A Very Deep Dive on Ski Boots, Part 2: Flex Patterns (Ep.55)


  • Flex Patterns & All the Feels (4:13)
  • “Progressive” vs. “Linear” flex patterns (5:10)
  • The mess that is Flex Ratings (13:13)
  • How Atomic arrives at their own flex ratings (17:24)
  • How brands make a “110” vs. “120” vs. “130” etc. (27:34)
  • Colorways & Geopolitics (34:20)
  • Why a standardized flex rating probably won’t happen (35:55)
  • The damping / “suspension” of a boot (42:32)
  • Weight & Damping (47:04)
  • Next areas of innovation for Atomic boots? (55:08)

In today’s episode we are bringing you part 2 of my conversation with Matt Manser, the product manager of Atomic Ski Boots. If you haven’t already listened to Part 1, you should do that first, because it is definitely relevant to this episode’s discussion about the flex patterns of ski boots.

Here in Part 2, we talk about the messy world of flex ratings; progressive vs. linear flex patterns, and the sometimes kinda shady seeming differences between a brand’s 110 and 120 rated boot, or their 120 and 130. We also talk about how Atomic arrives at their own flex ratings, issues of weight and damping, and we touch on the next areas of innovation that Atomic is focused on.

But our work here is not yet done. So we are going to be doing a Part 3 with Matt, and we invite you to submit some other questions you have below, and we’ll address some of them in our next conversation.

Also, if you are enjoying these discussions about materials and plastics, then you have to go check out our latest episode of our new bike podcast, Bikes & Big Ideas. I was just in Denver earlier this week sitting down with the founders, chief engineer, and director of composites of Guerrilla Gravity, and while their name might sound like they are some garage band, these guys are working on some cutting-edge stuff in the world of materials and manufacturing, and they are the only bike manufacturers using what I’m now calling carbon 2.0.




When you go to buy a ski boot, you will often find that the boot is said to have a flex rating of “100” or “110,” etc. and these ratings generally range from around “70” (this would be a very soft-flexing ski boot for adults — kid’s ski boots will sometimes be listed around “40”) all the way up to “170” or so, which would represent an extremely stiff and powerful World Cup race boot.

But here’s the thing: these stated flex ratings are not tied to any specific measurement. This is important, because it means that there is no universal standard for flex ratings. A “130” from Company X can — and will — be different than a 130 from Company Y.

Flex ratings are determined by each manufacturer to rank the boots within their own line, usually by stiffness and definitely by price point. So be wary, because sometimes a company will simply change a ski boot liner or attach a slightly different power strap, then give the boot a higher flex rating — which usually goes hand in hand with a price increase.


First, brands would potentially oppose any standard, because it could be costly. It’s extremely expensive to create a boot mold, so for a model to ultimately miss the rating it was intended to achieve could throw off the whole line and be financially disastrous for a brand.

Second, flex patterns are partially determined by the foot that the boot is designed to receive, which isn’t easily standardized.

Third, it would likely require that all boots of a certain flex rating have similar general constructions and shapes in order to be comparable, which would cause brands to align and be quite similar to each other. So, the standardization process could potentially slow down innovation.


When starting to design a boot, they tend to start with the 130. They then purchase the 130 boots from all relevant competitors, and measure each of their flex patterns. They measure them using in-store and field tests, as well as a custom-built robot (below is a graph of flex pattern data collected from a single boot design).

Having ranked the competitors, they then aim for their boot to land in the middle, in order to be the “average” stiffness among the 130 boots out there.

After experimenting with materials and design and arriving at the desired stiffness, they create the less stiff (120, 110, 100, etc.) models by blending in percentages of softer materials.


When a boot is said to have a “progressive” flex pattern, this means that the more you lean into it and the deeper that boot moves into its flex pattern or ‘travel,’ the stiffer it gets — just like the suspension on a mountain bike fork that gets progressively stiffer the deeper it gets into its travel.

(Conversely, coil shocks on a mountain bike generally have a more linear design.)

The chart below shows a progressive flex, in that as the angle increases, the rate of torque increases, showing a curved line (instead of a straight — or “linear” — one). This helps more efficiently transmit energy from you to the ski. A more linear flex will cause you to work harder.

