The Structure That Does Not Actually Exist - Part No. 2

 

Dimensional Relationship Between the Groove and the Brush

The difference between the ground profile and the functional profile is clearly illustrated by the geometric relationship between the groove and the brush fibres.

Consider a symmetrical V-shaped groove with a pitch of 500 micrometres and a depth of 50 micrometres. At half its depth, it is approximately 250 micrometres wide. A steel fibre with a diameter of 120 micrometres corresponds to the groove width at a depth of approximately 38 micrometres. This model deliberately disregards elastic deformation of both the fibre and the wax and therefore represents a rather optimistic estimate of the attainable depth.

Below this level, the groove is narrower than the fibre itself. The fibre therefore cannot directly enter the bottom approximately 12 micrometres, or roughly the lower quarter of the model groove. Actual access to the bottom will also be influenced by the applied pressure, the contact angle and whether the fibre is in fact guided in the direction of the groove.

The brush therefore cannot remove wax equally from all depth levels. The peaks and the upper portions of the sidewalls are worked intensively, while an area remains in the lower part into which the fibre cannot geometrically fit.

Actual Structures Are Even More Complex

The model is based on a single regular linear V-shaped groove. Real racing structures, however, are often cross-hatched, chevron-shaped, interrupted, combined or multi-level. Multi-level patterns may contain several overlapping groove systems, so that access to one part of the profile is blocked by surrounding peaks or sidewalls.

They contain local depressions, intersections, sidewalls and tighter angles that are less accessible to the brush. The more complex the geometry, the less uniform wax removal can be expected to be.

A pitch of 500 micrometres is also only an approximate midpoint. Finer structures may have pitches of 300, 200 or 150 micrometres. Although they tend to be shallower, their cross-section narrows towards the bottom, and the diameter of a conventional steel fibre may be comparable to a significant portion of the groove’s total width. In structures with a pitch of 200 or 150 micrometres, direct contact with the lower regions may therefore be limited to a very small part of the profile or may not occur at all.

The nominal fibre diameter is not the only decisive factor. With use, the fibre tip becomes rounded and is often worn asymmetrically in the brushing direction. The actual contact geometry is therefore less capable of penetrating narrow lower regions than a simple calculation would suggest. Moreover, a worn tip does not create a single point of contact, but a broader asymmetric contact area that bears against the groove sidewalls sooner.

The fibres also work in bundles, interacting with one another, bending and deflecting. They do not always enter the grooves individually or at right angles, and in fine or multi-level structures they may pass over the upper portions without reaching the bottom. Although the flexibility of the bundle allows some adaptation to the surface, it also limits the precise guidance of individual fibres into narrow and irregular depressions.

The Brush Does More Than Remove Wax

After scraping and brushing, the wax is therefore not removed uniformly from the full depth of the structure.

The peaks and upper groove sidewalls are exposed most strongly, whereas more wax remains in the lower regions. Depending on the structure geometry, the type and wear of the brush, and the properties of the wax, the lower quarter or third—or, in fine or complex structures, an even larger portion of the original depth—may be less accessible. This is therefore not merely a passive residue of wax, but also the result of its mechanical redistribution during the preparation process.

Where the fibres can no longer penetrate deeper, they do not necessarily only remove wax. They may also act as smoothing or compacting tools: they displace the wax by shear, smooth it along the sidewalls and partly press it into inaccessible areas.

Compared with the geometry created by the stone, the functional profile is consequently shallower, more rounded and less pronounced. The peaks remain largely exposed, the transitions are softened, and the lower parts of the grooves are filled to varying degrees with residual wax.

In coarse, open and simple linear structures, the difference may be smaller. In fine, cross-hatched or multi-level structures, it may be substantially greater because their lower and lateral regions are less accessible to a conventional brush. The functional profile also continues to change with every subsequent waxing cycle, cleaning procedure and skiing session, so even after the initial break-in it is not a definitively fixed state.

Conclusion: The Ground Structure Is Only the Beginning

The central thesis is therefore confirmed: the structure created by the grinding stone and the structure that actually interacts with the snow while skiing are not identical.

The grinding machine creates only the initial geometry in the UHMWPE. Brushing, waxing, scraping, break-in and wear transform it into a functional profile whose shape results from the original grooves, the material properties, the geometry and wear of the brush fibres, the properties of the wax and other preparation procedures.

The finer, more complex and more multi-level the original structure, the greater the difference may be between the structure stored in the grinding machine’s program and the surface that ultimately glides over the snow.

Examining the structure immediately after grinding therefore captures only part of the reality. Measurements of groove pitch, depth and shape describe the geometry created by the stone, but not yet the surface after waxing, scraping, brushing and break-in. For meaningful comparisons of structures, the conditions of the subsequent preparation process should therefore be defined and reproduced as accurately as possible.

To understand how a structure functions, we must examine its condition after the complete preparation process. Otherwise, we are analysing the structure produced by the grinding machine, not the structure on which the skis actually glide. Any examination that does not take the effect of wax into account therefore reveals only part of the truth.

The Structure That Does Not Actually Exist - Part No. 1

 

The Structure That Does Not Actually Exist

A great deal has been written about structures—perhaps so much that it has become difficult to make sense of them. Let us therefore look at them from a different perspective and show that, at least on well-prepared skis, a structure does not actually exist as a separate and unchanging geometric form.

