Ski boot flex posts

THE SKI BOOT FLEX INDEX INSTABILITY PROBLEM

It has been known for decades that an unbalanced moment of force or torque will be present on the outside ski when the center of pressure of the load applied to the ski by a skier is acting along the center of the transverse axis of the ski where it is offset from GRF acting along the inside edge. Ron LeMaster acknowledges the existence of an unbalanced moment of force on the ouside ski in both The Skier’s Edge and Ultimate Skiing (Edging the skis). LeMaster states in Ultimate Skiing;

The force on the snow is offset from the center of the skier’s and creates a torque on it that tries to flatten the ski.

Ron didn’t get the mechanics right. But he correctly shows the unbalanced torque acting on the ankle joint. LeMaster tries to rationalize that ice skates are easy to cut clean arcs into ice with because the blade is located under the center of the ankle. While this is correct, ice skaters and especially hockey players employ the Two Stage Heel-Forefoot Rocker to impulse load the skate for acceleration. Hockey players refer to this as kick.

In his comment to my post, OUTSIDE SKI BALANCE BASICS: STEP-BY-STEP, Robert Colborne said:

…..In the absence of this internal rotation movement, the center of pressure remains somewhere in the middle of the forefoot, which is some distance from the medial edge of the ski, where it is needed.

The load or weight of COM is transferred to distal tibia that forms the ankle joint. This is the lower aspect of the central load-bearing axis that transfers the load W from COM to the foot. What happens after that depends on the biomechanics. But the force will tend to be applied on the proximate center of the stance foot. This is a significant problem in skiing, (one that LeMaster doesn’t offer a solution for) when the ski is on edge and there is air under the body of the ski. The unbalanced torques will move up the vertical column where they will manifest at the knee against a well stabilized femur.

But this unbalanced torque creates another problem, one that is described in a paper published in 2005 by two Italian engineers (1.) that describes how this load deforms the base of the boot shell.

The Italian study found large amounts of deformation at mean loads of up to 164% body weight were measured on the outer ski during turning. The paper suggests that the ski boot flex index is really a distortion index for the boot shell. The lower the flex index, the greater the distortion potential.

For the ski-boot – sole joint the main problem is not material failure, but large amounts of local deformation that can affect the efficiency of the locking system and the stiffness of the overall system.

Values of drift angle of some degree (>2-3°) cannot be accepted, even for a small period of time, because it results in a direct decrease of the incidence of the ski with the ground.

My post GS AND KNEE INJURIES – CONNECTING THE DOTS (2.) cites studies that found that knee injuries are highest in GS in the shortest radius turns where peak transient forces are highest.

As shown in Figure 2a FR (sum of centrifugal and weight forces) and F GROUND (ground reaction force) are not acting on the same axis thus generating a moment MGR that causes a deformation of the ski-boot-sole system (Figure 2b) leading to a rotation of the ground reaction force direction. The final effect is to reduce the centripetal reaction force of the ground, causing the skier to drift to the outside of the turn (R decreases, causing the drift event).

An imperfect condition of the ski slope will emphasize this problem, leading to difficulties maintaining constant turning radius and optimal trajectory. The use of SGS ski-boot in competitions requires a particular focus on this aspect due to the larger loads that can be produced during races.

I have added a sketch showing that the moment arm M R created by the offset between the F Ground and F R is in the plane of the base of the ski where it results in an Inversion-lateral rotation torque.

The importance of sole stiffness is demonstrated with a simplified skier model…..…ski boot torsional stiffness with respect to ski longitudinal axis in particular is very important as it deeply influences the performance of the skier during turning…. A passage over a bump or a hollow may generate a sudden change in ground reaction force that may lead to a rapid change in the drift angle delta. The ski boot must be as stiff as possible going from the lower part of the boot to the ski (i.e. lower shell-joint-sole system)

As explained in the method section using the simplified model, values of some degree cannot be accepted, even for a small period of time, because the skier stability and equilibrium could be seriously compromised especially when the radius of curvature is small. A non perfect condition of the ski slope will emphasize the problem, leading to big difficulties for maintaining constant turning radius and optimal trajectory.

This excellent paper by the two Italian engineers concludes with the following statements:

Authors pushed forward the integration of experiments and modeling on ski-boots that will lead to a design environment in which the optimal compromise between stiffness and comfort can be reached.

