1. Slip Resistance Depends on the Combination of Shoe and Floor
When walking in school slippers, a freshly wiped floor can feel slippery. This "slip resistance" is not decided by the shoe alone or the floor alone, but by the combination of the shoe sole and the floor. The numbers mentioned at the start are an example of this. On the same bare concrete floor, rubber heels measured 0.72 to 0.73, while leather measured 0.38 to 0.46 [1]. A higher number means it is harder to slip.
The basis for these numbers is the coefficient of friction. It is expressed by dividing the force needed to pull the shoe sideways to move it by the force pressing the shoe against the floor. For example, if the pressing force is 1 and it starts moving with a force of 0.7, the number is 0.7 (this is a simplified number for explanation, not a measured value from the data). The combinations that require a large force to move are said to be less slippery.
This is the basic idea of the coefficient of friction. The measurement method differs depending on the device, and the way specific numbers are calculated is not the same either. For instance, the pendulum-type measuring machine mentioned later calculates the number from how much the height decreases when swinging up, while JIS tests calculate it by dividing the horizontal force by the pressing force.
So, when and with what tool were these numbers first made measurable? [1] (Note: The pendulum method calculates from the decrease in swing height, while JIS A 1454 and safety shoe tests calculate from the ratio of horizontal load to vertical load.)
2. How a Pendulum With a Heel Piece Measures Floor Slipperiness
The device created for this report was a portable "floor slip measuring machine." A piece of a shoe heel (4 cm square) is attached to the end of a pendulum and slid 10 cm across the floor. The slip resistance coefficient is then calculated from the decrease in the height it swings up on the opposite side [1]. The report states that before this, there was no way to express the degree of floor slip with numbers. This was a step forward from an era when slipping was discussed only by feeling.
The measurement method also considers walking movements. When walking, slipping is most likely to occur immediately after the heel of the forward foot touches the floor. It was known from worn-down shoe heels that this contact angle is 10 to 30 degrees [1]. Therefore, the measuring machine can attach the sole piece at 10, 20, and 30 degrees.
The idea was to capture the moment of falling on the floor using the movement of a pendulum. But what happens when the floor is wet with water or oil?
3. How Water on the Floor Can Make Slip Resistance Numbers Drop
Even with the same floor and the same shoe sole, values can change depending on where it is measured. If the floor surface gets wet with water or oil and a thin layer forms, it is said to be much more slippery than a dry floor [1].
Why does friction decrease when water gets in? A study investigating rubber friction (focused on tires and roads, introduced by a science news article) provides a clue. When rubber is pressed against a rough surface, the rubber catches on small bumps and deforms. Energy is lost inside the rubber each time this happens. This lost energy makes up a large part of the friction.
It is explained that when a small amount of water pools in the indentations and blocks them, the rubber cannot reach the bottom of the indentations. This deformation component disappears, and friction decreases [2].
Because this is research on roads and tires, we cannot say it applies directly to shoe soles. However, it is one way to think about why numbers might drop suddenly on wet floors. If the amount of water and the way it is pulled differ, the numbers will vary with each measurement. How do official tests prevent this?
4. How JIS A 1454 Measures the Force Needed to Start a Slip by Pulling a Weight
Currently, the representative Japanese test for floor slip resistance is JIS A 1454. A "slip piece" (contact area 70 × 80 mm), which acts as a substitute for a shoe sole or foot sole, is attached to the bottom of an 80 kg weight. It is pulled at a rate of 785 N (80 kgf) per second at an angle of 18 degrees upward. The value C.S.R is obtained by dividing the maximum pulling force at the moment it starts moving by the 785 N vertical load [3]. This method is said to reproduce the action of a person kicking off the floor when walking.
This test machine is a large device weighing over 150 kg in total. Because it is difficult to use on-site, the inventor, Mr. Hidetetsu Ono (Professor Emeritus at Tokyo Institute of Technology), developed a portable version. It has been confirmed that data with the same trends as the JIS test machine can be obtained (the portable version's values are written as C.S.R' to distinguish them) [3].
There are also guidelines for the numbers. The recommended values (draft, 2008) from the Architectural Institute of Japan state that for indoor corridors, stair treads, and floors in toilets and washrooms where footwear is worn, C.S.R should be 0.4 or higher, and for guest room floors, 0.3 or higher [4]. However, this is a draft, not a legal standard. Values change if water or sand is present, or if the material of the shoe sole changes.