What We Mean by Structure

By structure, we mean a system of grooves of different shapes, pitches and depths ground into the surface of the ski base.

The resulting pattern may be linear, cross-hatched, chevron-shaped, multi-level or otherwise combined. It is produced by a defined relative movement between the grinding stone and the ski base; its form is determined primarily by the stone-dressing method, stone rotation, ski feed speed, contact pressure and other grinding parameters.

We will use a simple linear structure as a reference. The same principles apply to more complex patterns, although their geometry and the interaction between individual grooves are more complicated.

Groove pitch and depth are generally related. The transverse profile often resembles a V, but it may also be rounded, asymmetric or irregular. A greater distance between the peaks is usually associated with a deeper structure, while finer structures tend to be shallower. In foreign-language materials, the ridges are referred to as PEAKS and the distance between them as PITCH.

For the structures considered here, the ratio of pitch to depth is approximately between 10:1 and 12:1. A groove with a pitch of 0.5 mm may therefore be about 40 to 50 micrometres deep. This is not a universal constant, however. In practice, both parameters may also be affected by the grit and condition of the stone, the ski-base material and the settings of the particular grinding machine.

The word “ground” is crucial. We are not referring to a structure that has been pressed or engraved into the surface, but to geometry created by removing material, most commonly by stone grinding. This distinguishes it from imprinted structures (IMPRINTS), which are produced mainly by plastic deformation of the surface.

The ground structure is the initial, relatively stable geometry. Its actual form is subsequently altered by brushing, waxing, thermal and mechanical loading, ski break-in and wear. This article therefore deals exclusively with stone-ground structures.

Structure Does Not Exist Without Material

A structure cannot be described accurately without considering the material into which it is ground. The same geometry created in different materials does not necessarily produce the same surface or the same functional properties.

We will limit the discussion to bases made of UHMWPE, or ultra-high-molecular-weight polyethylene, used particularly in high-quality cross-country skis. In addition to UHMWPE, they may contain carbon black and other graphitic, paraffinic or oil-based components.

Carbon black does not consist solely of perfectly separated primary nanoparticles, but also of aggregates and larger agglomerates. The properties of the ski base are therefore determined not only by the declared particle dimensions, but also by their concentration, dispersion and the nature of the interface with UHMWPE. In racing bases, molecular weight is typically around 5 million g/mol or higher, although the specific formulations used by individual manufacturers vary considerably.

Formation of Fibrils During UHMWPE Grinding

UHMWPE has exceptionally long and highly entangled molecular chains. During machining, shear deformation, chain orientation and chain pull-out occur, together with the separation of parts of the semicrystalline structure and, to some extent, chain scission.

This may result in fine surface fibrils formed by bundles of oriented chains. Higher molecular weight and greater entanglement density promote the formation of longer and mechanically more stable fibres.

Carbon-black aggregates may act as local inhomogeneities and as sites where fracture initiates or propagates. They can therefore alter the way the material separates and limit the pull-out of polymer fibrils. There is consequently no simple rule according to which a higher carbon-black content automatically produces a better surface.

With a suitable concentration, uniform dispersion and optimum aggregate size, a ski base may therefore exhibit a less “hairy” surface after grinding. Larger agglomerates, however, may instead act as defects and reduce homogeneity.

A Ground Structure Is Not Yet a Functional Structure

A freshly ground surface is not the final functional surface ready for skiing. In addition to the intended grooves, it may contain fibrils, burrs, sharp peaks, locally deformed material and irregular transitions.

Before the skis—especially racing skis—are used, the surface must therefore be mechanically finished, cleaned and stabilised. The first stage is thorough brushing, which removes loose particles and fibrils, cleans the grooves and softens the sharpest edges. It does not change the basic ground geometry, but it does change its actual surface profile.

This is followed by repeated hot-wax applications, scraping and brushing. These mechanical cycles continue to remove protruding micro-irregularities, while heat and molten wax affect the immediate surface layer of the ski base. Racing skis may then be broken in on clean, moderately abrasive snow. Contact with snow crystals gradually abrades the highest protruding micro-irregularities and stabilises the surface under real gliding conditions.

When molten wax is applied, the grooves are filled to varying degrees. After scraping and brushing, most of the excess wax is removed, but not equally from every part of the profile. The peaks and upper portions of the grooves are the most exposed, whereas more wax remains in deeper areas. At the same time, the wax does not penetrate all locations equally, and its subsequent removal is limited by the accessibility of the individual parts of the profile.

The degree of removal depends on groove geometry; the stiffness, length and diameter of the brush fibres; the shape of their tips; the applied pressure; the brushing direction; and the properties of the wax. The functional profile is therefore no longer identical to the UHMWPE profile immediately after grinding: the valleys are partly filled, sharp transitions are covered or softened, and the peaks are gradually rounded. This change can generally be described as partial flattening and rounding of the functional structure profile.

A distinction must be made between the more permanent alteration of the UHMWPE surface caused by brushing, scraping, heat and break-in, and the variable alteration caused by an unevenly distributed layer of wax.