The possibility of measuring accurately the skier kinematics on the ski slope, not addressed in the presented study, could represent a further step in the understanding of skiing dynamics and thus could provide even more insightful ideas for the ski-boot design process.

I first recognized the shell deformation, boot board instability issue in 1980, at which time I started integrating rigid structural boot boots into the bases of boot shells I prepared for racers. The improvement in ski control and balance was significant. The instability of  boot boards associated with shell/sole deformation with 2 to 3 degrees of drift at modest loads of up to 164% body weight has significant implications for footbeds.


  1. AN INNOVATIVE SKI-BOOT: DESIGN, NUMERICAL SIMULATIONS AND TESTING – Stefano Corazza 􀀍 and Claudio Cobelli Department of Information Engineering – University of Padova, Italy – Published (online): 01 September 2005 – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3887325/
  2. http://wp.me/p3vZhu-zx

UPDATE ON THE FREEMOTION SKI BOOT PROJECT

On December 9, 2016,  Simon Zachhuber from Austria posted the following comment on my blog.

Dear David! While researching for a university-project (FH-Salzburg, Austria) I discovered your blog! Clearly you are an expert on ski boots, and I thought maybe you can provide some feedback! At least it’s worth a try

I’m a product designer and our task is to analyze a specific ski boot and try to figure out ways to improve it! The ski boot we work on has a new concept of dealing with the occuring forces. Instead of providing stability and flex with plastic shells, the forces are applied to a metal spring. So the area of the tong and the shin doesn’t have to be covered with hard plastic, but with leather or fabric. You can see the concept on their website http://www.freemotion.cc

I am currently working on a project as a student for the FH Salzburg, Austria! We have the task of analyzing a specific ski boot and implementing improvements! Since this is a short project and we are not experts on ski boots, I wanted to ask you for profound feedback on this boot! It can be seen on the website http://www.freemotion.cc

The boot’s concept is that instead of having a hardplastic-shell to deal with the forces, it uses a metal spring that starts at the forefoot and goes through the ankle-axis all the way back over the heel! We already tested it and were quite surprised, how well it worked, but maybe you can add a few thoughts that come to your mind when you check the boot?
I know, it’s very hard to give feedback without having the boot to test, but maybe you can still give some feedback? Thank you very much!!!

With greetings from Austria
Simon Zachhuber


I replied to Simon by email

Dear Simon,

Greetings from Whistler, BC Canada.
I would be glad to assist you in any way I can with your project.

Do you know of the work of Dr. Martin Pfeiffer of the University of Salzburg? He was committed to the development of a ski boot designed along anatomical principles. Two Canadian radiologists of Austrian descent made me aware of Dr. Pfeiffer’s work in 1988 when they gave me a copy of Der Schu Im Sport. Part 6, The Ski Boot, features a chapter by Dr. Pfeiffer called Kinematics of the Foot in the Ski Boot. Dr. Martin Pfeiffer was a source of valuable knowledge that influenced my work. I had personal communication with him in the early ‘90s.

Dr Pfeiffer concluded his chapter by stating, “This goal (a ski boot designed along anatomical principles) has not yet been achieved”. I do not know whether he is still with us. But I would appreciate you recognizing his contribution to skiing by making his vision a reality and by recognizing his work.

Best regards,
David


Due to the urgency of Simon’s deadline (which was fortunately later extended to January 17, 2017), I made providing my assistance a priority with the following series of posts.

NEW AUSTRIAN SKI BOOT: THE FREEMOTION – http://wp.me/p3vZhu-1CB

SIMON RESPONDS TO MY PRELIMINARY OBSERVATIONS ON THE FREEMOTION SKI BOOT – http://wp.me/p3vZhu-1D9

POTENTIAL SOLUTIONS FOR THE FREEMOTION SKI BOOT – http://wp.me/p3vZhu-1Dt

THE FREEMOTION SKI BOOT:  FLEXURAL BEHAVIOUR – http://wp.me/p3vZhu-1FB

FREEMOTION FLEX CURVE OPTIONS – PART 1 – http://wp.me/p3vZhu-1Gj

FREEMOTION FLEX CURVE OPTIONS – PART 2 – http://wp.me/p3vZhu-1Gy

FREEMOTION FLEX CURVE OPTIONS – PART 3 – http://wp.me/p3vZhu-1GI


On January 18, 2017, the following video was posted on YouTube on the FreeMotion site with the text below translated from German.