Atomic's global product manager for ski boots, Matthew Manser, discusses ski boot flex ratings and more on Blister's GEAR:30 podcast.
Measuring Progressive Flex: This graph shows the forward and rearward flexes and returns of a single boot design with three differently stiff PU base materials.

As for “linear” flex patterns in ski boots … it is doubtful that any ski boots out there have a truly linear flex pattern. I.e., some boots will be easier to flex than others right off the bat, and some will be stiffer / harder to flex at the start. But virtually all ski boots on the market will ramp up in stiffness — to some degree — as the boot moves deeper into its travel; some boots just ramp up less — or less quickly — than others.


  1. The materials used. Some plastics are harder or produce more rebound than others, though they can be blended to achieve precise levels of stiffness. Also, based on how hard they are and how they behave when poured, certain plastics are also capable of forming different structures / designs.
  2. The design of the boot. How thick or thin the walls of the plastic are in certain locations plays a very big role.
  3. Your foot. This is an often overlooked factor. How much volume of the ski boot a person’s foot takes up will affect leverage and ultimately how stiff or soft a ski boot feels. Generally speaking, the less volume of a ski boot your foot takes up, the softer the boot is going to feel.
  4. There are some other factors that we will be discussing with Matt in our upcoming “Part 3” episode.

45 comments on “A Very Deep Dive on Ski Boots, Part 2: Flex Patterns (Ep.55)”

  1. When is somebody going to develop a blown in liner as lite as an Intuition? My Hawx Ultra 130’s are 4.5 pounds in a mondo 30 with booster strap. The ultra liner is great but heavy.

    • Hey Jeff, Unfortunately, this information won’t help you out since you are a size 30 but new for this coming fall, we’ve got a super updated liner for the Hawx Ultra XTD 130 that will be available aftermarket (sizes 22 – 29). This will drop the weight of an Ultra by about 150g per boot but retain the “Ultra” fit. For someone in your size, you will most likely have to go the Intuition route.


    • Ok… I loved all this, absolutely awesome… is there a plan to address in part 3 how the buckles and liner affect the “feels”???

      It seems that these are as huge a variable as much of the other technology you have spoken about…


      • Yep! I’m talking with Matt a little later today, and buckles and liners are on the agenda. “Part 3” will then be out this Friday.

  2. Thanks for the thorough notes. Looking forward to listening. If it hasn’t been covered, I’m interested in the effect, if any, of cuff height on flex and the perceived ablility to flex a boot.

    • Hi David, Taller cuffs offer move leverage & support (especially rearward) and will change how the boot flexes for sure. Our cuff height is more determined by fit/comfort vs flex, and there will be a limit as to how tall the cuff should be in a given size before the fit and flex get weird. Going too low with the cuff height can cause pain at the shin and going to high can cause pain in the calf area. We like to strike a good balance with maybe being a mm or two higher than normal.


      • Thanks for all of the responses, Matt. So informative.

        In the 80s, I was a tall, rail-thin, big-footed kid — the type GregL described in his question, and a pair of Nordica Polaris boots allowed me to flex forward in a way that I could not in my Langes that, because of my completely undefined, wiffle-ball bat calves, I had to crank down until the cuffs buckled and warped. Too stiff. I always ended up backseat with burning quads.

        If there was a pair of Polarises that wasn’t completely UV-degraded (there isn’t; they’ve all become brittle deathtraps), I’d love to see how they felt. I wonder if they’d diffuse the intense boot-top pressure that can occur when skiing fast in rough terrain. But I worry they might limit my ability to loosen my ankles and smear a turn.

        In any case, as a manufacturer, it’s got to be tough to arrive at a cuff diameter/height that works for most calf shapes. I can’t imagine trying to close those odd, odd boots if I’d actually had big leg muscles.

  3. Hey Jonathan, I would be interested to hear about Matt’s choices for his own personal ski boots. Atomic brand only? Flexes? Different boots for every possible scenario or a quiver of one? Prototypes? Really enjoying this latest podcast series!

    • Hi Peter, I only ski Atomic (except when doing competitor testing) and depending on the time of year in our development cycle, you will usually see me in a prototype of the boot we are currently developing or testing out an existing boot in a new/different plastic. When I’m not testing, I will be in a Redster Club Sport 130 or Hawx Ultra XTD 130 depending on the terrain we are skiing.


