Zeppa

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

A FOLLOWER OF THE SKIER’S MANIFESTO COMMENTS ON BOOT BOARD/BINDING RAMP ANGLE

The universal boot truisms that David puts forth in his blog is a ‘blueprint’, ‘computer program’, what ever you want to call it which is applied on an INDIVIDUAL basis. Each individual is measured with the system and then the results are applied to the boot. Unlike ‘one boot fits all skiers’ which is the current system the industry provides. I can’t believe that anyone would buy a boot based on a ski magazine test but that probably is hardly worse than the info one gets in the ski shop which gave me nerve damage in my feet (gratefully healed now since I gave up on ‘race fit’). David talks about using flat shims of varying thicknesses to fine tune ramp angle just like canting shims or duct tape are used for lateral experimentation; 2.5-6 is his starting point while I would start with zero but with current ski bindings only millimeters of fine tuning can be done when some individuals need centimeters from the current setups. Obviously if one changes the ramp angle the shank angle may have to be adjusted also which is why skiing is believing, David gives a perfect example in his reply on what lower ramp angle did for his skiing. Also the extremely high starting point on ramp angle makes it impossible for many skiers to loosen their cuff for normal forward flex because they need to be clamped tight to prevent falling one their noses; that’s where I got fooled for a couple of decades.

Having built an adjustable plate for ramp/delta in the early 2000s I can tell you one thing for sure; the skier knows instantly if things are better or worse. That by no means indicates an optimum net ramp because there are so many other aspects of the boot that are factors such as toe crunch (race fit) and ankle flex restricted to virtually zero. I started from the wrong end with ramp/delta whereas David starts in the boot first which is what I would do but took me about a decade to loosen my boot cuff significantly enough to make a difference; that due to a hip joint that was killing me from skiing. 2 months after loosening the boot cuff and removing the power strap which is only good for carrying the boots (my opinion), I was introduced to The Skier’s Manifesto and learned from that how to create an ankle glide path, free the toes, free the arches, etc. What amazes me is 2 things; first that when I first decided to build the BalancEnhancer as I call it (due to a friend’s prodding), that it actually worked , and second, how hard it is to even get skiers to try something different and the number that do try it and then don’t even try to modify their own equipment to their needs based on what had made there skiing better!!

  • Michael Pupko

BOOT BOARD (ZEPPA) RAMP ANGLE VS. BOOT SIZE

It is becoming clear, the angle the boot board (zeppa) establishes for the skier’s foot relative to the ground, is vitally important to the ability to balance and function on skis. Therefore, knowing boot board angle (ramp angle) and skier preferences should become part of every boot setup and purchase. Yet there appears to be a fundamental error in the understanding of ramp angle in boots. This is evident when someone states, for example: “The head Raptor has a ramp angle of 4.5 degrees”. The statement may only true if the angle is linked to the boot size.

There are production controls applied to boots just as controls and standards are applied to all other things mass produced. In boots, it means the first prototypes are designed to a specific size (generally Mondo 26). All other sizes are scaled up or down from it. Each Mondo size is a change of one centimeter. Zeppas are fixed in both rear foot and forefoot height in the prototype standard. Only the zeppa length changes as boot size changes.

It means; if the prototype size is twenty six, the zeppa of a twenty three is three centimeters shorter with the same toe and heel heights. Therefore, the ramp angle of the zeppa of a twenty three is steeper than the ramp angle of the zeppa of a twenty six. Since many women’s boots are scaled from the twenty-six Mondo standard, boot set-up problems can be more difficult to solve for women than for men. This is the reason women are more adversely affected by boot configuration than men. The graphic below compares the boot board (zeppa) ramp angles of larger and smaller boots to the standard Mondo 26 boot.

Zeppas Mondo 26

 

Bindings obviously confer the same effect, since with most models heel height is greater than toe height. As the heel and toe change distances from each other according to boot size, binding angle (delta) changes and its angle is additive with the boot ramp angle to determine gross equipment angle as shown in the graphic below. Binding delta has a double effect, since as delta increases boot cuff angle relative the ground also increases.

Zeppas Mondo 26 bindings

When talking about boot boad ramp, we should include the boot size or always use the ramp of the Mondo 26 as a known reference.


Lou Rosenfeld has an MSc. in Mechanical Engineering with Specialization in Biomechanics earned at the University of Calgary Human Performance Laboratory. His research was titled, “Are Foot Orthotic Caused Gait Changes Permanent”.

While at HPL, he assisted with research on the effects of binding position for Atomic, and later conducted research for Nordica that compared Campbell Balancer established binding position to the Nordica factory recommended binding position.

Lou is one of the invited boot-fitters on the EpicSki forum “Ask the Boot Guys” and has authored articles on boot fit, balance, alignment and binding position for Ski Canada, Ski Press, Super G, Calgary Herald, and Ski Racing, USA. He is a CSIA Level 2 instructor and CSCF Level 1 coach. He currently resides in Calgary where he owns and operates Lou’s Performance Centre. A selection of his articles may be found at www.Lous.ca.

THE BOOT BOARD FACTOR

While the Ottawa researchers did not explore this aspect, they correctly identified that equipment, including custom insoles, technical skills and ski technique might explain why the pressures recorded under the heel and the head of the first metatarsal of some instructors were much higher than the pressures seen in the same locations in other instructors.  The University of Ottawa studies are the only ones I am aware where the researchers considered the effect  of what is known in research as uncontrolled variables on their findings. Poor technique and interference with the function of the foot and leg caused by the ski boot can ensure that COP remains under the heel.

Although boot board ramp angle and shape have an undeniable impact on the function of the feet and lower limbs, as evidenced by the photographs below of a sampling of boot boards, there does not appear to be any continuity, let alone any standard for boot board ramp angle and the form of the surface that interfaces with the sole of the foot.

1

 

2

IMG_5560

3

IMG_6187

4

IMG_0144

When the effect of  binding ramp angle, which appears to have even more variation than boot board ramp angle, is added to ramp angle equation to arrive at Net Ramp Angle, the possible combinations that make up Net Ramp Angles becomes unlimited and can range from as little as two to as much as ten degrees.

As if the lack of any apparent standard for boot board and binding ramp angles were not causing enough of an impact on skier/racer performance, there is a factor that appears to be compounding the issue by introducing a layer of inconsistency; boot base shell deformation under loads typical of recreational skiing.

I will discuss boot base shell  deformation in a future post. In my next post I will propose a starting point for a boot board standard.