Kooperation Freemotion mit der FH Kuchl

The results of our successful cooperation with FH Kuchl are here! We are delighted about the great input of the students and professors. We are already working on the implementation of the ideas. Once again a big thank you to the students #Feelgoodskiboot

 


I sincerely hope that my efforts have assisted the students of FH Kuchl. The design of the ski boot needs fresh young thinking and a new direction with design based on the functional requirements of the human system. The most satisfying aspect of the design exercise at FH Kuchl was to see the large female component with students Marlene Arabjan and Evelyn Obermuller taking an active role.

Gut gemacht Simon  Zachhuber und die anderen studenten von FH Kuchl!

Viel Gluck und Best Wunsche

FREEMOTION FLEX CURVE OPTIONS – PART 3

A plausible reason why the FreeMotion boot has failed to gain acceptance is that the spring flex concept has not been tested in a medium that allows all the critical variables discussed in my last post to be adjusted to the individual skier. An essential prerequisite to this process is a validated physiological model; one that explains the physiologic processes of skier balance as well as the mechanics and biomechanics associated with the 3-dimensional physical environment of the activity.

While the Birdcage allowed adjustments specific to individual skier requirements to be made and data to be acquired that showed the effects on skier performance, in particular, skier balance, insufficient time and budgetary constraints limited the study of flex curve requirements.

Figures 52 and 55 A through D below show the flex assembly for the Birdcage. The details are disclosed in the section of the patent associated with aforementioned figures.

fig-52fig-55

Figure 56 below shows the different components of the Birdcage shaft adjustment and flex system resistance curve. The details are disclosed in the section of the patent associated with Figure 56.

fig-56

The photo below shows the actual flex system on the rear of the Birdcage. The nut under the rear stop lug allows for adjustment of the forward lean angle of the shaft. In this photo, the shaft is in the rearmost (hard stop) position with the lug seated against the rear stop nut on the spine.

img_1664

The photo below shows the gap between the lug and the washer-cyclinder assembly that presses on the coil spring after a pre-set number of degrees of forward flexion.

Experiments were done with several types of coil springs, including compound coil springs where a small coil spring is nested inside a larger spring. The rubber donut at the top of the assembly prevents the ‘brick wall’ deceleration effect that occurs when the coil spring is fully compressed. Different types and assemblies of rubber donuts were tried in conjunction with different coil spring configurations.

img_1666

In the photo below, the shaft of the Birdcage has rotated forward through the constant low resistance travel segment and is about to compress the coil spring assembly. This position is associated with isometric contraction of the triceps surace (calf muscles) in the SR Stance.

img_1665

The photo below shows the coil spring in the initial stages of compression.

img_1667

The effect of flexural qualities of the shaft of a ski boot on skier function and balance require structured studies conducted with instrumentation and data acquisition. Fortunately, papers on such studies have recently been published. I will discuss them in my next post.

FREEMOTION FLEX CURVE OPTIONS – PART 2

In The Shoe in Sport, the detached, objective assessment of the conventional ski boot articulated in Part 6, The Ski Boot, by preeminent authorities on biomechanics and safety served to support my conclusions of the past 10 years that little of what forms the basis of knowledge in skiing can be defended. This made me acutely aware of the dangers of going forward with a concept for a new ski boot that could not defended with a validated hypothesis based on principles of applied science and especially data captured during actual ski maneuvers that could be readily and consistently reproduced by others.

Key critical comments made in Part 6 of The Shoe in Sport, (first published in 1987 in Germany as Der Schuh im Sport – ISNB 0-8151-7814-X ) follow below as a prelude to the discussion of the Birdcage format as a basis for flex options for the FreeMotion ski boot and ski boots in general.


