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.

Hand or Rotary Brush - That Is the Question?

To understand the fundamental differences between hand brushing and rotary brushing when removing excess wax from the surface of a ski base, we must first describe how wax is deposited on the base and how it interacts with the base material.

1. Three Types of Wax Reservoir in the Ski Base

After hot waxing, the wax is not present on the ski base as a single homogeneous layer. To assess the effect of brushing, it is useful to distinguish three basic reservoirs.

1.1. Macroscopic Excess Wax

The first reservoir consists of a continuous wax layer above the surface of the ski base and wax filling the ground structure.

This layer is predominantly a processing surplus. It covers the structure, may increase friction and restrict water drainage. It should therefore be removed with a plastic scraper followed by brushing.

1.2. Thin Film in the Surface Microstructure

The second reservoir consists of a very thin wax film adhering:

• to the peaks and walls of the ground structure,
• in fine grooves and surface defects,
• between UHMWPE fibrils and microscopic surface irregularities.

This layer is no longer merely a coarse excess and may contribute to the frictional and wetting properties of the ski base.

This is precisely where the difficult-to-define boundary lies between the desirable removal of excess wax and the undesirable removal of functional wax.

1.3. Wax Absorbed in the Uppermost UHMWPE Layer

The third reservoir consists of wax that, when heated, penetrated into the uppermost polymer zone of the ski base.

UHMWPE is not porous like a sponge. It is a semicrystalline polymer, and wax can penetrate primarily into its amorphous regions, interphase spaces and surface defects.

This wax cannot simply be 'brushed out' of intact polymer. It may, however, be removed together with the uppermost UHMWPE layer if the brush begins to wear it mechanically.

2. How a Fine Hand-Held Steel Brush Removes Wax

As a reference hand brush, we will assume:

• a wire diameter of approximately 0.12 mm,
• a wire length of approximately 25 mm,
• wires grouped into bundles approximately 8 mm in diameter,
• slow manual movement along the length of the ski base.

2.1. Basic Mechanism

Under pressure, the long and flexible wire bends, partially lies down in the direction of travel and slides over the surface along a relatively long path.

A hand brush therefore works predominantly by:

• slow and continuous shear,
• gradual loosening of particles,
• mechanical separation of wax from the surface,
• partial penetration into accessible parts of the structure.

It first removes loose flakes and fragments left after scraping. It then disrupts thinner wax residues by shear and gradually releases them from the walls and upper parts of the grooves.

A single stroke usually does not remove the wax completely. The first pass disrupts or shifts it, the next breaks it into smaller pieces, and subsequent strokes remove it. Hand brushing is therefore a cumulative process.

2.2. Penetration into the Structure

A 0.12 mm wire can enter only sufficiently wide and open grooves. Its actual penetration also depends on:

• the shape and depth of the structure,
• wire flexibility,
• applied pressure,
• the contact angle,
• wear of the wire tip.

A long wire has time to conform to the surface, but it often cannot reach into fine, oblique or multi-level structures.

2.3. Effect on the Individual Wax Reservoirs

A hand brush:

1. removes macroscopic excess very effectively,
2. gradually reduces the thin film in the microstructure,
3. does not directly remove absorbed wax from intact UHMWPE.

However, if the steel wire continues to contact the polymer itself, plastic deformation, micro-ploughing, removal of fibrils and a very small abrasive loss of UHMWPE may occur. Wax absorbed in the polymer may then be removed together with the polymer.

2.4. Characteristics of Hand Brushing

A hand brush is a slow and readily controllable tool. The applied pressure, number of strokes and area of action can all be adjusted continuously.

Its main weakness is non-uniformity. Hand pressure, the angle of travel and the number of contacts are not perfectly identical over the entire ski-base surface.

3. How a Fine Rotary Steel Brush Removes Wax

3.1. Specific Design of a Rotary Brush

A rotary brush is functionally similar to a hand brush, but its design and mode of action differ substantially.

For SWIX and TOKO brushes, we will assume:

• a wire length of approximately 10 mm,
• a wire diameter of approximately 0.10 mm,
• an overall brush diameter of approximately 70 mm,
• an operating speed of approximately 800-1,000 rpm.

For the RED CREEK brush:

• a wire length of approximately 6 mm,
• a wire diameter of approximately 0.07 mm,
• an overall brush diameter of approximately 62 mm,
• an operating speed of around 1,000 rpm.

Despite their smaller wire diameter, the rotary-brush wires are approximately eight times stiffer than the reference 25 mm hand-brush wire because of their very short free length.

