Static coefficient of friction (SCOF) was formerly used to measure the slip resistance of a wet floor in the USA, but the test method (ASTM C1028) was withdrawn by the ASTM in 2014. Experts in the USA now know to use dynamic coefficient of friction (DCOF), as the rest of the world has been using it for decades to get good information on the real-world slip resistance of a floor. So what’s the difference between SCOF vs. DCOF?

For centuries scientists have known that the force, F, needed to overcome friction and drag one object over another is approximately proportional to the normal force N (that is, the force of gravity or whatever else, often expressed in pounds) pressing the two surfaces together:

F = µN

where µ is called the coefficient of friction (COF). The lower limit to µ is zero, but there is no upper limit; it can exceed 1.0 by a wide margin in some cases. Usually, however, it is less than 1.0.

Now to SCOF vs. DCOF. If the two objects are at rest, µ is called the static coefficient of friction, or SCOF. If one surface is moving, µ is the dynamic coefficient DCOF. Frequently under dry conditions SCOF is greater than DCOF, which is typically about 20 percent lower.

When there is water or another lubricant (oil, dust, grease, etc.) on the interface between the two objects, the SCOF vs. DCOF  situation can be drastically different. For one thing, there’s the possibility of hydroplaning causing a much lower DCOF because the two solid surfaces may not be touching; the upper object can “skimboard” or “water ski” along the liquid film with almost zero friction. For a heel touching down lightly on the floor, hydroplaning wholly or partially can happen at a very low speed. On the other hand, under static conditions hydroplaning can’t happen. Thus with lubrication SCOF vs. DCOF can show a very big difference between the two.

For some years wet SCOF was used in a misguided attempt to assess slip safety of walking surfaces. A common test method was American Society for Testing and Materials ASTM C1028. In 2014 ASTM finally recognized that this was counterproductive and withdrew C 1028 with no replacement.

Static friction is appropriate to a pedestrian standing still on a surface, but most slipping injuries happen when the pedestrian is moving. Thus, worldwide, dynamic friction is used by experts to assess pedestrian safety, and SCOF vs. DCOF is not an issue.

The most widely used dynamic test method in the world is the pendulum DCOF skid tester. The digital tribometer BOT-3000E also assesses dynamic friction. The latest American National Standards Institute (ANSI) method using the BOT-3000 is ANSI A326.3, although we don’t recommend using the BOT-3000E alone to assess the slipperiness of a floor. The ANSI A326.3 test method itself says it shouldn’t be used for that purpose, but rather as a “comparison of surfaces”.

Perhaps more important than initial slip resistance of flooring, when it comes out of the factory, is long-term slip resistance. Many initially slip-resistant floors lose their wet slip resistance within a few weeks or months of being installed. That’s why McDonalds Restaurants developed their Sustainable Slip Resistance test to evaluate the effect of wear. It involves laboratory testing with the pendulum before and after a standard abrasive wear protocol. Property owners want their floors to be safe throughout a reasonable economic lifetime of the flooring, and this test helps them achieve that. Just testing as the flooring comes out of the box does not.

Safety Direct America offers both laboratory and field slip resistance testing of flooring and tiles using the pendulum DCOF tester and/or the BOT-3000E DCOF AcuTest. Sustainable slip resistance testing is also a frequently requested option that we provide.