Biomechanical Considerations of the Ski Boot (Alpine)

Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland

  • Correct flexibility (of the boot shaft) is not the whole answer. The functional anatomy of the ankle joint must also be an important consideration. If the axis of flexion of the ankle joint is not precisely in line with the axis of flexion of the shaft, an unfavorable lever ratio will ensue, and the advantages of flexibility will be lost.
  • If flexion resistance stays the same over the entire range of flexion of the ski boot, the resulting flexion on the tibia will be decreased. With respect to the safety of the knee, however, this is a very poor solution. The increasing stiffness of the flexion joint of the boot decreases the ability of the ankle to compensate for the load and places the entire load on the knee.
  • Improvements in the load acting on the ankle make it biomechanically very likely that the problems arising in the rather delicate knee joint will increase.
  • From a technical (skiing) point of view, the ski boot must represent an interface between the human body and the ski. This implies first of all an exchange of steering function, i.e., the skier must be able to steer as well as possible, but must also have a direct (neural) feedback from the ski and from the ground (snow). In this way, the skier can adapt to the requirements of the skiing surface and snow conditions. These conditions can be met if the height, stiffness, angle and functions (rotational axes, ankle joint (AJ)/shaft) of the shaft are adapted, as well as possible to the individual skier.
  • The modern ski boot must be designed from a functional point of view, i.e., the design must take into consideration the realities of functional anatomy (axes etc.).
  • It (the design) should not make compromises at the expense of other joints (length of shaft, flexibility and positioning).
  • It (the ski boot) must represent the ideal connecting link between man and ski (steering and feedback).

Kinematics of the Foot in the Ski Boot – Professor Dr. M. Pfeiffer – Institute for the Athletic Science, University of Salzburg, Salzburg, Austria

  • The shaft of the boot should provide the leg with good support, but not with great resistance for about two-thirds of the possible arc, i.e., (12 degrees) 20 to 22 degrees. Up to that point, the normal, physiologic function of the ankle should not be impeded.
  • Because of its effects on the foot, the arc described by the shaft is divided into a “lead segment” and into a later “lever segment”.
  • Forward sliding of the foot should not be possible. There should also be no loss of contact of the sole and no decrease in the “feel”.
  • Previous misconceptions concerning its (the boot shafts) role in absorbing energy must be replaced by the realization that shaft pressure generates impulses affecting the motion patterns of the upper body, which in turn profoundly affect acceleration and balance.
  • Correct positioning of the foot is more important than forced constraint and “squeezing” the foot. This will prevent the misuse of the ligaments and weakening, particularly of the fibular musculature and ligaments. (This will also explain why even competitive skiers can suffer ankle sprains while engaged in light athletics or even just in everyday activities.)
  • When the lateral stability of the shaft (the leg) is properly maintained, the forces acting in the sagittal direction should not be merely passive but should be the result of active muscle participation and tonic muscular tension. If muscular function is inhibited in the ankle area, greater loads will be placed on the knee.
  • The ski boot and it’s shaft must be adapted to the technical skill of the skier, and the technical skills of the skier must be adapted to the preexisting biomechanical functions of the leg and the foot.
  • The medical requirements with respect to sports should not be construed as criticism of the boot industry. It is hoped that they are contribution to the development of a ski boot designed along anatomical principles. This goal has not yet been achieved.

Ski-Specific Injuries and Overload Problems – Orthopedic Design of the Ski Boot –  Dr. med. H.W. Bar, Orthopedics-Sportsmedicine, member of GOTS, Murnau, West Germany

  • Few forms of athletics place as high demands on the footwear used in their performance as alpine skiing. It (the ski boot) functions as a connecting link between the binding and the body and performs a series of difficult complex tasks.
  • Investigations by Pfeiffer have shown that the foot maintains some spontaneous mobility in the ski boot. Thus the total immobilization by foam injection or compression by tight buckles are unphysiologic.
  • The boot must assure freedom of mobility to the toes. This is accomplished by having a large enough inner shoe. Only in the case of major congenital or post traumatic deformities should foam injection with elastic plastic materials be used to provide a satisfactory fixation of the foot in the boot.

Sports Medical Criteria of the Alpine Ski Boot – W Hauser, P. Schaff, Technical Surveillance Association, Munich, West Germany

  • Many alpine skiers have insufficient mobility in their knees and ankle. The range of motion, particularly in the ankles, is much too small. This results in a static, stiff run. It does not correspond at all to the ideal of a wide range of mobility in the area of the knee and ankle, which was proposed and taught during early alpine ski lessons. Even the best diadactic (patronizing) methodology is not always successful in imparting to the student the full range of motion. The lack of proper technique seem so often is not due to a lack of ability, but to an unsatisfactory functional configuration of the shaft in so many ski boots. This is particularly true in models designed for children, adolescent and women.
  • In the future, ski boots will be designed rationally and according to the increasing requirements of the ski performance target groups.

