Encore: Ski Boot Stiffness Ratings — Is the Number Scale BS?

Post by WildSnow.com blogger | June 9, 2011      

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.

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 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.

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21 Responses to “Encore: 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,

  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


    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

  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.

    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.

  13. Albert Luongo December 31st, 2010 5:37 pm

    I just spent hours researching my first set of ski boots. I have found higher flex rating always translates to higher price point. Like most items (almost), professional marketers dictate what we buy and how much we pay, period! 😐 . (taught to me in MBA marketing course)……..

  14. Steve A January 2nd, 2011 8:54 pm

    Looking around for some info related I found this article very good and realistic and attacking the most of the aspects. The filling and stiffness also depends on the design of the boot. I found that some race boots (eh tested some extremes) have a more generous design like Atomic or Fisher but others like Rosi or Salomon very tight even for wearing. For the lasts once you are in no problem in my case (my foot is very standard) but a pain to get out or in!

  15. Ryanb June 9th, 2011 9:48 am

    Speaking of behind the scenes web stuff, I think their ia a bug with your RSS feed. It showed a Best of article that hasn’t been posted yet yesterday and its done the same thing before. I see some enticing sounding blurb in google reader but the link is broken then a few days/weeks later the article shows up on the site for real.

  16. Lou June 9th, 2011 10:20 am

    Ryan, THANKS! I think what causes that is that I sometimes actually publish something but I date it ahead in the future to keep it semi-private, so I can review with someone who doesn’t have admin access. I’m not sure if I can fix my RSS feed to prevent that. It’s like a vacuum cleaner (grin).

    It sounds like it might be my methods that are off. I’ll look at not using that technique for privatizing posts.


  17. Lou June 9th, 2011 12:49 pm

    Ryan, after testing, it appears that what happens is that sometimes I make a mistake and accidentally publish something I’m working on. It then appears in the RSS feed until I schedule for a future date. For the end user, it probably remains in their RSS feed till they reload or refresh their browser. Sorry for any confusion this has caused, I’ll try to be more careful with accidentally publishing, which is pretty poor form (grin). Thanks, Lou

  18. Oscar June 9th, 2011 4:34 pm

    I usually make it easy for myself and ask my local dealer for tips and tricks. Much easier to ask someone with a lot of knowledge about the different boot brands, different plastic behaviours and a general idea of how the boots are built. I suppose that might be a problem if you’re not blessed with the presence of a god-given boot guru at your local ski shop though (the best part is that he’s a tourer himself, so I never need to worry about that part either!)…

    But yes, I have the same experience as you, Lou, the flex scale is largely BS. The sad part is that not very many people tend to listen to you when you try to explain that to them and try to convince them that they should see a ski shop with someone that is somewhat of a boot guru…

  19. Tom June 9th, 2011 4:54 pm

    I think they are BS, just skied Refer Gully on Ice Mtn w/o the tongue on my TLT Mtn and didn’t even notice till I found them in my pack. I would have put them in if I had thought about it but that fact that I didn’t even notice and had a great ski tells me that if you are a decent skier your skill will more than make up for any deficiencies in the gear. And I am not an “expert” skier, I consider myself an “advanced beginner”

    If the boot fits just ski it, being able to stay comfortable for 8 hours or so is more important than any flex rating, just another marketing angle that causes unnecessary angst.

  20. jwolter7 June 10th, 2011 12:07 am

    Hans go to work for Surefoot and you”ll get some of those measurements. Exact length, and width. They then measure the foot and have a formula for picking the correct shell. They’re still working on instep hight.
    Jay White who had a company called Comp U Fit from Bend OR would take a shell and fill it with silicone let it set up and then cut the shell away and he had the best 3D measurements of ski boot shells I have even seen. Heel width midfoot width forefoot width, Instep hight, girth, ankle circumference, and calf circumference. He also had the best method of measuring the foot to closely match the foot date with know boot info , but it took a while.

  21. Gary Olsen December 21st, 2014 12:14 am

    this is perhaps old news. I agree with what Lew said. However, it is important to understand flex and fit as it pertains to control. For example, every world cup racer has a low volume tight fitting boot that is stiff as hell. Why? At race speeds when body position must be maintained precisely, on a really icy turbulent course, soft boots wont cut it. Take this into consideration for your own skiing. Frankly, in the BC, speed has always been the last of my worries, but there are times when control is mandatory. But we typically dont need World Cup boots when you are climbing for your turns.

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