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Surface State Decouples Normal Detachment and Slip Initiation in Grip- Optimized Rubber Contacts

This study demonstrates that surface preparation of grip-optimized climbing shoe rubber decouples normal detachment resistance from tangential slip initiation, revealing that pull-off performance cannot reliably predict friction behavior and that both surface state and counter-surface texture must be controlled for accurate performance evaluation.

Original authors: Michal ŽŮREK, David REBENDA, Martin HARTL, Martin VRBKA

Published 2026-10-26

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CC BY 4.0

Original authors: Michal ŽŮREK, David REBENDA, Martin HARTL, Martin VRBKA

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a climber hanging by a single toe on a tiny edge of rock. Their safety depends entirely on the rubber of their shoe gripping that surface without sliding. This grip is not a single, simple force; it is a complex battle between two different kinds of resistance. One is the effort required to pull the rubber straight away from the rock, a vertical separation. The other is the resistance to the rubber sliding sideways along the rock, a horizontal slip. For decades, engineers and scientists have often assumed that if a rubber is hard to pull off, it must also be hard to slide. They treated these two behaviors as two sides of the same coin, believing that measuring one would automatically tell you everything about the other.

A team of researchers at Brno University of Technology in the Czech Republic has challenged this long-held assumption. They set out to test a specific type of high-performance rubber used in climbing shoes, asking a simple but profound question: does the way a rubber surface is prepared change how it pulls away from a surface in the same way it changes how it resists sliding? To find the answer, they did not just look at the rubber as it came from the factory. They took the same batch of rubber and created four distinct surface conditions. They kept one exactly as it was molded. They cut another to expose the fresh material hidden just beneath the surface. They ground two others with sandpaper to create rough textures, one moderately rough and one very rough. They then tested each of these four surfaces against two different types of rigid, spherical pins: one perfectly smooth and one textured to mimic the rough surface of a real climbing hold.

The results revealed a surprising disconnect. The researchers found that the ability of the rubber to resist being pulled straight off and its ability to resist sliding sideways are not governed by the same rules. In fact, the surface treatments that made the rubber hardest to pull off often made it easier to slide. For instance, when the researchers ground the rubber to be very rough, it became significantly harder to pull away from the smooth pin, requiring much more force to separate the two. Yet, that same rough rubber slid more easily across the smooth pin than the original, smooth factory surface did. The rougher the rubber got, the less it resisted sliding, even as it held on tighter when being pulled straight up.

The most striking discovery came from comparing the original factory surface with the cut surface. The cut surface, which exposed the inner material of the rubber, had almost exactly the same smoothness and texture as the original factory surface. By every standard measurement of roughness, they were twins. Yet, their behavior was worlds apart. When tested against the smooth pin, the cut surface required more than three times the force to pull off and absorbed nearly seven times more energy during separation than the original surface. Despite this massive increase in resistance to being pulled away, the cut surface actually slid more easily than the original. This proved that the secret to the rubber's grip was not just about how bumpy or smooth the surface looked. Something deeper was happening. The act of cutting away the outer layer exposed a different kind of material underneath, one that bonded more strongly to the pin but offered less resistance to sideways movement.

The texture of the surface the rubber touched also played a critical role in how these forces behaved. When the rubber was tested against the rough, textured pin, the patterns changed again. The roughest rubber surface, which had been ground with the coarsest sandpaper, showed the highest resistance to being pulled off, likely because the rubber fingers had sunk into the valleys of the textured pin and locked in place. However, even this heavily locked-in rubber still slid more easily than the original factory surface. The researchers observed that the difference between the force needed to start sliding and the force needed to keep sliding was much larger on the textured pin. This suggests that while the rubber can mold itself perfectly into the nooks and crannies of a rough surface while standing still, that perfect fit breaks down the moment it starts to move, causing a sudden drop in grip.

These findings suggest that the old method of judging climbing rubber by how well it sticks when pulled straight off is insufficient. A rubber that is excellent at resisting a direct pull might be mediocre at preventing a slip, and vice versa. The study indicates that the condition of the rubber's surface—whether it is the original molded skin, a freshly cut layer, or a ground texture—alters the physics of the contact in ways that roughness measurements alone cannot predict. The researchers concluded that to truly understand how a climbing shoe will perform, one must test both the pull-off resistance and the sliding resistance separately, and one must always consider the texture of the rock or hold the shoe is touching. The grip of a climbing shoe is not a single number; it is a complex relationship between the state of the rubber, the texture of the rock, and the direction of the force.