The comment Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland 30 years ago has turned out to be prescient:

  • Improvements in the load acting on the ankle make it biomechanically very likely that the problems arising in the rather delicate knee joint will increase.

Unfortunately, Stussi’s warning does not appear to have been heeded.

In an article called Getting Serious About Skier’s Knee (Ski Tech, October 1993), Andy Bigford states, “Horror stories about ACL injuries are a dime a dozen” and “Victims tell of the cost involved (the whole package can run to $50,00 per injury), the difficulties of rehab, the lost work time and the fear of never being able to ski again”. Bigford goes on to point out that not everyone is convinced that the stiff boot-aggressive ski combination is totally at fault; that ACL injuries in the late ’80s and early ’90s came when softer (flex) and ‘more forgiving’ rear entry boots were popular.

Bigford ends his article by stating that the (knee) problem needs to be solved without scarring away skiers, but “If something isn’t done, they (skiers) will have plenty of reason to be scared”.

More than 20 years later, the cover heading What’s New With Knees (December 2016 Ski Canada magazine) is the lead-in to a feature article called Tear and Repair. The article header  states, “With knee injuries so commonplace, especially among skiers, the medical world is constantly updating procedures and surgical techniques for the big fix. The take away message from Tear and Repair  appears to be that the solution to knee injuries is not addressing the cause, it is repairing the resulting damage.

In the same magazine, a comment under Short Turns (Save Your Knees) states: “If you haven’t had a serious knee injury, you likely know someone who has”.  The take away message appears to be that knee injuries are part of skiing. Get used to it!

If my next post, I will discuss how the knowledge gleaned from The Shoe In Sport influenced my thinking on the design criteria for a new ski boot, in particular, shaft configuration and resistance to the rotation while creating an awareness of the need to be able to defend my design criteria with principles of applied science data acquired during actual ski maneuvers.

 

FREEMOTION FLEX CURVE OPTIONS – PART 1

For those who are new to the Skier’s Manifesto, welcome.

I became involved in an effort to design a new ski boot at the request of Crazy Canuck, Steve Podborski. Steve was (and I think still is) the only non-European to win the World Cup Downhill title. Steve also won a bronze medal at the 1980 Lake Placid Olympics. After he won the World Cup Downhill title he asked me if I could design a boot that would do for every skier what the boots I had built from components that used a new fit technology I had invented did for him. I saw this as an opportunity to advance skiing. I accepted.

I did not take on this project to make money. I took it on because I saw problems with equipment, especially ski boots, that were significantly affecting the enjoyment of the sport for the majority of skiers. I wanted to try and solve these problems and contribute to the betterment of a great sport.

In 1978, I started down the road to try and improve the ski boot by working with world class racers such as Steve Podborski. Today, my focus and mission remains unchanged. I am still working with skiers and racers and I am still learning. When Simon Zucchuber asked for my assistance with the Freemotion ski boot project, I did not hesitate to offer my assistance.

You can learn more about me under the HOME heading on the opening page.


Over the past week, I spent time going through my US Patent 5,265,350 trying to recall the events that influenced my thinking.

The first patent awarded to me was US 4,534,122. It was filed on Dec 1, 1983 and issued by the US Patent Office on August 13, 1985. The patent is for an innovative in-boot fit system that constrains the forefoot without obstructing the glide path of the ankle joint.

When I invented the fit system disclosed in the patent, I knew I was headed in the right direction. But I also knew that I did not have a full understanding, let alone a solution, for the flexural aspect of the ski boot. Between 1973 and 1983 I had come to understand that boot flex was affected by material stiffness, temperature and closure tension. But two of the biggest issues were that buckle boots flexed by deformation of what is a U-shaped tube (which made flex unpredictable) and the angle of the rear cuff that had minimal or no adjustment. This meant that the angle of the shank of a skier was determined by the mass of the calf muscle at the top of the shaft. Attempts by others to address flex had typically focussed on one issue at the expense of another or even caused new problems.

Devising a system for boot flex that would solve all issues and especially one that did not rely on shell deformation led me to the exo-skeleton format around 1987. A patent for this format was not filed until April 25, 1989 because of the time it took to work with lawyers and try and figure out how to define and describe the technology so it would meet the novel requirement for a patent.