  4. Matt, you talk about the importance of filling the shell with the correct volume foot in order to retain structural integrity and achieve full potential of the flex, and I’ve certainly found that to be true re: the shell not collapsing. On the other hand, I’ve found the opposite to be true with cuff diameter. A skier with a really skinny lower leg and calf, with the ladders all the way in and the buckles cranked will find the flex (at least the initial 10-15 degrees) quite a bit stiffer due to friction than the skier with a super big lower leg and calf who can barely get the cuff to cover the liner tongue. You get this with teens whose feet have attained full growth but their bodies are still rail thin – they want the “core” 130 boot but they can’t even bend it in the shop. The Clydesdale dude with the high volume foot and the huge calf finds the initial flex in the same boot underwhelming. Comments?

    • Hi Greg, Cuff closure tightness definitely has an affect on how stiff a boot is and a lot of that has to do with how much the cuff flaps overlap each other. More overlap will yield a stiffer flex, less overlap a softer flex. And when more overlap is combined with a tight buckle closure, it will be even stiffer.


        • Ha. (Also, my brief random review of Westworld is: I can’t think of a more promising, potentially more societally relevant premise for a TV show than Westworld. And the 1st and 2nd episode made me think that the show might achieve this. And then things went wildly off the rails, and the show badly lost its way and just became … not illuminating at all of our near-future world of humans & robots? Black Mirror seems to be doing a much better job of this?)

  5. Great series! How does a 3rd party ski liner like Zipfit affect the flex? For example, I have super low volume feet/legs & need to use Zipfit to take up volume. Zipfit hugely changed my relationship w/ the cuff [for the better].

    • Hi Joel, Generally speaking, softer and/or thinner liners will lead to a softer flexing boot and harder and/or thicker liners will lead to a stiffer feeling boot. I can definitely imagine your Zipfit liner reducing volume and leading to a stiffer feel.


  6. Similar to other comments. First, really great series thanks for taking the time to talk about this stuff.
    Second, what are the trade-offs to blown-in liner? Could it be desierable to have a “single size” boot last and rely on a liner to fill the space and help to control the flex or is that not how it works? i.e. if you have a 102 last boot and have liner fill it is quite different from the actual foot filling in down to the liner size? Also, why so much variation in liners from different boot manufacturers? They seem to run the gammit and that makes it hard to know how much is due to plastic/forming and how much to the stock liner v. intuition vs. Surefoot or others. Any thoughts and comments there would be great to hear about.

    • Hi Bob, I’m not sure I know what a blown-in liner is… is that something like a foam injection liner, where foam is pumped into an “empty” liner, or more like an EVA/Intuition-style liner? First option is highly dependent on who is making it, has a relatively high chance that the foam does not fill the liner evenly/balanced, and is quite heavy (if that matters to you). The second option is easier to form, is lighter, but will often pack out faster due to the EVA foams compressing over time. Both have their pros/cons depending on what is important to you.

      As for having one last and use the liners to change this fit, this is something I’d really like to avoid. When there is too much a gap between the foot and the shell, your skiing will suffer and the liner will pack out faster because your foot can move around more. So, the ideal (but way more costly) scenario is the have more lasts/molds and liners that fit according to each last/mold.

      Re: liner variations – Each brand has their idea of what constitutes the ideal liner, so they will have their own interpretation of that. Different R&D teams, different levels of know-how, different factories, different end goals, etc. all contribute to variations among the brands.


      • Matt, thanks for the reply. I mean the foam injection type liner.
        I can see your point about having as little space as possible between the foot and the hard shell. This also answers the question (for me) of why boot-fitters always suggest packing your foot into the smallest mondo size and then punching out rather than having some room there – you want as little material in there as possible. I can understand why many possible lasts and shapes would help solve that but would cost $$.

        Can the entire problem be inverted? Can you make a very thin flexible liner and a totally heat mold-able shell? Or the heat required to mold that would cook your foot (hence the current punch-out routine)?

        Also, an off-the-wall question but is there any way to get more “feel” or “contact” with the ski itself on the bottom of the boot or the stiffness on the bottom is part of having something that you can walk in without breaking and effects the boot flex overall?

