Humans are a funny lot. We flock to Starbucks and pick from more than 19,000 drink combos. Then we shop for ski boots and want it all reduced to a few flex rating numbers.
While using flex (AKA stiffness) as one factor in comparing ski boots is valid, the present trend of presenting boot stiffness with a detailed numbered scale, and implying the scale works across brands or even within a brand, is mostly BS.
Backstory: Many boot makers rate the flex of their shoes on an informal numeric scale from something like 30 to 130 (presumably starting at zero and being open ended). No official standard exists for this; all ratings are at the whim of manufacturers and mostly intended as a method for makers to compare models within their own lines (somewhat valid in that sense if the maker is honest and you disregard the fact that how a boot is fitted can change the flex).
Any resemblance of boot flex rating to an industry standard is disingenuous — this is NOT a way of comparing between brands any more than a very general guideline. In other words, when shopping between brands, boot stiffness numbers are no better than calling boots “stiff” “medium” and “soft.”
Nonetheless, many skiers have latched on to the numeric system (I use that term loosely) as way of comparing boots between brands — frequently with the simplistic view a “stiffer” boot will ski better. Granted, some of us want or need a beefy boot. But using these numbers to make fine comparisons between “stiff” boots is a poor way to go about shopping even if you do need a beefy shoe, and may even be highly misleading.
Boots vary in stiffness for any number of reasons — not just how stiff the shell is when it comes out of the mold at the factory.
Boots flex stiffer or softer based on temperature of the plastic. Adding to that confusion, some plastics change more with temperature, some less. Thus, you could “carpet demo” two pair of boots and pick one as stiffer, then take that same comparo out on the slopes and find the “softer” one was actually the stiffest in winter, but softer when used for spring skiing.
On top of that, the actual performance stiffness of a boot is influenced by how tightly it’s buckled, and by how the liner is fitted. Heck, even how you shell-size the boot changes how stiff it feels. For example, if you are large boned, your liner will compress more when fitted, thus being denser and causing the boot to feel “stiffer.” For this reason, using a smaller shell size can make a boot “stiffer.” Even the size and tightness of your power strap influences how “stiff” a boot feels, as does how tired you are.
Thus, what I’m saying is if you want a soft boot, just ID those marketed as “soft” and compare fit and feel within that group. Ditto for beefy boots. If you want a stiff boot, don’t fall in the trap of thinking the Brand X 130 is actually stiffer than the Brand Y 120 and will ski better. Depending on any number of factors, the opposite could be true.
Instead of a number system that implies accuracy and fine divisions that actually do not exist, what factors could be actually used to compare boots? How about internal width at ball of foot? Ramp angle? Forward lean when locked? Amount of rocker in sole? Height of cuff? Density of foam in liner? Weight? User maintainable buckle attachment? Dimension of heel pocket? Any of these numbers and items could be provided by makers or measured by us and charted out. Useful things. And yes, we can rate the stiffness of boots to some extent, perhaps by just pointing out what the stiffest obviously is in a particular brand, and then making subjective comparisons when we ski in the boots or even get them on the carpet.
Sometimes we’ll go ahead and mention boot flex ratings in our reviews, but be advised that they’re pretty much the level of stiffness the maker wants the boot to be perceived as by the market and where they think it fits within their own brand line, and not necessarily any comparative or accurate placement on any sort of global scale. Also, for the reasons stated above, I believe the existing system with more than 100 levels of stiffness is extremely overdone and more typical of some sort of marketing hype (my boot is a ONE HUNDRED and THIRTY, what’s YOURS?) than physical reality. A scale of 1 to 5 would work fine. Or how about “soft; medium; stiff?”
One other thing. We’re talking about backcountry skiing boots here: shoes a user might spend scads more time walking uphill in than they do going down. So why not a TOURING flex rating? Again, numbers 1 to 5 would be fine, or even Super; Medium; Limited, no need for a hundred and fifty fine divisions.