Ski Boot Flex With Expert Boot Fitter Brent Amsbury

All ski boots have a flex rating but what exactly does this mean for you and your skiing. In this podcast chat with expert boot fitter Brent Amsbury, we find out some important facts about boot flex.
What changing the bolts in the back of the boot does and why it’s possibly not the best place to make permanent changes to flex from.
How U cuts have a better impact on the ski boot flex range.
That you should always try and get within the right flex rating to begin with. ie dont buy a stiff boot and expect to soften it considerably.
How a skinny leg vs a thicker leg will alter a boots flex.
A look at an innovative ski boot design from UPZ that takes ideas from snowboarding!
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[@howardgee8765]

7 minutes & Brent takes a sip of wine… respect.

[@Landwy1]

A hint to eliminate stiction is to use one sided teflon tape on the interior of the shell. First of all the teflon surface allows you to get the liner into the shell easier. An added benefit is that the stiction is eliminated. If you grind your shells the teflon surface will smooth the ground areas, so the liner doesn't bind or stick to the shell.

[@ab-nz2mm]

you mean teflon tape between the lower clog and upper cuff? on the front of the clog between the cuff?

[@jamesgarcia9123]

thanks tom', the sleleton' down hill carver' logos cool! keep on sking! peace

[@쉐틀]

Very very helpful. My right foot is small than left. I understand better why it’s hard to flex my right boot than left when I tighten my right boot. I use liner sleeve to increase the volume of my right foot instead of tightening the buckles. Thanks again.

[@Bigpictureskiing]

Great to hear!

[@richjones2767]

If you've got slim calves and the boot is too stiff its worth checking how the cuff lower edge meets the top of the boot lower. Because of the amount or wrap needed to get a good cuff fit, the bottom edge can make early contact with the lower shell which takes all the flex range out of the boot. I've had to relieve the cuff lower edge on boots with this issue before.

[@chadridsdale9970]

I love a 120 flex boot but I typically buy a 130 flex to get longer life out of them. 130 feels a bit stiff for a while, then it softens up. I spoke with a boot mfg and was told that if you ski hard carving, moguls etc... that the plastic can really soften after 50 days and be well below the new stiffness. What are your thoughts?

[@bambaramba6509]

Great video Tom! 😊 One question please… Is it normal when flexing a boot that the front part of the cuff (where the tongue is) moves more than the back part of the cuff thus creating a, let’s say, 2cm gap between your calf and the back part of the liner? Thanks a lot! 🙏🏻

[@carterfan80]

This is such an important video for people to watch. A boot fit can be modified long after it's been bought, but most people are really not going to adjust the flex on their boots. I wish I had seen this before I bought my boots. I feel like I made an $800 mistake going up to a 130 Flex. My experience echoed his exactly. The 130 felt pretty good on trail, but I had similar issues in chop , moguls and trees (my favorite terrain). 2 questions 1. Can stiff boots also fatigue your quad muscles more quickly? Since I purchased these boots my legs have been screaming almost every time I've been out on the hill. This really never happened to me in the past. 2. Is it possible to get my Mach 1 130 closer to a 110 Flex? These boots fit pretty well and are in excellent shape. (My top two buckles are usually on the second to last notch. From what it sounds like this could also be an issue) I've lost my confidence in bootfitters. Many are just not that experienced. I bought these boots from an extremely reputable shop in Tahoe and I sat with their top guy. It still didn't work out. I don't think the right questions were asked by the fitters. The fit worked, but the application was wrong. I think they saw a 40 yo dad and assumed I skied groomer. Meanwhile I'm in the air and wood as much as possible!! Is 3 piece cabrio a better fit for my style? Don't think k2/FT will work for my foot shape unfortunately.

[@SportNut1]

I have the exact same issue as you. Bought the same pair of boots last year and facing everything you described including the buckling and application. Did you find the solution?

[@carterfan80]

@SportNut1 I moved the Buckle location on my top two buckles. Now I'm buckling on the 1st or 2nd Notch instead of the last or second to last notch. I also removed one of the rivets from my T drive ( bottom rivet). T drive is the carbon reinforcement on the back of the boot. It does feel like the boot feels a touch softer. I wouldn't say it made a big difference, and it could be all in my head, but I took them out yesterday and they felt pretty good. However it was early season and most of the skiing was on trail. I think it may be worth trying these two modifications. I also tried on the 120 Flex of the Mach One. In the store it really didn't feel any different than the 130. After speaking to a couple of more bootfitters, they both generally agreed that the tecnica Flex is a little bit blocky, and not very progressive. I think I may try something with a different Flex pattern as opposed to a different Flex rating next time. Lange Shadow is supposed to feel much smoother and more Progressive. Hopefully it fits my foot. Maybe next year I'll pick that one up

[@jiyang-chen]

@@carterfan80 Have you tried the Harald Harb method? Unbuckle top buckle and use the power strap. If you have trouble getting forward in the moguls and in rough terrain in transition, try this first before getting a new boot.