        • Hi Bob, I don’t think the technology is available yet where we can make a super thin flexible liner that can span multiple sizes of shells (and cuff heights!). BUT we are working on some new liner technologies that will definitely improve the customization potential of your boots. Stay tuned…

          RE: ski feel thru the bottom of the boot- this is something we experiment with very heavily from a wall thickness standpoint, shape of the bottom of the boot, and length of the heel/toe lugs. All of these factor in to ski & snow feel, and some of our racers go so far as to carve away material (or sometimes add material) from the sole to achieve a certain feel. But I definitely wouldn’t recommend that anyone try that tactic at home ;)


  7. Hey Matt—I’ve got a couple nerdy questions about the boot design process:

    – How do you make initial prototypes for a new boot designs, before cutting tools? I imagine making even aluminum molds would be expensive and take forever. Are there 3D print materials you can use to dial in fit and performance?

    – Is FEA/simulation used much in the boot design process? Do you have whole-boot simulations that can predict how a certain boot will flex?

    • Hey Asquier, 3D printing is not there yet where we can print a boot and ski it (or even just wear it). 3D printing allows us to create the boot 1:1 in terms of its proportions so we can judge is industrial design, geometry, and overall aesthetic but we cannot ski them yet. So, in order to ski a prototype or do fit testing, we have to open a mold, which is very expensive. This means we really have to do our homework before starting the CAD and giving the mold maker the go ahead to make the mold. Ski boot molds are made of aluminum so they can be changed and altered, so we often add thickness or remove thickness in certain areas of the boot once we test the prototypes. But less of that is always better from a cost and mold longevity standpoint. It’s always better to grind away part of the mold (to make the plastic thicker) than it is to weld material into the mold (to make the plastic thinner).

      We use a lot of different simulations in the boot development process, but the ultimate tests are always done with real prototypes. We can run all of the simulations we want (which are indeed helpful) but our experience combined with lab & real world testing always will be the best way to determine what materials will work the best and how the boot will flex.


  8. Regarding flex characteristics of various plastics: Why is the plastic shell material relied on for the desired flex characteristics of a ski boot? Wouldn’t it be advantageous to construct very rigid shell sections or even a 3 dimensional exoskeleton with carbon fiber, for example, with discrete, adjustable/variable forward and lateral flex and/or damping mechanisms?

    One can envisage a boot that is quite light yet perform with adjustable flex curve and damping for differing conditions, temperature ranges, body weight (days skiing with a heavy pack vs. no pack) and/or just sheer personal preference. I’ve had a modicum of success customizing some Dynafit Vulcan ski touring boots and modifying the ski/walk mode mechanism so that in forward flex the upper rear cuff canada arm buckle/lever locking alu knob squeezes a rubber plug to improve progressive forward flex and damping, while retaining rigid, firm rear support. It’s still in rough experiment stage and rubber durability is the limiting factor, a.t.m. With the Vulcan at least, it seems like a large portion of the forward flex stiffness and character is derived from the rigid carbon rear cuff so perhaps the mod is more specific to this boot. An interesting unintended positive benefit due to the increase in general damping when forward pressure was on the shins, was less ski edge chatter/edge grip particularly notable on overnight spring refreeze snow and frozen bulletproof chickenheads during coastal bushwhack ski tours in cold arctic outbreak conditions.

    Having felt that level change improvement in forward flex feel, it got the brain thinking about a different approach to ski boot shell design…just curious if there’s been any experimentation in any truly unconventional approaches to ski boot shell design and if results hold any promise.

    • Hey Swiss, Even with suspension elements added to rigid structure, the base materials still add a lot of damping characteristics on their own. For example, take the same dual suspension mountain bike & suspension elements but make one frame out of aluminum and a second one out of carbon- (when done properly) the carbon version will offer better damping characteristics than the full aluminum one. This example is even more clear in the road bike world, with aluminum bikes vs carbon bikes. But to specifically answer your question- simpler is often better in the ski boot world and adding suspension elements typically adds a level of complexity where things can go more wrong, more quickly. That’s not to say it will not happen in the future (and some really crazy ideas have already been tried) but for now we will continue down our current path.


  9. edit: near the end of second paragraph, the sentence should read: “An interesting unintended positive benefit due to the improvement in general damping and suspension when forward pressure was on the shins: Less ski edge chatter and enhanced/increased edge grip. All testing was performed in backcountry conditions but particularly notable on typically chattery overnight spring refreeze snow and frozen bulletproof chickenheads during coastal bushwhack ski tours/descents in cold arctic outbreak conditions.