This is a test that focuses on the "floor." Tests that focus on the shoe have different methods.
5. How Safety Shoes Are Tested on a Glycerin-Coated Steel Plate
The slip resistance test for safety shoes under JIS (T 8101) focuses on the shoe. The shoe is placed on a stainless steel plate with a surface roughness of 0.16 μm or less, coated with glycerin. The shoe sole is moved horizontally at 0.22 m per second while being pressed down with 500 N. Dividing the horizontal load from 0.4 to 0.8 seconds after the measurement starts by the vertical load gives the dynamic coefficient of friction. A value of 0.20 or higher is the standard for slip resistance [5].
It is important to note that this is a test for safety and work shoes. It is not a standard for school indoor slippers.
Do machine numbers really connect to how slippery it is for people walking? Research by the National Institute of Occupational Safety and Health verified this. They conducted experiments walking on slippery slopes and stepping forward and backward with 5 types of shoes with dynamic coefficients of friction between 0.12 and 0.64. The results showed that the standard of 0.20 is appropriate for preventing slipping during forward and backward movements. Furthermore, when the dynamic coefficient of friction increased by 0.1, the angle at which slipping started on the slope increased by about 2 degrees [5].
This result was also reported, and 0.30 was adopted as a higher standard in the JIS revision [5]. The flow was to verify machine numbers with human walking before setting the standard. However, this experiment involved 19 adult male participants and was conducted under strict conditions with a stainless steel plate and glycerin. It needs to be read as a separate issue from how children walk or water on school corridors.
By the way, there were children who tried to mimic the thinking behind these tests by hand. (Note: The research by the National Institute of Occupational Safety and Health (JNIOSH-SRR-No.47, 2017) tested walking on slippery slopes and forward/backward steps with 5 shoe types having dynamic friction coefficients of 0.12 to 0.64.)
6. How a 6th Grader Standardized Test Conditions With a Spray Bottle and Metronome
A Tokyo Metropolitan Government presentation includes a free study by a 6th grader from Nakano Ward [6]. They attached rubber, cork, polyurethane, and silicone to the bottom of hinoki cypress boards and pulled them with a spring scale on a resin board to compare the force needed to start moving.
Wet floors were made by spraying water 10 times with a spray bottle. The pulling speed was standardized by matching it to the sound of a metronome. They took the average of 3 trials each [6].
In this experiment, silicone showed good slip resistance whether dry or wet. Polyurethane was less slippery when dry, but became suddenly slippery when wet [6]. This is the result of this specific experiment, not a general conclusion about the materials.
They also compared patterns. On wet floors, among the patterns compared in this experiment, the round indentation pattern was the least slippery for both stepping out and pushing off. However, when the grooves filled with water, it suddenly became slippery. Adding drainage grooves stopped the sudden slipping. The discussion also touches on the point that the grooves reduce the area where the rubber touches the floor [6]. On sandy surfaces, a checkered pattern similar to hiking boots was less slippery for stepping out. They concluded that the best pattern depends on the situation [6].
The idea of standardizing the amount of water and pulling speed in this experiment is the same as JIS tests that standardize the pressing force and speed. To compare numbers, it is important to "standardize conditions and measure multiple times."
7. How to Compare Shoe Soles at Home With a Rubber Band
You can try a similar comparison method with things at home. However, do not try this on walking surfaces or wet floors, as falling is dangerous.
Place indoor slippers sole-down on a desk, hook a rubber band on them, and pull slowly horizontally. Measure the stretch of the rubber band just before it starts moving with a ruler. Measure 3 times and take the average.
Next, with a guardian, wet the desk with one spray of water from a spray bottle at a position where the slipper will not fall off, and do the same thing to see how the stretch changes. Wipe the desk when finished. The greater the stretch, the more force was needed to start moving, which is a guide for being less slippery. When comparing with other shoes, the key is to standardize conditions, such as changing only the desk material while using the same shoe.
This is the same idea as the elementary school students standardizing pulling speed with a metronome. If the numbers change depending on the desk material, it is a small confirmation of the view that slip resistance is determined by the combination of shoe sole and floor. [6]