Figure 1 below is from the initial patent filing for the patent that was eventually issued on November 30, 1993 as US 5,265,350. This figure and the material in the application established a priority date for the length of the eventual patent. This initial patent was later abandoned in favour of newer iterations.  All of the ‘improvements’ are described in the patent which can found by searching the patent number US 5,265,350 in the US Patent or Google Patent web sites.

 

fig-1

The device is a exo-skeleton arrangement with a tube for the leg attached to the base by arms on each side that rotate about an axis (23) on the base structure (11). A single wide band secures the front portion of the tube (shaft) about the leg of a user to the rear portion.

A bendable spring (40) is affixed to the base on the outside (lateral aspect) of the base (11). An adjustment (42) allows the spring to be moved closer or further away from the two contact arms (43 and 46). The contact arms slide up and down in a channel on the arms so as to allow for an amount of low consistent low resistance cuff rotation before higher resistance is introduced or allow spring resistance to be introduced earlier.  Contact arm (46) can be adjusted up or down the arm so as to change the resistance curve.

An adjustment means (generally shown at 30) allows the angle of the cuff to be adjusted. This enables a user to obtain the correct forward lean angle for the shank which I knew by then was critical (see the posts on SR Stance).

Figure 1 is a rough or what is called a schematic concept of the exo-skeleton system. The next step was to try and come up with a design with aesthetic qualities. Figure 5, below, shows the exo-skeleton of Figure 1 with a soft liner. The attachment for flex spring has been incorporated into the axis journals for the arms of the exo-skeleton.

fig-5

About 1989, I was approached by a husband and wife radiology team. They taught radiology at a university. They were both keen skiers. They heard about my project to develop a ski boot based on anatomical principles and offered their assistance. They presented me with a copy of a recently published book called The Shoe in Sport – Supported by the Orthopedic/Traumatologic Society for Sports Medicine (OTS).  The Shoe in Sport was initially been published in Germany in 1987 as Der Schu Im Sport. They were of German background. That was how they knew about the book.

I found the knowledge contained in The Shoe in Sport invaluable, especially the article the ‘Kinematics of  the Foot in the Ski Boot’ by Professor Dr, M. Pfeiffer of the Institute for Athletic Sciences at the University of Salzburg, Salzburg, Austria. The information contained in The Shoe in Sport helped crystallize many of the issues I had been struggling with and profoundly influenced the thinking behind the Birdcage and the Birdcage experients conducted in the July of 1991 on Whistler Mountain’s glacier.

For the first time, I felt I was on solid ground with my thinking. I was ready to go boldly forward and break new ground.

… to be continued in Part 2.

 

 

 

 

 

 

 

THE FREEMOTION SKI BOOT: FLEXURAL BEHAVIOUR

The innovative aspect of the FreeMotion ski boot appears to be a U-shaped spring flex-system for the shaft of the boot that is minimally affected by temperature and buckle closure tension and an exo skeleton shaft system that does not deform significantly under load. The arms of the U-spring running along both sides of the shell lower appear to act like rails in transferring force applied to the shaft to the shovel of a ski. Given the stated importance of ski boot flex and the universally accepted position that flexing the shaft of a boot applies force to the shovel of a ski to make a ski turn, the FreeMotion should have been hailed as a breakthrough technology and widely embraced. But this does not appear to be the case.

Simon’s request for assistance, in conjunction with a recently published paper on the  flexural behaviour of ski boots has provided an opportunity to explore this aspect in detail.

The design of ski and ski touring boots should consider three key elements: performance, safety and comfort. The performance of a ski boot is often equated with its (forward) flex index (my emphasis added). A parameter used by nearly every manufacturer ranging from 50 (soft) up to 150 (very stiff). Despite the widespread usage (of the flex index) there is no regulation on how to measure these stiffness indices and it is up to the manufacturer to test and rate their models. 

Whereas industry and special interest magazines  regularly perform and publish ski performance tests, very few systematically derived knowledge is available on ski boots. This is surprising as ski and boot are influencing each other’s mechanical behaviour and should therefore be treated as a system (my emphasis added).