  10. Matt,

    Really interesting podcast. As a Mech Engineer and race coach I can relate very well to what you are saying. I was not that familiar with the specific plastics you use in boots, now I am and Thx.

    I modified the flex of my Lange RS boots and was surprised to learn how the rear “spine” or to describe it another way “the part of the lower shell that extends up your achilles and mates with the upper shell, or cuff” was a major design feature in boot stiffness. When I flexed my boots at room temp I could see the lower shell deforming by bowing out and the cuff that is bolted to the spine bending the spine. The entire “flex” of the boot is definitely complicated. I was adding booster straps and wanted to understand boot flex.

    I hope you touch on the role the spine plays and how a booster strap interacts with that in round 3.

    • Hi Scott, As you found out, managing ski boot flex is indeed a big puzzle with lots of pieces in play. The rear spine of the boot (its height, thickness, etc.) definitely plays a big part in the overall for sure. In this area, and in the sides of the shell throat, a lot can be changed to alter a boot’s flex pattern.


  11. PS: I get a lot of requests to answer the question “Do Booster straps go on the outside of the cuff or inside” and why. I prefer mine on the inside because it adds more cushion to my boot tongue. On the outside I didn’t feel like it was doing much.

  12. Is Matt at liberty to discuss the biomechanic science behind zeppa – delta angle / forward lean choices in ski boot design?

    • Hi George, great question and one that could be it’s own podcast (hint: @jonathan ellsworth ;) ). There is a lot of discussion from the various boot brands as to which forward lean angle is the best or most natural and my opinion is there isn’t one that works for everyone. This is why almost all of our ski boots offer (in some way) adjustable forward lean. We think that a 15° forward lean angle combined with a 4° ramp angle is a very good basis for most people, and all of our Hawx boots are delivered in this combo. But due to different ankle ranges of motion, calf sizes, etc. this 15° could be too much or too little for someone and this is why we created our “Power Shift” which is found throughout our Hawx range. This feature allows the boot-fitter to change the forward lean of the boot to 13° or 15° or 17° without performing major surgery to the boot. The same forward lean changes can be made to our Hawx Ultra XTD, Backland, and Redster boots, but in slightly different (but still easy) ways.


  13. Can you speak more to what material or construction elements make a shell heat moldable (i.e. oven moldable)? Why didn’t Atomic/Amer chose to pursue some sort of pressure molding system for their Memory Fit boots similar to Fisher’s Vacuum system? Does oven molding a shell change any of the flex or durability properties of a shell?

    Terrific information on the recent ski boot episodes; Gear:30 is certainly one of my favorite podcasts. I hope to hear an third boot episode and more engineering oriented episodes in general.

    • Hi Rome, I can’t divulge too much when it comes to what makes our plastics more moldable than others, but it is something we look for when talking with our plastic suppliers. But basically, we want plastics that mold easily and also retain their molded shape long term (i.e. not shrinking back over time). Having both of these attributes is what makes our Memory Fit process so successful.

      We didn’t go down the compression molding route because frankly 95% of boot-fitting is about expansion (when starting from a proper fit to begin with). More often than not, most people need more room in their boots, not less room. Or worded another way, more people complain about pain rather than looseness.
      But if you are concerned with having a snug, über-responsive fit, then you should go down the route of spot stretching/grinding rather than molding the entire boot. This will always yield a higher performance fit, but it will also take longer to arrive at a comfortable fit (i.e. more refits at your local boot-fitter).

      With the high-end plastics that we use in our Memory Fit boots, you won’t notice any change in flex or durability in the plastic after heating & molding. With lower-end plastics you would, but we don’t put those in any of our Memory Fit boots.


  14. Great Blister podcast as usual guys and hats off to Matt explaining plastics technologies.

    My question is: with the widespread use of heat moldable shell technologies and plastics, why haven’t ski boot manufacturers explored a completely heat moldable shell (clog & cuff) technology whereby the entire shell (except the sole) is heat molded under positive outside pressure around the foot and using a liner that has a uniform thickness of high density cushioning material in all areas of the foot? I’m thinking this could eliminate most boot fitting work as the boot itself (clog & cuff) could mirror the skiers foot providing a secure, fully customized fit and possibly reduce weight. Modern ski boot shells do not closely resemble the shape of the human foot, requiring the liner to fill voids and/or create pressure points that sub-optimize the fit of almost every ski boot. Thanks

  15. Hi Matt, I accept your explanations about why there isn’t, and may never be, a universal standard for ski boot flex ratings. It’s great that you & Atomic do your own indoor measurements “by feel”, on-hill measurements “by feel”, and robot measurements.

