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Buying Boots . . .
. . . Two numbers to know

Ski boots are the most commonly replaced piece of equipment because when they stop working, they get your attention by hurting like hell. When your feet are telling you it’s time for an upgrade, there are two numbers you should learn: how wide a boot needs to be to comfortably hold your forefoot, measured in millimeters; and how stiff the cuff should feel when you flex forward, as indicated by a flex index number.

For example, if you have a medium-width foot, you may fit a 98mm shell, defined as the widest part of the lower shell. If you’re a good male skier of average build, you might prefer a flex index of 110 or 120, based on a scale that usually keys off a 150-point maximum for the most brutal of plug race boots and scales down from there in 5- or 10-point increments.

Neither of these numbers are ironclad, immutable facts.  The flex index isn’t tied to any universal standard, so a Salomon 110 and an Atomic 110 aren’t obliged to have the same flex resistance. Indeed, even within a brand, a 3-piece shell, a classic overlap and a model with a “hike” position may all have the same flex index in their name and all behave differently. While this sort of defeats the whole idea of a meaningful index, the flex index number is still the only guideline we get, so you have to be as flexible as the index itself when seeking your perfect match.

Here’s how manufacturers rationalize their free-for-all “system:”

Within each family of ski boots the flex indices decline rationally, i.e., the stiffest model in the genre is anointed a 130 flex, the next model down a 120 and so forth down to 80 or 90 (for men).  They’re not insisting that all 130’s are created equal, only that all lower numbers shall be softer flexing than all higher numbers within a particular collection that all use the same shell architecture.

Despite a somewhat muddled methodology, flex index numbers are accurate enough that we feel comfortable advising lighter or less aggressive skiers to use boots with a lower flex index (e.g., 80 or 90), and heavier, more athletic or more powerful skiers indices from 110 and up. But you can’t really know a boot by its flex index. You have to first be in the right shell before you can even consider which flex is optimal for you within that shell. And picking the right shell entails a lot more complicated analysis of your needs than just picking the right flex, but if you know the flex you prefer, this at least gives your bootfitter a clue as to which boot you ultimately belong in. 

The forefoot measurement, mercifully, isn’t as wildly subject to interpretation as the flex index number. While it’s a physical fact and not a marketing tool, the forefoot measurement is still variable in that where the boot is widest is just as important as the measured width. If your “sixth toe” metatarsal bulge occurs behind the widest point in the boot, just where the shell starts to narrow, the fact that the boot has a wide zone doesn’t help you much. Still in most cases it’s useful to know, for example, that a 98mm shell fits you comfortably and 100mm shells feel roomy. But even the hard fact of a width measurement turns mushy in light of new technologies from Fischer and Salomon that allow the shell to contract or expand so the width you end up with probably won’t be the width you started with. But as with the flex index, some idea of your width requirement is better than no idea at all, and will help your bootfitter narrow the search for your perfect boot.

Small variables make a big difference in how boots fit, which is why no set of numbers can provide a foolproof guide.  Only by putting your foot in the boot can you tell if all the numbers add up to the right fit and performance that will optimize your skiing.

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