There are no less than 14 different phases of the human gait cycle, plus some five ranges of motion for the associated joints. Thus it would be impractical, if not impossible, to create a device capable of simulating the walking characteristics of most human beings. While slipping and falling on a surface during walking may seem like a simple thing to analyze, it involves much more than meets the eye. As long as the available friction of a walkway surface is greater than the required friction needed for the ambulation process, no slipping should occur. The problem is that different people have different requirements in different circumstances.

Experiment: Visit any large donation store that sells used footwear. Turn over half a dozen or so random pairs of shoes. You should observe a variety of wear patterns on the soles and heels. Some may even exhibit different wear patterns between the left and the right shoes! Take a close look at your own shoes. You may be surprised at your own wear patterns. These "contact patches" are the vital areas of the shoes where the most interaction with the walking surface occurs. As you can see in the photo below, they are much smaller than the entire sole. The heel of a worn shoe is usually slightly rounded.

It is our belief that trying to simulate human ambulation with a mechanical device is the wrong approach to tribometry (measurement of friction). We do believe however that it is practical to correlate a test device to a method or model that includes humans in the measurement loop. The BOT-3000 has established a statistically significant wet DCOF correlation coefficient (0.879) to the German ramp (DIN 51130), and a 0.926 correlation coefficient to the GMG-100 (DIN 51131) using SBR rubber during a study performed at Wuppertal University (Germany) in 2008 using many types of flooring.

More information is available in Dr. Jens Sebald's book “The System Oriented Concept for Testing and Assessment of the Slip Resistance of Safety, Protective and Occupational Footwear”, available from this website.

Hydroplaning (DCOF)

The effects of hydroplaning may come into play if there is insufficient normal force (gravitational pull) in proportion to a mass moving over a horizontal wet surface. This is why your heavy car does not hydroplane at 5 or 10 mph even with the baldest of tires. The BOT-3000 has sufficient mass and low enough drive speed to avoid any hydroplaning action, which is also known as the "hydrodynamic lubrication" portion of the Stribeck curve. It operates within the "boundary lubrication" portion of the curve, ensuring adequate contact with a wetted (water) test surface.

Stick-slip (DCOF)

There is a phenomenon known as “stick-slip” which occurs between certain materials during continuous friction producing interactions. This may occur in both wet and dry conditions. Stick-slip is what you hear when a violin plays, a wet windshield wiper blade squeaks, or a car tire skids. Sometimes the resulting frequency is inaudible or muted.

“Stiction” (SCOF)

According to Websters, it is a coined word made from the combination of static and friction. Many tend to use the term to describe other physical stress manifestations related to "elastic contact" effects which may occur when a resilient material (such as rubber) first conforms to the microscopic surface profile features of a harder material (such as ceramic tile) under pressure (usually vertical). As the shear force increases (usually horizontal), the resilient material may become slightly distorted as mechanical energy is being stored, and eventually released. This only happens once the horizontal force overcomes the vertical force, and now sliding begins. Stiction effects may be time and/or temperature sensitive, as resilient material can take a while to conform or distort depending upon its physical properties. Stiction may be compounded by suction effects exhibited in certain instances, especially where flat surfaces mate. Sometimes this, or similar adhesive effects are described as "cold welds". These adhesive effects may require additional force to separate the intimately joined surfaces. The BOT-3000 minimizes the adhesive effects of stiction by minimizing dwell or "residence" time with the surface being tested, and by employing a relatively small, relatively hard, convex shaped test foot (slider).