I want my hiking boots and other outdoor footwear to last a long time, even with regular use and abuse. So one of the most important features I look for (after a good fit, of course) is a durable sole that won't wear down before the rest of the shoe. It's not an easy thing to identify, however, especially since manufacturers never provide precise specifications for the hardness (and consequent durability) of the rubber used in the shoe sole.
First off, some basic background info. As these pictures illustrate, there are many different types of rubber soles (or, technically, "outsoles") available in today's outdoor footwear. As a general rule, a softer rubber provides better grip and traction underfoot than a harder alternative, especially on smooth and potentially slippery surfaces. But a softer rubber also wears down much faster, which shortens the lifespan of the shoe before it will need to be retired or, potentially, resoled.
Harder rubbers last longer, potentially much longer, than their softer kin but won't provide super-sticky grip. In my experience, however, this loss of traction only becomes significant if you're involved in an activity, or out in conditions, that requires maximum grip. I'm thinking of activities like rock climbing, trail running, or traveling over smooth, slippery, and possibly wet or icy surfaces. For most general uses (most hiking conditions, walking around town, etc.), I've found that a harder rubber performs just fine.
But how do you know whether a shoe sole features a softer or harder rubber? There is an exact industry specification, known as a durometer hardness rating, which precisely identifies the harness of a rubber by measuring its resistance to indentation using standard testing procedures.
You can learn more about the durometer rating and methods here, here, or here, but the short story is this: Shoe rubber is measured using a durometer rating system known as "Shore A," which ranges from 0A to 100A. On the softer end of the spectrum, a rubber band measures in around 20A and a pencil eraser at 40A. At the other end are shopping cart wheels (100A) and inline skate wheels (80 to 85A). Most shoe rubber falls somewhere in the 70A to 80A range.
But here's the bad news: Footwear manufacturers never list this seemingly important spec on their products! So now what?
If you're shopping for shoes in the store, you can get a relative sense of the shoe rubber hardness by pushing a fingernail into the sole. If it pushes in easily, and leaves a brief indent behind, the rubber is on the softer end and will likely wear out after a season of hard use. Also try pushing the raised treads from side-to-side. If they move easily, it's again a softer rubber. I generally look for harder shoe soles that do neither. (If you're shopping online, watch out for product descriptions that emphasize how wonderful or grippy the traction is.)
When it comes to durability, I've sometimes heard the misleading advice that you should just "look for a Vibram sole." While it's true that Vibram makes some very durable soles, they also offer nearly 20 different versions of shoe rubber, so again, you should try and test the rubber first before buying if possible.
Learn more about outdoor footwear and fit in these recent posts:
New Boots? How to Check (and Adjust) For Fit (2011)
How to Prevent Heel Blisters (2010)
A Sticky Situation: Tincture of Benzoin and Blister Treatment and Prevention (2011)
Fitting Hiking Boots for Bunions and Other Wide Foot Issues (2009)
Fun Foot Factoids (2010)
"Equipped” is an AMC Outdoors blog, written by Matt Heid.