[@carterfan80]

@jiyang-chen I have not ....but inwill look into it. Thanks!

[@jbarryclkrec]

What about when we all were using Derby Flex and risers? Talk about increasing the dead spot on a ski. Those mounting points were centimeters in front and behind the binding mounts.

[@gretabird7305]

hopefully this isn't too late to ask - but could shin bang come from too high / too low flex? I've had what I can only call "shin-bang" for around 7 years now. Pain location is on the outside of the shin, on the meaty part. Depending on the boot somewhere between the top of the tongue and first buckle - usually under the power strap. The pain doesn't go away with time. After a whole offseason, I can locate the spot by pressing my thumb into my shin or running the palm of my hand down my leg. During the season I have to be extremely careful about walking in my alpine boots lest I aggravate the shin bang, which will get to the point where I end up crying even with pain pills. Any idea if I should experiment going higher or lower with flex? For reference it started when I was in a 95 Flex Technica that was a bit too high volume and no booster straps. I think I started that season in Japan too loose, and then ratched them down far too tight... Background: - I'm 5'8, F, 145-150 lbs. Long legs, short torso, have relatively low-insertion calves - Had 4 different 2-4 buckle boots in the 7 years. Men's & women's oriented boots. Flex range 95-115 - Have always had custom insoles, always fit by fitters - Boots sized down 2 sizes - Have Booster straps on my alpine boots - Use the thinnest merino socks possible, 3/4 length baselayers - Light park use, nothing crazy - Ski mostly forward-center. Rarely back seat, but the more it hurts, the less I am able to press into the front of the boot - I climb and run and have relatively strong legs. don't have shin splints when running - Using sidas gel inserts helps with alpine pain, but they shift / roll up on the uphill - I constantly (3-10 times / day adjust buckles as the boots warm up / cool down etc.

[@scottb6047]

Based on my experience here are my suggestions: no long underwear inside of boots, I rolled my longies up and just thin smooth sock touching my tongue and much less irritation. Second, the inner flaps of the shell might need to cut down, they can just be in the wrong place for your foot/leg/shin shape. Third, your boot and more specifically your instep tongue area has to fit your leg shape so forward lean force is not concentrated on one spot, instead it is spread out along your entire shin. Atomic boots have changeable tongue thicknesses to get this as dialed as possible. I had similar issue until I got 3/4 baselayer like you and got a boot that fit my shin area and spread the force over the entire tongue. Atomics have 5 different boot widths (over different models) and 3 tongue widths. They seem to lead the industry with boot fit options.

[@ricardobelcredi2475]

The Dalbello flex control is from de 70s, it is Raichle design

[@scottb6047]

Brent certainly knows his stuff and I don't debate anything he says about the UPZ boots, but I think he has over simplified the shorter BSL pressure effect. Sure the pressure (pounds per square inch) go up with a shorter BSL, if you are standing on a flat surface or just considering the surface area of the bottom of the boot. Standing on a ski, you are physically in contact with it only on the AFD in the front and the heel pad in the back. He weighs the same, so the force and pressure load on those pads won't change since their area isn't changing. What does change is the location of the force (weight) from your foot and the ski boot. This will shift "center of pressure" in the fore/aft direction. That will effect how the ski performs. Another thing most likely going on is he left his front binding mount point the same, and adjusted his heel for the shorter BSL. This changes his center of pressure on the ski as well. I suspect that is what is really causing the change in feeling he senses. Sorry, the engineer in me just couldn't let this go unstated when you do such a great tech deep dive, Tom, into all things ski related. Think about going to extremes with BSL, would a short, small pad in the middle of the boot be optimal? or does a really long BSL make a ski perform poorly? I am in a proper fitting 29.5 boot, size 13 feet (clyde), would I ski much better and get better edge grip if I had feet 3 sizes smaller? I don't think so, personally, but I know from experience that when I move my binding mount position around the ski will perform much better as I get closer and further away from the optimum (for me) mount point. And for someone much smaller on the same ski, the mount point is different for sure.

