A recent paper reported repeatability and bias test results for the variable incidence tribometer (VIT) and for the portable inclineable articulated strut slip tester (PIAST). These two tribometers approximate initial pedestrian heel-contact during walking, a critical consideration when attempting to quantify walkway slip resistance in a representative manner. Horizontal/vertical force-ratio settings on the tribometers were compared to measured ratios detected by a force plate. When these detected ratios were assessed mathematically by standard statistical methods, both tribometers displayed a high degree of repeatability and a low degree of bias. However, when assessed by graphical means, both tribometers exhibited patterns of indicated bias. An effort is made to identify the sources of the indicated bias. Results of the VIT analysis are consistent with its indicated bias being related to the influences of inertial forces. The PIAST appears systematically affected by the presence of elastomeric friction in its testing regime. Under elastomeric friction conditions, the ratio of the friction force developed divided by the applied normal force decreases as the applied normal forces increase. Under these conditions, the applied normal force and developed friction force are not directly proportional. The relevance of such conditions to pedestrian ambulation is discussed.
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