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Ski Boot Stiffness Ratings — Is the Number Scale BS?

Bookmark and Share            By Lou

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, 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).

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. 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.

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.

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.

We’re getting excellent support from the boot makers, so perhaps we’ll start going over boots this way and getting the details out to you, as we try to do with bindings.

In summary:
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. A scale of 1 to 5 would work fine. Or how about “soft; medium; stiff?”

Comments on!

Posted by Lou on September 3, 2008 | Filed Under Backcountry Skiing 

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Comments

12 Responses to “Ski Boot Stiffness Ratings — Is the Number Scale BS?”

  1. Chris September 3rd, 2008 12:55 pm

    Lou -

    You must have been reading my mind this morning as I left a comment for a much older post on almost this precise topic. I’ll repeat the comment here as this seems to be a more appropriate place…

    Polyurethane boots are said to get stiffer as the temperature cools. If really true, how much of a difference are we talking about – 10%, 50% ? Also, how, in general, does that change in stiffness affect the fit – if at all? The last thing I want is a boot (I am considering the Zzero 4 U-TF) that fits well in the shop but then feels different on snow. I am a bit weary of this since this is something I wouldn’t be able to know until after I already shelled out hard-earned money for the boot.

    Finally, any general insight into the durability of a polyurethane boot versus a pebax boot would be nice to hear.

    Thanks a lot,
    Chris

  2. Tom G September 3rd, 2008 1:48 pm

    I think the flex ratings are usefull as they do give one a sense of relative stiffness, but Lou is correct that you can’t assume one manufacturer’s 120 boot is for certain softer than another’s 130 boot. Nevertheless, I like to see the flex index as it gives me some idea of stiffness. I also know that, at least in alpine boots, all top end race models are going to be somewhat similar in stiffness regardless of whether they are labelled 130 or 150, thus I can calibrate my expectations. I would love to see a standardized procedure for rating flexes. I was watching the boot video on Black Diamonds site and saw their machine for measuring flex. Surely they have tested lots of other boots and could tell us how their top boot ranks relative to an alpine race boot and some well known touring boots. This would be really interesting to see. There is also a really good user ranking of AT boot flexes on one of the TGR forums.

  3. Jordo September 3rd, 2008 2:43 pm

    Chris,

    I might be wrong, but I suspect various maufacturers use different mixes of plastics but refer to them by the same names. I.e company X’s PU may not be = to Company Y’s PU. Lou or anybody: do you think there is a standard “recipe” for these things?

    However, with regards to Scarpa: I have a PU version of the Tornado with about 15 days of use on them. They look brand new. I also bought a pair of Pebax Spirit 4s at the end of last season. I used them five times but they have LOTS of gauges and nicks on them. Not a big deal for me but I’m sure it will add up to a shorter overall lifespan at some point, and there are photos floating attesting to the marked increase in wear and tear in Scarpa Pebax vs their old PU.

  4. Nick Thomas September 3rd, 2008 10:41 pm

    For a simple flex index to have any meaning at all the flex of the boot must be approximately linear (double the force required to double the amount it is flexed). I doubt that is anywhere close to reality with most boots.

  5. Lou September 4th, 2008 6:09 am

    Nick, exactly. That’s also the problem with my binding flex ratings having any more than a general meaning. The only thing that would really tell the true story would be to graph the flex of each boot or binding, through a range of motion. Couloir magazine used to do that with tele stuff (another thing Couloir was probably under-appreciated for, sadly.)

    I should have mentioned in my blog essay that a huge difference between boots that might be rated as similar flex is how that flex performs in a range of motion. Does it stop abruptly? Smoothly progress? Blend of the two? More, are there easy mods that a boot fitter can do to change the flex for individual preference?

    All above causes the present system to not pass the smell test (in my ever humble opinion).

  6. Hans September 4th, 2008 2:29 pm

    The only stat that really matters for boot reviews is fit. If a boot is too wide, it won’t work period, and because AT boots can be harder to punch and grind, too narrow can be a problem too.
    I’d love to see some dimensional information- not numbers but an indication of the shape of the boot, width, instep height, and overall volume.
    I appreciate the detailed reviews of features etc, but w/o fit info, it’s pretty irrelevant. The best features in the world don’t make a boot fit right, and if it doesn’t, you can’t ski.

  7. Lou September 4th, 2008 4:01 pm

    Hans, I agree, we’ll try to figure out a way to present dimensional info. I’ve got some ideas for cool tricks using some hardware store engineering. We shall see.

  8. JW September 4th, 2008 6:29 pm

    A great post and comments Lou and others, thanks
    jw

  9. Lou September 4th, 2008 6:31 pm

    Indeed, this has been good and I thank all you guys!

  10. Hans September 4th, 2008 9:13 pm

    Overall volume can be measured pretty easily, and just knowing forefoot & rearfoot width plus some indication of instep height is enough to get a good idea about fit compatibility for most people.

    As far as flex, isn’t stiff-medium-soft good enough?

  11. Core Shot September 5th, 2008 10:40 am

    Good points, but why not try? By your logic, the ski descent ratings systems would never be worth your efforts. And yet, you have proceeded with them on a peer-review consensus-building basis.

    Here is the TGR AT boot flex thread. It is a great place to start for AT boot stiffness feedback. Member feedback has made it pretty accurate.

    http://www.tetongravity.com/forums/showthread.php?t=78437

    Personally, I think a thread like that is great.
    You are right that manufacturer ratings are often wrong and not consistent, but if a new boot comes on the market, it is nice to know how it skis compared to other boots I have owned or tried, so I know whether to even bother getting excited about the new boots. Getting feedback from you, LeeLau, and other beta testers is more important to me than a manufacturers flex #.

  12. Phil Kessel April 2nd, 2009 6:23 pm

    I just bought a pair of Dalbello Proton 8s with labeled at 90 flex. it puzzled me that thew flex rating had no units, eg… Lbf Ft/Degree. Now I understand why. I bought a soft flex boot because I want to ski Moguls and reasoned that soft flex like soft tips and tails will throw me around a lot less. What is the down side? I often see hot telemark skiers doing moduls and their effective flex rating must be zero. Some salesmen disguised as boot fitters say that a stiffer boot gives you more power. Since your ‘power’ is limited by your body I don’t understand what the boot contributes.

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