[@ab-nz2mm]

I've dropped from 300 to 290 bsl and the control difference is very noticeable, response is quicker and more direct

[@gregcsc]

I agree that the AFD pad and the boot heel pad are the pressure points. That being said this distance still becomes shorter on top of the ski. A shorter boot binding interface mounted on top of a ski indeed changes the flex pattern. As mentioned, manufacturers have addressed this in every way possible to help get ski flex perfected on their skis. Interesting to note is that the UPZ boot is shortened in the back only. Therefore mounting it on a current ski with the heel moved forward keeps your mass in the same place (the ball of your foot does not move). If you downsize your boot on a current ski and move the heel up you do move your foot forward and boot/foot center moves forward 1/2 the overall change in boot length. Typically about 5mm. This does indeed change the ski a bit by moving your mass forward on the ski. Very complicated subject. What fun!

[@ab-nz2mm]

it doesn't matter if the forefoot doesn't move. Transmitting the same force & torque through a 287mm last is more efficient than through a 295mm last. Everyone can feel it skiing, it's obvious.

[@jdthood]

Suppose the skier leans forward and applies a torque to the ski boot, which is balanced by forces applied to the boot by the binding at the toe and the heel, keeping the boot on the ski. The strength of these forces is inversely proportional to the boot sole length. The shorter the boot sole length, the greater these forces must be in order to keep the boot in place. This is probably what Brent is referring to. The shorter the bsl, the smaller the section of ski to which the torque is applied and, according to Brent, the more he feels his ski bend.

[@L3uX]

Super informative and especially relevant for someone who has been diving into the technicals and fine tuning of boots. Picked up end of season deal on 120S WCR. General fit and everything is amazing, however, I need to crank down to the last few rungs.. not ideal and I’m getting that bind at closer end of ROM. Going to reduce volume near calves. Thanks for the information.

[@L3uX]

Update: put thin spoilers at front and back of liner, holy shit what a difference. My calves are average in size, but the extra space taken up is huge bc and I’m no longer at the end rungs.

[@Bigpictureskiing]

Awesome!! 😎

[@Coconut_54]

Very informative, I learned a lot 👍

[@Bigpictureskiing]

Glad to hear it!

[@africanjew1596]

Wouldn’t anyones boot being locked into any binding change the flex of the ski anyway? I know the mounting point of the pivots could be superior to others but this is all interesting stuff so thanks.

[@scottcursons1036]

Really interesting. Am a chicken leg lightweight. Sticktion definitely defines what I face. I'll try not doing my upper buckle up so tight in the future. What are your thoughts on the power strap? I swapped out the Fisher RC4 strap for an elastic booster strap as recommended by my boot tech.

[@Steph-iw3hr]

Hi guys Brent , I have same BOA pro Fischer like you I need to get more flex as I am pushed back with those boots I removed one screw only behind and much better ; easy to ski and make turns What about grinding a bit the flap of the cuff to give more room to flex ? Internal one is touching the lower shell . Tom knows what I mean 😊 I live in France , thanks to give me a boot fitter you might recommend Thanks

[@DCU88]

I have always wondered why manufactures do not make a full featured boots in the 120 range, or maybe I should say I have always been annoyed by manufactures not having a 120 full featured boot

[@73gratia]

Yes. A person weighting 90kg or more.. compared to a person at 60kg with same strength and biodynamic angles.. will theoretically need different flex; right? Choosing a liner that is soft or hard flexed also contributes to the responce/flex of the boot. Its complex.

[@gregcsc]

Yes. Super annoying. The smaller skier is punished in quality by buying a softer boot. I blame it on marketing. The Fischer Podium GT series did offer the same features and materials for a few years. The 130 flex seemed priced right. The 110 flex was much more expensive than competitors boots and didn’t sell well. What a great boot that was in 110! Maybe the answer is to make smaller boot sizes softer in a 130 flex? Therefore a 140lb 5’5” man and a 210lb 6’2” man had flex and features keyed to advanced skiers by size of boot? Just thinking here. Maybe that is already done to an extent? Brent?

[@Stomperproductions]

Can you explain why softer boots for pow? Why not the same stiffness but more forward lean instead? That way you have the precision but are already in the optimal position without having to flex into it (can you really ever flex in properly deep pow??)

[@samwils1]

We want the full range of motion to use the fore aft. We want to be flexed forward before we enter the fall line in order to get the rebound aft as we accelerate in the fall line. More forward lean does not let you move your center of mass aft to the center of your foot as you accelerate.

[@Windy1867]

How would you rate the Daleboot infinitesimal adjustable flex system through their screw/spring design?