Flexural behavior of ski boots under realistic loads – The concept of an improved test method – Michael Knye, Timo Grill, Veit Senner

  • Technical University of Munich (TUM), Sport Equipment and Materials, Boltzmannstraße 15, D-85748 Garching, Germany – 11th conference of the International Sports Engineering Association, ISEA 2016

The authors of the above cited paper note that usually boots with high flex indices are used by more experienced and skilled skiers whereas for beginners softer boots are recommended.

Based on what we have been told for decades, this makes perfect sense. More experienced and skilled skiers have stronger muscles and are more precise than beginners. Stiff boots allow more experienced and skilled skiers to make better turns because stiff boots enable them to apply more pressure to the shovel of a ski to start it turning.

Studies cited by the authors have shown high activation levels for the m. triceps surea and m. gastrocnemius were measured for various skiing situations.

The triceps surae (aka the calf muscle) is a 3-headed muscle comprised of the m. soleus and the m. gastrocnemius. These two muscles form the major part of the muscles of the (lower leg). The two muscles share the Achilles tendon that inserts into the calcaneus.

The graphics below show the m. soleus and the m. gastrocnemius.

triceps-surae

Based on the studies cited by the authors, it seems obvious that the m. soleus and the m. gastrocnemius muscles are instrumental in flexing the shaft of a ski boot.

But then the authors cite an apparent paradox when they state:

Muscular activity of the lower leg is also affected by the boots flexural behavior showing a higher activation with softer boots.

Why would the muscles of the triceps surae show a higher activation with softer boots than stiffer boots? In the current paradigm, this doesn’t make any sense. If the muscles of the triceps surae are responsible for flexing the shaft of a ski boot, shouldn’t they show a higher activation with stiff boots than with soft boots?

One explanation for the apparent paradox is that the paradigm of boot flex is just plain wrong.

…. to be continued.

POTENTIAL SOLUTIONS FOR THE FREEMOTION SKI BOOT

The  3 main features that appear to be limiting the performance of the FreeMotion boot are the lack of a hard rear stop and forward lean adjustment for the exo cuff and an adjustment means for the U spring that would allow a range of low resistance rotation of the cuff before the resistance provided by the U spring is introduced. It is also important to have a monoplanar boot board with a ramp angle in the order of 2.5 to 2.7 degrees or, preferably, the ability to substitute boot boards with different ramp angles to allow experimentation to determine the optimal angle or range.

In researching the history of the FreeMotion ski boot, it appears to have evolved out of the Kneissl Rail soft boot introduced around 2002. Perhaps Simon can confirm this.

The Kneissl Rail is shown in the graphic below.kneissl-rail

Like the Freemotion, the Kneissl Rail does not appear to have a hard rear stop for the exo cuff. But the Rail appears to have a  dial on the spine that suggests some sort of adjustment for the U spring that might permit a range of low resistance rotation before it is introduced or perhaps a tension adjustment for the u spring. The Rail also has a constraint plate over the instep that is secured with a buckle, a feature the FreeMotion shown below in Figure 1 from the patent, lacks.fig-1Since the investment in prototypes and production molds is substantial, aesthetic considerations and production costs typically take priority over functional considerations.

Because of this, my preferred option is to use purely functional, low cost prototypes that are easily modified as research vehicles to prove out the functional aspects of a technology. Prototypes such as the Birdcage (shown below) can be designed and fabricated at minimal cost compared to the costs of sophisticated aesthetic and production prototypes.

birdcage

The photo below is of an early research prototype called the Lab Rat that was developed for a recent project. The open architecture of Birdcage and Lab Rat formats permit instrumentation to be incorporated and visible observation of the effects of the technology on the foot and leg to be conducted, something that is difficult, if not impossible in aesthetic prototypes, especially ski boots.

1

The photo below shows a second generation version of the Lab Rat call The Fit. It is more compact and much lighter than the Lab Rat.

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The 4 photos below show the modifications I made to address structural inadequacies and interface issues of the first generation mold generated post Birdcage prototype called the P1. The instep was reinforced with a formed stainless structure and the internal plastic components were replaced with reinforced fibreglass and Tig welded stainless steel components. While these modifications did not lead to a marketable prototype, they validated the conclusions of the analysis that explained why the prototype failed to deliver the expected performance.

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In my next post, I will offer some suggestions for potential grafted-on modifications for the FreeMotion ski boot that may clarify the options required to address the issues that I flagged that were confirmed by Simon. It would be helpful if Simon can provide his comments on whether this approach is viable from his perspective.