At 800-1,000 rpm, the tips of the SWIX/TOKO brush wires move at a circumferential speed of approximately 2.9-3.7 m/s.

A rotary-brush wire is therefore:

• significantly shorter,
• stiffer,
• less able to lie along the surface for an extended distance,
• loaded at a many-times higher speed.

3.2. Basic Mechanism

In a rotary brush, each wire moves along a circular path. It enters into contact with the ski base at high speed, bends briefly, shears across the surface and then springs back to its original position.

One working cycle comprises:

1. a brief dynamic contact or micro-impact,
2. rapid bending of the wire,
3. a brief high-speed shear,
4. elastic release,
5. displacement or ejection of a particle.

Each individual contact is very brief, but it is immediately followed by other wires. The surface is therefore worked by a large number of short, overlapping contacts.

3.3. Wax Removal

In the first stage, the rotary brush removes very quickly:

• flakes left after scraping,
• loose wax dust,
• residues of a thicker layer,
• wax from the upper and accessible parts of the structure.

The wire disrupts, fragments or separates the wax by impact and shear. As it springs back, and under the action of centrifugal force, it may eject particles from the contact area.

A short wire can enter wide grooves. In a finer structure, however, it often acts mainly on the peaks and upper parts of the groove walls. It removes wax not in one continuous stroke, but through repeated fragmentation.

3.4. Effect on the Thin Film and UHMWPE

After the macroscopic excess has been removed, the brush continues to work on the thin wax film by the same mechanism.

The short dynamic contacts may:

• shift the film by shear,
• tear it,
• separate it from the surface,
• break it into fine particles,
• remove it progressively.

The brush does not distinguish between excess wax and functional wax.

As the film diminishes, the steel wires come increasingly into direct contact with UHMWPE at peaks, edges and fibrils. They may plastically deform it, micro-plough it, score it or detach microscopic particles from it.

This is unlikely to constitute uniform removal of a continuous layer across the entire surface. Material loss will instead be local and concentrated on the highest and most heavily loaded areas. Even very slight abrasion, however, may reach the thin wax-enriched surface zone.

3.5. Characteristics of Rotary Brushing

A rotary brush is fast, effective and generally more uniform than a hand brush.

Its defining characteristic, however, is the very rapid transition between the individual stages:

1. removal of loose wax,
2. opening of the structure,
3. reduction of the thin functional film,
4. direct contact with UHMWPE,
5. possible micro-abrasion of the wax-enriched polymer layer.

A rotary brush works through a combination of micro-impact, brief shear and a very high number of repeated contacts.

Rotary brushing is faster and more uniform, but it leaves less time to stop the process at the point when the excess has been removed but the functional surface layer has not yet been significantly affected.

4. Hand or Rotary Brush - That Is the Question

Hand-held and rotary steel brushes serve the same basic purpose: to remove excess wax, open the ground structure and prepare the ski base for gliding. The way in which they achieve this result, however, is fundamentally different.

A hand brush works predominantly through slow, continuous shear by long, flexible wires. Its effect develops gradually, stroke by stroke, and the operator can continuously adjust the pressure, speed and number of repetitions. Its disadvantages are the longer working time and lower uniformity of treatment.

A rotary brush works through brief dynamic contacts by substantially shorter and stiffer wires. At a circumferential speed of several metres per second and with a high number of repeated contacts, it can remove excess wax extraordinarily quickly and uniformly. Once the excess has been removed, however, the same mechanism continues without interruption to work on the thin functional film and subsequently on the highest parts of the UHMWPE itself.

The rotary brush cannot determine whether it is currently removing:

1. undesirable macroscopic excess,
2. a still-desirable thin wax film,
3. or the wax-enriched uppermost polymer layer itself.

The boundary between these stages is not sharp and may be crossed at different times in different areas of the ski base. The high speed of a rotary brush also significantly shortens the time available to the operator to stop the process.

From this perspective, a hand brush may be considered the safer tool. Although it is considerably slower and the result may be less uniform, it offers greater control and a wider time margin between removal of excess wax and the onset of more intensive treatment of the functional 

surface layer.

A rotary brush has undeniable advantages, especially in professional servicing, where speed, productivity and repeatability are decisive. These advantages, however, come at the cost of greater sensitivity to correct rotational speed, pressure, feed speed and number of passes.

The decisive question therefore cannot be merely which brush removes wax faster. More important is which tool makes it possible to stop brushing more reliably at the point when the processing excess has already been removed while the functional wax and polymer surface zone remains preserved. From this perspective, the hand brush is the more controllable tool and offers a greater tolerance for error.