[@eagsalazar]

Sticktion can also be minimized by just loosening the tightness of your cuff buckles. I honestly don't know why, with a pair of boots that fit reasonably well, you need to cinch down your cuff buckles unless the entire point is to increase stiffness.

[@Bigpictureskiing]

Yep this is how approach it too.

[@kuanjuliu]

@4:39 Brent's description of the "bounce" on that Fischer boot is reminiscent of Eric Schlopy saying Fischer's vacuum plastic was far too reactive for him? I wonder how Eric's impressions of Fischer boots might change if Brent U-modified one for him. Or are the plastics' physical properties too correlated to the subtle behavior of boot flex to be modified at all?

[@Bigpictureskiing]

That would be a cool experiment 🔬

[@carterfan80]

What is your opinion on the Tecnica Mach 1 130's stiffness? (Compared to some other brands 130s)

[@johnward654]

They are equivalent to a 70 flex boot in my opinion

[@ArpadDr.Hamori]

Have Mach 1 130 as well. Sized properly. They are a way softer, than they have to be as a 130 flex.

[@Landwy1]

P.S. Forgot to add that boot boards should also have one sideed teflon tape put on them as well.

[@robinjohnston7376]

why?

[@wjames2000]

There is something about skiers and their 911 and I love mine.

[@jdthood]

According to Tom (14:00), "John" has narrow legs and can flex the boot; "Jim" with thicker legs has trouble flexing the boot. Brent explains using the concept of "stiction" (i.e., static friction) why people with narrow legs have trouble flexing boots. According to this, John should be the one that has trouble flexing the boot. Did you guys get John and Jim confused or what? EDIT: My reply to samwils1 got rejected so I will say here that I agree, Brent's point is clear. But the confusion of John and Jim is important because one might actually expect the lower volume foot to experience a softer flex pattern than a higher volume foot. (In my reply I provided a reference for this, but this may be the reason the comment got rejected.) So is Brent providing an explanation for what happens or for the opposite of what happens? Confusing.

[@samwils1]

Brent seemed to be clear that a person with narrow legs would have more “stiction” from the same boot since more of the boot materials would be in contact. Tom seems to have misspoken or had the wrong assumption initially when he framed the question.

[@beninstone1824]

The thickness of the material does not reflect its mass or weight and having a shorter sole length has no affect on the psi pushing on the ski, the only waay you can increase the psi on the ski is to use a binding with a smaller foot print.

[@fmbjmf]

That is precisely what they were saying. The boot they were discussing is built to have a smaller footprint and would require the heel and toe pieces to be mounted closer together.

[@beninstone1824]

Moving them closer doesn't increase the pressure just where the pressure is applied. You would need to make the surface area of the heel and toe piece smaller to increase then psi.

[@fmbjmf]

@@beninstone1824 Hmmm. That's an interesting POV I had to think about it for a minute, but I think if we look at it from Tom's example you can consider the entire sole of the boot to be pressing on the area defined by the front and rear contact points. The ski will bend from the contact point of the toe forward, and from the contact point of the heal rearward. The closer those two points are together, the more the ski will bend.

[@beninstone1824]

Correct but he stated it iincreased the psi on the ski which it doesn't.

[@beninstone1824]

Also bindings do have some built in give to them to allow the ski to flex under the boot

[@johannorberg4174]

Unless you are close to zero in FIS points I cant take it anywhere serious what you think you feel. Getting more edgehold because of 10mm BSL with most likely the exact same binding mount seems like wishful thinking and The placebo of new gear is probably a much larger factor. Come back with silly tech talk after you do the Schladming SL or the Streif.

[@gregcsc]

I think if you skied a look pivot and another binding on the same ski you would notice the difference. I do and my FIS points have never diminished…

[@johannorberg4174]

@@gregcsc that is not the same question though is it? The question is pretty much all else equal would 10mm shorter bsl matter? Answer is not really. Most bindings are mounted on a plate either way to the plate design is way more influential

[@abdulullah587]

Too long of a video with too much technicality’s not for people getting into skiing and beginners. Drags on too much gets boring confusing and tiring

[@Bigpictureskiing]

No not for people getting into skiing, m,y audience is ski addicts!

[@joelknutson9971]

@@Bigpictureskiing Great discussion about the nuts and bolts of a good fit. And a good boot fit makes one's best skiing happen, in my experience.

[@fox156]

Disagree, we (skiing addicts) want all the details, caveats, the whole story!

[@ricardobelcredi2475]

Great video I am an Instructor with 45 years of experience, and clarify me a lot of information

[@ricardobelcredi2475]

The Dalbello Flex control is from de 70s, it was Raichle