    Are you willing to publish for us all your flex test results for all those boots from different brands? If you don’t have time to polish up a nice presentation of the data, it would be sweet even if you could just hurriedly dump all your data on a webpage in a sloppy format for us. I think nerds like me would enjoy seeing all that data. Thanks.

  16. If a temp stable/linear plastic is selected for a 130 rated flex boot, would that boot cuff not be less flexible at room temp when entering the boot?

    If the flex does not change as much in those materials with temp change then you would have to start out with a stiffer boot to enter at room temp to end up with the same 130 flex of a PU at 10° F

    • Hi Jeff, That is true and is what we experience with a boot like Hawx Ultra. And these boots are a little bit harder to get on at room temperature partly because of this. A Hawx Ultra 130 will fill very similar on snow to how it feels in the store, which can be seen as a benefit too. You won’t have to guess or be surprised by how differently the boot will flex when skiing.

  17. Thanks for publishing the string of Q & A on boots. I see Matt responded his boot of choice to ski in when not testing is the Atomic Redster Club Sport 130. Why? LV boots not snug enough? I’ve been searching for boots for long narrow feet (28.5) and after buying/wearing inside/returning 3 pair of LV (Lange, K2, Tecnica) I’ve also settled on the Redster Club Sport 130. Spend most my time skiing powder and I’m I my 60’s….any regrets on their stiffness?

  18. Hi matt. i have a low volume foot, and the redster club sport is a good fit. my problem, i usually prefer about a 120 flex in head and nordica boots. if a buy atomic club sport boots, should i buy the 110 flex and add a cuff bolt, or but the 130 flex, and remove a cuff bolt? thanks

  19. Hi Matt, I am a 69 YO female pro skier (pro patroller CO. 26 yrs) and I think my current boots are too stiff- Lange RX 110 100mm last 23.5 w/ Intuition Luxury liners. Besides being old, I,m 5’1″ and about 105#. Feel like I have to constantly be thinking to drive my legs forward- never used to think about it; I just went skiing. Read about heel lifts- some pro, some con; although seem to be a better idea for women ( Jennie Thoren, PSIA ), although, they say it softens the flex and less rebound (?). My Langes have a short cuff, aslthough I have very skinny legs and calves. I recently purchased a pair of Nordica Strider W 95 (walk/hike cuff) 23.5. quite comfy w/ my Intuitions, and boy, are they quite a bit softer! I guess my main question is: I s the flex right for you if you can visibly see the the upper cuff flex forward when wearing in a warm indoor 68 degree land enviroment? They feel good; i.e., the flexing feels good and I’m not bottoming them out and, of course, they will be much stiffer in the cold. The cuff is a smidge higher than my Langes, but I don’t feel that they are too tall. Any advice/feedback that you can provide will be greatly appreciated

  20. Yes, I would say that a Lange RX 110 flex is definitely too stiff for you. I am same age as you and my weight is 190 lbs, I ski a 110 or 120 flex boot. And, a 110 flex all mountain boot is plenty stiff for me. Also, keep in mind that flex numbers are not standardized between makers. My experience is that Nordica boots tend to ski a bit stiff for their flex ratings. Also, intuition liners often add stiffness to a boot, although an intuition luxury liner is not suppose to add stiffness. Have you tried skiing the stock strider liner? its a quality liner. I think that you have the correct stiffness with the 95 flex. Keep in mind that boots with the hike function are not as structural sound as a boot with a bolted cuff spine, and it will soften faster as the plastic gets fatigued

  21. 155 lbs. , male, 71 years, skinny lower legs. I’ve been skiing in a Nordica Promachine 120 which fits great but wonder if a 110 flex would be better? Any flex suggestions or Atomic suggestions? Easy to find 130 or 120 boots in shops, not so much 110 for high performance in a 98 width. Thanks

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