ultimately, the careful analysis of the specs of any shoe should be in the service of selecting the right shoe for your specific conditions and preferences. Under most conditions, I personally prefer a firmer shoe like the Fireblade. But I’ve also found that my body appreciates a little extra cushion if I’m doing a lot of steep, longish descents, especially on a punishing surface like, say, sandstone. So, for example, I used my Fireblades more than any other shoe this past summer, but there were many days I was extremely glad to lace up a pair of Vasque Celerators, a shoe with a profile similar to the Wildcat. Of course, this last bit is just my experience and is offered only as an illustration, as was the side-by-side Wildcat/Fireblade comparison.
The point I hope you take away is that drilling down into the specs of a given shoe far enough to have a good grasp of sole thickness and, especially, density will tell you a great deal about how a shoe will feel underfoot. When it comes to trail running shoes, it turns out George McFly’s malaprop is actually spot on: density pretty much is destiny.
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