Whole-Shoe Traction Tester
Last updated March 28, 2008 by William English
Although there have been nearly 100 tribometric devices mentioned in the scientific literature over the past half-century, the vast majority have been designed primarily to meter the slipperiness of walking surfaces, and are only secondarily used to evaluate the traction properties of shoe bottom materials and surface treatments. The XST was the first shoe tester to apply a whole shoe to a test surface much like the human foot does in walking, and it can do it right on the workplace floor.
Because of its explicit purpose as an evaluator of shoe traction, it is called the English XST Shoe Traction Tester.
What Was the Need for Such a Tester
Although ASTM's F13 Committee on Safety and Footwear Traction has as its primary role the development of means of measuring shoe traction performance against a reference value, over the 20 years that I have been an observer of its activities, there has never been a satisfactory device for testing the performance of a shoe product in the presence of various contaminants in which shoes are required to function. (For discussion of various whole-shoe testers in use around the world, including their limitations for wet testing, see the Second Edition of Pedestrian Slip Resistance, Chapter 13, "Traction Testing of Footwear.")
At the first F-13 Committee meeting I attended, I recall Charles Irvine saying that our primary mission was to develop a means of ranking shoes for traction performance in various commonly encountered walkway lubricating contaminants (such as water, grease or antifreeze) so they could be labeled in a way that would permit the purchaser to select shoes that would be safe for their intended purpose.
Today there is still no ranking or rating system and no labeling standard. Many shoe sellers are advertising footwear for slip resistance under various conditions and various claims are being made for shoe performance in the presence of lubricating contaminants without objective substantiation. Some shoes now on the market do in fact perform very well, but others perform poorly despite the seller's claims.
Further complicating the pedestrian safety climate is the fact that testing laboratories are doing a lot of evaluation of both shoe bottom assemblies and shoe bottom material samples using invalid test methods. Some of these improper methods are even identified as "recognized" US testing standards.
Even F13 has had a standard for the testing of shoe bottom materials based on the James Machine and the Horizontal Pull Slipmeter--neither of which can be used to measure traction under contaminated conditions, and ASTM standards for their use say so. Over my objection these obsolescent standards have been renewed again for the alleged reason that the subject is so important that we need to keep the old standards in place to sort of hold their place until a valid tester can be perfected.
The prototype XST was demonstrated at the July 11, 1995 F13 meeting near Washington, DC where my negative vote on the James Machine standard was overridden. Since then the production version of the XST has been developed. One was sold to the University of Washington for research work. Since there was no commercial market for ranking shoes in order of traction performance in the workplace, I have one production version of the XST available for sale.
Knowledgeable observers will notice immediately that the XST's action looks something like the miniature floor slipmeter that has been available as the English XL Variable Incidence Tribometer. That is, it is pressure-operated; it causes the shoe to contact heel-first in a manner that avoids the sticktion factor that has disqualified earlier static testers (including the James Machine and the famous SATRA tester.) Its unique feature is that it has a "live" ankle joint that lets the shoe come down into a flat position on the floor, and its ankle joint is pneumatically loaded in such a way as to resist its rotation into the flat position--not unlike what the tendons do in a human foot during ambulation.
Other unique features of the XST are its unparalleled controllability of the dynamics involved in the application of the shoe to the test surface. The force application is controlled by adjusting operating pressure, and the velocity of impact is controllable by adjusting flow rate. The hold-back force applied to the ankle motion is adjustable by controlling operating pressure to the secondary cylinder independently of the primary stroke cylinder. In addition to the variability of mast angle, the angle of shoe contact with the test surface is also mechanically adjustable.
Although there is still no shoe-testing standard (under the auspices of ASTM or ANSI) in place, several shoe sellers have commissioned testing of their products (here in my lab) against those of competitors, and a couple have used the XST test results in their advertising materials.
There is no particular need for a standard, however. The XST can compare any number of shoes head-to-head on contaminated surfaces and rank them in order of traction performance under the test conditions. For example, if a safety engineer wants to select the most slip-resistant shoe for a particular high-hazard operation, the XST can test shoes under consideration, either in the lab or on the actual shop floor in the workplace, and select the best performer.
Applications for the XST
This ergonomic shoe tester can be used to rank any number of shoe products in order of their traction performance on any test surface and under any contaminating conditions. Because it is even lighter than a Brungraber Mk II, it can be taken anywhere there is a supply of compressed air available; or if there is no plant air, a portable compressor can be used. It has been used repeatedly in the laboratory facilities of William English, Inc. to rank shoe products for manufacturers of work shoes and general footwear on several test surfaces and with several contaminants.
The production version of the XST has been evaluated on a triple-axis forceplate in the ambulation labs of the major sports footwear manufacturer, and the machine's dynamics were found to be similar to a pedestrian's on several parameters, both by forceplate traces and high-speed digitized video.
The selling price of the XST is $12,000USD, including the tribometer, carrying case and instruction of the user (here at my shop). If you are interested in buying one, come here for instruction and carry it home with you on the plane as checked luggage. The gross weight is less than a PAST or PIAST.
For the ultimate specialized shoe traction tester, the new SST, see http://englishxl.com/sst.html.
Call me at 239/728-3254 with your questions.
© William English 2008 - All Rights Reserved Worldwide. You may print off any information on this Web Site for your personal use, but no part of these documents may be otherwise reproduced in part or in full in other publications without the express, written approval of W. English.. The XL slipmeter and the XST Shoe Traction Tester are protected by US patent.