Technical Summary: Surface State Decouples Normal Detachment and Slip Initiation in Grip-Optimized Rubber Contacts

Problem Statement
High-friction applications, such as climbing shoes, rely on soft rubber contacts that must resist slip under limited normal loads. While rubber friction is traditionally interpreted through adhesive and hysteretic contributions, the relationship between normal detachment resistance (often used as a proxy for adhesion) and resistance to tangential slip initiation remains complex. Previous studies have shown mixed correlations between work of detachment and static friction, often complicated by variations in material composition and surface conditions. This study addresses the specific question of whether the surface state of a grip-optimized climbing shoe rubber influences normal detachment and tangential slip initiation through the same interfacial mechanisms, or if these responses can be decoupled.

Methodology
The study utilized a single rubber compound (Cat μ1.5, 80.2 ± 0.9 Shore A) commonly used in climbing shoes to isolate the effects of surface preparation. Four distinct surface states were prepared:

P0: The original moulded surface, cleaned with IPA.
CUT: A surface exposing subsurface material, created by splitting the rubber sheet and bonding it back together, preserving the original roughness amplitude but removing the moulded skin.
P100: A surface ground with P100 abrasive paper.
P60: A surface ground with P60 abrasive paper (rougher than P100).

These rubber specimens were tested against two rigid spherical polyester pins (radius 20 mm) supplied by a climbing hold manufacturer:

A smooth pin ( $S_a \approx 2.72$ µm).
A textured pin ( $S_a \approx 67.69$ µm), designed to match commercial climbing hold textures.

Two primary experimental protocols were conducted using a universal mechanical tester (UMT TriboLab) under controlled laboratory conditions (24.2 °C, 30% RH):

Pull-off Experiments: A pin was indented to 45 N, stabilized for 1 s, and retracted at 1 mm/s. Metrics included maximum pull-off force ( $F_p$ ) and work of detachment ( $W_{det}$ ).
Slip Initiation Experiments: A pin was loaded to 50 N, stabilized for 1 s, and moved laterally at 1 mm/s. Metrics included the static friction coefficient ( $\mu_s$ ), steady sliding friction coefficient ( $\mu_d$ ), and the static-to-steady friction drop ( $\Delta\mu$ ).

Surface topography was characterized using optical microscopy to determine areal roughness parameters ( $S_a$ , $S_q$ ).

Key Results
The study found that surface preparation significantly altered both detachment and friction, but the responses were not directly coupled:

Decoupling of Detachment and Friction: Conditions that exhibited higher resistance to normal detachment did not consistently exhibit higher static friction.
- CUT vs. P0: Despite having nearly identical arithmetic mean roughness ( $S_a \approx 1.01$ µm for P0 vs. $1.02$ µm for CUT), the CUT surface showed approximately 3.1 times higher pull-off force and 6.8 times higher work of detachment against the smooth pin compared to P0. However, CUT exhibited lower static friction than P0 against both pin types.
- Ground Surfaces (P100, P60): Generally, ground surfaces showed higher detachment resistance (particularly against the textured pin) but lower static and steady sliding friction compared to the original moulded surface (P0).
Influence of Counter-Surface Texture:
- Against the smooth pin, static friction decreased monotonically with increasing rubber roughness (P0 > CUT > P100 > P60).
- Against the textured pin, the trend was less uniform. While P0 still had the highest static friction, the ground surfaces (P100, P60) showed static friction values closer to P0 than they did against the smooth pin. The static-to-steady friction drop ( $\Delta\mu$ ) was substantially larger against the textured pin for all conditions, suggesting stronger interface development during stationary contact followed by a larger reduction during sliding.
Work of Detachment vs. Pull-off Force: The work of detachment ( $W_{det}$ ) and maximum pull-off force ( $F_p$ ) did not always scale identically, indicating that surface topography influences both the peak resistance and the progression of interfacial failure.

Significance and Claims
The authors conclude that resistance to normal detachment cannot be used as a sole predictor for resistance to tangential slip in grip-optimized rubber contacts. The study demonstrates that:

Surface State is Multifaceted: Surface preparation affects performance through mechanisms beyond simple roughness amplitude ( $S_a$ ). The CUT condition, which exposed subsurface material without significantly altering $S_a$ , produced markedly different detachment and friction responses compared to P0. This suggests that the removal of the moulded surface layer (potentially exposing different interfacial chemistry or mechanical properties) plays a critical role.
Complementary Testing: Pull-off tests and slip-initiation tests provide complementary, not interchangeable, information. A surface optimized for high pull-off force may not necessarily offer superior grip (static friction) in a climbing scenario.
Context Dependence: The performance ranking of rubber surfaces depends heavily on the texture of the counter-surface. Rankings obtained against smooth surfaces may not translate to rough climbing holds.

The paper posits that the decoupling arises because normal detachment is governed by crack propagation across the interface, whereas tangential slip initiation depends on processes within the contact area, including real contact area, viscoelastic deformation, and geometrical interlocking. Consequently, evaluating climbing shoe rubber performance requires controlling and reporting specific surface preparation methods and representative counter-surface textures, rather than relying solely on roughness metrics or detachment force alone.

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